Conventional Field Testing Methods and...

20-10-2015

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Conventional Field Testing Methods and Issues

Amit Prashant Indian Institute of Technology Gandhinagar

Short Course on

Geotechnical Investigations for Structural Engineering 15– 17 October, 2015

IITGN Short Course on Geotechnical Investigations for Structural Engineering

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Conventional Field Testing

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In-situ shear strength tests Standard Penetration Test (SPT) Cone Penetration Test (CPT) Dynamic Cone Penetration Test (DCPT) Vane Shear Test (VST) Dilatometer Test (DMT) Pressure meter Test (PMT)

Settlement test Plate Load Test

Conventional Field Testing


Conventional Field Tests

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www.fhwa.dot.gov

http://www.fhwa.dot.gov/engineering/geotech/pubs/05037/04b.cfm

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Standard Penetration Test

Kovacs, William D.; Salomone, Lawrence A. & Yokel, Felix Y. Energy Measurement in the Standard Penetration Test.



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http://www.tecopsa.com/

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Components Drilling Equipment

Inner diameter of hole 100 to 150 mm Casing may be used in case of soft/non-cohesive soils

Split spoon sampler IS:9640-1980 Drive weight assembly

Falling Weight = 63.5 Kg Fall height = 75 cm

Others Lifting bail, Tongs, ropes, screw jack, etc.

Procedure The bore hole is advanced to desired depth and bottom is cleaned. Split spoon sampler is attached to a drill rod and rested on bore

hole bottom. Driving mass is dropped onto the drill rod repeatedly and the

sampler is driven into soil for a distance of 450 mm. The number of blow for each 150 mm penetration are recorded.


Procedure (Cont….) N-value

First 150 mm penetration is considered as seating penetration The number of blows for the last two 150 mm penetration are

added together and reported as N-value for the depth of bore hole.

The split spoon sampler is recovered, and sample is collected from split barrel so as to preserve moisture content and sent to the laboratory for further analysis.

SPT is repeated at every 750 mm or 1500 mm interval for larger depths.

Under the following conditions the penetration is referred to as refusal and test is halted

a)50 blows are required for any 150 mm penetration b)100 blows are required for last 300 mm penetration c) 10 successive blows produce no advancement


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Precautions during SPT

The ht. of free fall Must be 750 mm The fall of hammer must be free, frictionless and vertical Cutting shoe of the sampler must be free from wear & tear The bottom of the bore hole must be cleaned to collect

undisturbed sample When SPT is done in a sandy soil below water table , the

water level in the bore hole MUST be maintained higher than the ground water level. Otherwise: QUICK condition!! Very Low N value


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SPT Corrections Correction for Overburden Pressure :

Correction for Dilatancy :

[ ]

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SPT Hammer Safety Hammer

Donut Hammer

cmeco.com

geotechpedia.com


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SPT Value

http://cmeco.com/drills/rubber-tire-atv-mounted-drills/cme-750x-optional-equipment/

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SPT Test Data

No. of blows per 0.30m

Data from different bore holes


Interpretation from SPT: Cohesionless Soils

N f Dr (%) consistency

0-4 25-30 0-15 very loose

4-10 27-32 15-35 loose

10-30 30-35 35-65 medium

30-50 35-40 65-85 dense

>50 38-43 85-100 very dense

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0.689

0.193'

NOCR

p

MN/m2

Interpretation from SPT: Cohesive Soils

N cu (kPa) consistency visual identification

0-2 0 - 12 very soft Thumb can penetrate > 25 mm

2-4 12-25 soft Thumb can penetrate 25 mm

4-8 25-50 medium Thumb penetrates with moderate effort

8-15 50-100 stiff Thumb will indent 8 mm

15-30 100-200 very stiff Can indent with thumb nail; not thumb

>30 >200 hard Cannot indent even with thumb nail

not corrected for overburden 6.25. in kPauc N

Mayne and Kemper (1988)


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Shear Strength from SPT-value

Peck, Hansen, and Thornburn (1974)

&

IS:6403-1981 Recommendation

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Total Settlement from SPT Data for Cohesionless soil

Multiply the settlement by factor W'


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Allowable Bearing Pressure from SPT value

Terzaghi and Peck (1967):

2

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0.31.37 3

2n w D a

Bq N R R S kN m

B

0.5 1 1w fw w

f

D DR R

D

1 depth correction factor 1 0.2 1.2fD

DR

B

aS Permissible settlement in mm. (25 mm)

Sa in mm and all other dimensions in meter.

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Cause Effects Influence on SPT N Value

Inadequate cleaning of hole SPT in disturbed soil. Soil gets trapped in sampler and may be compressed as sampler is driven, reducing recovery

Increases

Failure to maintain adequate head of water in borehole

Bottom of borehole may become quick Decreases

Careless measure of drop Hammer energy varies Increases

Hammer weight inaccurate Hammer energy varies Increases or decreases

Hammer strikes drill rod collar eccentrically Hammer energy reduced Increases

Ungreased sheaves/shaft, new stiff rope on weight, more than two turns on cathead, incomplete release of rope each drop

Hammer energy reduced Increases

Sampler driven above bottom of casing Sampler driven in disturbed, artificially densified soil

Increases greatly

Careless blow count Inaccurate results Increases or decreases

Use of non-standard sampler Correlations with standard sampler invalid Increases or decreases

Coarse gravel or cobbles in soil Sampler becomes clogged or impeded Increases

Use of bent drill rods Inhibited transfer of energy of sampler Increases 19

Kulhawy and Mayne, 1990 Issues during SPT test


Cone Penetration Test (CPT) IS: 4968 (Part –III)

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CPT Cones

Drive Cone Dutch Cone Electrical Piezocone


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Push the sounding rod with cone into the ground for some specified depth. Then push the cone with friction sleeve for another specified depth (> 35 mm). Repeat the process with/without friction sleeve.

Pushing rate = 1 cm/s Mantle tube is push simultaneously such that it is always above the

cone and friction sleeve. Tip Load, Qc = Load from pressure gauge reading + Wt. of cone +

Wt. of connecting sounding rods Tip resistance

With friction sleeve add its self weight as well Qt = Qc + Qf

Frictional resistance

Friction Ratio

cc

c

Qq

A

x-sectional area off cone = 10 cm2

surface area of friction sleeve t c

ff

Q Qq

A

fr

c

qf

q Typical range

0%

10% Cohesive

Granular

CPT Procedure

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Failure Modes around Advancing Cone


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CPTU

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Typical Measurements with CPTU


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CPT Interpretations

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CPT Interpretations


Interpreted Soil Profile EQ Drain Test Area 1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

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Dep

th (

m)

Sand

Silty sand/sand

Silt and Sandy Silt

Sand to Silty Sand

Cone Tip

Resistance, qc

(MPa)

0 2 4 6 8 1012

Fricton Ratio, Fr

(%)

0 1 2 3 4 5 6

Relative

Density, Dr

0 0.2 0.4 0.6 0.8 1

Pore Pressure, u (kPa)

-100 0 100 200

CPT Profile for Piezocone

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CPT Versus SPT

CPT: Advantages over SPT provides much better resolution, reliability versatility; pore water pressure, dynamic soil

properties

CPT: Disadvantages Does not give a sample Will not work with soil with gravel Need to mobilize a special rig


0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10 12 14

CPT Cone Resistance, qc1

(MPa)

MeanMean-SDMean+SD

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30

SPT Blow Count, N1(60)

(Blows/300 mm)

0

1

2

3

4

5

6

7

8

9

10

0 20 40 60 80 100

Relative Density, Dr

(%)

From CPT

From SPT

Interpreted Soil Profile

0

1

2

3

4

5

6

7

8

9

10

0 0.2 0.4 0.6 0.8 1

De

pth

Be

low

Ex

cava

ted

Su

rfa

ce (

m)

Interbedded Fine SandandSilty Sand(SP-SM)

Fine Silty Sand (SM)

Gray Silty Clay (CL)

Sand (SP)

Fine Sandw/ Shells(SP)

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CPT-SPT Correlation

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Downhole Seismic Piezocone Penetration Test (SCPTU)

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CPT Correlations Soil Profiling and Soil Type Equivalent SPT N60 Profiles Soil Unit Weight Undrained Shear Strength (su) Soil Sensitivity Stress History - Overconsolidation Ratio (OCR) In-Situ Stress Ratio (Ko) Friction Angle Relative Density (Dr) Stiffness and Modulus Modulus from S and P Wave Velocity Hydraulic Conductivity (k) Consolidation Characteristics

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Refer the given handout


Continuous Sampling on the Side of CPT

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http://www.alfakat.gr/en/projects/16_Geo_Boula.htm

http://geopractica.co.za/portfolio/namoya-mine-drc/

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Dynamic Cone Penetration Test (DCPT)

Components: 1) Cone (dia = 50 mm)

~usually made of steel

IS: 4968 (Part – I, II)

SPT

DCPT

Hollow (split spoon)

Solid (no samples)

2) Driving rods/drill rods

~marked at every 100 mm


DCPT Procedure

Cone – drill rod – driving head assembly is installed vertically on the ground and hammer is dropped from standard height repeatedly

The blow counts are recorded for every 100 mm penetration. A sum of three consecutive values i.e. 300 mm is noted as the dynamic cone resistance, Ncd at that depth.

The cone is driven up to refusal or the project specified depth.

In the end, the drill rod is withdrawn. The cone is left in the ground if unthreaded or recovered if threaded.

No sample recovered

Fast testing – less project cost / cover large area in due time

Use of bentonite slurry is optional, which is used to reduce friction on the driving rods.

• Modified cone is used in this case: diameter = 62.5 mm

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For clays, and mainly for soft clays.

Measure torque required to quickly shear the vane pushed into soft clay.

torque undrained shear strength cu

Typical d = 20-100 mm.

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Vane Shear Test (VST)

vane

undrained

bore hole

soft clay

measuring (torque)

head

vane h2d

d


Vane Shear Test

Test in Progress Failure surface

2

2.

. . .

13.

u

Tc

D HD

H

30.273u

Tc

D

Interpretation:

Undrained shear strength -

For H = 2.D

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60 mm dia. Flexible membrane

Insert DMT using SPT drilling equipment to the desired depth and pressure the cell

Measure pressure when the membrane is flushed with plate and when it enters ground by 1.1 mm.

Decrease the pressure & measure the pressure when membrane is again flushed with plate.

Determined:

Elastic Modulus

Soil Type and state

Dilatometer Test (DMT)


Pressure meter Test (PMT)

Determined: Elastic Young Mod, E Shear Mod, G Undrained shear strength, Su

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Pressure meter Test (PMT)

Measurements: 1. Fluid Pressure 2. Fluid volume change


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Plate Load Test

This test is used to estimate the Elastic Modulus and Bearing Capacity of soils which are not easily sampled.

Modulus Estimation: The load is applied to the plate in increments of one fifth of the design load. Time-settlement and load-settlement curves are then produced to estimate modulus of subgrade reaction (K) at 1.25 mm settlement.


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Geophysical Methods

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P or Primary Waves

longitudinal, primary or compressional wave

Material particles oscillate about a fixed point in the direction of wave propagation by compressional and dilatational strain.


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S or Secondary Waves

transverse, secondary or shear wave

Particle motion is at right angles to the direction of wave propagation and occurs by pure strain.

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Rayleigh Waves (used in MASW)


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Love Waves

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Wave Velocities

P-wave velocity – Vp

Shear Wave velocity – Vs

Vp > Vs

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Soil Properties from Wave Velocity

Shear Modulus

Constrained Modulus,

Young’s Modulus,

Poisson’s Ratio, 50

2. sG V

Density of soil

2. pM V

2 2 2

2 2

3 4s p s

p s

V V VE

V V

2 2

2 2

2

2p s

p s

V V

V V

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Typical Wave Velocities in Geomaterials

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1. Geophone

2. Cable

3. Hammer (Source)

4. Processing and Control Unit

Seismic Measurement-Systems

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Seismic Methods

Seismic Reflection Method Seismic Refraction Method Cross-Hole Test Down Hole Test & Up-Hole Test Spectral Analysis of Surface Wave (SASW) Multichannel Analysis of Surface Waves

(MASW) method

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Waves from point source

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Snell’s Law

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Critical Angle of Refraction

1 1

2

sinV

AV


Seismic Refraction Method

http://www.geologicresources.com/seismic_refraction_method.html

Depths less than ~ 30 m Cost Effective as compared to Reflection method (<3to5 times) Used for computation of layer thickness of soil

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Measurement at a Geophone

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Shot Record – uniform deposit

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Shot Record – real deposit

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Tim

e (s

) Source


Cross-Hole Test

Sensors are placed at one elevation in one or more boring. Source is triggered in another boring at the same elevation. S wave travels horizontally from source to receiving hole, and the arrivals of S waves are noted Shear wave velocity (Vs) is calculated by dividing the distance between the bore holes and the travel time. 60

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Cross-Hole Record

61 http://www.structuremag.org/article.aspx?articleID=994


Down Hole Test

http://www.geophysics.co.uk/mets3.html

Sensors are placed at various depths in the boring. Source is located above the receivers, at the ground surface. Only one bore hole is required. A source rich in S wave should be used (P wave travels faster than S wave)

Up-Hole method: source of energy is deep in boring and the receiver is at the ground surface 62

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Seismic Cone Penetration Test (SCPT) A Down-Hole Test

http://geoprobe.com/how-seismic-cone-penetration-equipment-works

Seismic cone is pushed into the ground Shear wave is generated at the top and the time required for the shear wave to reach the seismometer in the cone is measured Computer in the SCPT rig collects and processes all the data & shear wave velocity is measured

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SPT Velocity (m/s) Time (s)

http://www.belirti.com/english/downhole.htm

Down-Hole Test Record

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Conventional Field Testing Methods and...

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