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TECHNICAL PAPER

Challenges due to problematic soils: a case study at the crossroadsof geotechnology and sustainable pavement solutions

Khaled Sobhan1

Received: 4 May 2017 / Accepted: 2 June 2017 / Published online: 5 July 2017

� Springer International Publishing AG 2017

Abstract Geotechnical characteristics of subsoils should

be adequately incorporated in the rehabilitation strate-

gies of existing pavements which have performed poorly

due to problematic subsurface conditions. However,

there appears to be a disconnect between the advances in

our understanding of the mechanics of soft or problem-

atic soils and the rehabilitation design of the overlying

pavement structure, leading to repeated cycles of pre-

mature distresses, underperformance, and failures. A

case study is presented for the rehabilitation of a flexible

pavement built over soft organic soils in Southeastern

Florida, USA. The study incorporates forensic investi-

gation of the deteriorated pavement structure, subsurface

investigations with cone penetration testing, design and

construction of reinforced overlays in field test sections,

and long-term performance monitoring with non-de-

structive dynamic tests. Efforts are made to correlate site

characteristics with pavement performance. Based on the

secondary compression behavior of the organic soils,

cement deep mixing criteria are proposed for a more

durable and sustainable solution.

Keywords Sustainability � Pavement � Problematic soils �Geosynthetics � Rehabilitation � CPT

Introduction

In his 36th Rankine Lecture, Professor Stephen Brown

stated [6]: ‘‘Application of soil mechanics principles to the

design of pavement foundations, the design of complete

pavements and to their structural evaluation ‘in-service’

has lagged some way behind knowledge accumulated

through research’’. Since the delivery of the famed Rankine

Lecture 20 years ago, pavement geotechnics has gradually

emerged as an important subdiscipline within geotechnical

engineering bridging the geotechnical aspects of the

pavements (mechanics of the foundation layers and sub-

soils) with the performance of the pavement superstructure

(i.e., the asphalt or concrete). The principles of soil

mechanics relevant to pavement engineering mostly

include the resilient and permanent deformation responses

of the underlying granular materials and the subgrade soils

to repeated traffic loading, along with their moisture and

environmental conditions. Consideration of the geotechni-

cal principles can be readily implemented during the design

phase of a new construction (e.g., by incorporation of the

non-linear stress–strain behavior of the granular layers and

soil subgrade). However, it is widely recognized that most

of the efforts are now directed towards the rehabilitation of

existing pavements rather than construction of new pave-

ments in both urban and rural settings. A pavement reha-

bilitation process typically includes the full or partial

removal and reconstruction of the deteriorated asphalt (or

near-surface) layers. When engineers encounter pavements

that have been built over soft and problematic soils, they

must also take into account the characteristics and the

variability of the underlying natural soils to correctly

determine the root causes of premature pavement dis-

tresses, and to identify the correct strategy for a durable

and sustainable solution.

This paper was selected from GeoMEast 2017—Sustainable Civil

Infrastructures: Innovative Infrastructure Geotechnology.

& Khaled Sobhan

[email protected]

1 Center for Marine Structures and Geotechnic, Florida

Atlantic University, Boca Raton, FL 33498, USA

123

Innov. Infrastruct. Solut. (2017) 2:40

DOI 10.1007/s41062-017-0070-y

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Many times, the general class of problematic soils

include naturally occurring soft clays and silts, expansive

clays, sensitive and collapsible soils, highly organic soils,

peat deposits, etc. characterized by one or more of the

following attributes: high compressibility, low bearing

capacity, high void ratios, high water content, and high

spatial variability. Unfortunately, the geotechnical proper-

ties of the problematic natural soils are generally not a

direct input to the design of a pavement overlay, although

engineers remain fully aware of the difficult site conditions.

Accordingly, the inherent weaknesses of the original

pavement linked to the underlying problematic soils con-

tinue to exist in the newly rehabilitated pavement system

[25], leading to the recurring cycles of premature distresses

(cracking, rutting and differential settlements) and costly

rehabilitation.

It follows from the above discussions that the effects of

naturally occurring problematic soils underneath a pave-

ment cannot be mitigated simply by constructing thicker

and thicker pavements during each rehabilitation cycle

with the hope of ‘‘isolating’’ the soft soils. For example, in

case of organic soils and peats, the increased dead weights

from the thicker pavements will cause higher secondary

compression settlements, resulting in more (and possibly

faster) occurrences of cracking and rutting in the overlying

asphalt layers. To effectively deal with the situation,

engineers have few choices: (1) near-surface measures,

such as reinforcing the asphalt and base layers with

geosynthetics thus reducing the potential (and the rate) of

cracking and/or rutting; (2) deep-seated measures, such as

chemical stabilization, cement deep-mixing columns and

other ground modification techniques; and (3) a combina-

tion of (1) and (2). Adopting any of these choices can result

in higher initial costs, but may lead to substantial

improvement in overlying pavement performance. Only a

life cycle cost analysis can determine the actual long-term

benefits of the selected strategy.

The current paper describes the rehabilitation case history

of SR15/US98 located in the northwestern part of Palm

Beach County, Florida, running along a section of the

perimeter of Lake Okeechobee. The existing SR15/US98

roadway consisted of a thick AC layer averaging almost

330 mm (13 in.), a base layer averaging 305 mm (12 in.),

and a sand fill subgrade averaging almost 1 m (40 in.),

overlying organic soils (silty muck and peat) ranging from 3

to 5 m (9–16 ft) in thickness. USDA soil survey data indi-

cate that the surficial soils of the site are mapped as Torry

muck and Adamsville sand, organic subsoil variant over

hard limestone. The distressed conditions of the roadway

prior to rehabilitation, and the depth and type of problematic

soils are shown in Fig. 1. The organic soils had the fol-

lowing properties [22]: the organic contents range from 25

to 92%, moisture content ranges from 160 to 650%, void

ratio ranges from 3.2 to 13.9, undrained shear strength

ranges between 17 and 40 kPa, and the Ca/Cc ratio ranges

between 0.028 and 0.051, where Ca and Cc are the primary

and secondary compression index, respectively. Frequent

and costly rehabilitation is necessary to maintain the func-

tionality of these roadways which often experience prema-

ture distress in the form of cracking, rutting, and differential

settlement. In the Fall of 2008, 24 experimental pavement

sections were constructed along the roadway alignment with

various geosynthetic reinforcing products embedded in the

asphalt overlay, and a comprehensive field testing and

monitoring program was undertaken (near-surface mea-

sures). Various components of this project including

geotechnical characterization, forensic investigation, recon-

struction, long-term monitoring, and the development of a

deep-mixing criterion (deep-seated measures) for the

organic soils and peats are summarized in this paper. More

details can be found in the references[22–26]

Objectives

The broad objectives of this study are to present a perfor-

mance-based case history of a flexible pavement (before

and after rehabilitation) built over soft organic soils and

peat. Efforts are made to identify the linkage between

problematic soils properties and the performance of the

overlying pavement structure, and thereby, highlight the

importance of geotechnical characterization of underlying

soils in developing appropriate pavement rehabilitation

strategies. The investigation was carried out in the fol-

lowing three Phases:

1. Phase I Geotechnical Characterization based on

Piezocone Penetration Testing and coordinated labo-

ratory consolidation experiments;

2. Phase II Performance of Experimental Test Sections in-

corporating various reinforcing products in the asphalt

overlay; and

3. Phase III Development of Cement Deep-Mixing

Criteria, based on time–stress–compressibility rela-

tionships for cement-stabilized organic soils.

Relevant studies

A significant body of literature exists on the laboratory

compression behavior of organic soils and peat,[12, 15, 18]

and on Piezocone Penetration tests for in situ characteri-

zation of clayey soils [1, 2, 14]. Laboratory Time–Stress–

Compressibility relationship was developed for Florida

organic soils following the procedures outlined in the lit-

erature [17]. In addition, the strength, modulus and

40 of 18 Innov. Infrastruct. Solut. (2017) 2:40

123

deformation characteristics were also interpreted directly

from the cone tip resistance data [11, 14, 20, 21].

Sobhan and Tandon [27] conducted laboratory model

tests and ABAQUS finite element based numerical

investigations to study reflection crack propagation in

geogrid reinforced asphalt overlays. Cafiso and Di Gra-

ziano[8] reported that asphalt pavement test sections

reinforced with steel reinforcement meshes placed at two

different depths had improved the remaining life 5 years

after construction. In a coordinated study involving the-

oretical analysis and laboratory model tests it was con-

cluded that the steel reinforcement extended the asphalt

fatigue life by 2.5 times, whereas the improvement for

polypropylene and glass fiber grids ranged from 1.2 to 1.8

times compared to control specimens[7] . Several other

relevant laboratory and field studies are available in the

literature[3, 13, 29]

Phase I: geotechnical characterization

Background

The current study evaluated the use of Piezocone Pene-

tration Tests (CPTu) as a versatile tool for subsurface

investigations when soft organic soils are encountered.

Accordingly, eleven different severely distressed locations

were chosen along the alignment of SR 15/US 98 for

conducting the piezocone penetration tests, and collection

of ‘‘undisturbed’’ Shelby tube samples at various depths for

subsequent laboratory testings. The primary objective was

to evaluate the strength and compressibility characteristics

of the organic soils in situ and through laboratory consol-

idation and secondary compression testing.

Field testing program

A preliminary geotechnical site investigation was carried

out previously along the alignment of SR 15/US 98 at the

project location [9]. This included 93 Standard Penetration

Test borings (numbered B-1 to B-93) up to a depth of 6 m,

with borehole locations spaced at 150 m intervals. Based

on the observed distress conditions of the roadway, and an

analysis of the available data, 11 different locations (named

Site 1 through 11) were carefully selected for field testing

and/or retrieval of undisturbed soil samples. These site

selection strategies are described elsewhere [19] In general,

CPTu is gaining nationwide acceptance as a versatile tool

for subsurface geotechnical investigations. A Piezocone

Penetrometer is a CPT device equipped with a pore pres-

sure transducer which measures the pore water pressure

(ue) in the proximity of the cone. This feature enables the

on-site estimation of the time rate of consolidation char-

acteristics of the soft layer. The cone tip resistance (qc) and

the sleeve friction (fs) are also measured for the estimation

of soil classification/stratification, and in situ strength and

modulus. Details of the field experimental program are

available in the Ref [9]. A summary is provided below.

Fig. 1 a, b Distressed condition

along SR 15/US98; c silty muck

layer from 1.5 to 3.0 m;

d fibrous peat layer from 3.0 to

6.0 m

Innov. Infrastruct. Solut. (2017) 2:40 of 18 40

123

At a typical location, the upper 152 cm of pavement

layers was first augered using a Mobile B-31 rig. All CPTu

tests were conducted using Hogentogler CPT equipment

(10-ton digital 4-channel subtraction cone) approximately

in accordance with ASTM D 5778 methodology [4]. At a

depth of 3.5 meters (11.5 ft), the cone was stopped, and the

dissipation of excess pore water pressure with time was

monitored, called Porewater Dissipation Test discussed

elsewhere [23]. The CPTu sounding was again continued

until practical refusal was met at a depth of about 5.5 m.

Adjacent to the CPTu location, a Central Mine Equipment

Model 75 drilling rig was employed to obtain Shelby tube

samples from 2 different depths at each site using a

hydraulically operated piston sampler (Acker Gregory

Undisturbed Sampler) in accordance with ASTM standard

methodology [5]. All boreholes were finally backfilled with

the soil cuttings and surfaced with asphalt cold patch.

Laboratory consolidation tests

Shelby tube samples were collected from two different depths

at each of the 11 sites. It was found that soils at shallow depth

contains dark brown organic sandy silt (organic content

25–40%), which is underlain by pre-dominantly dark, fibrous

organic soils resembling peat (organic content 70–92%). The

moisture contents in the organic layers range between 160 and

650%, with initial void ratios varying from 5.25 to 11.67.

Therefore, the laboratory tests were conducted on samples of

both the organic silt and the peat materials for each site (total

of 22 soil types and 44 specimens). The specimen is incre-

mentally loaded to the desired pressure (r0), and allowed to

undergo secondary compression at constant stress for

2–4 weeks. For 50% of the sites, the constant stress level,

defined by the ratio of applied pressure to the pre-consoli-

dation pressure (rv/rp), was 0.30–0.60, which corresponded

to the in situ pressure due to the overlying pavement layers.

These specimens were therefore in the recompression range,

while the remaining sites were subjected to a constant stress

level rv/rp of 1.0–1.15, implying a stress state corresponding

to the normally consolidated range. Details of this laboratory

testing program are available elsewhere [19].

Results of consolidation tests

Typical void ratio versus effective stress behavior is shown

in Fig. 2a, and the Compression Indices are plotted in

Fig. 2b, which is a compilation of available data on the

variation of Cc with natural water contents [18]. It is found

that the Cc values for Florida soils fall within the accept-

able ranges of other similar soils. The Pre-consolidation

pressure, r0p was found to vary within the range of

73–83 kPa. Taylor’s square-root-of-time standard plots

were constructed to estimate the time to end-of-primary

(EOP) consolidation, which was found to be approximately

1 min for most laboratory samples [22]. Similar values

were reported in the literature for peat and organic soils

under similar stress levels [18].

Secondary compression behavior

Secondary compression tests were conducted on 22

undisturbed specimens representing all 11 sites using the

loading scheme described earlier. Typical behavior shown

in Fig. 3 demonstrates that during the secondary phase, the

variation of e with log time is approximately linear. The

slope of the curve is called the secondary compression

index, Ca, and is defined as follows:

Ca ¼Delog t

tp

¼ DeD log t

; ð1Þ

where tp is the time to end-of-primary (EOP) consolidation,

and t is any time t[ tp. In this study, Ca was calculated

during the first log cycle after the EOP consolidation.

Time–Stress–Compressibility relationships (Ca/Cc concept)

Mesri and Godlewski [17] postulated that for any given soil,

there is a unique relationship between Ca ¼ De=Dlogtð Þ andðCc ¼ De=Dlogr0Þ, that holds true at all combinations of time

(t), effective stress (r0), and void ratio (e). At any given

effective stress, the value of Ca from the first log cycle of

secondary compression and the corresponding Cc value

computed from the EOP e-log r0 curve are used to define therelationship between Ca and Cc. It is to be noted that Cc

denotes the slope of e-log r0 curve throughout the recom-

pression and compression ranges. These values are plotted in

Fig. 4 to develop the unique Ca/Cc relationship for Florida

organic soils. Also shown in Fig. 4d is the Ca/Cc relationship

developed forMiddleton peat [18] for comparisonpurposes. It

is found from Fig. 4 that the Ca/Cc ratio for Florida organic

soils range from 0.028 to 0.051. Compilation of worldwide

existing data for peat, fibrous peat, and amorphous to fibrous

peat from the literature shows that the value for theCa/Cc ratio

varies within the range 0.035–0.1 [18]; these values are con-

sistent with the values obtained in the current investigation.

Organic factor (Forg) and soil properties

A site-specific geotechnical parameter termed the organic

factor (Forg) was introduced during this Phase of this

investigation to serve as a guide for determining the

appropriate locations for the experimental test sections for

Phase II [22]. The Organic Factor, Forg is the theoretical

weight of the pure organic material per unit area of the


123

organic layer, and is expressed in terms of the organic

content (OC), moisture content (MC), total unit weight

(cT), and organic layer height (hm), as follows:

Forg ¼hmcT � OC

ð1þMCÞ : ð2Þ

Due to large variability in organic and peat layers, Forg can

change within relatively short distances along the roadway.

Note that the product of {cT/(1 ? MC)} (which is the dry unit

weight of solids including organics) and OC gives the weight

of the pure organic solids per unit volume of the soil. Multi-

plying this weight by the volume of a column of the organic

soil with unit area results in theweight of organic solids in that

column of the foundation. With this interpretation, it is rea-

sonable to expect that larger the Forg, the poorer will be the

support for the overlying pavement structure.

The Organic Factor was determined at 35 boring loca-

tions along SR15/US98 as shown in Fig. 5, with the most

heavily distressed areas indicated by yellow color. Based

on this data, two locations were identified for the future test

sections: Location 1, spanning boreholes B-33 through

B-41 (Station 155 ? 00.75–Station 170 ? 98.01) with an

average Forg (theoretical organic weight) of 86 kg; and

Location 2, spanning boreholes B-54 through B-70 (Station

227 ? 02.48–Station 258 ? 97.75) with an average Forg of

61 kg. Pre-construction visual distress survey indicated

that locations with higher Forg corresponded to higher level

of pavement deterioration.

Although based on limited data, reasonable correlations

were found between the organic factor and in situ Elastic

Modulus, E, and the organic factor and secondary compres-

sion index, Ca, as shown in Fig. 6. Elastic modulus was

estimated from cone tip resistance data [14], with details

described in Sobhan [22]. These preliminary relationships

involving both stiffness and compressibility behavior show

some expected trends, such as decreasing moduli and

increasingCa valueswith increasingForg. SinceForg is related

to both the modulus (and, in turn the strength) and the com-

pressibility of the organic soils, and since the strength and

deformation (settlement) properties of the foundation layer

can be assumed to have a strong influence on the performance

of the pavement structure (cracking, rutting and ride quality),

it is reasonable to hypothesize that a site-specific parameter,

such as Forg will also be related to pavement performance.

Accordingly, the visual distress survey coupled with Forg

provided some ‘‘geotechnical guidance’’ in selecting the

appropriate locations for the experimental test sections along

the SR15/US98 roadway (Phase II).

Lessons learned from Phase I

The site-specific soil conditions (moisture content, organic

content, thickness and unit weight of the organic layer) were

expressed by an organic factor (Forg), which was correlated

with pertinent soil properties important for pavement per-

formance. It was found that the (Ca/Cc) ratio for Florida

organic soils and peat at any stress level has constant values

ranging from 0.028 to 0.051, which are consistent with the

values reported in the literature for similar soils. Considering

the inherent difficulty in sampling and laboratory testing of

undisturbed soft organic soils, Piezocone penetration tests

showed promise as an efficient tool for relatively rapid

in situ characterization of subsoil strength, modulus, and

compressibility, all of which may be used for forensic

interpretations of pavement failures, mechanistic analysis,

and validation of pavement performance models.

Void

Rat

io,e

Cc

16

SiltyFibrous

12

Clay and Silt Deposits 10 Peats

Florida Organic Sandy SiltFlorida Peat

81

4

010 100 1000

Applied Pressure, kPa

0000100101

Wo, %

(a) (b)

Fig. 2 a Consolidation behavior of Florida organic soils; b compression index for Florida soils relative to other similar soils (after [18])


123

Phase II: performance of test sections

Background

Based on the analysis of geotechnical data gathered in

Phase I, two locations with distinctly different average

subsoil conditions were selected for the construction and

monitoring of 24 test sections. This included 8 control

sections, and 16 reinforced asphalt overlay sections

incorporating four different asphalt reinforcing products:

(1) PetroGrid 4582, which is a composite of a glass fiber

structural grid bonded to a paving fabric; (2) GlassGrid

8511, which is a fiberglass mesh with a 25 mm 9 25 mm

aperture size and an elastomeric polymer coating; (3)

PaveTrac MT-1, which is a coated steel mesh consisting of

a twisted woven hexagonal wire netting reinforced in the

Fig. 3 Compression behavior with respect to time (r0v=r0p = 0.6): a organic silts; depth = 2.1 m; b fibrous peat; depth = 3.66 m


123

transverse direction at regular intervals by flat, alternately

twisted reinforcing bars; and (4) Asphalt Rubber Mem-

brane Interlayer (ARMI), which is composed of a separate

application of asphalt rubber binder ARB-20 covered with

a single application of aggregate constructed per FDOT

specification. The test site locations were each 915 m

(3000 ft) long, separated by 1280 m (4200 ft) and were

subdivided into six test sections each 152.5 m (500 ft) long

covering both the northbound and southbound travel lanes.

The first and last sections in each lane were designated

control sections with no reinforcement. Figure 7 shows the

layout and details of the test sections.

Prior to the rehabilitation project, series of falling

weight deflectometer (FWD) tests were conducted at

every 15.2 m (50 ft) along the proposed test section

alignment for evaluating the existing pavement capacity,

and statistically determining the site variability among

the test sections. Six months after the reconstruction

project, FWD tests were repeated at the same locations

for characterizing the test sections. The stiffness prop-

erties of the composite pavement structures were deter-

mined directly from the load–deflection data for

evaluating the relative performance of the reinforced

pavement sections. A major objective was to quantify

the benefits (gain in stiffness) of using reinforcing

products in asphalt overlays based on FWD test data by

comparing the performance of reinforced and control test

sections. Details of this work are available in the liter-

ature [25, 26]. A summary is provided below.

Pre-construction baseline investigation

To fully characterize the existing pavement distress and

the uniformity of the current bearing strength of the

pavement, dynamic nondestructive tests using the FWD,

Sec

onda

ryC

omp.

Inde

xS

econ

dary

Com

p.In

dex

Cα/

(1+e

0)S

econ

dary

Com

p.In

dex

0.20

0.16

(a)0.20

0.16

(c)

0.12

0.08

0.04

Cα/Cc= 0.029R2= 0.96

0.12

0.08

0.04

Cα/Cc= 0.028R2= 0.98

0.000 2 4 6 8

Compression Index, Cc

0.000 2 4 6

8 Compression Index, Cc

0.08

0.06

(b)0.05

0.04

(d)

0.04

0.02

Cα/Cc= 0.051R2= 0.95

0.03

0.02

0.01

Cα/Cc= 0.052

0.000 0.4 0.8 1.2 1.6 2

Compression Index, Cc

0.000 0.2 0.4 0.6 0.8 1

Cc/(1+e0)

Fig. 4 Ca/Cc Relationships—a all Florida specimens; b Florida organic silts; c Florida peat; d Fibrous peat (after [18]


123

and pavement rut measurements were conducted at every

15.2 m in each lane, compiling ten deflection and ten rut

measurements per test section. Statistical analysis of the

FWD data using t tests at a 99% confidence level was

conducted, and every section was statistically compared

with every other section in the same lane and same test

location. This method assured that pre-construction and

post-construction comparisons would be made among

sections for which the initial conditions are well defined

and directly comparable.

In this study, the Impulse Stiffness Modulus (ISM) was

used as a direct quantitative measurement of pavement

responsewhen subjected to FWD loading. ISM is defined by:

ISM ¼ F

D0

; ð3Þ

where F = vertical dynamic load &40,000 N (9000 lb);

and D0 = deflection at the center sensor. The deflections

under the center of the load plate were adjusted to a ref-

erence asphalt temperature of 20 �C (68 �F) per the

200

Yellow signifies pavements under significant distress, as determined by site investigation

150

100

50

0

Boring Log No.

Org

anic

Fac

tor,

F org

(Kg)

B-0

2

B-0

6

B-0

9

B-1

2

B-1

5

37.9 45

.8

45.0

44.1

60.6

B-2

0

B-2

6

B-3

0

B-3

1

B-3

2

B-3

3

B-3

4

B-3

5

B-3

6

B-3

7

B-3

8

B-4

0

B-4

1

B-4

4

B-4

8

B-5

4

29.8 37

.4

21.0

35.1 41

.6

26.3

53.0 56

.2

56.1

52.7

64.1

98.4 10

1.8 11

0.0

142.

9

188.

2

B-5

5

B-5

7

B-6

0

B-6

2

B-6

5

B-6

7

B-7

0

B-7

6

B-7

9

B-8

3

B-8

6

32.3 35

.6

24.8

46.1

66.6

57.3 64

.4

56.6

57.8 60.4 69

.3

B-8

8

B-9

1

B-9

28.

3

19.7

36.4

Fig. 5 Organic factor at various borehole locations

5 0.2

4 0.16

3 0.12

2 0.08

1 0.04

0 0

0 40 80 120 160 0 40 80 120 160Organic Factor, Forg (kg) Organic Factor, Forg (kg)

Elas

tic M

odul

us, E

(MPa

)

Seco

ndar

y C

omp.

Inde

x, C

α

Fig. 6 Variation of a Elastic

Modulus and b Ca with organic

factor (1 psi = 6.894 kPa;

1 lb = 4.44 N)


123

procedures given in Federal Highway Administration

Publication no. FHWA-RD-98-085 [30]. The results of the

pre-construction data analysis are summarized in Fig. 8,

indicating consistency of the test sections through a color

coding scheme, defining sections that were significantly

different from two, three or more other sections in the same

lane and test location. It was observed that the southbound

travel lanes (STL) of both test locations were more con-

sistent (uniform) compared to northbound travel lanes

(NTL). The rut data indicated good consistency in the

northbound lane of location 1 while the ISM data indicated

good consistency in the northbound lane of test location 2.

Construction of test sections

The existing pavement was milled 11.4 cm (4.5 in.), and a

25 mm (1 in.) overbuild was placed on the milled surface.

In each test section, the reinforcing material was placed on

the overbuild layer according to the installation require-

ments of each material. A 64 mm (2.5 in.) structural layer

was placed on top of the reinforcement materials and

finally a 25 mm (1 in.) friction course brought the finished

road surface back up to approximately the original level. A

typical longitudinal view of the test pavement is shown in

Fig. 9.

Post-construction tests and analysis

Comparison of ISM values

Six months after the reconstruction, a second set of FWD

data was gathered, using the same survey baseline mea-

suring the pavement deflections at nearly the same points

Test Location 1Stn: 155+00 160+00 165+00 170+00 175+00 180+00 185+00

STL Control Petrogrid GlassGrid PaveTrac ARMI Control STL

NTL Control Petrogrid GlassGrid PaveTrac ARMI Control NTLSection: 1.0 1.1 1.2 1.3 1.4 1.5

Test Location 2Stn: 227+00 232+00 237+00 242+00 247+00 252+00 257+00

STL Control Petrogrid GlassGrid PaveTrac ARMI Control STL

NTL Control Petrogrid GlassGrid PaveTrac ARMI Control NTLSection: 2.0 2.1 2.2 2.3 2.4 2.5

Fig. 7 Layout of test locations

and reinforcement materials

used for rehabilitation

Test Location 1Station: 155+00 160+00 165+00 170+00 175+00 180+00 185+00

STLISM Rut STL

NTL ISM Rut NTL

Section: 1.0 1.1 1.2 1.3 1.4 1.5Test Location 2

Station: 227+00 232+00 237+00 242+00 247+00 252+00 257+00

STLISM Rut STL

NTL ISM Rut NTL

Section: 2.0 2.1 2.2 2.3 2.4 2.5Key

Section is statistically dissimilar to not more than one other section Section is stiffer or less rutted than two other sections Section is stiffer or less rutted than three or more other sections Section is softer or more rutted than two other sections Section is softer or more rutted than three or more other sections Note: All comparisons are between sections in the same lane and location

Fig. 8 Pre-construction data

analysis for determining

uniformity of test sections


123

as in the pre-construction tests. The data was compiled,

adjusted to a 20 �C (68 �F) reference temperature, and is

presented in Figs. 10 through Fig. 11. Rut measurements

were also made at the time of the FWD tests and were

found to be less than 1.5 mm (0.06 in.) at all points on the

resurfaced road, and were considered negligible. An

interesting observation in Figs. 10 and 11 is the clear

matching of peaks, valleys and trends in the two sets of

data despite the data being separated by 19 months (time

between pre- and post-construction FWD tests), and a very

significant amount of work having been done on the

roadway. Local maxima and minima occur at virtually the

same locations along the abscissa in all of the test sections

and in many instances the increasing and decreasing trends

seen in the pre-construction and post-construction data also

match. This provides some degree of confidence that the

FWD data sets can be readily compared to one another.

In most cases, the post-construction FWD data yielded

ISM values greater than the pre-construction values. The

post-construction FWD data was compared to the pre-

construction data using F tests and t tests to ascertain that

the change in ISM values were statistically significant.

Comparison of post-construction FWD test results with

pre-construction results showed that PetroGrid, GlasGrid

and PaveTrac reinforcement improved the pavement stiff-

ness in 11 out of 12 test Sections (95% confidence level).

By contrast, only three of eight control sections showed

statistically significant improvement in stiffness.

Contribution of reinforcing products

To isolate and quantify the contribution of the reinforce-

ment materials alone in the overall improvement of pave-

ment stiffness, the following methodology was developed:

1. The mean of the ten ISM values in each section was

found for both the post- and pre-construction mea-

surements and designated as follows:

ISMM-POST = the mean ISM value for a section (post-

construction)

ISMM-PRE = the mean ISM value for a section (pre-

construction)

2. The difference in the mean ISM value post- versus pre-

construction was found and designated as DISMM:

DISMM ¼ ISMM�POST � ISMM�PRE ð4Þ

Thus DISMM is the increase in mean ISM due to

reconstruction.

3. The DISMM values for the two control sections in each

lane of each test location were averaged and desig-

nated DISMM-C. Therefore,

DISMM-C = average increase in mean ISM for two

control sections

DISMM-R = increase in mean ISM for one reinforced

test section.

4. The difference between the DISMM for each section

and the average of the two DISMM values for the two

Fig. 9 Typical longitudinal

view of test section [25]


123

control sections in the same lane and test location are

calculated. This is a direct comparison of the change in

stiffness of each test section compared with the

average change in stiffness of the corresponding

control sections. So the actual gain in stiffness due to

reinforcement alone, GR, is defined as follows:

GR ¼DISMM�R � DISMM�C ð5Þ

The results of this analysis are presented in Fig. 12, which

shows that in case of all three reinforcing materials used in test

sections 1, 2 and 3, the increase in pavement stiffness was both

statistically significant and greater than the average change in the

two related control sections (true for 11 out of 12 sets of data).

The ARMI sections showed either a non-significant increase in

ISM or had a ‘‘negative’’ change (or decline in stiffness values)

compared to that of control sections. The increase in stiffness

observed with three reinforcing grids, however, was more pro-

nounced and statistically significant than that observed by the

other researchers [8].

Lessons learned from Phase II

The organic factor, Forg, introduced to quantify soft organic

pavement foundations is shown to be a satisfactory param-

eter for characterizing thick deposits of soft organic soils

encountered in the south Florida area. Comparison of post-

construction FWD test results with pre-construction results

shows that PetroGrid, GlasGrid and PaveTrac reinforcement

improved the pavement stiffness in 11 out of 12 test sections

(95% confidence level). The spatial consistency of FWD test

results suggests that it (FWD) can be effectively employed

for direct, reliable and fast characterization of the stiffness

Fig. 10 ISM values at Test Location 1, post-construction vs. pre-

construction Fig. 11 ISM values at Test Location 2, post-construction vs. pre-

construction


123

properties (such as ISM) of reinforced asphalt pavement

sections. Gain in stiffness,GR, calculated directly from ISM

data, can be used as a performance-based parameter for

long-term evaluation of reinforced test sections, and the

estimation of pavement remaining life, which is required for

a Life Cycle Cost Analysis (LCCA).

Phase III: cement deep mixing criterion

Background

In this Phase, efforts are made to explore a deep-seated

solution to the existence of problematic organic soils and

peats underlying a pavement structure based on time–

stress–compressibility concepts. It focused on cement sta-

bilization of organic soils and peat (with organic contents

ranging from 67 to 90%) obtained from the SR 15/US 98

test sections to evaluate if the compressibility characteris-

tics of these problematic soils can be fundamentally

improved for the long-term preservation and sustainability

of the roadway structure. It is further expected that the

optimized mix-design developed in this Phase could pro-

vide some criteria for the design of Deep Mixing Methods

in organic soils.

Shelby tube samples were retrieved from SR 15/US 98

test sections with the help of a truck-mounted drill rig

provided by the Florida Department of Transportation

Fig. 12 Gain in stiffness due to

reinforcement N/S not

statistically significant


123

(FDOT) State Materials Office. Samples were taken from

depths ranging from 7.5 to 14 ft measured from the road-

way surface targeting the lower soil stratum of fibrous peat

which had the highest organic and moisture contents, and

was prone to significant long-term secondary compression

under sustained loading. Upon visual inspection, the soil

appeared to be a mixture of brown to light brown and red in

color, with vast amounts of fibers from dead vegetation

oriented in a vertical fashion. In addition, it was found to be

easily deformable to the touch, low plasticity, and with a

spongy feel.

From a mechanistic standpoint, the primary consolida-

tion process in organic layers is quite rapid, followed by

significant secondary compression stages under sustained

overburden pressure due to the dead weight of the pave-

ment structure and granular fill. Results of the primary and

secondary compression tests were presented earlier in

Figs. 2 and 4 (Phase I). Although the passage of traffic may

initiate short pulses of primary consolidation processes in

the organic layer, the major component of the deformation

in the organic layer is due to the long-term continuing

secondary and tertiary compression phases under the con-

stant pavement dead weight leading to premature distress,

differential settlement, and failure.

Experimental program

Specimen preparation

The sample preparation consisted of four main tasks

including: mixing, compacting, curing, and the control of

any swelling pressure during the curing period. The mixing

process was carried out at the soil’s natural moisture con-

tent and the stabilizing agent was introduced in a dry state

in an effort to simulate field conditions (Dry Soil Mixing

technique) practiced in Deep Mixing Method (DMM).

Type I Portland cement was added by small increments

followed by a mixing period of 2 min. The mixing process

was achieved with the aid of laboratory spatula and spoon

for achieving a homogeneous mixture. The soil was com-

pacted in three equal layers to reach a representative unit

weight close to the in situ conditions (about 11 kN/m3).

Once the samples were mixed and compacted, they were

inundated with water and the treated soil was allowed to

cure for a period of 7 days.

Methodology

The testing program was composed of two series of

experiments, each series consisting of six simultaneous

consolidation tests.: (a) Test Series I: The soil utilized for

Test Series I consisted of organic silts (sometimes referred

to as ‘‘muck’’) with an organic content of 67.0% and a

preconsolidation pressure of about 58 kPa. It was incre-

mentally loaded up to 192 kPa and then allowed to undergo

secondary compression for 14 days; and (b) Test Series II:

The soil utilized for series II was a peat with an organic

content of 89% and a preconsolidation pressure of 12 kPa.

It was incrementally loaded up to 48 kPa and then allowed

to undergo secondary compression for 14 days. In this

study, a stress level was defined as the ratio of vertical

applied pressure to the preconsolidation pressure ðr0v=r0pÞ.

Experimental results

Effect on void ratio (e)

Figure 13 shows the variation of void ratio with cement

content and stress levels for both test Series I and II

specimens It was found that with increasing cement con-

tent, (1) the void ratio decreased at the same stress level;

and (2) the total change in void ratio from zero to final

stress level was drastically reduced, or in other words the

volume change tendency was effectively stabilized. For

example, in Test Series I, the change in void ratio between

the stress levels of 0.00 and 3.33 is significantly higher at

0.00% cement content (&4.0) than it is as 56.48% cement

(nearly zero). For the case of Test Series II, the change in

void ratio between the stress levels of 0.00 and 4.167 is

appreciably higher at 0.00% cement content (&3.0) than it

is at 89.68% cement (nearly zero). The success of the

stabilization process was clearly evident, and it was par-

ticularly pronounced with cement contents in excess of

35% for Test Series I, and 50% for Test Series II.

Effect on compression index (Cc)

By definition, Cc is the slope of the e versus log (r0v) curve½Cc ¼ De=Dlog r0vÞ

� �and is typically computed at the virgin

zone of compression. Because the Ca/Cc concept applies

both to the compression and recompression zones [16], Cc

was acquired from the entire range of the curve and subse-

quently used in the compressibility analysis. Figure 14

shows the variations ofCc with different cement percentages

at each stress level. It was found that the specimens with

56.48% cement from Test Series I and 89.68% cement from

Test Series II underwent no deformation until the applica-

tion of 1.0 and � tsf, respectively. Therefore, values for Cc

could only be calculated beyond these stress levels. It was

found that with increasing cement content, (1) the com-

pression index decreased at the same stress level; and (2) the

total change in compression index from initial to final stress

level drastically reduced, or in other words, the potential for

large primary consolidation settlement was effectively sta-

bilized. Based on the observed behavior, the stabilization


123

effects were again found to be maximized with cement

contents exceeding the 35 and 50% thresholds for Test

Series I and II, respectively.

Effect on secondary compression index (Ca)

Ca is defined as the change of void ratio with respect to the

log of time [De/Dlog(t)], and was obtained from the void

ratio versus time (in minutes) semi-log plots at a constant

stress level. Variations in Ca with cement content and

different stress levels are shown in Fig. 15. Trends were

found to be similar as found in the case of void ratio and

compression index, reported previously. This is significant

because it implies that the long-term and continuing set-

tlement of organic soils due to sustained load from the

infrastructure elements in the field may be effectively

stabilized with cement treatment at appropriate dosages.

Evaluation of the Ca/Cc ratio

Figures 16 and 17 shows the variations of Ca with Cc for

cement-treated Florida organic soils. The Ca/Cc ratios thus

obtained, were plotted against cement content in Fig. 18 to

investigate any possible correlations. It is found that the

Ca/Cc ratio decreases with increasing cement. As the Ca/Cc

ratio decreases, the soil engineering behavior is known to

shift from that of peaty soils, to organic clays and silts, to

inorganic clays and silts, to shale and mudstone and finally

to a granular material [28]. Following these guidelines

available in the literature, the possible fundamental trans-

formation of specimens belonging to Test Series I and II

are shown in Fig. 18.

It is found that the Ca/Cc ratios reached a desirable level

(close to granular soils) at a cement content of about 35 and

60% for Test Series I and II, respectively. With cement

additions in excess of these dosages, no noticable

improvement were observed. Unlike Test Series I, the peat

(Test Series II) did not enter the granular range, and was

rather stabilized in the shale and mudstone range. For

comparison purposes, the transformation of another soil

(organic content between 50 and 60% and moisture content

between 240 and 289%) due to cement addition is also

shown here from the literature [10]. These results are

encouraging, since the compression behavior of organic

soils appears to be fundamentally changed (by cement

stabilization) to that of an approximate granular soil, which

is considered to be an excellent foundation material by the

geotechnical and pavement engineers.

Fig. 13 Void ratio with varying cement content for Test Series I (left) and II (right)

Fig. 14 Cc with different cement dosages for Test Series I (left) and II (right)


123

Lessons learned from Phase III

Both the compression index, Cc, and the secondary com-

pression index, Ca, can be significantly reduced by cement

stabilization, and as a result, the engineering behavior

(analyzed by the Ca/Cc ratio concept) of highly organic

soils can be modified to a more desirable performance

close to that of a granular material. The optimum cement

Fig. 15 Ca with different cement dosages for Test Series I (left) and II (right)

Fig. 16 Ca vs. Cc for all cement contents for Test Series I


123

dosages needed for this desirable soil performance was

found to be 35% for the silty organic soil, and 55% for the

peat soil, both by dry weight of the soil.

Summary and conclusions

Flexible pavements built over soft compressible soils pose

a serious challenge to engineers involved with pavement

maintenance and rehabilitation. The traditional solutions

such as milling and resurfacing are often ineffective and

costly due to frequent repairs or reconstructions because

the root causes of the problem are linked to the question-

able and uncertain properties of the naturally occurring soft

soils. The rich body of knowledge on problematic soils

accumulated in geotechnical engineering is not generally

incorporated in the design and planning of rehabilitation

strategies. The current study was undertaken to explore

near-surface and deep-seated measures to deal with the

highly organic and peat soils underlying SR15/US98 in

southeastern Florida. A case study is presented on the

performance of experimental test sections incorporating

various reinforcing products to mitigate the impact of soft

soils in the form of differential settlements and excessive

cracking of the asphalt layer. Geotechnical characterization

of the organic soils through piezocone penetration testing,

borehole exploration, and laboratory investigations of

consolidation and secondary compression behavior pro-

vided some guidance to the design and construction of

experimental sections. The time–stress–compressibility

relationships developed in this study for the organic rich

soils were directly used to optimize mix-design criteria for

Fig. 17 Ca vs. Cc for all cement contents for Test Series II


123

cement deep mixing in order to effectively control sec-

ondary compression behavior responsible for excessive

settlement. A combination of the near-surface and the

deep-seated solution, coupled with life cycle cost analysis,

can be an excellent choice for the long-term preservation

and sustainability of the pavement infrastructure when

problematic soils are encountered in practice.

Acknowledgements The Phases I and II of the project were partially

funded by grants from the Florida Department of Transportation. This

support is gratefully acknowledged.

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