1

Geotechnical Testing Methods II

Ajanta SachanAssistant ProfessorCivil EngineeringIIT Gandhinagar

2

FIELD TESTING

2

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 undisturbed conditions

Disadvantage:

Testing conditions are not controlled

Time dependent phenomenon are difficult to control due to large scale

Measurements/instrumentation is tricky and rather a difficult task

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

Field Test (In-situ Test)

3

5

Common In Situ Testing Devices

In bore holes

DMTVST

SPT

CPTPMT

DCPT

6

Standard Penetration TestIS: 2131-1981

4

7

Standard Penetration Test

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

Standard Penetration Test

5

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

Correction for Overburden Pressure :

N' = Corrected value of observed N

CN = Correction factor for overburden pressure

' .NN C N

Peck, Hanson and Thornburn (1974)

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

SPT Corrections

6

SPT Corrections

Correction for Dilatancy :

[ ]

Correction for Overburden Pressure : (Alternative)

Alternative -

If the stratum consists of fine sand and silt below water table, for N' > 15, the dilatancy correction is applied as

SPT Hammer Energy Correction

Energy is dissipated in some fraction during the impact, and the output energy is usually in the range of 50% to 80% of energy input.

For rope pully system with safety hammer

The N-value is standardized for 60 % energy output. For other hammers, the N-value may be corrected in ratio of their energy input

Although IS 2131-1981 is silent on this issue, the correction may be applied as per the requirement of the project.

60%out

in

EE

60

%.

60

out inE EN N

7

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

8

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)

Cone Penetration Test (CPT)

IS: 4968 (Part –III)

9

17

CPT Procedure

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 sleevet c

f

f

Q Qq

A

f

r

c

qf

q Typical range

0%

10%Cohesive

Granular

CPT Results & Soil Classification

10

Typical CPT Data

0 2 4 6 8 10 12 14

CPT Cone Resistance, qc1

(MPa)

Mean

Mean-SDMean+SD

0 10 20 30

SPT Blow Count, N1(60)

(Blows/300 mm)

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

De

pth

Be

low

Ex

ca

va

ted

Su

rfa

ce

(m

)

Interbedded

Fine Sand

and

Silty Sand

(SP-SM)

Fine Silty

Sand

(SM)

Gray Silty

Clay (CL)

Sand (SP)

Fine Sand

w/ Shells

(SP)

11

Interpreted Soil Profile

EQ Drain Test Area 1

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Depth

(m)

Sand

Silty sand/sand

Silt and Sandy

Silt

Sand to

Silty Sand

Cone Tip

Resistance, q c

(MPa)

0 2 4 6 8 1012

Fricton Ratio, F r

(%)

0 1 2 3 4 5 6

Relative

Density, 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 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

12

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

13

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.

25

Vane Shear Test (VST)

vane

undrained

bore hole

soft clay

measuring (torque)

head

vaneh2d

d

Vane Shear Test

Test in Progress Failure surface

2

2.

. . .

13.

u

Tc

D H

D

H

30.273u

Tc

D

Interpretation:

Undrained shear strength -

For H = 2.D

14

27

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, EShear Mod, GUndrained shear strength, Su

15

29

Pressure meter Test (PMT)

Measurements:1. Fluid Pressure 2. Fluid volume change

Plate Load Test

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

Bearing Capacity Estimation: The load is applied such that the rate of penetration remains constant. A load-settlement curve is produced. Equations have been developed to obtain undrained shear strength from ultimate bearing capacity.

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 soil from the test results.

16

ROCK TESTING

Rock Testing

Unconfined Compression Test

Brazilian Test

Point Load Test

Direct Shear Test

Slake Durability Test

Schmidt Rebound Hardness Test

Sound Velocity Test

In-situ stress measurements in rocks

17

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

Polishing & Lapping machine

Core drilling machine:Rock core preparation For regular and irregular Samples. 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 strength Very brittle

Limestone: Medium stiffness Medium strengthMedium brittleness

Shale: Low stiffness Low strengthDuctile

18

Unconfined Compression Test

This test is performed to obtain the unconfined compressive strength (UCS) of intact rock cores (slenderness ratio = 2).

UCS is the maximum stress that that rock specimen can sustain.

Rock specimen is kept in a loading frame, and if required heated to the desired test temperature.

Axial load is continuously increased on the specimen until peak load and failure are obtained.

Brazilian test: Tensile strength of Rock

Brazilian test is performed to obtain the tensile strength of rock mass.

Tensile strength of rock is imp to know for drilling, blasting of rocks, failure of roof and floor of tunnels, chambers & underground roadways; often weak rocks fail in tension exhibiting splitting mode of failure.

In this test, a disc/cylinder is subjected to a line load, and fracture should initiate at the centre and progress towards periphery. If opposite, the test is discarded as considered that it did not fail in tension.

19

Point Load Test: compress. strength of irregular rock sp.

When regular cores could not be obtained; only irregular pieces are available from the rock excavation, Point load test is performed to obtain the compressive strength of rock mass.

The roughly chiseled spherical mass with dia. ranging between 30-50 mm is tested between 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 of stress normal to the sheared plane.

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

20

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

Triaxial cells for testing rocks are designed to withstand a confining pressure 150 Kg/cm2.

Mostly triaxial tests on rock specimens are performed under no volume change conditions.

Stress-strain curve is obtained using deviator stress and axial strain. The modulus and failure deviator stress are estimated. Shear strength parameters (c & f ) are calculated adopting similar methods as in soils

If the strain gauges are attached to measure the lateral strain, poisson’s ratio (n) also can be obtained.

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

Rock fragments of known weight placed in rotating drum apparatus, and rock pieces (approx 10 pieces, each 40-60gm weight) are circulated through wet and dry cycles.

Re-weigh the rock fragments to determine the slake durability index (SDI).

Mostly, this test allows the rock mass to get exposed up to two cycles of wetting and drying.

21

Schmidt Test: Hardness of rock

Schmidt test is performed to determine the rebound hardness of rock.

The plunger of the hammer is pressed against the specimen and the height of rebound of the plunger on a scale is taken as the measure of hardness.

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

It is non-destructive test and performed to determine the velocity of elastic wave propagation through rock in the laboratory.

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

A transmitter and a receiver are attached at sides of rock specimen (a thin layer of grease is applied on the specimen’s ends to have proper contact with transducers).

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

22

In-situ stress measurements in rocks

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

23

In-situ stress measurements in rocks:Flat Jack Test

In-situ stress measurements in rocks:Hydrofracturing Test

24

Thank You