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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 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)
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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 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
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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
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
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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
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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)
Cone Penetration Test (CPT)
IS: 4968 (Part –III)
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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
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
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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)
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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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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
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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
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
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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, EShear Mod, GUndrained shear strength, Su
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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.
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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
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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 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
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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.
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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
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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.
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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.
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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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Thank You