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Table of Contents

Spencer J. Buchanan 1

Donors 3

Spencer J. Buchanan Lecture Series 6

Agenda 7

Biographies

Dr. Craig Benson, 2014 Spencer J. Buchanan Lecturer

Dr. Arvid Landva, Guest Lecturer

Dr. Maria Caterina C. Santagata, Guest Lecturer

8

“Landfill Covers: water balance, unsaturated soils, and a pathway

from theory to practice”

Dr. Craig Benson

12

“Non-textbook soil mechanics: peatland soils and municipal

waste”

Dr. Arvid Landva

46

“Geo-chemical control of pore fluids for liquefaction mitigation” Dr. Maria Caterina Santagata

48

Pledge Information 55

SPENCER J. BUCHANAN

Spencer J. Buchanan, Sr. was born in 1904 in Yoakum, Texas. He graduated from Texas A&M University with a degree in Civil Engineering in 1926, and earned graduate

and professional degrees from the Massachusetts Institute of Technology and Texas A&M University.

He held the rank of Brigadier General in the U.S. Army Reserve, (Ret.), and

organized the 420th Engineer Brigade in Bryan-College Station, which was the only such unit in the Southwest when it was created. During World War II, he served the U.S. Army Corps of Engineers as an airfield engineer in both the U.S. and throughout the islands of the

Pacific Combat Theater. Later, he served as a pavement consultant to the U.S. Air Force and during the Korean War he served in this capacity at numerous forward airfields in the combat zone. He held numerous military decorations including the Silver Star. He was

founder and Chief of the Soil Mechanics Division of the U.S. Army Waterways Experiment Station in 1932, and also served as Chief of the Soil Mechanics Branch of the Mississippi

River Commission, both being Vicksburg, Mississippi.

Professor Buchanan also founded the Soil Mechanics Division of the Department of Civil Engineering at Texas A&M University in 1946. He held the title of Distinguished Professor of Soil Mechanics and Foundation Engineering in that department. He retired

from that position in 1969 and was named professor Emeritus. In 1982, he received the College of Engineering Alumni Honor Award from Texas A&M University.

He was the founder and president of Spencer J. Buchanan & Associates, Inc., Consulting Engineers, and Soil Mechanics Incorporated in Bryan, Texas. These firms were

involved in numerous major international projects, including twenty-five RAF-USAF airfields in England. They also conducted Air Force funded evaluation of all U.S. Air

Training Command airfields in this country. His firm also did foundation investigations for downtown expressway systems in Milwaukee, Wisconsin, St. Paul, Minnesota; Lake Charles, Louisiana; Dayton, Ohio, and on Interstate Highways across Louisiana. Mr.

Buchanan did consulting work for the Exxon Corporation, Dow Chemical Company, Conoco, Monsanto, and others.

Professor Buchanan was active in the Bryan Rotary Club, Sigma Alpha Epsilon

Fraternity, Tau Beta Pi, Phi Kappa Phi, Chi Epsilon, served as faculty advisor to the Student

Chapter of the American Society of Civil Engineers, and was a Fellow of the Society of American Military Engineers. In 1979 he received the award for Outstanding Service from the American Society of Civil Engineers.

Professor Buchanan was a participant in every International Conference on Soil Mechanics and Foundation Engineering since 1936. He served as a general chairman of the International Research and Engineering Conferences on Expansive Clay Soils at Texas

A&M University, which were held in 1965 and 1969.

Spencer J. Buchanan, Sr., was considered a world leader in geotechnical engineering, a Distinguished Texas A&M Professor, and one of the founders of the Bryan Boy’s Club. He died on February 4, 1982, at the age of 78, in a Houston hospital after an

illness, which lasted several months.

The Spencer J. Buchanan ’26 Chair in Civil Engineering

The College of Engineering and the Department of Civil Engineering gratefully recognize the

generosity of the following individuals, corporations, foundations, and organizations for their part in helping to establish the Spencer J. Buchanan ’26 Professorship in Civil Engineering. Created in 1992 to honor a world leader in soil mechanics and foundation engineering, as well as a

distinguished Texas A&M University professor, the Buchanan Professorship supports a wide range of enriched educational activities in civil and geotechnical engineering. In 2002, this professorship became the Spencer J. Buchanan ’26 Chair in Civil Engineering.

Donors

Founding Donor

Mr. C. Darrow Hooper ‘53

Benefactors ($5,000+)

Mr. Douglas E. Flatt ’53

ETTL Engineers and Consultants, Inc.

Fugro Consultants, Inc.

Patrons ($1,000-$4,999)

Aviles Engineering Corporation

Dr. Dionel E. Aviles ‘53

Dr. Rudolph Bonaparte

Dr. Mark W. Buchanan

Mr. & Mrs. Spencer J. Buchanan, Jr. ‘53

The Dow Chemical Foundation

Dr. Wayne A. Dunlap ‘51

Br. Gen. John C.B. Elliott

ExxonMobil Foundation

Mr. Perry G. Hector ‘54

Dr. Allen Marr

Dr. Jose M. Roesset

Spencer J. Buchanan Associates, Inc.

Dr. Kenneth H. Stokoe

Dr. Lyle A. Wolfskill ’53

Fellows ($500-$999)

Mr. & Mrs. John R. Birdwell ‘53

R.R. & Shirley Bryan

Mr. Joe L. Cooper ‘56

Mr. George D. Cozart ’74

Mr. & Mrs. Peter C. Forster ‘63

Harvey J. Haas, Col USAF (Ret) ‘59

Mr. Conrad S. Hinshaw ‘39

O’Malley & Clay, Inc.

Mr. Robert S. Patton ‘61

Mr. Donald R. Ray ‘68

Mr. Alton T. Tyler ‘44

Members ($100-$499)

Adams Consulting Engineers, Inc.

Lt. Col. Demetrios A. Armenakis ‘58

Mr. Eli F. Baker ‘47

Mr. & Mrs. B.E. Beecroft ‘51

Dean Fred J. Benson ‘36

Mr. & Mrs. Willy F. Bohlmann, Jr. ‘50

Mr. Craig C. Brown ‘75

Mr. Donald N. Brown ‘43

Ronald C. Catchings, LTC USA (Ret) ‘65

Mr. Ralph W. Clement ‘57

Coastal Bend Engineering Associates

Mr. & Mrs. James T. Collins

Mr. John W. Cooper, III ‘46

Mr. George W. Cox ‘35

Mr. Murray A. Crutcher, Jr. ‘68

Dodd Geotechnical Engineering

Mr. Donald D. Dunlap ‘58

Enterprise Engineers, Inc.

Edmond L. Faust, Jr. Col (Ret) ‘47

Mr. David T. Finley ‘82

Mr. Charles B. Foster, Jr. ‘38

Mr. Benjamin D. Franklin ‘57

Mr. Thomas E. Frazier ‘77

G.R. Birdwell Construction, L.P.

Mr. William F. Gibson ‘59

Dr. Anand V Govindasamy '09

Mr. Cosmo F. Guido ‘44

Joe G. Hanover, Gen (Ret) ‘40

Mr. & Mrs. John L. Hermon ‘63

William & Mary Holland

Dr. W. Ronald Hudson ‘54

Mr. Homer A. Hunter ‘25

Ms. Lyllis Lee Hutchin

Mr. & Mrs. Walter J. Hutchin ‘47

Mr. & Mrs. Shoudong Jiang ‘01

Brig. Gen. Hubert O. Johnson, Jr. ‘41

Lt. Col. William T. Johnson, Jr. ‘50

Mr. Homer C. Keeter, Jr. ‘47

Mr. Richard W. Kistner ‘65

Mr. Donald W. Klinzing

Andrew & Bobbie Laymay

Dr. YanFeng Li ‘04

Frank Lane Lynch, Col USA (Ret) ‘60

Marathon Oil Company

Major General Charles I McGinnis ‘49

Mr. Jes D. McIver ‘51

Mr. Charles B. McKerall, Jr. ‘50

Morrison-Knudsen Co., Inc.

Dr. James D. Murff ‘70

Mr. & Mrs. Frank H. Newnam, Jr.

Mr. & Mrs. Jack R. Nickel ‘68

Northrup Grumman Foundation

Mr. Roy E. Olson

Dr. Nicholas & Martha Paraska ‘47

Mr. & Mrs. Daniel E. Pickett ‘63

Pickett-Jacobs Consultants, Inc.

Mr. Richard C. Pierce ‘51

Mr. & Mrs. Robert J. Province ‘60

Mr. David B. Richardson ‘76

LTC David E. Roberts ‘61

Mr. Walter E. Ruff ‘46

Mr. Weldon Jerrell Sartor ‘58

Mr. Charles S. Skillman, Jr. ‘57

Soil Drilling Services

Mr. Louis L. Stuart, Jr. ‘52

Mr. Ronald G. Tolson, Jr. ‘60

Mr. & Mrs. Hershel G. Truelove ‘52

Mr. Kenneth C. Walker ‘78 Mr. & Mrs. Thurman Wathen Mr. Donald R. Wells ‘70

Williams Gas Pipelines-Transco

Mr. Andrew L. Williams, Jr. ‘50

Dr. James T.P. Yao

Associates ($25-$99)

Mr. & Mrs. John Paul Abbott

Mr. & Mrs. Charles A. Arnold ‘55

Bayshore Surveying Instruments Co.

Colonel Carl F. Braunig, Jr. ‘45

Mrs. E. D. Brewster

Mr. Robert P. Broussard

Mr. & Mrs. Norman J. Brown ‘49

Mr. Kenneth W. Korb ‘67

Dr. & Mrs. George W. Kunze

Mr. Larry K. Laengrich ‘86

Mr. Monroe A. Landry ‘50

Lawrence & Margaret Laurion

Mr. & Mrs. Charles A Lawler

Mrs. John M. Lawrence, Jr.

Mr. Stewart E. Brown

John Buxton, LTC USA (Ret) ‘55

Caldwell Jewelers

Lawrence & Margaret Cecil

Mr. & Mrs. Howard T. Chang ‘63

Mrs. Lucille Hearon Chipley

Mr. & Mrs. Joseph R. Compton

Caroline R. Crompton

Harry M. & Josephine Coyle

Mr. Robert J. Creel ‘53

Maj Gen Robert E. Crosser ‘49

Mr. O. Dexter Dabbs

Guy & Mary Bell Davis

Robert & Stephanie Donaho

Mr. Charles A. Drabek

Mr. & Mrs. Stanley A. Duitscher ‘55

Mr. & Mrs. Nelson D. Durst

Mr. George H. Ewing ‘46

Virginia & Edmond Faust

First City National Bank of Bryan

Mr. & Mrs. Neil E. Fisher ‘75

Mr. & Mrs. Albert R. Frankson

Maj. Gen Guy & Margaret Goddard

Lt. Col. John E. Goin ‘68

Mr. & Mrs. Dick B. Granger

Col. Howard J. Guba ‘63

Halliburton Foundation, Incorporated

James & Doris Hannigan

Mr. Scott W. Holman III ‘80

Lt. Col. & Mrs. Lee R. Howard ‘52

Mrs. Jack Howell

Col. Robert & Carolyn Hughes

Mr. & Mrs. William V. Jacobs ‘73

Mr. Ronald S. Jary ‘65

Richard & Earlene G. Jones

Stanley R. Kelley, COL USA (Ret) ‘47

Mr. Elmer E. Kilgore ‘54

Mr. Kenneth W. Kindle ‘57

Mr. Tom B. King

Lt. Col. Walter A. Klein ‘60

Lockwood, Andrews, & Newman, Inc.

Lt. Col. Linwood E. Lufkin ‘63

Robert & Marilyn Lytton

W.T. McDonald

James & Maria McPhail

Mr. & Mrs. Clifford A. Miller

Minann, Inc.

Jack & Lucille Newby

Leo Odom

Mr. & Mrs. Bookman Peters

Mr. Charles W. Pressley, Jr. ‘47

Mr. & Mrs. D.T. Rainey

Maj. Gen. & Mrs. Andy Rollins

Mr. J. Jack Rollins

Mr. & Mrs. John M. Rollins

Mr. & Mrs. J.D. Rollins, Jr.

Brig. Gen. Allen D. Rooke, Jr. ‘46

Mr. Paul D. Rushing ‘60

Schrickel, Rollins & Associates, Inc.

William & Mildred H. Shull

S.K. Engineering

Mr. Milbourn L. Smith ‘60

Southwestern Laboratories

Mr. & Mrs. Homer C. Spear

Mr. & Mrs. Robert F. Stiles ‘79

Mr. & Mrs. Robert L. Thiele, Jr.’63

W. J. & Mary Lea Turnbull

Mr. & Mrs. John P. Tushek

Mr. Edward Varela ‘88

Troy & Marion Wakefield

Ms. Constance H. Wakefield

Mr. & Mrs. Allister M. Waldrop

Mr. Robert R. Werner ‘57

Mr. & Mrs. William M. Wolf, Jr. ‘65

Mr. & Mrs. John S. Yankey III ‘66

Dr. Congpu Yao ‘13

H.T. Youens, Sr

Mr. William K. Zickler ‘83

Mr. Ronald P. Zunker ‘62

Every effort was made to ensure the accuracy of this list. If you feel there is an error, please contact

the Engineering Development Office at 979-845-5113. A pledge card is enclosed on the last page

for potential contributions.

Spencer J. Buchanan Lecture Series

1993 Ralph B. Peck “The Coming of Age of Soil Mechanics: 1920 - 1970”

1994 G. Geoffrey Meyerhof “Evolution of Safety Factors and Geotechnical Limit State Design”

1995 James K. Mitchell “The Role of Soil Mechanics in Environmental Geotechnics”

1996 Delwyn G. Fredlund “The Emergence of Unsaturated Soil Mechanics”

1997 T. William Lambe “The Selection of Soil Strength for a Stability Analysis”

1998 John B. Burland “The Enigma of the Leaning Tower of Pisa”

1999 J. Michael Duncan “Factors of Safety and Reliability in Geotechnical Engineering”

2000 Harry G. Poulos “Foundation Settlement Analysis – Practice Versus Research”

2001 Robert D. Holtz “Geosynthetics for Soil Reinforcement”

2002 Arnold Aronowitz “World Trade Center: Construction, Destruction, and Reconstruction”

2003 Eduardo Alonso “Exploring the Limits of Unsaturated Soil Mechanics: the Behavior of

Coarse Granular Soils and Rockfill”

2004 Raymond J. Krizek “Slurries in Geotechnical Engineering”

2005 Tom D. O’Rourke “Soil-Structure Interaction Under Extreme Loading Conditions”

2006 Cylde N. Baker “In Situ Testing, Soil-Structure Interaction, and Cost Effective Foundation

Design”

2007 Ricardo Dobry “Pile response to Liquefaction and Lateral Spreading: Field Observations

and Current Research”

2008

Kenneth Stokoe

"The Increasing Role of Seismic Measurements in Geotechnical

Engineering"

2009 Jose M. Roesset “Some Applications of Soil Dynamics”

2010 Kenji Ishihara “Forensic Diagnosis for Site-Specific Ground Conditions in Deep

Excavations of Subway Constructions”

2011 Rudolph Bonaparte “Cold War Legacy – Design, Construction, and Performance of a Land-

Based Radioactive Waste Disposal Facility”

2012 W. Allen Marr “Active Risk Management in Geotechnical Engineering”

2013 Andrew J. Whittle “ Importance of Undrained Behavior in the Analysis of Soil-Structure

Interaction”

2014 Craig H. Benson “Landfill Covers: Water Balance, Unsaturated Soils, and a Pathway from

Theory to Practice”

The texts of the lectures and a DVD’s of the presentations are available by contacting:

Dr. Jean-Louis Briaud

Spencer J. Buchanan ’26 Chair Professor

Zachry Department of Civil Engineering

Texas A&M University

College Station, TX 77843-3136, USA

Tel: 979-845-3795

Fax: 979-845-6554

E-mail: Briaud@tamu.edu

https://ceprofs.civil.tamu.edu/briaud/buchanan%20web/lecture_series.htm

AGENDA

The Twenty-second Spencer J. Buchanan Lecture

Friday, November 7, 2014

College Station Hilton

2:00 p.m. Introduction by Jean-Louis Briaud

2:05 p.m. Katherine Banks – Dwight Look College of Engineering

2:10 p.m. Robin Autenrieth – Zachry Department of Civil Engineering

2:15 p.m. Gary Gregory – ASCE Geo Institute

2:20 p.m. Introduction of Arvid Landva by Jean-Louis Briaud

2:25 p.m. “Non-Textbook Soil Mechanics: Peatland Soils and Municipal

Waste”

Lecture by Arvid Landva

2:55 p.m. Introduction of Maria Caterina Santagata by Jean-Louis Briaud

3:00 p.m. “Geo-Chemical Control of Pore Fluids for Liquefaction

Mitigation”

Lecture by Maria Caterina Santagata

3:30 p.m. Introduction of Craig Benson by Jean-Louis Briaud

3:35 p.m. “Landfill Covers: Water Balance, Unsaturated Soils, and a

Pathway from Theory to Practice”

The 2014 Buchanan Lecture by Craig Benson

4:35 p.m. Discussion

4:50 p.m. Closure with Jean-Louis Briaud

5:00 p.m. Photos followed by a Reception at the home of Jean-Louis and

Janet Briaud

BBiiooggrraapphhiieess

Craig H. Benson, Ph.D., PE, NAE Wisconsin Distinguished Professor

Director of Sustainability Research & Education, UW-Madison Chair, Civil & Environmental Engineering, Geological Engineering

Dr. Benson is a Distinguished Professor at the University of

Wisconsin-Madison (UW-Madison), where he serves as

Director of Sustainability Research and Education and Co-

Director of the Office of Sustainability. He is also Chair of

the Departments of Civil and Environmental Engineering

and Geological Engineering. He is an international expert

in geoenvironmental engineering and a member of the US

National Academy of Engineering. Dr. Benson has a BS

from Lehigh University and MSE and PhD degrees from the

University of Texas at Austin, all in Civil Engineering with

an emphasis in geoenvironmental, geotechnical, and

geological engineering.

Dr. Benson has been conducting experimental and

analytical research related to protection of the

environment for nearly three decades, with a primary focus on beneficial use of industrial

byproducts; environmental containment of solid, hazardous, radioactive, and mining wastes;

and sustainable infrastructure. His research includes laboratory studies, large-scale field

experiments, and computer modeling. In the Office of Sustainability, Dr. Benson is responsible

for coordinating all sustainability related education on the UW-Madison campus and for

facilitating the interdisciplinary sustainability research enterprise across the UW-Madison

campus.

Dr. Benson has received several awards for his work, including the Ralph Peck Award, the

Huber Research Prize, the Alfred Noble Prize, and the Croes (twice), Middlebrooks Award

(twice), Collingwood Award, and Casagrande Award from the American Society of Civil

Engineers and the Award of Merit and the Best Practical Paper Award from ASTM

International. Dr. Benson is a former Editor-in-Chief of the Journal of Geotechnical and

Geoenvironmental Engineering. He is a past President of the ASCE Geo-Institute and past

Vice Chair of the Executive Committee of ASTM Committee D18 on Soil and Rock. Dr. Benson

is a member of the Academy of Distinguished Alumni at the University of Texas at Austin.

Arvid O. Landva, Ph.D. Professor Emeritus

University of New Brunswick, Canada

B.Sc., University of Strathclyde, Civil Engineering

Dr.ing., Norwegian Institute of Technology and

Norwegian Geotechnical Institute, dissertation on the

geotechnical behaviour and testing of soft quick clays

(supervisors Dr. L. Bjerrum, Dr. N. Janbu and Dr. R.

Selmer-Olsen)

Ph.D., Université Laval, dissertation on the geotechnical

behaviour and testing of peats (supervisor Dr. P. La

Rochelle, external advisor Dr. N.W. Radforth,

University of New Brunswick)

Main research topics

The geotechnical behaviour of -

Norwegian and Canadian quick clays

peats and organic soils

bonded New Brunswick clayey silt of constant or increasing water content with

depth

incinerated waste as compacted embankment fill

municipal solid waste

hazardous wastes

Maria Caterina C. Santagata, Ph.D. Associate Professor of Civil Engineering

Purdue University

Dr. Santagata is an Associate Professor in the School of

Civil Engineering at Purdue University. She holds a “Laurea in Ingegneria Civile” from the University of Ancona in Italy, and MS and PhD degrees in Civil and

Environmental Engineering from MIT. Dr. Santagata’s research interests and contributions are founded on fundamental studies of the behavior of a broad range of

soils and of their interaction with other materials, relying on experimental investigations that probe the materials

of interest at different scales. Recent research efforts have focused on: earthquake induced soil liquefaction, rheology and microstructure of clay dispersions,

lubrication properties of drilling fluids, mineral surface-water interactions in clay minerals, behavior and improvement of soils with high organic content, stress-strain-strength response of sensitive

carbonatic clays. At Purdue, Dr. Santagata routinely teaches the introductory geotechnical engineering course, and graduate courses devoted to the fundamentals of soil behavior and geotechnical testing.

Dr. Santagata is the Chair of the Committee on Soil Properties and Modeling of the Geo-

Institute of ASCE, and serves on the G-I’s Awards Committee. She is a member of the Committees on Soils and Rock Instrumentation and on Cementitious Stabilization of TRB. She served as an Associate Editor for ASCE’s Journal of Geotechnical and Geoenvironmental

Engineering between 2005 and 2013, and is currently a member of the Editorial Board of Rivista Italiana di Geotecnica.

Dr. Santagata received the National Science Foundation CAREER Grant in 2007 and Purdue University’s Faculty Fellowship for Study in a Second Discipline in 2009. At Purdue her work has been recognized with the Roy E. & Myrna G. Wansik Excellence in Research

Award (2007), the Edmund M. Burke Outstanding Professor Award from Chi Epsilon (2006), the Ross Judson Buck '07 Memorial Award for Undergraduate Counseling (2006) and the

Harold Munson Award for Outstanding Teacher in the School of Civil Engineering (2004). Dr. Santagata is a registered professional engineer in Italy.

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LANDFILL COVERS: WATER BALANCE, UNSATURATED SOILS, AND A

PATHWAY FROM THEORY TO PRACTICE

By Craig H. Benson1 and Christopher A. Bareither

2

1 Wisconsin Distinguished Professor and Director of Sustainability Research and Education,

University of Wisconsin-Madison, Madison, WI USA 53706 USA, chbenson@wisc.edu 2 Asst. Professor, Civil & Environmental Engineering, Colorado State Univ., Fort Collins,

CO, USA 80523 USA, bareither@wisc.edu

ABSTRACT

This paper describes key steps in the design process for water balance covers using a

case history at a municipal solid waste landfill in Missoula, Montana, USA as an

example. The intent is to illustrate how state-of-the-art concepts can be applied in the

state-of-the-practice. The process begins by understanding the design objective

(including regulatory requirements) and investigating lines of evidence indicating that

a water balance cover is likely to function satisfactorily at the design location. Data

from two other instrumented water balance covers in the region are used to evaluate

efficacy along with historical meteorological data and information contained in a

prior unsuccessful submittal for a water balance cover at the Missoula landfill. Site

characterization is conducted to define properties of the soil resources and vegetation

at the site for preliminary cover sizing and numerical modeling of the water balance.

A numerical model is used with various design metrological conditions as input to

evaluate whether the cover will meet the design goal under realistic conditions. The

final design consists of a monolithic cover comprised of a 1.22-m storage layer

overlain by a 0.15-m topsoil layer. A field test with a fully instrumented lysimeter

was constructed and monitored to confirm that the cover performs as anticipated in

the design.

INTRODUCTION

Final covers for waste containment that rely on principles of variably saturated flow

to control percolation into underlying waste have become accepted as a viable

methodology for long-term isolation of waste, particularly in semi-arid and arid

regions where precipitation is favorably balanced by the energy available for

evaporation (Khire et al. 2000, Zornberg et al. 2003, Albright et al. 2004, Malusis and

Benson 2006). These covers are referred to using various names, including water

balance covers, store-and-release covers, evapotranspirative (or “ET”) covers, and

alternative covers. The nomenclature “water balance cover” is used by the authors

because this term represents the basic principle on which these covers function – the

ability to balance storage of water corresponding to an acceptable level of percolation

with the ability of plants and the atmosphere to remove stored water and replenish the

water storage capacity of the cover profile. The authors specifically do not use the

“ET cover” nomenclature because evapotranspiration is the predominant component

of the water balance in nearly all cover systems, and thus is not particularly

descriptive. The balance between water balance quantities is the feature that makes

water balance covers unique.

Monolithic barriers and capillary barriers are the most common forms of water

balance covers (Benson 2001, Ogorzalek et al. 2007, Bohnhoff et al. 2009) (Fig. 1).

Monolithic barriers consist of a thick layer of fine-textured soil that is engineered and

placed with appropriate compaction specifications so that the cover stores infiltrating

water with little drainage while unsaturated. Capillary barriers generally consist of a

two-layer system comprised of an overlying fine-textured storage layer similar to a

monolithic cover that is underlain by a clean coarse-grained layer that provides a

contrast in unsaturated hydraulic properties. The “capillary break” formed at the

interface of the two layers enhances the storage capacity of the fine-textured layer

(Stormont and Morris 1998, Khire et al. 2000), and can also promote lateral diversion

of water in the fine-textured layer (Stormont 1995). For both types of covers,

thickness of the storage layer is selected to have adequate capacity to store infiltrating

water during the wet season while ensuring the cover meets the design percolation

rate (Benson 2001). A surface layer of topsoil normally is placed over the storage

layer of either type of cover to provide a hospitable environment for the plant

community. Storage capacity of the topsoil layer generally is ignored during design.

Waste Waste

Capillary

Barrier

Monolithic

Barrier

Coarse Soil

Fine

Textured

Soil

Fine

Textured

Soil

Top Soil Top Soil

FIG. 1. Schematic of monolithic and capillary barriers.

Water balance covers are designed to be compliant with natural hydrologic

conditions and to rely on hydrologic processes comparable to those in the

surrounding landscape. Natural hydrologic controls are employed in lieu of

engineered hydraulic barriers because, in most cases, final covers must function for

decades to centuries, and in some case millennia. A system engineered to be

compliant with nature is more likely to function over these long periods compared to

a system that employs engineered hydraulic barriers not commonly found in nature.

However, because natural hydrologic controls are employed, water balance covers

may not be appropriate for all climates (e.g., controlling percolation to minute

quantities in a humid region with high precipitation may not be practical).

Understanding this limitation is important, and the engineer must resist the temptation

to force-fit water balance covers into applications where they are not appropriate.

Sustainability is an intrinsic principle in water balance covers. These covers

employ natural hydrologic processes congruent with the surrounding landscape,

which reduces long-term maintenance requirements. On-site materials (soils and

plants) are employed, which minimizes transportation requirements, and

straightforward construction methods are employed that can be implemented by local

personnel. As a result, energy consumption and emissions are reduced, natural

resource consumption is limited, and local economies are supported. In the current

regulatory climate, a tacit assumption is made that the waste being contained is

stabilized and has minimal or no value as a resource. This assumption may not be

realistic and requires examination in the context of sustainability.

Over the last two decades, the senior author (Benson) has been intimately involved

in research focused on exploring the mechanisms controlling performance of water

balance covers, developing measurement methods to characterize the engineering

properties needed for design (e.g., ASTM D 6093 and D 6836, Suwansawat and

Benson 1998, Khire et al. 1995, Meerdink et al. 1995, Albrecht et al. 2003, Wang and

Benson 2004, Benson et al. 2007a, 2007b, 2011a, Schlicht et al. 2010), developing

methods and models for sizing covers and predicting performance (e.g., Khire et al.

1997, 1999, 2000; Benson and Chen 2003; Shackelford and Benson 2006; Albright et

al. 2010; Ogorzalek et al. 2007; Benson 2007, 2010; Bohnhoff et al. 2009; Smesrud et

al. 2012), and defining methods to confirm field performance (e.g., Benson et al.

1994, 1999, 2001, 2011b; Kim and Benson 2002; Benson and Wang 2006, Waugh et

al. 2008, 2009). Connecting theory and practice as well as coupling bench-scale to

field-scale have been threads throughout this research program. This research effort

has been sponsored by a broad set of stakeholders concerned with long-term waste

containment, including the National Science Foundation, the US Environmental

Protection Agency’s (USEPA) Alternative Cover Assessment Program (ACAP), the

US Department of Energy’s (DOE) Environmental Management (EM) and Legacy

Management (LM) programs, and the US Nuclear Regulatory Commission. Dr.

William Albright (Desert Research Institute) and Dr. William “Jody” Waugh (Stoller

Corporation and DOE-LM) have been collaborators in this effort since 1999.

In 2008, USEPA commissioned a guidance document summarizing the knowledge

gained from these two decades of research, development, and practice. This

document evolved into the book Water Balance Covers for Waste Containment:

Principles and Practice, by Albright, Benson, and Waugh, which was published by

ASCE Press in 2010 (Albright et al. 2010). The objective of the guidance document

and book was to facilitate the transition from state-of-the-art to state-of-the-practice.

Practitioners and environmental regulatory agencies in the US and abroad have

adopted the principles and strategies described in this book.

A case history is described in this paper where state-of-the-art principles described

in the book were employed in the state-of-the-practice to evaluate, design, and

demonstrate the viability of a water balance cover for an operating municipal solid

waste (MSW) landfill in Missoula, Montana. While this case study applies to MSW

containment in a semi-arid climate, the principles are universal and can be (and have

been) adapted to design covers for other types of waste and in other climates. The

discussion contained herein is brief to meet publication constraints; more detailed

discussion of each of the issues is covered in the book. Moreover, the extensive

citations common in academic scholarship have been forgone. The book serves as the

primary reference, and within the book numerous citations are included.

PROCESS

The procedure for designing and evaluating a water balance cover consists of five

steps, which can be summarized as follows (Albright et al. 2010):

1. Preliminary assessment – determine the performance goal and seek lines of

evidence that a water balance cover may be successful at the proposed location.

Understand the expectations of the overseeing regulatory authority and

constraints required by the owner.

2. Site characterization – characterize the soils and vegetation available for the

water balance cover.

3. Storage assessment – estimate the required thickness of the water balance cover

by determining the amount of water that must be stored and the capacity of the

cover to store the water.

4. Water balance modeling – predict the performance of the cover identified in the

storage assessment for realistic meteorological data using a numerical model that

simulates variably saturated flow and root water uptake in a multilayer system

with a climatic flux boundary at the surface; refine the cover thickness if

necessary.

5. Performance demonstration – conduct a performance demonstration to validate

that the design meets the performance goal by instrumenting the actual cover or

constructing a full-scale test section.

Each of these steps was conducted when evaluating, designing, and demonstrating the

water balance cover for the site in Missoula.

PRELIMINARY ASSESSMENT

The preliminary assessment addresses two fundamental questions:

What is the design goal for the project?

What evidence exists that the design goal can be achieved with the climate, soils,

and vegetation at the site?

Both of these questions need to be addressed during the preliminary assessment.

The first question seems obvious, but is often overlooked until the project is far

along. The second question is particularly important. If strong evidence is not

available indicating a water balance cover will be successful, the engineer must

carefully consider whether the design process should continue.

Discussions with the Montana Department of Environmental Quality (MDEQ, the

regulatory agency with jurisdiction over the site), and review of MDEQ’s Draft

Alternative Final Cover Guidance (v. 9-11), indicated that the water balance cover

for the Missoula landfill must be hydraulically equivalent to the conventional cover

required in Montana for MSW landfill cells containing a composite liner. The

conventional cover consists of a composite barrier with a compacted soil barrier

having a saturated hydraulic conductivity no more than 10-5

cm/s overlain by a

geomembrane and a vegetated surface layer (Fig. 2).

0 mm

300 mm

600 mm

900 mm

1200 mm

1500 mm

1800 mm

Helena

(ACAP)

Polson

(ACAP)

Conventional

Cover

2002

Missoula

Cover

SMSC

ML

SM

GP

SC

-

CH

SC

2100 mm

Topsoil/ Vegetative Layer

Storage Layer

Interim Cover

Silty sand

GravelGP

Compacted Soil Barrier

Geomembrane

SM

SM

FIG. 2. Cover profiles at Missoula (conventional cover and 2002 Missoula

Cover) and water balance cover profiles in Polson and Helena, MT. Soil

classification based on Unified Soil Classification System.

MDEQ does not stipulate statewide equivalent percolation rates for conventional

covers. Owners are required to propose an equivalent percolation rate for

consideration and possible concurrence by MDEQ. For this project, the design

percolation rate recommended by ACAP for conventional composite covers (3 mm/yr

average percolation rate, Benson 2001) was proposed, and accepted by MDEQ as the

design goal. The rate is reported in units of length/time, which corresponds to units

of volume/time per area. The equivalency criterion for this project is similar to, but

slightly less than the recommendation in Apiwantragoon (2007) for covers with

composite barriers (4 mm/yr). North Dakota also stipulates 4 mm/yr for an

equivalent percolation rate.

Supporting Evidence

Evidence was sought to determine if a design percolation rate of 3 mm/yr was

realistic for the Missoula landfill. Field data from other projects in similar climates

and with similar soils and vegetation generally comprise the best evidence. In this

case, water balance covers had been evaluated at MSW landfills in Polson and

Helena, Montana as part of ACAP (Albright et al. 2004, Apiwantragoon 2007).

These sites are 115 km north (Polson) and 185 km east (Helena) of Missoula. Index

and hydraulic properties of the cover soils were available for both sites (Benson et al.

2011a) as well as data from an ACAP-style lysimeter (Apiwantragoon 2007) used to

characterize the water balance and verify the percolation rate. At both landfills, the

water balance cover evaluated with the ACAP lysimeter was deployed as final cover.

Profiles of the water balance covers evaluated in Polson and Helena by ACAP are

shown in Fig. 2. Both included a capillary break. At Polson, however, the contrast

between the layers was modest because the underlying sand contained fines. At

Helena, the storage layer was thick and had relatively high air entry pressure.

Consequently, the water balance covers at Polson and Helena functioned like

monolithic covers, even though they included a break in soil texture (Apiwantragoon

2007). For this analysis, the covers at Polson and Helena were assumed to function

as monolithic covers.

The covers at Polson and Helena functioned remarkably well, with average

percolation rates of 0.5 mm/yr (Polson) and 0.0 mm/yr (Helena) during the ACAP

monitoring period (2000-04) (Apiwantragoon 2007). Thus, these covers provided a

good benchmark for assessing the viability of a water balance cover in Missoula.

That is, if the climate in Missoula is sufficiently similar to the climates in Polson and

Helena, then a water balance cover for Missoula should function comparable to the

covers in Polson and Helena, provided the cover in Missoula has similar available

water storage capacity relative to the required storage capacity.

Additional information was available from an application made by the landfill

owner in 2002 (Miller 2002) to deploy a water balance cover with a 0.91-m-thick

storage layer (Fig. 2). This application was based primarily on findings from a

numerical modeling exercise using on-site soil hydraulic properties. The modeling

indicated that percolation would be nil for typical meteorological conditions. The

application was not approved because the site characterization was limited, wetter

than normal conditions were not assessed, and no provision was made to demonstrate

performance at full scale. This proposed cover is referred to henceforth as the “2002

Missoula Cover,” although a cover was never constructed based on this design.

The data from Polson and Helena were evaluated in the context of the conditions in

Missoula to determine if these sites could be used as analogs, and to determine if the

design goal for Missoula was realistic given the meteorological conditions at the site,

the soil resources available, and the local vegetation. Soil hydraulic properties

reported in Miller (2002) were used for this preliminary assessment.

Climate Assessment

Meteorological data for Missoula were obtained from the National Weather Service.

Climate type, average annual precipitation, and average high and low temperatures

for Missoula, Polson, and Helena are summarized in Table 1. The data correspond to

the period 1952-2010, for which a complete record of precipitation and temperature

was available for all three sites.

Missoula and Helena have semiarid climates based on the definitions in UNESCO

(1999), whereas Polson is subhumid. Polson has the highest average annual

precipitation (380 mm) and the highest ratio of annual precipitation (P) to annual

potential evapotranspiration (PET). The ratio P/PET is a measure of the amount of

water to be managed (a fraction of P) relative to the energy available to manage water

via evapotranspiration. Lower P/PET corresponds to greater aridity and higher

confidence in managing precipitation with minimal percolation; i.e., the likelihood of

achieving a percolation goal increases as annual P/PET decreases. Annual

precipitation at all three locations was close to average during 2000-04, when data

were collected from the ACAP-style lysimeters at Polson and Helena (Table 1).

Thus, the water balance data from Polson and Helena during this period represent

typical conditions.

Table 1. Climatic data for Missoula, Helena, and Polson, Montana.

Site Climate

Avg.

Annual

Precip.

(mm)

Avg.

Annual

P/PET

Avg.

Annual

Precip.

During

ACAP

(mm)

Avg. Air Temperature

(°C)

High with

Month

Low with

Month

Missoula Semi-

arid 337 0.40 333 30 (July) -10 (Jan.)

Helena Semi-

arid 289 0.44 270 29 (July) -13 (Jan.)

Polson Sub-

humid 380 0.58 362 29 (July) -7 (Jan.)

Daily average precipitation is shown in Fig. 3a for Missoula, Polson, and Helena.

The annual precipitation pattern is similar for all three sites, with the wettest period

occurring at the end of spring and beginning of summer, followed by a much drier

period in mid summer and a wetter period in late summer and early fall. Fall and

winter are the driest seasons. Daily precipitation at Missoula is more similar to

precipitation in Polson than Helena. However, Missoula is drier than Polson,

particularly in the spring. Missoula is wetter than Helena in the winter and spring,

and comparable in the summer (Fig. 3a).

Daily average minimum and maximum air temperatures at all three sites (Fig. 3b)

show similar seasonality. Missoula has slightly higher maximum daily air

temperatures than Polson and Helena, except in late fall and winter. The daily

average minimum air temperature tends to be cooler in Missoula than Polson and

Helena during late spring, summer, and early fall (Fig. 3b), which is indicative of a

clearer sky and lower humidity during the summer. Polson exhibits the least seasonal

variation in temperature of the three sites, and a slightly smaller difference between

daily maximum and minimum temperature, due to buffering provided by Flathead

Lake (adjacent to Polson).

0.0

0.5

1.0

1.5

2.0

2.5

Daily Aveage

Precipitation

1971 - 2000

MissoulaHelenaPolson

Aver

age D

aily

Pre

cip

itati

on (

mm

)

(a)

SpringWinter Summer Fall

-20

-10

0

10

20

30

40

0 50 100 150 200 250 300 350 400

Daily Average

Temperature

1971 - 2000

Missoula

HelenaPolson

Av

erag

e D

aily

Max

imum

and M

inim

um

Tem

per

ature

(oC

)

Julian Day

(b)

SpringWinter Summer Fall

FIG. 3. Daily average precipitation (a) and daily average maximum and

minimum temperature (b) between 1971 and 2000 for Missoula, Helena,

and Polson.

Annual daily average solar radiation, relative humidity, and wind speed (all

affecting ET) for the period 1991-2005 for Missoula, Polson, and Helena (period for

which complete data are available for all three sites) are shown in Fig. 4 using box

plots. The centerline of the box is the median, the outer boundaries represent the

interquartile range (i.e., 25th

and 75th

percentile), and the upper and lower whiskers

represent the 10th

and 90th

percentiles of the data. Outliers are shown as individual

data points above or below the whiskers (e.g., Fig. 4c). Solar radiation at each site is

comparable since the three locations are at similar latitude. Polson is the most humid

site, due to the proximity of Flathead Lake. Helena is the least humid and windiest

site (Fig. 4b and c). Missoula falls between Polson and Helena for all three

meteorological parameters.

Soil Resource Assessment

Index properties of the soil proposed by Miller (2002) for the 2002 Missoula Cover

are summarized in Table 2 along with the properties of the storage layers in Polson

and Helena. The Missoula soil is a broadly graded silty sand with gravel (SM). The

storage layer at Polson has an upper layer of lean clay (CL-ML) over a lower layer of

silty sand (SM). At Helena, the storage layer is silty clay (SC). The soils at Polson

and Helena contain less gravel ( 6%) than the Missoula soil reported by Miller

(2002) (33% gravel, Table 2).

Table 2. Composition and classification of storage layer soils at Missoula

(Miller 2002) and Polson and Helena (Albright et al. 2004).

Site Unified Soil

Classification

Particle Size Distribution (%) Atterberg Limits

Gravel Sand Fines 2 m

Clay

Liquid

Limit

Plasticity

Index

Missoula SM 33 34 33 NR NR NR

Helena SC 2 54 44 30 67 47

Polson SM 6 54 42 5 NP NP

CL-ML 0.8 6.1 93.2 18 28 7

Notes: NR = not reported; NP = non-plastic as defined in ASTM D 2487; particle sizes based on

definitions in the Unified Soil Classification System (ASTM D 2487): gravel > 4.8 mm, 4.8 mm >

sand > 0.75 mm, fines < 0.75 mm

The silty clay at Helena has a similar distribution of predominant particle sizes as

the silty clay at Polson (Table 2), but the Helena soil has moderately plastic fines and

a larger clay fraction (30% vs. 5%). Although Atterberg limits were not reported for

the Missoula soil by Miller (2002), the high percentage of sand and gravel and the

relatively high saturated hydraulic conductivity (see subsequent discussion), suggests

that the Missoula soil probably is non-plastic.

Saturated and unsaturated hydraulic properties reported by Miller (2002) for the

Missoula soil are in Table 3 along with properties for covers in Polson and Helena.

The hydraulic properties at Polson and Helena were measured during construction as

well as 9 yr after the covers had been in service (cited as the “in-service” condition

0

50

100

150

200

250

300

350

Daily Averages: 1991- 2005

Glo

bal

Rad

iati

on

(W

/m2)

(a)

40

50

60

70

80

90

Rel

ativ

e H

um

idit

y (

%)

(b)

0

1

2

3

4

5

Missoula Helena Polson

Win

d S

pee

d (

m/s

)

(c)

FIG. 4. Box plots of daily average global radiation (a), relative humidity (b),

and wind speed (c) from 1991 to 2005 for Missoula, Helena, and Polson.

henceforth). The saturated hydraulic conductivities (Ks) of the Missoula soil and the

as-built silty sand at Polson (4.2x10-5

to 4.9x10-5

cm/s) are comparable. The silty

clay at Polson and clayey sand at Helena were less permeable when constructed (Ks =

1.5x10-7

to 4.0x10-7

cm/s). However, samples collected after 9 yr of service indicated

that pedogenesis had altered the soils (Ks = 2x10-6

to 8x10-6

cm/s), making them

nearly as permeable as the Missoula soil. (Benson et al. 2007, 2011a).

Soil water characteristic curves (SWCC) for the soils at Polson, Helena, and

Missoula (as described in Miller 2002) are shown in Fig. 5. van Genuchten’s (1980)

equation was used to describe the SWCC:

m

n

rs

r

)(α1

1

θθ

θθ

ψ (1)

where and n are fitting parameters, s is the saturated volumetric water content, r

is the residual water content, and m = 1-1/n. The fitted parameters are summarized in

Table 3 for as-built conditions in Polson and Helena, and laboratory-compacted

conditions for the Missoula soil as reported by Miller (2002). The SWCCs in Fig. 5

correspond to the as-built and in-service conditions for Polson and Helena, and the

laboratory-compacted Missoula soil reported by Miller (2002).

Table 3. Hydraulic properties of storage layers at Polson and Helena and

for 2002 Missoula Cover.

Site Soil Hydraulic Properties

Ks (cm/s) (kPa-1

) n s r

2002 Missoula

Cover SM 4.9x10

-5 0.052 1.28 0.34 0.00

Helena SC 1.5x10-7

0.0018 1.19 0.34 0.00

Polson SM 4.2x10

-5 0.0010 1.40 0.35 0.00

CL-ML 4.0x10-7

0.0027 1.27 0.30 0.00

The as-built soils for Polson and Helena and the Missoula soil have similar s and r

(Table 3), but the parameter reported by Miller (2002) for the Missoula soil is more

than one order of magnitude larger than the as-built for the soils at Polson and

Helena (0.0010-0.0027 1/kPa). However, the in-service for both Polson and Helena

(0.01-0.05 1/kPa, Benson et al. 2011a) after 9 yr of service is similar to for the

Missoula soil reported by Miller (2002). All of the soils have similar n (1.19 to 1.40)

in the as-built and in-service conditions (Benson et al. 2011a).

Storage Assessment

Available Soil Water Storage Capacity. Available soil water storage capacity (SA)

was computed for the covers in Polson and Helena along with the profile proposed

previously for the 2002 Missoula Cover (Miller 2002) using:

SA = L (FC – WP) (2)

10-1

100

101

102

103

104

105

2002 Missoula Cover

Laboratory Compacted

Su

ctio

n (

kP

a)

= 1500 kPa

= 33 kPa

(a)

10-1

100

101

102

103

104

105

Helena

As BuiltIn Service

Su

ctio

n (

kP

a)

= 1500 kPa

= 33 kPa

(b)

10-1

100

101

102

103

104

105

0.0 0.1 0.2 0.3 0.4 0.5 0.6

Polson

SM - Initial

SM - In Service

CL-ML - As Built

CL-ML - In Service

Su

ctio

n (

kP

a)

Volumetric Water Content

CL-ML

SM

= 1500 kPa

= 33 kPa

(c)

FIG. 5. Soil water characteristic curves (SWCCs) for Missoula (a), Helena (b),

and Polson (c).

where L is the thickness of the storage layer, FC is the field capacity, and WP is the

wilting point (Albright et al. 2010). Available soil water storage capacity is reported

in units of length (volume per unit area = length). Field capacity (FC) is the water

content at which drainage becomes negligible under gravity and is estimated from the

SWCC as the volumetric water content ( at = 33 kPa. The wilting point (WP),

the water content at which transpiration ceases, was estimated as at = 1500 kPa.

In semi-arid regions such as western Montana, wilting points can be as high as 5-7

MPa (Apiwantragoon 2007). Thus, defining WP based on 1500 kPa underestimates

available storage (a conservative estimate).

The following SA were computed: Polson – 90/166 mm, Helena – 82/164 mm, and

2002 Missoula – 134 mm (the latter SA for Polson and Helena correspond to in-

service conditions). The increase in SA at Polson and Helena is due to pedogenic

changes in the SWCC, which are known to increase s and (Benson et al. 2007a,

2011a). For these soils, the increase in available storage due to the increase in s was

more significant than the reduction in available storage due to the increase in .

When the in-service condition is considered for Polson and Helena, the 2002

Missoula Cover has approximately 30 mm less storage than the covers in Polson and

Helena, and therefore could transmit more percolation.

Required Soil Water Storage. Required soil water storage (SR) was computed for

the 2002 Missoula Cover and the covers in Polson and Helena using Eq. 3 and the

procedures outlined in Albright et al. (2010):

6 12

r FW,m SS,m

m 1 m 7

S S S (3)

where SFW,m is monthly accumulation of soil water storage in fall and winter (m = 1-

6 corresponding to October through March):

FW,m m FW m FWS P PET (4)

and SSS,m is monthly accumulation of soil water storage in spring and summer (m =

7-12, April through September).

SS,m m SS m SSS P PET (5)

In Eqs. 4 and 5, Pm is monthly precipitation and PETm is monthly PET for the mth

month, ij is the ratio of ET to PET for fall-winter (FW) or spring-summer (SS)

conditions, and ij is the water balance residual (runoff, percolation, and internal

lateral flow, if any) for fall-winter (FW) or spring-summer (SS) conditions. For sites

with snow and frozen ground, FW = 0.37, SS = 1.00, FW = 0, and SS = 168

(Albright et al. 2010). PET was computed using methods presented in Allen et al.

(1998). For months when P/PET was less than 0.51 (fall and winter months) or 0.32

(spring and summer months), the monthly accumulation was set at zero, as

recommended in Albright et al. (2010). In addition, if SFW,m or SSS,m computed

with Eqs. 4-5 was less than zero for any m, the monthly accumulation was set to zero

for that month as recommended in Albright et al. (2010).

Required storage for each site during 2000-04 (period when Polson and Helena were

monitored) is summarized in Table 4. The required storage for Missoula during this

period (35-156 mm, annual average = 92 mm) is comparable to the required storage

in Polson (49-134 mm, annual average = 89 mm), and appreciably more than the

required storage in Helena (14-42 mm, annual average = 12 mm).

Table 4. Required and available storage for ACAP monitoring period (2000-

04) for 2002 Missoula Cover and covers in Polson and Helena.

Site

Available

Storage

SA (mm)

Year Precip.

(mm)

PET

(mm)

Required

Storage SR

(mm)

SR/SA

2002

Missoula

Cover

134

2000 314 861 156 1.17

2001 337 846 95 0.71

2002 258 783 35 0.26

2003 370 875 106 0.80

2004 386 826 66 0.49

Helena 164

2000 213 1038 42 0.26

2001 273 1105 6 0.04

2002 319 1004 0 0.00

2003 238 1093 0 0.00

2004 308 990 14 0.08

Polson 166

2000 382 822 134 0.81

2001 341 856 81 0.49

2002 356 812 49 0.29

2003 343 898 80 0.48

2004 386 777 103 0.62

Historical meteorological data for Missoula were used to compute required storage

for a typical year (year with annual precipitation closest to long-term average, 1984 –

338 mm) and the more challenging design scenarios recommend in Albright et al.

(2010): the wettest year on record (1998, 556 mm) and the 95th

percentile

precipitation year (1975 – 469 mm). Required storage for these cases is summarized

in Table 5.

The required storage for Missoula computed from the historical data varies by a

factor of four between the typical year (1984 – SR = 51 mm) and the 95th

percentile

precipitation year (1975, SR = 204 mm). Moreover, the required storage is higher for

the 95th

percentile precipitation year (SR = 204 mm), even though the wettest year on

record (1998, SR = 133 mm) received more precipitation. This unexpected difference

in SR reflects differences in the temporal distribution of precipitation. In 1998, large

precipitation events were received during summer when ET was high, whereas the

large precipitation events in 1975 occurred in late fall and winter, when ET was low

and water was accumulating in the cover. A wetter winter also occurred in 1975.

Thus, the wettest year is not necessarily the worst-case scenario for Missoula.

Table 5. Required storage and required-available storage ratio for 2002

Missoula Cover for wet-year scenarios cited in Albright et al. (2010).

Quantity

Meteorological Year

Wettest 95

th

Percentile Typical

Year 1998 1975 1984

Precipitation (mm) 556 469 338

PET (mm) 851 762 953

Required Storage, SR (mm) 133 204 51

SR/SA (SA = 134 mm) 0.99 1.53 0.38

Relative Storage. Each cover was evaluated using the relative storage ratio SR/SA,

which describes the required storage relative to the available storage in the cover

profile for a given meteorological year. When SR/SA is << 1, negligible percolation

is anticipated because the cover has adequate capacity to store infiltrating

precipitation. Percolation is anticipated when SR/SA is ≈ 1 or > 1 because the water to

be stored is comparable to or larger relative to the storage capacity available in the

cover. For SR/SA > 1, the annual percolation rate can roughly be estimated as SR-SA.

A summary of SR/SA is in Table 4 for Polson, Helena, and the 2002 Missoula Cover

for the 2000-04. For the covers in Polson and Helena, SR/SA is < 1 for each year,

which is consistent with the very low percolation rates measured for these covers (< 1

mm/yr). For the 2002 Missoula Cover, SR/SA is < 1 for all years except 2000.

However, the average SR/SA for the 2002 Missoula Cover (0.69) during 2000-04 is

larger than for the covers in Polson (0.54) and Helena (0.08).

Ratios of SR/SA for the 2002 Missoula Cover computed using historical

meteorological data are summarized in Table 5; SR/SA = 0.38 for a typical year, 0.99

for the wettest year on record, and 1.53 for the 95th

percentile precipitation year.

Overall Assessment

Comparison of the meteorological data from Missoula, Polson, and Helena

indicates that a water balance cover in Missoula should function comparably as the

water balance cover in Polson, and maybe as well the water balance cover in Helena

(both Polson and Helena had very low percolation rates during 2000-04), provided

the cover in Missoula has adequate storage capacity and the vegetation at all three

sites has similar ability to remove water from the profile. This is supported by the

relative magnitudes of the average annual P/PET, which is lower in Missoula than in

Polson and Helena. A wheatgrass blend similar to the vegetation at Polson and

Helena is present in the grasslands surrounding the Missoula landfill (Table 6). Thus,

the vegetation at Missoula should have similar ability to remove stored water as the

vegetation at Polson and Helena.

Table 6. Vegetation at Missoula, Polson, and Helena as reported in Miller

(2002), Roesler et al. (2002), and Albright et al. (2004).

Site Vegetation

Missoula Critana thickspike, sodar streambank, and pryor slender wheatgrass,

sheep fescue, yellow sweetclover.

Helena Bluebunch, slender, and western wheatgrass, sandburg bluegrass, sheep

fescue, blue gamma, green needlegrass, needle-and-thread.

Polson

Thickspike, bluebunch, slender, and crested wheatgrass, mountain

brome, Idaho fescue, prairie junegrass, needle-and-thread, meadow

brome, Canada and Kentucky bluegrasses, yarrow, fringed sagewort,

alfalfa, rubber rabbitbrush, prickly rose, arrowleaf, balsamroot, dolted

gayfeather, lewis flax, silky lupine, cicer milkvetch.

The 2002 Missoula Cover profile should be adequate under typical conditions,

which is consistent with the predictions made by Miller (2002). However, a thicker

cover profile is needed to provide sufficient storage capacity for wetter conditions.

For example, SR/SA exceeds 1 for the Missoula cover when the meteorological data

from 2000 are used to compute SR (Table 4). For this same year, SR/SA = 0.81 for

Polson and 0.26 for Helena. The year 2000 was wetter than the other years during the

2000-04 monitoring period in Polson and Helena, but was not exceptionally wet.

Thus, in wetter years, the 2002 Missoula cover has a higher likelihood of transmitting

percolation than the covers at Polson or Helena. MDEQ agreed with this assessment,

and concurred that a thicker cover was necessary.

Path Forward

Based on the preliminary assessment, MDEQ was satisfied that a properly sized

water balance cover could function satisfactorily at Missoula. However, discussions

with MDEQ indicated that approval for a water balance cover would require more

comprehensive analysis and design, including (i) more comprehensive evaluation of

soil resources, (ii) site-specific assessment of vegetation properties, (iii) additional

modeling to evaluate wetter conditions, and (iv) a full-scale demonstration to validate

that the cover functions as designed. These requirements from MDEQ are consistent

with their Draft Alternative Final Cover Guidance (v. 9-11) and the approach

recommended in Albright et al. (2010). The following steps were conducted to

addresses these concerns:

a soil resource evaluation was conducted and the saturated and unsaturated

hydraulic properties were measured for potential cover soils,

vegetation at the site was sampled and the leaf area index (LAI) and root density

function were measured,

data describing the phenology of the wheatgrass blend in the region were obtained

from the literature,

preliminary design was conducted to estimate the required thickness of the

storage layer under wetter conditions (wettest year on record and 95th

percentile

precipitation year) using soil hydraulic properties from the soil resource

evaluation,

percolation from the cover identified in preliminary design was predicted using a

numerical model employing site-specific meteorological data, soil properties, and

vegetation properties as input; meteorological data for typical and much wetter

conditions were employed,

a test section was constructed so that the water balance (particularly the

percolation rate) could be measured at field scale to confirm the sufficiency of the

design.

SOIL RESOURCES

A soil resource evaluation was conducted to determine the suitability of the on-site

soils. Ten soil samples were collected from test pits excavated at four sites (Sites 1-

4) where soil was available for the cover. Sites 1-3 were soil stockpiles excavated

from previous cell construction. Native ground was sampled at Site 4, and at Site 1 in

an area adjacent to the soil stockpile at Site 1. Topsoil was sampled at Sites 3 and 4.

The site has an abundance of soil and availability of soil is not an issue. Thus, the

characterization focused on identifying soils that were suitable for the cover and not

the volume of each soil that was available.

Test pits were excavated with a backhoe at each sampling site. Each test pit was

inspected visually to assess homogeneity of the borrow source. Disturbed samples

were collected with hand tools and placed in 20-L buckets. All buckets were sealed

with plastic lids containing rubber gaskets.

Particle Size Distribution

Particle size analyses were conducted on each sample following ASTM D 422. The

particle size distributions are shown in Fig. 6. Atterberg limits were not measured.

The average particle size fractions for the stockpile soils (gravel = 39%, sand = 27%,

and fines = 32%) are comparable to those for the Missoula soil cited in Miller (2002).

Five soils were selected for additional testing (solid symbols and solid lines in Fig.

6). These soils included three stockpile soils, one at each of Sites 1-3 (S1-XX to S3-

XX; XX is an identifier), the topsoil at Site 3 (S3-TS), and the finer-textured native

ground from Site 4 (S4-NG-1). Topsoil at Site 3 was selected to define topsoil

properties for use in water balance modeling. The topsoil at Site 3 was coarser than

the topsoil at Site 4, and was expected to provide a conservative representation of

topsoil available for the cover. The three stockpile soils constitute a broad range in

particle size distribution (Fig. 6), and were selected to define a range of anticipated

hydraulic properties. The fine-grained native ground soil (S4-NG-1 in Fig. 6) was

selected for comparison with the stockpile soils.

Compaction Properties

Compaction tests were conducted on the five soils using ASTM D 698 (standard

Proctor). All soils were scalped on a 9.5-mm sieve, and a coarse-fraction correction

was applied to account for particles larger than 9.5 mm using the procedure in ASTM

D 4718. Compaction curves for the three stockpile soils were comparable, with

maximum dry unit weight (dmax) ranging between 18.3 and 20.1 kN/m3 and optimum

water content (wopt) ranging between 8.2 and 11.4% (Benson and Bareither 2011).

The compaction curve reported by Miller (2002) for the 2002 Missoula Cover also

falls in this range (dmax = 19.9 kN/m3, wopt = 9.6%).

0

20

40

60

80

100

0.010.1110100

S1-SP-1

S1-SP-2

S1-NG

S2-SP

S3-TS

S3-SP-1

S3-SP-2

S4-TS

S4-NG-1

S4-NG-2

Per

cent

Pas

sing (

%)

Particle Size (mm)

FIG. 6. Particle size distribution curves for 10 soils sampled at Missoula landfill.

Solid symbols correspond to soils selected for hydraulic property

characterization; SP = stockpile, TS = topsoil, and NG = native ground.

Hydraulic Properties

Saturated hydraulic conductivity was determined on each of the five soils following

the procedure in ASTM D 5084. Specimens were compacted to 85% of dmax at wopt

per ASTM D 698 in 150-mm-diameter molds. Relatively low compaction is used to

ensure that the cover soils provide a hospitable environment for root growth (Albright

et al. 2010). The effective stress was set at 15 kPa and the hydraulic gradient at 10 to

represent conditions existing in a cover.

Soil water characteristic curves (SWCCs) were measured on specimens prepared to

the same compaction conditions as specimens for the hydraulic conductivity tests.

Procedures described in ASTM D 6836 were followed. The wet end of each SWCC

was measured using a pressure plate extractor and the dry end with a chilled mirror

hygrometer. Eq. 1 was fit to the SWCC data using non-linear least-squares

optimization (Fig. 7).

10-1

100

101

102

103

104

105

0.0 0.1 0.2 0.3 0.4 0.5 0.6

S1-SP-1S2-SPS3-TSS3-SP-1S4-NG-1

Suct

ion (

kP

a)

Volumetric Water Content

Symbols:

Solid = pressure plate

Open = hygrometer

FIG. 7. SWCCs for stockpile soils from Sites 1, 2, and 3, Site 3 topsoil, and Site 4

native ground.

Saturated hydraulic conductivities and van Genuchten parameters for the SWCCs

are summarized in Table 7. Saturated hydraulic conductivity of the stockpile soils

varies in a narrow range from 3.7x10-6

cm/s to 6.0x10-5

cm/s. The SWCCs for the

stockpile soils are also comparable (Fig. 8), with ranging from 0.0958 to 0.145

1/kPa, n ranging from 1.27 to 1.28, and s ranging from 0.35 to 0.41. Similar

hydraulic properties were reported in Miller (2002) for the storage layer of the 2002

Missoula Cover (Table 3).

VEGETATION

Vegetation samples were collected for measurement of site-specific properties for

input to the numerical model. Four locations in the surrounding grassland were

selected that had mature vegetation representative of the area surrounding the landfill.

A test pit was excavated in each location for root samples and a sampling area was

selected for collecting surface biomass. Root samples were collected at 150-mm

intervals from the sidewall of each test pit using the modified Weaver-Darland

method described in Benson et al. (2007b) and placed in evacuated re-sealable plastic

bags. Samples of surface biomass were collected from four 1-m2 areas by removing

all biomass with shears. Surface biomass samples were placed in evacuated plastic

bags that were sealed in the field. All of the samples were stored in a refrigerator at 4 oC prior to analysis.

Table 7. Saturated hydraulic conductivity and unsaturated hydraulic

properties for soils sampled from Missoula Landfill.

Sample Sampling

Site KS (cm/s) (1/kPa) n s r

S1-SP-2 1 6.0x10-5

0.126 1.27 0.36 0.0

S2-SP 2 3.7x10-6

0.0958 1.28 0.35 0.0

S3-TS 3 2.8x10-6

0.0496 1.33 0.50 0.0

S3-SP-1 3 1.2x10-5

0.145 1.28 0.41 0.0

S4-NG-1 4 1.6x10-6

0.115 1.24 0.45 0.0

2002 Miss.

Cover - 4.9x10

-5 0.520 1.28 0.34 0.0

Notes: KS = saturated hydraulic conductivity; and n = fitting parameters for van Genuchten equation

(Eq. 1); s = saturated volumetric water content; r = residual volumetric water content; SP = stockpile;

TS = topsoil; NG = native ground.

Leaf area of the clippings was measured using a LI-COR LI-3100C leaf area meter

and leaf area index (LAI) was computed as the quotient of the total leaf area and the

sampling area (1 m2). The following LAIs were obtained: 1.46, 1.61, 1.86, and 1.99.

Root densities were measured by soaking each root sample in tap water for 48 h,

separating the roots from the soil particles, and air drying the root mass as described

in Benson et al. (2007b). Normalized root density profiles for the four pits are shown

in Fig. 8. The root profiles in each pit were remarkably similar, and a single root

density function was fit to the combined data set from all four pits using least-squares

regression.

STORAGE ANALYSIS

A storage analysis was conducted to determine the required thickness of the storage

layer (L). This consisted of equating SR (Eqs. 3-5) and SA (Eq. 2) and solving for L:

L = SR/(FC – WP) (6)

SWCCs corresponding to the combinations of s, r, , and n yielding the highest and

lowest available storage capacity were used to define FC and WP. Computations

were made using two required storage capacities (SR): (1) SR = 133 mm for the

wettest year on record and (2) SR = 204 mm for the 95th

percentile precipitation year.

Accounting for pedogenesis increased SA for the Polson and Helena sites. Thus,

pedogenesis was not included when computing the storage layer thickness with Eq. 4.

In addition, pedogenesis is known to make cover soils more similar (Benson et al.

2007a, 2011a), and the hydraulic properties from the soil resource evaluation are

similar to the in-service soils for Polson and Helena. Thus, adjusting the hydraulic

properties of the Missoula soils obtained from the soil resource evaluation for

pedogenesis probably would have been unrealistic.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.0 0.2 0.4 0.6 0.8 1.0

Pit 1

Pit 2

Pit 3

Pit 4

Dep

th -

z (

m)

Normalized Root Density - R

R = 3.12·e(-16.32·z)

+ 0.002

FIG. 8. Normalized root densities from the four test pits at Missoula landfill.

The required storage layer thicknesses (Table 8) ranged from 740-950 mm for SR =

133 mm (average = 840 mm) and from 1190-1560 mm for SR = 204 mm (average =

1370 mm), with thicker layers required for the soil with lower storage capacity.

Based on this analysis, a preliminary design was selected with a 1.22-m-thick storage

layer overlain by a 0.15-m-thick topsoil layer. This cover was expected to provide

acceptable storage capacity under most meteorological conditions for typical soils on

site. A thicker cover could have been proposed to address worst case conditions, but

the assumptions were conservative and increasing the thickness would have raised

construction costs an unacceptable amount.

Table 8. Storage layer thicknesses for required storage (SR)

representing wettest year on record (SR = 133 mm) and

95th

percentile precipitation year (SR = 204 mm).

Storage Layer Capacity Storage layer thickness (mm)

SR = 133 mm SR = 204 mm

Lower Bound Storage Capacity1 950 1560

Upper Bound Storage Capacity2 740 1190

1 = 0.145 1/kPa, n = 1.27, s = 0.35, r = 0.0;

2 = 0.096 1/kPa, n = 1.28, s =

0.41, r = 0.

WATER BALANCE MODELING

The variably saturated flow model WinUNSAT-H was used to predict the water

balance for the proposed water balance cover. WinUNSAT-H (and its DOS

counterpart UNSAT-H), is the most widely used numerical model for simulating the

hydrology of water balance covers (Benson 2007). When properly parameterized,

WinUNSAT-H provides a reliable prediction of the water balance of covers, and over

predicts the percolation rate modestly in most cases (Khire et al. 1997, Ogorzalek et

al. 2007, Bohnhoff et al. 2009). WinUNSAT-H simulates variably saturated flow,

root water uptake, and climatic interactions (Benson 2007, 2010).

Soil Properties

Hydraulic properties used in the design were selected so that the percolation would

not be under-predicted, and likely would be over predicted. The topsoil layer was

assigned the hydraulic properties associated with Soil S3-TS (Table 7), except the

saturated hydraulic conductivity was increased one order of magnitude to account for

pedogenesis and to ensure that runoff comprised no more than 10% of the annual

water balance, as recommended in Albright et al. (2010). Saturated hydraulic

conductivity of the storage layer was set at 6x10-5

cm/s, the highest of the saturated

hydraulic conductivities (Soil S1-SP-2, Table 7) measured during the site

characterization. The SWCC was defined using the combination of van Genuchten

parameters measured during site characterization yielding the lowest storage capacity.

Vegetation

The vegetation was assigned the minimum LAI (1.46) and the root density function

(Fig. 8) obtained from the site characterization. Phenology and water stress

parameters described previously (from Roesler et al. 2002) were input.

Meteorological Data

Four meteorological data sets were used for the simulations: the typical year (1984),

the wettest year on record, the 95th

percentile precipitation year, and the 10-yr period

with the highest precipitation (1977-1986). The 10-yr period with the highest

precipitation is recommended for design in MDEQ’s Draft Alternative Final Cover

Guidance.

The simulations were conducted in two phases. The first phase consisted of a 5-yr

simulation using meteorological data for the typical year for each year of the

simulation (i.e., typical year repeated 5 times). This simulation had two purposes: (i)

to create a realistic initial condition for the simulations conducted with the 10-yr

record with the highest precipitation and (ii) to define a ‘typical’ percolation rate for

the cover, as defined in Albright et al. (2010). The second phase followed

immediately after the first phase, and consisted of one of the following: (i) the wettest

year on record run 5 times sequentially, (ii) the 95th

percentile precipitation year run 5

times sequentially, and (iii) the complete 10-yr record with the highest average

precipitation. All three of these scenarios are suggested in Albright et al. (2010).

The simulations with the wettest year and the 95th

percentile precipitation year are

relatively simple to conduct, and are expected to be very conservative (the likelihood

over 5 sequential very wet years is very small). Many engineers believe this design

strategy is unrealistic and too conservative. The record for the 10-yr wettest period is

realistic, but is more difficult and time consuming to simulate.

Water Balance Predictions

Annual water balances predicted by WinUNSAT-H are summarized in Table 9 for

each year of the 10-yr period with highest precipitation along with the average water

balance over the 10-yr period, the typical year, the wettest year on record, and the

95th

percentile precipitation year. Predictions for the five-year repetitive simulations

are for the final year. The maximum annual runoff was 6.6% (95th

percentile

precipitation year), indicating that nearly all precipitation reaching the surface

became infiltration. Thus, the water balance predictions met the runoff criterion (<

10% of annual water balance) suggested in Albright et al. (2010), which applies to

arid and humid climates.

Table 9. Predicted water balance quantities for water balance cover with 1.22-

m-thick storage layer and 0.15-m-thick topsoil layer.

Year

Annual Water Balance Quantity

Cum-

ulative

Precip.

(mm)

Cum-

ulative

Runoff

(mm)

Cum-

ulative ET

(mm)

Cum-

ulative

Percolation

(mm)

Avg. Soil

Water

Storage

(mm)

Runoff

(% Precip.)

1977 322 0.0 276 1.1 203 0.0

1978 299 0.0 330 7.1 242 0.0

1979 263 0.0 268 4.3 228 0.0

1980 483 16 446 5.7 246 3.3

1981 441 1.5 422 3.0 223 0.3

1982 390 7.8 367 15.9 270 2.0

1983 424 0.0 417 6.6 244 0.0

1984 339 0.1 387 4.2 224 0.0

1985 318 0.3 338 1.5 195 0.1

1986 425 0.0 406 1.5 206 0.0

Avg.

(77-86) 370 2.5 366 5.1 228 0.7

Typ.

(1984) 338 0.0 366 1.1 197 0.0

Wettest

(1998) 556 19.0 522 21.3 285 3.4

95th

%

(1975) 469 30.8 410 36.4 294 6.6

The annual percolation rate during the wettest 10-yr period ranged from 1.1 to 15.9

mm/yr, with an average of 5.1 mm/yr. The percolation rate was no more than 3.0

mm/yr for four years in the 10-yr period, and no more than 1.5 mm/yr for three years.

For the typical year, the percolation rate was 1.1 mm/yr. Much higher percolation

rates were obtained from the 5-yr repetitive simulations using the wettest year on

record (21.3 mm/yr) and the 95th

percentile precipitation year (36.4 mm/yr). The

higher percolation rate predicted for the 95th

percentile precipitation year is consistent

with the findings from the storage assessment.

The water balance graphs for the typical, wettest, and 95th

percentile precipitation

years are shown in Fig. 9. These graphs illustrate that differences in the total amount

of precipitation, as well as the time when precipitation is received, are responsible for

the wide range of percolation rates transmitted for these meteorological conditions.

During the typical year (Fig. 9a), most of the precipitation occurs in spring and

summer (≈ Julian days 100-300), when ET is high. All of the precipitation during

this period along with water stored in the cover is returned to the atmosphere, as

evinced by a nearly monotonic drop in soil water storage during this period. Storage

begins to climb again in mid-fall when ET begins to diminish (> Julian day 300), but

the increase in storage is modest because the precipitation is small.

During the wettest year, the majority of the precipitation occurs in spring and early

summer (≈ Julian days 90-190) (Fig. 9b). The precipitation between Julian days 130-

190 is so heavy at times that soil water storage increases periodically during this

period, even though ET is high. In mid-summer to early fall, however, the

precipitation rate is low again and the soil water storage diminishes to 223 mm,

although not to the very low storage (181 mm) in mid fall of the typical year. Heavy

precipitation in late fall raises the soil water storage at the end of the year to 302 mm,

106 mm more than soil water storage at the end of the typical year. This additional

storage, when combined with heavy precipitation during the first 25 d of the new

year, results in peak storage of 341 mm that persists through spring. At this level of

soil water storage, percolation is transmitted and continues through summer.

The 95th

percentile precipitation year received less precipitation than the wettest

year on record, but more precipitation (26 mm) was received during the fall than

occurred in the fall of the wettest year (Fig. 9c). As a result, soil water storage was

325 mm at the end of the year, i.e., higher than at the end of the wettest year on

record (302 mm). This high soil water storage, coupled with heavy precipitation

during the first 40 d of the new year (100 mm, vs. 50 mm during the same period

during the wettest year), resulted in a peak soil water storage of 380 mm at Julian day

49. Percolation began earlier in the year due to the high soil water storage at the end

of the previous year, increased significantly as the soil water storage reached its peak,

and continued through summer.

Comparison of the water balance graphs in Fig. 9 to the water balance graph in Fig.

10 for the 10-yr period with the most precipitation illustrates that high levels of soil

water storage at the end of the previous year, combined with wetter than normal

conditions in the winter and spring, consistently give rise to the highest percolation

rates. The highest percolation rate predicted in the 10-yr simulation was in 1982 (15.9

mm/yr), which occurred in response to a sustained period of frequent and less intense

precipitation beginning in late Fall 1981 and continuing into early Fall 1982. This

0

100

200

300

400

500

600

0

20

40

60

80

100

0 100 200 300 400

PrecipitationEvapotranspirationSoil Water Storage

RunoffPercolation

Pre

cip

itat

ion

, E

vapo

tran

spir

atio

n,

and

Soil

Wat

er S

tora

ge

(mm

) Perc

olatio

n an

d R

un

off (m

m)

(a) Typical Year

0

100

200

300

400

500

600

0

20

40

60

80

100

0 100 200 300 400

Pre

cip

itat

ion

, E

vap

otr

ansp

irat

ion

,

and

Soil

Wat

er S

tora

ge

(mm

) Perc

olatio

n an

d R

un

off (m

m)

(b) Wettest Year

0

100

200

300

400

500

600

0

20

40

60

80

100

0 100 200 300 400

Pre

cip

itat

ion

, E

vap

otr

ansp

irat

ion,

and

Soil

Wat

er S

tora

ge

(mm

) Perc

olatio

n an

d R

un

off (m

m)

Julian Day

(c) 95th

Percentile Year

FIG. 9. Predicted water balance quantities for fifth year of 5-yr analysis using

hydraulic properties corresponding to lower bound storage.

0

1000

2000

3000

4000

0

100

200

300

400

1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987

Precipitation Evapotranspiration Soil Water Storage Runoff Percolation

Cum

ula

tive

Pre

cip

itat

ion a

nd E

vap

otr

ansp

irat

ion (

mm

)S

oil W

ater Sto

rage an

d

Cum

ulativ

e Runoff an

d P

recipitaio

n (m

m)

FIG. 10. Water balance predictions for 10-yr period with highest average precipitation made with WinUNSAT-H using

hydraulic properties corresponding to lower bound on storage properties.

condition led to a very large increase in storage during the winter of 1982, even

though four years during the 10-yr period (1980, 1981, 1983, and 1986) had higher

annual precipitation than 1982.

The second highest annual percolation rate (7.1 mm/yr) occurred in 1978, even

though this year was drier than average (299 mm precipitation vs. 337 mm, on

average) and only one year during the 10-yr record had less annual precipitation (263

mm in 1979). Like 1982, percolation in 1978 occurred in response to a large increase

in soil water storage in late Fall 1977 and Winter 1978 that was caused by a sustained

period of frequent and less intense precipitation. Thus, the timing of precipitation and

the sequencing from one year to the next has a critical impact on the accumulation of

storage and the amount of percolation that occurs.

The timing and sequencing of precipitation is represented realistically using actual

multi-year time series, whereas repetitive simulations with the wettest year on record

or the 95th

percentile year probably are unrealistic. As shown in Fig. 10, very wet

years generally are not sequential, and the likelihood of five very wet years occurring

sequentially is very small.

Implications

None of the modeling predictions confirm that the design objective (3 mm/yr average

percolation rate) will be accomplished. The percolation rate for the typical year (1.1.

mm/yr) is lower than the design objective, but the average percolation rate over the

10-yr period with highest precipitation (5.1 mm/yr) exceeds the design objective.

Percolation rates greatly in excess of the design objective were obtained from the 5-yr

repetitive simulations, but these simulations are considered unrealistic.

The long-term percolation rate could be evaluated by simulating the entire 50-yr

meteorological record or a very long record generated with a synthetic weather

generator based on meteorological statistics for Missoula. However, either simulation

would be very time consuming and computationally costly, and probably would not

be practical for most projects. This type of simulation was considered impractical for

the Missoula landfill.

The average of the percolation rate from the typical year and from the 10-yr

simulation is 3.1 mm/yr, which is slightly larger than the design objective. Given

that the 10-yr record used in the simulation is the wettest decade on record, and that

periods of drought will also occur along with typical conditions, the long-term

average percolation rate is likely to be less than 3.1 mm/yr. Thus, the design

objective likely will be met, and MDEQ concurred that this conclusion is reasonable.

Based on this assessment, and the favorable monitoring data from the covers in

Polson and Helena, the final design consisted of a 1.22-mm-thick storage layer

overlain by 0.15 m of topsoil.

TEST SECTION

A field demonstration of the cover is being conducted using an ACAP-style test

section constructed following the methods described in Benson et al. (1999). The test

section slopes at 3% to simulate the actual top deck slope for the landfill, and faces

north to receive the greatest snow accumulation and lowest solar radiation (i.e., worst

case orientation). The test section includes a 10 m x 20 m pan-type lysimeter (Fig.

11) for direct measurement of the water balance, including surface runoff, soil water

storage, and percolation. The base and sidewalls of the lysimeter are comprised of

linear low-density polyethylene geomembrane, and the base is overlain with a

geocomposite drainage layer to protect the geomembrane and to transmit percolation

to a zero-storage sump. Diversion berms are placed on the surface to prevent run-on

and collect run-off.

Cover

Soils

Instrument

NestDiversion

Berm

Geomembrane

Liner

Geocomposite

DrainInterim

Cover

Soil

Root

Barrier

Surface

Runoff

Percolation

Diversion

Berm

FIG. 11. Schematic of test section for evaluating performance of the water

balance cover (not to scale).

Instrumentation

Percolation and surface runoff are routed by pipes to basins equipped with a pressure

transducer, tipping bucket, and float switch (triple redundancy) capable of measuring

flows with a precision better than 0.1 mm/yr (Benson et al. 2001). Soil water content

is measured in three nests located at the quarter points along the centerline. Each nest

contains 5 low-frequency (40 MHz) time domain reflectometry (TDR) probes in a

vertical stack. Soil water storage is determined by integration of the point

measurements of water content. Each probe includes a thermistor for monitoring soil

temperature along with water content. Soil-specific calibration of the TDR probes

was conducted using the method in Benson and Wang (2006).

Meteorological data (precipitation, air temperature, relative humidity, solar

radiation, wind speed, and wind direction) are measured with a weather station

mounted on the test section. Data are collected and recorded by a datalogger every 15

min and are stored on one-hour intervals. At times of intense activity (e.g., an intense

rain event with high surface runoff), data are stored at time intervals as short as every

15 s. Data from the test section are stored on a server equipped with a screening level

quality assurance (QA) algorithm. Detailed QA checks are conducted quarterly. At

the time this paper was prepared, the duration of the monitoring period was too short

to present the monitoring data in a useful manner.

Placement of Cover Profile

Prior to constructing the cover profile, a layer of soil simulating the existing interim

cover was placed on top of the geocomposite drainage layer. The interim cover layer

was overlain with a root barrier (thin nonwoven geotextile studded with nodules

containing trifluralin, a root inhibitor) to prevent root intrusion into the geocomposite

drainage layer and the percolation collection system. Inclusion of the root barrier

results in less water being transpired than might occur in an actual cover, where roots

can grow through the interim cover and into the waste (Albright et al. 2004).

However, the root barrier prevents plants from having access to water retained in the

collection and measurement system that would otherwise become deep drainage in an

actual application.

The storage layer was constructed with soil from Site 3, which was placed over the

root barrier using a bulldozer in three 0.41-m-thick lifts. The dry unit weight was

required to be 80-90% of maximum dry unit weight per standard Proctor and the

water content was required to be no wetter than optimum water content, as

recommended in Albright et al. (2010). Thick lifts were used to minimize the

potential for over compaction of the soil, as is common in practice during

construction of water balance covers.

Topsoil stripped from Site 3 was placed on the surface and fertilized to stimulate

growth. Seed was not added; the natural seed bank within the topsoil serves as the

source of seed. This “live haul” approach creates a more realistic and sustainable

plant community that is consistent with the surroundings.

SUMMARY AND CONCLUSIONS

The key steps required to design and demonstrate a water balance cover have been

illustrated in this paper using a case history where a cover was designed for a MSW

landfill in Missoula, Montana. This design process evolved from two decades of

research. The state-of-the-art developed through research is now being applied as

state-of-the-practice.

The process begins by understanding the design objective (including regulatory

requirements) and investigating lines of evidence indicating that a water balance

cover is likely to function satisfactorily at the design location. For the Missoula case

history, regional data from two instrumented water balance covers were used along

with meteorological data to illustrate that a water balance cover could be successful in

Missoula if sized properly. Site characterization was then conducted to define

properties of the soil resources and vegetation at the site for preliminary cover sizing

and numerical modeling of the water balance. A preliminary design was created using

semi-empirical analytical methods and the design was evaluated by simulation with a

numerical model using several design meteorological conditions as input.

The numerical modeling illustrated that predictions made with an actual multiyear

time series (e.g., wettest 10-yr period in the meteorological record) are more realistic

than predictions from 5-yr repetitive simulations using a worst case design year,

which have been recommended historically and have become common in practice.

The multiyear simulations preserve realistic sequencing of seasonal precipitation

patterns, which have a strong influence on soil water storage and percolation rate. The

drawback is that multiyear simulations are more cumbersome and time consuming to

conduct.

The final design for the Missoula landfill is a monolithic water balance cover

comprised of a 1.22-m storage layer overlain by a 0.15-m topsoil layer. A field

demonstration with a fully instrumented lysimeter was deployed in October 2011 to

confirm that the cover performs as anticipated in the design.

ACKNOWLEDGEMENT

The US Environmental Protection Agency, the National Science Foundation, the US

Department of Energy (DOE), the US Nuclear Regulatory Commission, Republic

Services, Inc., Waste Connections Inc., Waste Management Inc., Monsanto

Corporation, and Benson’s Wisconsin Distinguished Professorship have provided

financial support for Professor Benson’s research program on water balance covers.

A portion of the support from DOE has been provided through the DOE Landfill

Partnership within the Consortium for Risk Evaluation with Stakeholder Participation

(CRESP) awarded to Vanderbilt University. The opinions, findings, conclusions, or

recommendations expressed herein are those of the authors and do not necessarily

represent the views of the US Department of Energy, Vanderbilt University, or any of

the other sponsors. Republic Services Inc. provided financial support for the case

history described in this paper. GSE Inc. provided the geosynthetics for the test

section. William Albright, Jiannan Chen, Brad Lyles, Jose Llobell Ruvira, Jhan

Sorenson, and Xiaodong Wang assisted with construction of the test section.

REFERENCES

Albrecht, B., Benson, C., and Beuermann, S. (2003), Polymer Capacitance Sensors

for Measuring Soil Gas Humidity in Drier Soils, Geotech. Testing J., 26(1) 3-12.

Albright, W. H., Benson, C. H., and Waugh, W. J. (2010). Water Balance Covers for

Waste Containment, Principles and Practice, American Society of Civil Engineers,

ASCE Press, Reston, Virginia.

Albright, W. H., Benson, C. H., Gee, G. W., Roesler, A C., Abichou, T.,

Apiwantragoon, P., Lyles, B. F., and Rock, S. A. (2004). Field Water Balance of

Landfill Final Covers, J. Environmental Quality, 33(6), 2317-2332.

Allen, R. G., Pereira, L. S., Raes, D., and Smith, M. (1998). Crop Evapotranspiration

– Guidelines for Computing Crop Water Requirements, FAO Irrigation and

Drainage Paper 6, Food and Agriculture Organization of the United Nations, Rome.

Apiwantragoon, P. (2007). Field Hydrologic Evaluation of Final Covers for Waste

Containment, PhD Dissertation, University of Wisconsin, Madison, WI.

Benson, C. (2001), Waste Containment: Strategies and Performance, Australian

Geomechanics, 36(4), 1-25.

Benson, C. (2007), Modeling Unsaturated Flow and Atmospheric Interactions,

Theoretical and Numerical Unsaturated Soil Mechanics, T. Schanz, Ed., Springer,

Berlin, 187-202.

Benson, C. (2010), Predictions in Geoenvironmental Engineering: Recommendations

for Reliable Predictive Modeling, GeoFlorida 2010, Advances in Analysis,

Modeling, and Design, Geotechnical Special Publication No. 199, D. Fratta, A.

Puppula, and B. Muhunthan, eds., ASCE, Reston, VA, 1-13.

Benson, C., Abichou, T., Albright, W., Gee, G., and Roesler, A. (2001), Field

Evaluation of Alternative Earthen Final Covers, International J. Phytoremediation,

3(1), 1-21.

Benson, C., Abichou, T., Wang, X., Gee, G., and Albright, W. (1999), Test Section

Installation Instructions – Alternative Cover Assessment Program, Environmental

Geotechnics Report 99-3, Dept. of Civil & Environmental Engineering, University

of Wisconsin-Madison.

Benson, C., Albright, W., Fratta, D., Tinjum, J., Kucukkirca, E., Lee, S., Scalia, J.,

Schlicht, P., Wang, X. (2011a), Engineered Covers for Waste Containment:

Changes in Engineering Properties & Implications for Long-Term Performance

Assessment, NUREG/CR-7028, Office of Research, U.S. Nuclear Regulatory

Commission, Washington.

Benson, C. and Bareither, C. (2011), Design and Performance Demonstration of a

Water Balance Cover at Missoula Landfill in Missoula, Montana, Geotechnics

Report No. 11-21, Wisconsin Geotechnics Laboratory, University of Wisconsin,

Madison, WI.

Benson, C., Bosscher, P., Lane, D., and Pliska, R. (1994), Monitoring System for

Hydrologic Evaluation of Landfill Final Covers, Geotech. Testing J., 17(2), 138-

149.

Benson, C. and Chen, C. (2003), Selecting the Thickness of Monolithic Earthen

Covers for Waste Containment, Soil and Rock America 2003, Verlag Gluck auf

GMBH, Germany, 1397-1404.

Benson, C. and Khire, M. (1995), Earthen Covers for Semi-Arid and Arid Climates,

Landfill Closures, ASCE, GSP No. 53, J. Dunn and U. Singh, eds., 201-217.

Benson, C., Sawangsuriya, A., Trzebiatowski, B., and Albright, W. (2007a), Post-

Construction Changes in the Hydraulic Properties of Water Balance Cover Soils, J.

Geotech. Geoenvironmental Eng., 133(4), 349-359.

Benson, C., Thorstad, P., Jo, H., and Rock, S. (2007b), Hydraulic Performance of

Geosynthetic Clay Liners in a Landfill Final Cover, J. Geotech. Geoenvironmental

Eng., 133(7), 814-827.

Benson, C., Waugh, W., Albright, W., Smith, G., and Bush, R. (2011), Design and

Installation of a Disposal Cell Cover Field Test, Proc. Waste Management ‘11,

Phoenix, AZ.

Bohnhoff, G., Ogorzalek, A., Benson, C., Shackelford, C., and Apiwantragoon, P.

(2009), Field Data and Water-Balance Predictions for a Monolithic Cover in a

Semiarid Climate, J. Geotech. Geoenvironmental Eng., 135(3), 333-348.

Khire, M., Benson, C., and Bosscher, P. (1997), Water Balance Modeling of Earthen

Final Covers, J. Geotech. Geoenvironmental Eng., 123(8), 744-754.

Khire, M., Benson, C., and Bosscher, P. (1999), Field Data from a Capillary Barrier

in Semi-Arid and Model Predictions with UNSAT-H, J. Geotech.

Geoenvironmental Eng., 125(6), 518-528.

Khire, M., Benson, C., and Bosscher, P. (2000), Capillary Barriers: Design Variables

and Water Balance, J. Geotech. Geoenvironmental Eng., 126(8), 695-708.

Khire, M., Meerdink, J., Benson, C., and Bosscher, P. (1995), Unsaturated Hydraulic

Conductivity and Water Balance Predictions for Earthen Landfill Final Covers, Soil

Suction Applications in Geotechnical Engineering Practice, ASCE, GSP No. 48,

W. Wray and S. Houston, eds., 38-57.

Malusis, M. and Benson, C. (2006), Lysimeters versus Water-Content Sensors for

Performance Monitoring of Alternative Earthen Final Covers, Unsaturated Soils

2006, ASCE Geotechnical Special Publication No. 147, 1, 741-752.

Meerdink, J., Benson, C., and Khire, M. (1995), Unsaturated Hydraulic Conductivity

of Two Compacted Barrier Soils, J. Geotech. Eng., 122(7), 565-576.

Miller (2002), Alternative Final Cover Evaluation, Missoula Landfill, Missoula

Landfill, report prepared by Miller Engineers and Scientists, Sheboygan, WI.

Ogorzalek, A., Bohnhoff, G., Shackelford, C., Benson, C., and Apiwantragoon, P.

(2007), Comparison of Field Data and Water-Balance Predictions for a Capillary

Barrier Cover." J. Geotech. Geoenvironmental Eng., 134(4), 470-486.

Roesler, A., Benson, C., and Albright, W. (2002), Field Hydrology and Model

Predictions for Final Covers in the Alternative Cover Assessment Program – 2002,

Geo Engineering Report 02-08, University of Wisconsin-Madison.

Schlicht, P., Benson, C., Tinjum, J., and Albright, W. (2010), In-Service Hydraulic

Properties of Two Landfill Final Covers in Northern California, GeoFlorida 2010,

Advances in Analysis, Modeling, and Design, Geotechnical Special Publication No.

199, D. Fratta et al., eds., ASCE, Reston, VA, 2867-2877.

Shackelford, C. and Benson, C. (2006), Selected Factors Affecting Water-Balance

Predictions for Alternative Covers Using Unsaturated Flow Models, Geotechnical

Engineering in the Information Technology Age, D. DeGroot, J. DeJong, J. Frost,

and L. Baise, eds., ASCE.

Smesrud, J., Benson, C., Albright, W., Richards, J., Wright, S., Israel, T., and

Goodrich, K. (2012), Using Pilot Test Data to Refine an Alternative Cover Design

in Northern California, International J. Phytoremediation, in press.

Stormont, J. (1995), The Effect of Constant Anisotropy on Capillary Barrier

Performance, Water Resources Research, 32(3), 783-785.

Stormont, J. and Morris, C. (1998), Method to Estimate Water Storage Capacity of

Capillary Barriers, J. Geotech. Geoenviron. Eng. ASCE, 124(4), 297-302.

Suwansawat, S. and Benson, C. (1998), Cell Size for Water Content-Dielectric

Constant Calibrations for Time Domain Reflectometry, Geotechnical Testing J.,

22(1), 3-12.

UNESCO (1999), Map of the World Distribution of Arid Regions. MAB Tech. Notes

No. 7, United Nations Educational, Scientific and Cultural Organization, Paris

van Genuchten, M. T. (1980). A closed-form equation for predicting the hydraulic

conductivity of unsaturated soils, Soil Science Society of America J., 44(5), 892-

898.

Wang, X. and Benson, C. (2004), Leak-Free Pressure Plate Extractor for Measuring

the Soil Water Characteristic Curve, Geotech. Testing J., 27(2), 1-10.

Waugh, W., Benson, C., and Albright, W. (2008), Monitoring the Performance of an

Alternative Landfill Cover Using a Large Embedded Lysimeter, Proceedings,

Global Waste Management Symposium 2008, Penton Media, Orlando, 1-10.

Waugh, W., Benson, C., and Albright, W. (2009), Sustainable Covers for Uranium

Mill Tailings, USA: Alternative Design, Performance, and Renovation, Proc. 12th

International Conference on Environmental Remediation and Radioactive Waste

Management, ICEM2009, ASME, 11-15 October 2009, Liverpool, UK.

Zornberg, J., LaFountain, L., and Caldwell, J. (2003). Analysis and Design of

Evapotranspirative Cover for Hazardous Waste Landfill. J. Geotech. Geoenviron.

Eng., ASCE, 129(6), 427-436.

NNoonn--TTeexxttbbooookk SSooiill MMeecchhaanniiccss::

PPeeaattllaanndd SSooiillss aanndd MMuunniicciippaall WWaassttee

GGuueesstt LLeeccttuurreerr DDrr.. AArrvviidd LLaannddvvaa

Non-texbook soil mechanics: peatland soils

and municipal waste

Professor Arvid Landva

ABSTRACT:

A peatland soil could be anything from an extremely coarse-fibrous woody matting

containing an insignificant amount of inorganic constituents to a gyttja-like and extremely

soft soil containing up to 80% minerals (the Swedish term gyttja comprises dead

microscopic animals decomposed to a flocculent substance in which the mineral particles

are embedded). With such a large range of composition it follows that it is important to

describe the peatland soil in detail. A case record or specification for construction on

peatland is of little value without such detailed description. In a fibrous peat with little

inorganic content most of the primary compression takes place during construction.

Interparticle and intraparticle waters in the peat are expulsed simultaneously because the

peat structure is open at all common degrees of humification. The main geotechnical

problem in fibrous peat is settlement rather than strength. In gyttja-like soils the

consolidation is similar to that in soft clays; in addition, secondary consolidation is often

quite prominent. Another major problem in gyttja-like soils is their very low strength.

Municipal waste is another non-textbook material, generally highly fibrous (as placed),

heterogeneous, erratic, highly compressible, and displaying significant long-term

settlement caused by plastic creep and decomposition. Neither undisturbed sampling nor

field testing is practical. And despite the relatively widespread use of laboratory tests for

the purpose of determining strength properties, laboratory testing of artificially prepared

samples must also be considered questionable. This is so because the structure of the waste

in the field is really indeterminate and cannot therefore be reliably duplicated in the

laboratory. Also, sampling of undecomposed or partly decomposed waste can be expected

to lead to changes in the particle size distribution, while sampling of strongly decomposed

waste may not be possible because of caving problems. Another important consideration,

generally overlooked by the profession, is the fact that the waste is subject to decomposition

and that the stipulation of constant strength parameters like cohesion and friction may

therefore not be valid.

Recommendations are offered for what the author considers a rational approach to dealing

with the geotechnical behaviour and testing of non-textbook soils.

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LLiiqquueeffaaccttiioonn MMiittiiggaattiioonn

GGuueesstt LLeeccttuurreerr DDrr.. MMaarriiaa CCaatteerriinnaa CC.. SSaannttaaggaattaa

Geomechanics from Micro to Macro – Soga et al. (Eds)© 2015 Taylor & Francis Group, London, ISBN 978-1-138-02707-7

Building a nanostructure in the pore fluid of granular soils

Marika Santagata, Antonio Bobet, Alain El Howayek, Felipe Ochoa-Cornejo & Joseph V. SinfieldLyles School of Civil Engineering, Purdue University, West Lafayette, IN, USA

Cliff T. JohnstonDepartment of Agronomy, Purdue University, West Lafayette, IN, USA

ABSTRACT: The paper describes the nature and behavior of the pore fluid that is formed when controlledamounts of a highly plastic nano-clay – bentonite or laponite – are introduced in suspension form in the porespace of a granular medium. The delivery of the bentonite and laponite in the sand pore space and the structureand properties of the pore fluid are ultimately related to careful control of the geo-chemical factors affecting thenano-scale interactions in these complex colloid-water systems. Observations conducted using cryo-SEM showthat, at the concentrations examined, the bentonite and laponite suspensions have the percolated structure typicalof attractive gels, with characteristic dimension on the order of microns, several orders of magnitude greaterthan the size of the clay particles that form it. Dynamic oscillatory measurements performed on these gels show‘solid-like’ properties, with a predominantly elastic response up to shear strains of ∼10%. The formation ofthis nanostructure in the sand pore space is thought to restrain sand particle mobility, reducing the generationof excess pore pressure that is observed at the macro-scale in resonant column and cyclic triaxial tests duringundrained loading. This indicates that treatment with bentonite or laponite suspensions may be a viable solutionto soil improvement for liquefaction mitigation.

1 INTRODUCTION

Liquefaction is “the transformation of a granular mate-rial from a solid to a liquefied state as a consequenceof increased pore-water pressure and reduced effectivestress” (Marcuson 1978) under the effect of earth-quake shaking or other form of rapid loading. Itis observed in loose to medium saturated granulardeposits, or even finer non-plastic soils, which havethe tendency to contract under the application of cyclicshear deformations.

Evidence of soil liquefaction has been documentedin all recent earthquakes, including the 2007 Pisco,Peru, the 2011 Tohoku, Japan, the 2010 Maule, Chile,and the 2010 and 2011 Christchurch, New Zealandearthquakes. Given the significant damage associ-ated with this geo-hazard, there continues to be greatinterest in the geotechnical community in developingground improvement methods to eliminate or mitigateliquefaction.

Methods for improving the liquefaction resistanceof a deposit can be grouped in four general categories:those that reduce the void space between the grains(e.g. vibrocompaction, compaction grouting); thosethat supply a drainage path to facilitate excess porepressure dissipation (e.g. gravel drains); those thatrely on desaturating the soil (e.g. dewatering, air bub-ble injection); and finally those that reduce/eliminate

free grain movement (e.g. cement/microcement per-meation grouting, bio-cementation, colloidal silicainjection).

The selection of the treatment method depends ona variety of factors including soil conditions, costconsiderations and type of project, in particular thevolume of soil requiring treatment, and the proximityto structures and lifelines.

The approach described in this paper, which isinspired by the well-documented effect that the pres-ence of plastic fines has on the liquefaction resistanceof granular soils, falls in the last of the categoriesoutlined above. It has two key features.

First, in contrast to other methods, such as bio-cementation and cement grouting, which rely on thecreation of bonds between the grains, it involves themodification of the pore fluid between the grains(“pore fluid engineering”). Specifically, through theintroduction of controlled amounts of highly plasticclay nanoparticles – bentonite or laponite – a nanos-tructure is created in the pore space. This structure hassolid-like properties that restrict particle mobility dur-ing shaking. This occurs with no alteration of the soilskeleton.

Second, careful engineering of the colloidal inter-actions is required to delay the formation of thisnanostructure, and facilitate “delivery” of the clayparticles in suspension form inside the pore space.

1377

Figure 1. A multi-scale approach to liquefaction mitigation.

Figure 1 illustrates the different scales involved inthe problem: the nano-scale, at which the colloidalinteractions occur; the micron scale, which as dis-cussed below, describes the characteristic length scaleof the structure formed in the pore space; the mm sizescale of the pores; and finally, the cm to m scale atwhich liquefaction is observed. It also illustrates thevision behind this work, i.e. how a large-scale geotech-nical problem can be addressed by controlling thebehavior/interactions over a range of scales.

The following sections provide some detail on theproposed approach to soil improvement for liquefac-tion mitigation, highlighting the scale based effectsthat ultimately enable the observed macro behavior.After introducing the materials used in this study (Sec-tion 2), Section 3 describes how the concept of creatinga structured pore fluid is founded on fundamental con-siderations of colloidal interactions supported by novelobservations of the structure of clay gels, and mea-surements of the mechanical properties of these softmaterials. Section 4 goes on to illustrate how deliveryof the targeted amounts of bentonite or laponite insidethe pore space requires further exploitation of existingknowledge of colloidal interactions. Finally, Section5 briefly examines how the observations conducted atthe nano and micro scales relate to the macroscopicbehavior observed in element tests.

2 BENTONITE AND LAPONITE, HIGHLYPLASTIC NANO-CLAYS

Two nano-clays, both belonging to the smectite groupare discussed in this paper: bentonite and laponite.

Bentonite is a clay mostly formed by Na-montmorillonite that is widely used in geotechnicalapplications. The clay used in this study is a commer-cialWyoming sodium bentonite offered byVOLCLAY.It has at least 85% passing the No. 200 sieve, specificgravity of 2.65, and liquid and plastic limits equal to397% and 41%, respectively.

Laponite RD, herein referred to as laponite is asynthetic trioctahedral hectorite produced by South-ern Clay Products, Inc., Texas. Laponite particles havea fairly uniform disk shape, with 1 nm thickness, anddiameter of approximately 25 nm, 5–10 times smaller

than bentonite. The specific gravity of laponite isreported to be about 2.57 (Rockwood Additives Ltd.2011). Determinations of the liquid and plastic lim-its performed immediately after mixing the clay withwater yielded values of 1280% and 180%, respectively(El Howayek et al. 2011).

3 A CLAY BASED MECHANICALLYPERCOLATED PORE FLUID

3.1 Attractive gel state of clay-water suspensions

Clay-water suspensions are complex systems withstructure and properties that, for the same clay, dependon a variety of factors including solid concentra-tion, type and concentration of the cations and anionspresent in solution in the water, and pH. This is illus-trated in Figure 2, which presents “phase diagrams”previously obtained for a purified Wyoming Na-montmorillonite and laponite RD by other researchers.These figures show that for both of these materialsthere exists a region in which the suspensions form anattractive gel. As summarized by Tanaka et al. (2004),an attractive gel has the following characteristics:

– it is in a nonergodic state,– it has a percolated network structure that originates

from attractive interactions between the colloidalparticles,

– the characteristic length of the network is muchlarger than the size of the colloidal particle.

A schematic diagram of an attractive gel is shownin Figure 2c, in which the clay particles are repre-sented by black thick lines, while the gray regionsidentify the double layer. For comparison purposesFigure 2d shows a repulsive gel, a colloidal systemwhose response is controlled by caging effects, andwith characteristic length scale on the order of theinterparticle distance.

3.2 Direct observation of bentonite and laponiteattractive gel structure

The phase diagrams shown in Figure 2 were developedbased on rheological and/or osmometric measure-ments.

1378

Figure 2. Phase diagram of (a) Wyoming bentonite and (b)laponite RD; and schematics of (c) attractive gel and (d) repul-sive glass (adapted from Abend & Lagaly 2000 and Tanakaet al. 2004).

Direct observations of the percolated structure ofbentonite and laponite gels have been performed bythe authors using cryo-scanning electron microscopy(Cryo-SEM). In cryo-SEM the sample is cooledrapidly in nitrogen slush, sublimated at −85◦C toremove the unbound water and imaged under cryo-genic conditions (∼−130◦C). Thus it remains close toits natural state and dehydration is prevented. More-over, the rapid cooling process avoids the artifacts dueto water crystallization (Negre et al. 2004).This is idealfor materials such as the laponite and bentonite sus-pensions described here, which have very high watercontents (>1000%) and a delicate structure that wouldbe damaged as a result of volumetric changes to itswater fraction.

Figure 3 shows images obtained making use of theFEI NOVA nanoSEM in Purdue University’s Life Sci-ence Microscopy facility for two gels prepared withdeionized water: an 11% suspension of a rawWyomingbentonite and a 3% laponite RD suspension (concen-trations expressed as mass of clay over total mass ofthe suspension).

Note that the concentration of the raw bentonitesuspension used for the cryo-SEM (and for the testsbelow) is much greater than the values shown in Fig-ure 2a.This is because the phase diagram was obtainedfor a purified Na-montmorillonite (prepared from theraw bentonite following a multi-step treatment process

Figure 3. Cryo-SEM of (a)–(b) 11% Wyoming bentoniteand (c)–(d) 3% laponite gels (Solomon 2006, El Howayek2011).

that included sodium saturation, size fractionation, anddialysis to remove all impurities). In the case of a rawbentonite, the formation of an attractive gel is shiftedto much higher values of clay concentration (Huang2013).

The micrographs in Figures 3a, b show that theclay particles form a percolated network aroundcells occupied by water. Measurements by Solomon(2006) indicate that the average cell size is 3.9 µm(+1.9 µm). This dimension greatly exceeds the size ofthe bentonite particles, confirming that the suspensionstructure is that of an attractive gel.

A networked structure is observed also in the imagesof the laponite suspension. Figure 3c shows a structurein which parallel elongated cells are grouped into smalldomains. Each domain contains several cells that havethe same orientation (Figure 3c), which differs fromthat of the neighboring domains, resulting in an overallisotropic structure. Measurements by El Howayek etal. (2014) indicate that the width and the length of theelongated cells range from 5 to 10 µm, and 80 µm to200 µm, respectively. Both of these dimensions exceedthe size of the laponite particle by several orders ofmagnitude.

3.3 Rheology of attractive gel

Dynamic oscillatory measurements conducted usingan advanced rheometer (Physica MCR 301) revealthe mechanical response of the concentrated ben-tonite and laponite suspensions shown in Figure 3.In these tests, which measure the visco-elastic prop-erties with increasing shear strain (also referred toas “strain sweeps”), the suspension is subjected toan oscillatory shear strain (γ = γ0sin[ωt]) of increas-ing amplitude at constant frequency (ω = 1 Hz) whilemeasuring the resulting shear stress τ, which, in gen-eral, is shifted by a phase angle δ with respect tothe strain wave. τ can also be expressed as the sumof an elastic (solid) component in phase with the

1379

Figure 4. Results of dynamic oscillatory tests on concen-trated (a) bentonite and (b) laponite suspensions.

applied strain, and an out-of-phase viscous compo-nent: τ = γ0{G′sin(ωt) + G′′cos(ωt)}, where G′ andG′′ are termed the storage (elastic) and loss (viscous)moduli, respectively (e.g. Barnes et al. 1989).

Figure 4a shows an example of the results of a strainsweep test conducted on a 10% Wyoming bentonitesuspension 2 days after mixing (see Section 4 for adiscussion of the effect of time). The figure showsplots of storage modulus (G′), loss modulus (G′′),and phase angle (δ) as a function of shear strain γ .For γ as large as 1–2%, G′ remains basically con-stant. As reflected also by the value of δ very closeto zero, in this region G′ � G′′, i.e. the response ofthe material is essentially elastic, typical of solid-likebehavior, and consistent with the attractive gel struc-ture observed in the cryo-SEM images. Comparableresults are reported in Figure 4b for a 2 day old 3.25%laponite suspension, which, despite the ∼3 times lowerclay concentration, has similar mechanical propertiesto the 10% bentonite suspension.

For both suspensions, once the shear strain exceedsa critical value, the storage modulus drops signif-icantly, and the behavior of the material becomesincreasingly viscous. With continued shearing, thereis significant degradation in G’ (while the viscousmodulus, G′′ increases), but the material continues toexhibit essentially solid-like behavior (i.e. G′ > G′′) upto shear strains exceeding 10%.This suggests that up tothese magnitudes of the shear strain, the clay gel willbe able to restrain any movement of the sand grainsduring cyclic loading.

Moreover, Santagata et al. (2014) and El Howayek(2011) show that even after significant and repeatedstraining, as a result of their thixotropic nature, boththe bentonite and the laponite suspensions exam-ined in this work immediately recover their orig-inal solid-like response, and with time recuperatethe intact mechanical properties. This suggests that,if effective in mitigating soil liquefaction, the pro-posed treatment with laponite or bentonite suspensionswould be self-healing, i.e. would be expected to resist

Figure 5. Flow curve for a 3% laponite suspension 15minutes after mixing.

multiple earthquakes, even after undergoing localizedstraining.

4 ENGINEERING NANO-SCALEINTERACTIONS TO ENABLE TREATMENTDELIVERY

For bentonite and laponite to be practical solutions fortreating a sand deposit susceptible to liquefaction itbecomes necessary to find the means to deliver thesenano-clays inside the sand pores. However, deliveryof the amounts of bentonite or laponite identified inSection 3 in the sand pore space in suspension formposes significant practical challenges, as it requiresthat on the short term the suspensions have rheologicalproperties (no yield stress and low viscosity) and solrather than gel structure that allow their permeationinside a porous medium; but that once inside the sandpores, the suspensions regain the gel-like nature andsolid-like properties observed in Figures 3–4.

From this point of view, laponite offers an advan-tage over bentonite, in that, for concentrations up to∼3.25%, the suspensions display an initial Newtonianbehavior and a naturally retarded gelation process. Asan example, the data in Figure 5 from a constant shearrate ramp test on a 3% laponite suspension, performed15 minutes after mixing the clay with water, show thatthe behavior of the suspension is similar to that of aNewtonian fluid, with an almost constant value of theviscosity, about 4 times that of water. This reflects that,at this time, the suspension is in form of a sol.

With continued time, the suspension displaysincreasing viscosity (e.g. at 1 hour the viscosity is dou-bled), and after 1.5 hours, oscillatory measurementsshow a transition from sol to gel state. As a result,within this time window it is possible to permeate thesuspensions at least at the laboratory scale with noadditional treatment. Similar observations are reportedby El Howayek (2011) for a 3.25% laponite RD sus-pension, although the initial viscosity is higher (7 vs.4 mPa·s) and the sol-gel transition time is reduced to0.4 hours.

1380

Figure 6. Variation of storage modulus with time for ben-tonite and laponite suspensions.

The evolution of the mechanical response of thelaponite suspension is illustrated in Figure 6, whichreports the storage modulus (G′) measured at smallstrains as a function of time for concentrations of 3 and3.25%. At early ages, G′ falls below G′′ (not shown),reflecting the liquid-like behavior of the suspension.With time G′ increases and the suspension developsa solid like response, with a value of the phase anglevery close to zero (see Figure 4b for the behavior after2 days).

The above is not true for bentonite, and for this pur-pose, a methodology was developed (Santagata et al.2014) to chemically modify the rheology of bentonitesuspensions through the use of sodium pyrophosphatedecahydrate (Na4P2O7.10H2O), or SPP. The anionof SPP, which has a negative charge of −4, has ahigh affinity for the broken edges of clay minerals,and once sorbed onto these positively charged sites,imparts them a negative charge. This effectively limitsedge to face interactions, preventing the formation ofa gel and reducing the viscosity of the suspension.

This is illustrated in Figure 6, which shows thechange in storage modulus measured over time fora 10% bentonite suspension treated with 0.5% SPP(by mass of the clay). This is the minimum dosage ofSPP that allows permeation of the concentrated sus-pension inside a laboratory column of Ottawa sandwith height of ∼45 cm (El Mohtar et al. 2013). Forreference the figure also shows the variation in G′of an untreated 10% bentonite suspension. As shownin Figure 6, at very short times, the addition of SPPcauses the storage modulus (and the viscosity) of thesuspension to decrease by several orders of magni-tude, allowing permeation into a porous medium. Overtime, by a mechanism that is not completely under-stood, the effect of the SPP disappears. As a result, G′increases, eventually leading to the formation of a gelstructure with behavior similar to that of Figure 4a, andwith thixotropic properties that allow rapid healing ofthe gel structure, as described above for the untreatedlaponite and bentonite gels.

Compared to the laponite suspensions, the recov-ery of the storage modulus is much slower (with 7months required for the 0.5% SPP suspension to reachthe value attained by the 3% laponite suspension after7 days). Similar behavior is observed for suspensions

Figure 7. Results of resonant column tests on clean sandand laponite-permeated sand specimens.

Figure 8. Cryo-SEM image of laponite-permeated sand.

treated with higher dosages of SPP. The greater theSPP%, the greater the reduction in viscosity and stor-age modulus, but also the more delayed the formationof the gel, and the weaker structure of the gel formedat a given time.

5 RELATIONSHIP TO MACRO-RESPONSE

The effects of the presence of the bentonite or lapnitegel on the response of treated sand are illustrated inthe results of tests performed on specimens of Ottawasand permeated in the laboratory with concentratedsuspensions of laponite or bentonite.

Figure 7 shows data obtained from two resonantcolumns tests: one conducted on a specimen of cleanOttawa sand with Dr = 22%, the other on an identicalspecimen permeated with a 3.25% laponite suspen-sion. Both specimens were consolidated to 100 kPaand tested undrained. The laponite-permeated spec-imen was tested after 72 hours of aging to allowdevelopment of the gel structure. The data show thatthe laponite-permeated specimen exhibits a retardedgeneration of excess pore pressure and a less signifi-cant reduction in shear stiffness at any strain level.Thiscan be attributed to the restraining action provided bythe laponite gel, which, as shown in the cryo-SEMimage presented in Figure 8, forms a dense networkaround the sand particles.

Under cyclic shearing this effect is manifested asan increase in the number of cycles sustained under

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Figure 9. Results of cyclic triaxial tests (adapted from ElMohtar et al. 2013).

a given cyclic stress ratio (CSR). This is illustrated inFigure 9, which presents data for clean Ottawa sandspecimens and for specimens of the same sand and thesame relative density, permeated with a 10% bentonitesuspension treated with 0.5% SPP by dry mass of theclay, and aged for 20 days (to allow formation of thegel structure). It is seen that under a CSR of 0.11 thebentonite permeated specimen can sustain over 1000cycles, relative to only 5 for the untreated sand.

6 CONCLUSIONS

The paper has addressed some fundamental aspects ofa novel approach to soil improvement for liquefactionmitigation that is based on engineering the pore fluidof a sand through the introduction, in suspension form,of controlled amounts of two smectitic nano-clays:bentonite, a material widely used in the geotechnicalcommunity, and laponite a synthetic clay with diameter5–10 times smaller than that of bentonite.

The work presented demonstrates the influence ofnano and micro level interactions on the observedmacro behavior. At the core of the proposed approachis the formation in the sand pore space of an attrac-tive gel that has solid-like rheological characteristics.Its presence enables a clear reduction in the genera-tion of excess pore pressure during undrained loading,leading to an increased cyclic shear resistance, andindicating that this may be a viable approach to lique-faction mitigation. The overall benefits are believed tobe realizable in practice due to the demonstrated abil-ity to control the sol-to-gel transition of the nano-claysuspensions, enabling introduction of the engineeredpore fluid into the soil matrix with no disturbance.

ACKNOWLEDGEMENTS

This research was partially funded by the NationalScience Foundation, Geomechanics and Geotechni-cal Systems Program, under grants CMS-0408739 andCMS-0928679.

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