Soil Investigation

FHWA - NHI Subsurface

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Chapter 5

In-Situ Geotechnical Tests

NHI Course on Subsurface Investigations

Lesson 7

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FHWA - NHI Subsurface

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"In-Situ"

Latin: In its original position

Why perform in-situ testing?

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In-Situ Testing - Objectives

Select in-situ tests for augmenting, supplementing, and even replacing borings.

Realize the applicability of various in-situ methods to different soil conditions.

Recognize the complementary nature of in-situ direct push methods with conventional rotary drilling & sampling methods.

Recognize values for utilizing these methods and quality implications for their underuse.

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Outline of Geotechnical Site Characterization Methods

Drilling & Sampling In-Situ Tests

o Standard Penetration Test (SPT)o Cone Penetration Test (CPT + CPTu)o Flat Plate Dilatometer (DMT)o Pressuremeter (PMT)o Vane Shear (VST)

Geophysical Methodso Mechanical Waves (P-, S-, R-waves)o Electromagnetic (radar, resistivitity)

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In-Situ Geotechnical Tests for Soils

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Truck-Mounted Drill Rigs

Layne Drilling

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All-Terrain Drill Rigs

McLean, VA GT Campus, Atlanta, GA

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Track-Mounted Drill Rigs

Steele, Missouri

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Standard Penetration Test (SPT)

Split-Barrel Samplers

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Standard Penetration Test (SPT) Very common test worldwide

1902 - Colonel Gow of Raymond Pile Co.

Split-barrel sample driven in borehole

Conducted on 5-ft depth intervals (1.5-m).

ASTM D 1586 guidelines

Drop Hammer (140-lbs falling 30 inches) (63.5-kg hammer falling 0.76 meters)

Three-increments of 150-mm each; Sum last two increments = "N-value" (blows/ft)

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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 types Can perform in

weak rocks Available

throughout the U.S. (worldwide)

Obtain Sample + Number

Disturbed sample (index tests only)

Crude number for analysis

Not applicable in soft clays and silts

High variability and uncertainty

Corrections to SPT N-value

Nmeasured = Raw SPT Resistance (ASTM D 1586).

N60 = (ER/60) Nmeasured = Energy-Corrected N

Value where ER = energy ratio (ASTM D 4633). Note: 30% < ER < 100% with average ER = 60% in the U.S.

N60 CE CB CS CR Nmeas = Estimated corrected N

For Clean Sands: (N1)60 = CN N60 = Energy-

corrected SPT N-value normalized to an effective overburden stress level of one atmosphere:

(N1)60 = (N60)/(vo’)0.5 with stress given in atm. (Note:

1 atm = 1 bar = 100 kPa = 1 tsf).

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Standard Penetration Test (SPT)

4

6

8

10

12

14

16

0 10 20 30 40 50

Measured N-values

Dep

th (

met

ers)

Donut

Safety

Sequence

4

6

8

10

12

14

16

0 10 20 30 40 50

Corrected N60

Dep

th (

met

ers)

Donut

Safety

Trend

ER = 34 (energy ratio)

45

40

41

41

39

47

56

55

60

56

63

63

63

64

69

Data from Robertson, et al. (1983)

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ADSC Load Test Site at Georgia Tech Campus

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SPT Results at GT Campus

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SPT Results at GT Campus

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Cone Penetrometers

Cone Penetration Test (CPT)

Electronic Steel Probes with 60° Apex Tip ASTM D 5778 Procedures Hydraulic Push at 20 mm/s No Boring, No Samples, No Cuttings, No Spoil Continuous readings of stress, friction, pressure

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Cone Penetration Testing (ASTM D 5778)

Cone Penetration Vehicles

Mobile 25-tonne rigs with hydraulic pushing systems. Enclosed cabins to allow testing for all weather conditions

Cone Trucks

Cone Penetration Vehicles

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Electric Friction Cone Penetrometer

0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8

qT (MPa)

De

pth

(m

)

0

2

4

6

8

10

12

14

16

18

20

0 100 200 300

fS (kPa)

De

pth

(m

)

Georgia Tech Test Site

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Piezocone Penetrometers

Porewater Pressures Measured at Apex McClelland Penetrometer Design

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Cone Penetrometer Types

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Cone Penetration Test

Advantages

Disadvantages Fast and continuous

profiling of strata Economical and

productive Results not operator-

dependent Strong theoretical

basis for interpretation

Particularly suited to soft soils

High capital investment

Requires skilled operator for field use

Electronics must be calibrated & protected

No soil samples Unsuited to gravelly

soils and cobbles.

Corrections to CPT

*Need Type 2Piezo-Element

at Shoulderfor qc qt

Procedures for CPTu

Porous Element Materials•Sintered Metals•Ceramics•Plastics (disposable)

Saturation of Porous Elements:•Water•Glycerine•Silicone

Procedures:•Vacuum for 24-hours•Pre-saturated elements•Prophylactic to maintain fluids

Grease-Filled Slots - (no element)

CPTu Classification

Approximate Rules of Thumb:

Clean Quartz Sands

o tip stress qc ~ qt > 5 MPa (50

tsf)

o porewater pressures: u2 = ub ~

uo (near hydrostatic)

Soft to Firm to Stiff Intact Clays

o qt < 5 MPa (50 tsf)

o porewater pressures u2 different

than uo (usually greater)

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Geostratigraphy by Piezocone Tests, Blytheville, AR

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Geostratigraphy by Piezocone Tests, Blytheville, AR

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Geostratigraphy by Piezocone Tests, Blytheville, AR

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Geostratigraphy by CPTu at Univ. Mass-Amherst

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Vane Shear Test (VST)

Field Vane (FV) per ASTM D 2573 Performed at bottom of boring or by direct push placement of device Four-sided blade pushed into clays and silts to measure following:

suv (peak) = Peak Undrained Strength

suv (remolded) = Remolded Strength (after 10

revolutions)

Sensitivity, St = suv(peak)/suv (remolded)

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Vane Shear Test (VST)

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Vane Shear Devices

Scandinavian Vanes McClelland Offshore Vane

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Vane Shear Devices

Dutch Vane Equipment, Holland VST in Upstate NY

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Vane Shear Test

Advantages

Disadvantages Assessment of

undrained shear strength of clays

Simple test and equipment

Measure inplace sensitivity

Long history of use in practice, particularly embankments, foundations, & cuts

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

Slow & time-consuming

Raw suv needs empirical correction

Can be affected by sand seams and lenses

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Interpretation of Undrained Shear

Strength (suv) from

Vane Shear Test

HiDiDD

Ts

BTuv 6)cos/()cos/(

122

Height H

Width D

iT

iB

T = measured torque

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Interpretation of suv from Vanes with H/D =2

Geometries

33273.0

7

6

D

T

D

Tsuv

3265.0

D

Tsuv

3257.0

D

Tsuv

Rectangular

Nilcon

Geonor

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Results from Vane Shear TestsSan Francisco Bay Mud, MUNI Metro Station

0

5

10

15

20

25

30

0 10 20 30 40 50 60 70 80

Vane Strength, suv (kPa)

De

pth

(m

ete

rs)

Peak

Remolded

0

5

10

15

20

25

30

0 1 2 3 4 5

Sensitivity, St

De

pth

(m

ete

rs)

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Mobilized Strength: mob = Rsuv Correction Factor (Chandler, 1988)

0.5

0.6

0.7

0.8

0.9

1

0 20 40 60 80 100 120

Plasticity Index, PI (%)

Van

e C

orr

ecti

on

Fac

tor,

R tf = time to failure (minutes)

10

100

103

104

Correction for

Embankments Under

Normal Rates of

Construction

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Field Vane Equipment

Nilcon (mechanical) Geonor (mechanical) A.P. vanden Berg Geotech AB (electrovane) Envi (memovane)

Lab Vane EquipmentASTM D 4648

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Flat Plate Dilatometer Test

Direct push of stainless steel plate at 20-cm intervals; No borings; no cuttings.

Introduced by Marchetti (1980).

18o angled blade

Pneumatic inflation of flexible steel membrane using nitrogen gas

Two pressure readings taken (A and B) within about 1 minute

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Flat Plate Dilatometer Test (DMT)

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Dilatometer Test (DMT)

Advantages

Disadvantages Simple and Robust

Equipment

Repeatable and Operator-Independent

Quick and Economical

Theoretical Derivations for elastic modulus, strength, stress history

Difficult to push in dense and hard materials

Primarily established on correlative relationships

Needs calibration for local geologies

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Flat Plate Dilatometer

Marchetti Device (ASCE JGE, March 1980;ASTM Geot. Testing J., June 1986)

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Flat Dilatometer Test Calibrations: A, B (positive values)

Readings: contact pressure "A" and expansion pressure "B" with depth

Corrections for membrane stiffness in air: p0 A + A p1 = B -B

DMT INDICES:

ID = material index = (p1-po)/(po-uo)

ED = dilatometer modulus = 34.7(p1-po)

KD = horizontal stress index =(po-uo)/vo’

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DMT in Piedmont Residuum, Charlotte, NC

0

2

4

6

8

10

12

14

16

0 200 400 600 800

Modulus ED (atm)

0

2

4

6

8

10

12

14

16

0 500 1000 1500

Pressure (kPa)

De

pth

(m

ete

rs)

PoP1

0

2

4

6

8

10

12

14

16

0 1 10

Material Index ID

Clay Silt Sand

0

2

4

6

8

10

12

14

16

0 5 10 15

Horiz. Index KD

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Computerized DMT System

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Pressuremeter Test (PMT)

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Pressuremeter Test (PMT)

0

1

2

3

4

5

0 100 200 300 400 500 600

Volume Change (cc)

Pre

ssu

re (

tsf)

0

1

2

3

4

5

0 10 20 30 40 50

Creep (cc/min)

Pre

ssu

re (

tsf)

Prebored PMT data from Utah DOT project

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Pre-Bored Pressuremeter

Menard Pressure Panel Texam Monocell Probe

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Self-Boring Pressuremeter

Professor Jean Benoit, UNH

Cambridge-Type Probe

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CPTu in Piedmont Weathered Schist

23

34

71

34

56

67

50/6"

50/2"

50/3"

SPT-N (bpf)

Fugro Sounding at MARTA site, North Atlanta

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Dual-Element Piezocone

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Cone Penetrometers

Triple-Element Piezocone (Norwegian Institute of Technology)

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Cone Penetrometers

Quad-Element Piezocone (Oxford University)

Memocone (cableless system)

Memory chip in penetrometer. Sychronizewith depth wheel & data logger at surface

Audio- or Acoustic-cone (cableless system)

Uses audio signal to send CPTu data upthe inside of the rods. Decoder at surface

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Piezocone Penetrometers

Various Penetrometers Used at Georgia Tech

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SmallPortableTrack

CPT Rig

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GT Geostar Anchored Cone Rig

Mud Island, Mississippi River, Memphis, TNMud Island, Mississippi River, Memphis, TN

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CPT Track Truck

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Geophysical Methods Mechanical Wave Measurements

Crosshole Tests (CHT) Downhole Tests (DHT) Spectral Analysis of Surface Waves Seismic Refraction Suspension Logging

Electromagnetic Wave Techniques Ground Penetrating Radar (GPR) Electromagnetic Conductivity (EM) Surface Resistivity (SR) Magnetometer Surveys (MT)