Developing Subsurface

Exploration & Testing Programs Considering Geophysics & In-Situ

TestingTesting

Benjamin S. Rivers, P.E.Geotechnical Engineer

FHWA Resource Center

Purpose of a Site Investigation

• Assess Suitability of Site for Proposed Project

• Enable Adequate and Economical Design

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• Enable Adequate and Economical Design– No Failures - No Conservatism

• Foresee and Provide for Construction Problems that may Arise (Reduce Claims)

Who looks at the results and do they get what they want?

• Geotech Engineers/Designers• Better characterization of engineering properties, less

uncertainty• Contractors

• Better characterization of construction needs, less uncertainty• Owners

• Better value from engineers and contractors

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Value is Economics• Less expensive, longer lasting projects through better understanding of

engineering properties and their distribution• Less Uncertainty/Better Reliability in Characterization and Parameters• Faster investigations – “time is money”

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“If you do not know what you should be looking for in site

investigation, you are not likely to

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R. Glossop-8th Rankine Lecture

investigation, you are not likely to find much of value.”

Rational Approach

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GEC #5Figure 1:Soil & Rock

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Soil & Rock Property

Selection Flowchart

Planning Exploration & Testing Program

• Gather & Analyze Existing Information; Conduct Site Visit; Develop Preliminary Site Model

• Identify Material Properties required for Design & Construction; Estimate Scope of Field Program; Divide into Zones of Interest

• Develop Site Exploration Program

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• Develop Site Exploration Program• Conduct Exploration & Field Testing• Perform Descriptions and Laboratory Index Testing• Summarize Data & Develop Subsurface Profile

Are Results Consistent with Preliminary Model???

Planning Exploration & Testing Program

• Review Design Objectives and Initial Results; Identify “Critical Areas”

Additional data needs???• Identify Representative Samples for Performance

Testing.• Conduct Performance Testing, Review Test Data,

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• Conduct Performance Testing, Review Test Data, Summarize

Are Results Consistant & Valid?Is a Phase II Investigation Necessary?

• Select Material Properties and Final Model• Perform Design and Consider Constructability Issues

Planning Exploration & Testing Program

Identify Data Needs� Identify Design & Constructability Requirements� Identify Performance Criteria & Schedule Constraints� Identify Areas of Concern on Site and Potential

Variability� Develop Likely Sequence and Phases of Construction

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� Develop Likely Sequence and Phases of Construction� Identify Engineering Analyses to be Performed� Identify Required Engineering Properties &

Parameters� Evaluate Methods to Obtain Parameters� Evaluate Number & Locations of Tests/Samples

Needed(GEC #5: Table 1)

Planning Exploration & Testing Program

Exploration Tools� Undisturbed Sampling� Disturbed Sampling

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� In-situ Testing� Geophysical and Remote Sensing Methods

In-Situ Geotechnical Tests for Soils

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Standard Penetration Test (SPT)

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Standard Penetration Test

Advantages Disadvantages• Obtain Sample + Number• Simple & rugged device at

low cost• Suitable in many soil

• Disturbed sample (index tests only)

• Crude number for analysis• Not applicable in soft

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• Suitable in many soil types

• Can perform in weak rocks

• Available throughout the U.S. (worldwide)

• Not applicable in soft clays and silts

• High variability and uncertainty

Test Results

• N – SPT Resistance Value (blows/foot)• Consistancy/Relative Density• Soil Properties for sands to φ, E,

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• Soil Properties for sands to φ, E, liquefaction potential

• Must apply corrections for energy and overburden pressure for normalization

Standard Penetration Test (SPT)

4

6

8

0 10 20 30 40 50

Measured N-values

Dep

th (m

eter

s)

4

6

8

0 10 20 30 40 50

Corrected N60

Dep

th (m

eter

s)

Donut

Safety

Trend

ER = 34 (energy ratio)

45

40

41

55

60

56

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10

12

14

16

Dep

th (m

eter

s)

Donut

Safety

Sequence

10

12

14

16

Dep

th (m

eter

s)

41

41

39

47

56

63

63

63

64

69

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Cone Penetration Test (CPT)

Cone Penetration Testing (ASTM D 5778)

Geostratigraphy by Piezocone Tests, Blytheville, AR

0

5

10

0 10 20 30 40 50

qt (MPa)

0

5

10

0 100 200 300 400

fs (kPa)

0

5

10

15

0 1000 2000 3000

u2 (kPa)

Clayey Silt

Sand

15

20

25

30

35

40

Dep

th (m

)

15

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CleanSand

Clay

Cone Penetration Test

Advantages Disadvantages• Fast and continuous profiling

of strata• Economical and productive• Results not operator-

• Electronics must be calibrated & protected

• No soil samples• Unsuited to gravelly soils

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• Results not operator-dependent

• Strong theoretical basis for interpretation

• Particularly suited to soft soils

• Unsuited to gravelly soils and cobbles.

CPT/CPTu/SCPTu ResultsCommon to all:• Tip Resistance (Force/Area)• Sleeve Resistance (Force/Area)CPTu:• Pore-water PressureSCPTu:

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SCPTu:• Shear Wave VelocitySoil Properties:

Vs, Gmax, Emax, ρtot, eoSands - φ’, Dr, σho’, uo/water table elevationClays - su, σp’, ch, kh, OCR

Vane Shear Devices

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Scandinavian Vanes McClelland Offshore Vane

Vane Shear Test (VST)

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0

5

0 10 20 30 40 50 60 70 80

Vane Strength, suv (kPa)

Peak

Remolded

0

5

0 1 2 3 4 5

Sensitivity, St

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10

15

20

25

30

Dep

th (m

eter

s) 10

15

20

25

30

Dep

th (m

eter

s)

Vane Shear Test

Advantages Disadvantages• Assessment of undrained

shear strength of clays

• Simple test and equipment

• Limited to soft to stiff clays & silts with suv < 200 kPa

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• Simple test and equipment

• Measure inplace sensitivity

• Long history of use in practice for embankments, foundations, & cuts

• Raw suv needs empirical correction

• Can be affected by sand seams and lenses

Flat Plate Dilatometer (DMT)

Blade, Pressure Panel, Tubing, and Nitrogen

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Flat Plate Dilatometer Test (DMT)

DMT in Piedmont Residuum, Charlotte, NC0

2

4

6

8

0

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8

Dep

th (m

eter

s)

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10

12

14

160 200 400 600 800

Modulus ED (atm)

10

12

14

16

0 500 1000 1500

Pressure (kPa)

Dep

th (m

eter

s)

PoP1

10

12

14

16

0 1 10

Material Index ID

Clay Silt

10

12

14

16

0 5 10 15

Horiz. Index KD

DMT Results

• Pressure Readings (A, B, C)Soil Properties:Sands – φ’, E, D , m ,

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Sands – φ’, E, Dr, mv,Clays – σp’, Ko, su, mv, E, ch, kh

Dilatometer Test (DMT)

Advantages Disadvantages• Simple and Robust

Equipment

• Repeatable and Operator-

• Difficult to push in very dense materials and not in gravels.

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Independent

• Quick and Economical

• Theoretical Derivations for elastic modulus, strength, stress history

• Primarily established on correlative relationships

• Needs calibration for local geologies

Pressuremeter Test (PMT)

Pre-Bored Pressuremeters (ASTM D4719)

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Menard Pressure Panel Texam Monocell Probe

PMT Data - Utah DOT Project

3

4

5

Pre

ssur

e (t

sf)

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0 200 400 600Volume Change (cc)

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Pressuremeter Test (PMT)

Advantages• Theoretically sound in

determination of soil parameters

Disadvantages• Complicated

procedures – requires

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parameters• Tests larger zone of

soil mass than other in-situ tests

• Develop stress-strain-shear curves

procedures – requires high level of expertise

• Time consuming• Delicate – easy to

damage

PMT Results

• Pressure vs. ∆Volume or Volumetric Strain

Soil Behavior

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Soil Behavior• Load/Volumetric Displacement

Soil Properties E, G, mv, su

Geophysical Investigations• Initial Site Exploration/Preliminary Surveys

• Assist with Placement of Borings/In-Situ Tests• Difficult Locations

• Gravels, Cobbles, Boulders, Debris

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• Gravels, Cobbles, Boulders, Debris• Difficult Terrain• Contaminated Sites

• Supplementary Exploration• Observe Variations Between

Borings/Soundings/Outcrop, etc.• Locate Anomalies

Common Geophysical MethodsSurface Methods

• Siesmic Refraction

• Spectral-Analysis-of-Surface-Waves (SASW)

Borehole Methods• Crosshole/Downhole• Suspension Logger• Electrical Logging

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Surface-Waves (SASW)

• Electrical Resistivity

• Electromagnetics (EM)

• Ground Penetrating Radar (GPR)

• Microgravity

• Electrical Logging• Nuclear Logging• Optical and Acoustical

Televiewer

Geophysical Investigations

• Stratification of Subsurface Materials• Profile Top of Bedrock• Depth to Groundwater

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• Depth to Groundwater• Limits of Types of Soil Deposits• Rippability of Hard Soil and Rock• Locate Voids, Buried Utilities, Substructures• Shear Velocity and Modulus Properties

Geophysical Properties

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Resistivity Values (ConeTec & GeoProbe, 1997)

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Results from Seismic Refraction

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Electrical Resisitivity Measurements

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Electromagnetic Conductivity (EM)

Televiewer

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Source: Eliassen, et al

Summary• Value

– Reduce Uncertainty– Increase Reliability– Quicker

• Rational Approach– Develop Preminary Model

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– Identify Data Needs– Develop & Execute Appropriate Exploration & Testing Program– Re-evaluate Data Needs– Evaluate Data– Select Properties and Finalize Subsurface Model

GEC #5: Evaluation of Soil & Rock Properties

http://www.fhwa.dot.gov/engineering/geotech/library_listing.cfm

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Publication No. FHWA-IF-02-034

Highly Recommended

Reference on Geophysics

• Application of Geophysical Methods to Highway Related Problems (FHWA Manual DTFH68-02-P-00083; Sept. 2003)

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Manual DTFH68-02-P-00083; Sept. 2003)

www.cflhd.gov/geoTechnical

Subsurface Investigation Courses

• NHI Course 132031: Subsurface Investigations – Geotechnical Site Characterization

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Characterization• NHI Course 132079: Subsurface

Investigation Qualification – QC/QAwww.nhi.fhwa.dot.gov