Geotechnical Testign Methods II_AS

8/13/2019 Geotechnical Testign Methods II_AS

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Geotechnical Testing Methods II

Ajanta SachanAssistant ProfessorCivil EngineeringIIT Gandhinagar

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FIELD TESTING


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Field Test (In-situ Test)When it is difficult to obtain undisturbed samples.In case of Cohesionless soils, Sensitive clays, etc.Advantage:

Testing on natural soil under undisturbedconditions

Disadvantage:Testing conditions are not controlledTime dependent phenomenon are difficult tocontrol due to large scaleMeasurements/instrumentation is tricky andrather a difficult task

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

Settlement testPlate Load Test

Field Test (In-situ Test)


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Common In Situ Testing Devices

In bore holes

DMTVST

SPT

CPTPMT DCPT

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Standard Penetration TestIS: 2131-1981


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Standard Penetration TestComponents

Drilling EquipmentInner diameter of hole 100 to 150 mmCasing may be used in case of soft/non-cohesive soils

Split spoon sampler IS:9640-1980Drive weight assembly

Falling Weight = 63.5 Kg Fall height = 75 cm

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

ProcedureThe bore hole is advanced to desired depth and bottom is cleaned.Split spoon sampler is attached to a drill rod and rested on borehole bottom.Driving mass is dropped onto the drill rod repeatedly and thesampler is driven into soil for a distance of 450 mm. The number ofblow for each 150 mm penetration are recorded.

Procedure (Cont.)N-value

First 150 mm penetration is considered as seating penetrationThe number of blows for the last two 150 mm penetration areadded together and reported as N-value for the depth of borehole.

The split spoon sampler is recovered, and sample iscollected from split barrel so as to preserve moisturecontent and sent to the laboratory for further analysis.SPT is repeated at every 750 mm or 1500 mm intervalfor larger depths.Under the following conditions the penetration isreferred to as refusal and test is halted

a) 50 blows are required for any 150 mm penetrationb) 100 blows are required for last 300 mm penetrationc) 10 successive blows produce no advancement

Standard Penetration Test


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Precautions during SPTThe ht. of free fall Must be 750 mmThe fall of hammer must be free, frictionless and verticalCutting shoe of the sampler must be free from wear & tearThe bottom of the bore hole must be cleaned to collectundisturbed sampleWhen SPT is done in a sandy soil below water table , the

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

Correction for Overburden Pressure :

N' = Corrected value ofobserved N

C N = Correction factor foroverburden pressure

' . N N C N

Peck, Hanson and Thornburn(1974)

p' = Effective overburden pressure at a depthcorresponding to N-value measurement

SPT Corrections


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SPT Test Data

No. of blows per 0.30m

Data from different bore holes

Interpretation from SPT: Cohesionless Soils

N'' ' D r (%) 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'

N OCR

p

MN/m 2

Interpretation from SPT: Cohesive Soils

N c u (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)

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


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CPT ProcedurePush the sounding rod with cone into the ground for some specifieddepth. Then push the cone with friction sleeve for another specifieddepth (> 35 mm). Repeat the process with/without friction sleeve.Pushing rate = 1 cm/sMantle tube is push simultaneously such that it is always above thecone and friction sleeve.Tip Load, Q c = 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 + Q f

Frictional resistance

Friction Ratio

cc

c

Qq

A x-sectional area off cone = 10 cm 2

surface area of friction sleevet c

f f

Q Qq

A

f r

c

q f

q Typical range0%

10% Cohesive

Granular

CPT Results& SoilClassification


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TypicalCPT Data

0 2 4 6 8 10 12 14

CPT Cone Resistance, q c1(MPa)

Mean

Mean-SDMean+SD

0 10 20 30

SPT Blow Count, N 1(60)(Blows/300 mm)

0 20 40 60 80 1 00

Relative Density, D r (%)

From CPT

From SPT

InterpretedSoil Profile

0

1

2

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7

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9

10

D e p

t h B e

l o w

E x c a v a

t e d S u r f a c e

( m )

InterbeddedFine Sand

andSilty Sand(SP-SM)

Fine SiltySand(SM)

Gray SiltyClay (CL)

Sand (SP)

Fine Sandw/ Shells(SP)


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Interpreted Soil ProfileEQ Drain Test Area 1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

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D e p

t h ( m )

Sand

Silty sand/sand

Silt and SandySilt

Sand toSilty Sand

Cone Tip

Resistance, q c(MPa)0 2 4 6 8 1012

Fricton Ratio, Fr

(%)0 1 2 3 4 5 6

RelativeDensity, D r 0 0.2 0.4 0.6 0.8 1

Pore Pressure, u(kPa)-100 0 100 200

CPT Profile for Piezocone

CPT Versus SPT

CPT: Advantages over SPTprovides much better resolution, reliabilityversatility; pore water pressure, dynamic soilproperties

CPT: DisadvantagesDoes not give a sampleWill not work with soil with gravelNeed to mobilize a special rig


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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/drillrods

~marked at every 100 mm

DCPT ProcedureCone drill rod driving head assembly is installedvertically on the ground and hammer is dropped fromstandard 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 thedynamic cone resistance, N cd 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 theground if unthreaded or recovered if threaded.

No sample recoveredFast testing less project cost / cover large area in due timeUse 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 quicklyshear the vane pushed into soft clay.

torque undrained shear strength c uTypical d = 20-100 mm.

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Vane Shear Test (VST)

vane

undrained

bore hole

soft clay

measuring (torque)head

vane

h 2d

d

Vane Shear Test

Test in Progress Failure surface

2

2.

. . .

13.

u

T c

D H D H

30.273

u

T c

D

Interpretation:

Undrained shearstrength -

For H = 2.D


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60 mm dia.Flexiblemembrane

Insert DMT using SPTdrilling equipment to thedesired depth and pressurethe cellMeasure pressure when themembrane is flushed withplate and when it entersground by 1.1 mm.Decrease the pressure &measure the pressure whenmembrane is again flushedwith plate.

Determined:Elastic ModulusSoil Type and state

Dilatometer Test (DMT)

Pressure meterTest (PMT)

Determined:Elastic Young Mod, E

Shear Mod, GUndrained shear strength, S u


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Pressure meter Test (PMT)

Measurements:1. Fluid Pressure2. Fluid volume change

Plate Load Test

This test is used to estimate theElastic Modulus and BearingCapacity of soils which are noteasily sampled.

Bearing Capacity Estimation : Theload is applied such that the rate ofpenetration remains constant. Aload-settlement curve is produced.Equations have been developed toobtain undrained shear strengthfrom ultimate bearing capacity.

Modulus Estimation : The load isapplied to the plate in increments of onefifth of the design load. Time-settlementand load-settlement curves are thenproduced to estimate modulus of soilfrom the test results.


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ROCK TESTING

Rock Testing

Unconfined Compression TestBrazilian TestPoint Load TestDirect Shear TestSlake Durability TestSchmidt Rebound Hardness TestSound Velocity TestIn-situ stress measurements in rocks


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Core cutting &grinding machine:Cutting and grindingcylindrical rock specimenscore size: EX to NX

Polishing & Lapping machine

Core drilling machine:Rock core preparationFor regular and irregularSamples.core size: EX to 100mm

Specimen Preparation Equipments for

Rock Testing Rock Core sizes:EX = 21.46 mmAX = 30.10 mmBX = 42.04 mmNX = 54.74 mmMore: 35mm, 50mm,75mm, 100 mm

Rock Samples

Granite:High stiffness

High strengthVery brittle

Limestone:Medium stiffnessMedium strengthMedium brittleness

Shale:Low stiffnessLow strengthDuctile


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Unconfined Compression TestThis test is performed to obtainthe unconfined compressivestrength (UCS) of intact rockcores (slenderness ratio = 2).

UCS is the maximum stress thatthat rock specimen can sustain.

Rock specimen is kept in a

loading frame, and if requiredheated to the desired testtemperature.

Axial load is continuouslyincreased on the specimen untilpeak load and failure areobtained.

Brazilian test: Tensile strength of Rock

Brazilian test is performed to obtain thetensile strength of rock mass.

Tensile strength of rock is imp to knowfor drilling, blasting of rocks, failure ofroof and floor of tunnels, chambers &underground roadways ; often weakrocks fail in tension exhibiting splittingmode of failure.

In this test, a disc/cylinder is subjectedto a line load , and fracture shouldinitiate at the centre and progresstowards periphery . If opposite, the testis discarded as considered that it didnot fail in tension.


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Point Load Test: compress. strength of irregular rock sp.When regular cores could not be o btained;only irregular pieces are available fromthe rock excavation, Point load test isperformed to obtain the compressivestrength of rock mass.

The roughly chiseled spherical mass withdia. ranging between 30-50 mm is testedbetween two hard conical tips in a rigid

frame.

Direct Shear Test:Normal stress versus Shear stress response of rock mass

It measures peak and residual direct shear strength as a function ofstress normal to the sheared plane.

It can be used for testing for both: core & lump specimens.Shear box size: 300mm x 300mm x 100mm


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Triaxial Shear Test:shear strength parameters (c, f ) of rock mass

Triaxial cells for testing rocks aredesigned to withstand a confiningpressure 150 Kg/cm2.

Mostly triaxial tests on rockspecimens are performed under novolume change conditions.

Stress-strain curve is obtained usingdeviator stress and axial strain. Themodulus and failure deviator stressare estimated. Shear strengthparameters (c & f ) are calculatedadopting similar methods as in soils

If the strain gauges are attached tomeasure the lateral strain, poissonsratio ( n) also can be obtained.

Slake Durability Test:Resistance of rock mass to disintegration during wetting-drying

Rock fragments of knownweight placed in rotating drum

apparatus, and rock pieces(approx 10 pieces, each 40-60gmweight) are circulated throughwet and dry cycles .

Re-weigh the rock fragments todetermine the slake durabilityindex (SDI) .

Mostly, this test allows the rockmass to get exposed up to twocycles of wetting and drying .


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Schmidt Test: Hardness of rock

Schmidt test is performedto determine the reboundhardness of rock.

The plunger of the hammeris pressed against thespecimen and the height ofrebound of the plunger on

a scale is taken as themeasure of hardness .

Sound Velocity Test: P & S-wave velocity of rocks

It is non-destructive test and performed to determine thevelocity of elastic wave propagation through rock in thelaboratory.

slenderness ratio used for the test is usually 3. Test can beconducted on dry, moist or saturated specimens.

A transmitter and a receiver are attached at sides of rockspecimen (a thin layer of grease is applied on thespecimens ends to have proper contact with transducers).

The energy transmission between the transducers(transmitter and receiver) is used to determine thevelocities of P and S wave .


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In-situ stress measurements in rocks

In-situ stress measurements in rocks:Testing methods on Field


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In-situ stress measurements in rocks:Flat Jack Test

In-situ stress measurements in rocks:Hydrofracturing Test


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Geotechnical Testign Methods II_AS