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CE 601: In-situ shear strength testing

In-situ shear strength tests

Field Test: In-situ shear strength Testing

g Standard Penetration Test (SPT) Cone Penetration Test (CPT) Dynamic Cone Penetration Test (DCPT) Vane Shear Test (VST) Dilatometer Test (DMT) Pressure meter Test (PMT)

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Common In Situ Testing Devices

SPT

3In bore holes

DMTVSTCPT

PMTDCPT

Standard Penetration TestIS: 2131-1981

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Components Drilling Equipment

Inner diameter of hole 100 to 150 mm

Standard Penetration Test

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 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

Standard Penetration Test

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 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

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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 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 :

' .NN C N

SPT Corrections

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

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SPT Corrections

Correction for Overburden Pressure : (Alternative)

Correction for Dilatancy :

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

[ ]Alternative -

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

60%out

in

EE

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

60

%.

60out inE E

N 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'' ' D (%) i tN'' ' 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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Interpretation from SPT: Cohesive Soils

not corrected for overburden 6.25. in kPauc N

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

0.689

0.193'

NOCR

p

MN/m2

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

>30 >200 hard Cannot indent even with thumb nail

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.

h / 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

h f l dd lf h ll

cc

c

Qq

A

x-sectional area off cone = 10 cm2

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

Frictional resistance

Friction Ratio

surface area of friction sleevet c

ff

Q Qq

A

fr

c

qf

q Typical range

0%

10%Cohesive

Granular

0

1

2

0 2 4 6 8 10 12 14

CPT Cone Resistance, qc1

(MPa)

0

1

2

0 10 20 30

SPT Blow Count, N1(60)

(Blows/300 mm)

0

1

2

0 20 40 60 80 100

Relative Density, Dr

(%)Interpreted Soil Profile

0

1

2

0 0 .2 0 .4 0 .6 0 .8 1

Fine Sandw/ Shells(SP)

3

4

5

6

3

4

5

6

3

4

5

6

3

4

5

6

Be

low

Ex

cava

ted

Su

rfa

ce (

m)

Interbedded Fine SandandSilty Sand(SP-SM)

Fine Silty Sand

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8

9

1 0

MeanMean-SDMean+SD

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8

9

1 0

7

8

9

1 0

From CPT

From SPT

7

8

9

10

De

pth

(SM)

Gray Silty Clay (CL)

Sand (SP)

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Interpreted Soil Profile EQ Drain Test Area 1

0

1Sand

Cone Tip

Resistance, qc

(MPa)

0 2 4 6 8 10 12

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

2

3

4

5

6

7

8

9

Dep

th (

m)

Silty sand/sand

Silt and Sandy Silt

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10

11

12

13

14

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Sand to Silty Sand

CPT Results & Soil Classification

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T i l Typical CPT Data

Dynamic Cone Penetration Test (DCPT) Components:

1) Cone (dia = 50 mm)~usually made of steel

IS: 4968 (Part – I, II)

usually made of steel

SPTHollow (split spoon)

2) Driving rods/drill rods

~marked at every 100 mm

SPT

DCPT

Solid (no samples)

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DCPT Procedure Cone – drill rod – driving head assembly is installed

vertically on the ground and hammer is dropped from standard height repeatedlystandard 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 d if th d d d if th d dground 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

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

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CPTU

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Resistance calculation for CPTU

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Typical Measurements with CPTU

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DOWNHOLE SEISMIC PIEZOCONE PENETRATION TEST (SCPTU)

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(SC U)

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Failure Modes around Advancing Cone

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For clays, and mainly for soft clays. Measure torque required to quickly

h h h d f l

Vane Shear Test (VST)measuring (torque) head

shear the vane pushed into soft clay.

torque undrained shear strength cu

Typical d = 20-100 mm.

undrained

bore hole

vaneh2d y

28vane

soft clay

vane

d

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Vane Shear Test Interpretation:

Undrained shear strength -

2

2.

. . .

13.

u

Tc

D HD

H

T

For H = 2.D

Test in Progress Failure surface

30.273u

Tc

D

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

Dilatometer Test (DMT)

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

b i i fl h d

30

60 mm dia. Flexible membrane

membrane is again flushed with plate.

Determined: Elastic Modulus Soil Type and state

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Pressure meter Test (PMT)

Determined:Elastic Young Mod, EShear Mod, GUndrained shear strength, Su

Pressure meter Test (PMT)

Measurements:1. Fluid Pressure 2. Fluid volume change

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Thank You