conventional field testing & issues - · pdf file3 5 standard penetration test is:...

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Conventional Field Testing & Issues (SPT, CPT, DCPT, Geophysical methods)

Ajanta Sachan Assistant Professor Civil Engineering IIT Gandhinagar

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)

Field Test: In-situ shear strength Testing

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Common In Situ Testing Devices

In bore holes

VST

SPT

CPT DCPT

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Standard Penetration Test IS: 2131-1981

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.

Standard Penetration Test

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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 100 blows are required for last 300 mm penetration

Standard Penetration Test

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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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

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

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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

SPT Test Data

No. of blows per 0.30m

Data from different bore holes

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SPT Test Data

Interpretation from SPT

IS 6403-1981

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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

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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Total Settlement from SPT Data for Cohesionless soil

Multiply the settlement by factor W'

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

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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 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

DCPT – SPT Correlations for 50 mm dia. cone

Ncd = 1.5 N For depth < 3 m

Ncd = 1.75 N For depth 3 m to 6 m

Ncd = 2.0 N For depth > 6 m

DCPT – SPT Correlations for 62.5 mm dia cone

Without bentonite

slurry

Ncbr = 1.5 N For depth < 4 m

Ncbr = 1.75 N For depth 4 m to 9 m

Ncbr = 2.0 N For depth > 9 m

With circulating

bentonite slurry

Ncbr = N For all depths

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DCPT

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

Qq

A

x-sectional area off cone = 10 cm2

surface area of friction sleeve t c

f

f

Q Qq

A

f

r

c

qf

q Typical range

0%

10% Cohesive

Granular

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

15

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

CPT Results & Soil Classification

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Typical CPT Data

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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CPTU

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Typical Measurements with CPTU

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DOWNHOLE SEISMIC PIEZOCONE PENETRATION TEST (SCPTU)

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

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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

Plate Load Test

This test is used to estimate the Modulus of subgrade reaction and Bearing Capacity of soils.

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 of Subgrade Reaction 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 the

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Plate Load Test – IS:1888-1982

Bearing Plate:

Rough mild steel bearing plate in circular or square shape

Dimension: 30 cm, 45 cm, 60 cm, or 75 cm. Thickness > 25 mm

Smaller size for stiff or dense soil. Larger size for soft or loose soil

Bottom of the plate is grooved for increased roughness.

Concrete blocks may be used to replace bearing plates.

Plate Load Test – IS:1888-1982

Section

Plan

Test Pit:

Usually to the depth of foundation level.

Width equal to five times the test plate

Carefully leveled and cleaned bottom.

Protected against disturbance or change in natural formation

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IITGN Plate load test

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Plate Load Test: Bearing Capacity

In case of dense cohesionless soil and highly cohesive soils ultimate bearing capacity may be estimated from the peak load in load-settlement curve.

In case of partially cohesive soils and loose to medium dense soils the ultimate bearing capacity load may be estimated by assuming the load settlement curve so as to be a bilinear relationship.

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Plate Load Test: Bearing Capacity

A more precise determination of bearing capacity load is possible if the load-settlement curve is plotted in log-log scale and the relationship is assume to be bilinear. The intersection point is taken as the yield point or the bearing capacity load.

uf f

up p

q B

q BFor cohesioless soil

uf upq qFor cohesive soil

Geophysical Methods

Seismic Reflection Method

Seismic Refraction Method

Cross-Hole Test

Down Hole Test & Up-Hole Test

Spectral Analysis of Surface Wave (SASW)

Seismic Cone Penetration Test (SCPT)

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longitudinal, primary or compressional wave

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

P Wave (Compression/Primary Wave)

S Wave (Shear/Secondary Wave)

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)

Love Waves

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Wave Velocities

P-wave velocity – Vp

Shear Wave velocity – Vs

Vp > Vs

Soil Properties from Wave Velocity

Shear Modulus

Constrained Modulus,

Young’s Modulus,

Poisson’s Ratio,

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

2

p 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 Reflection Method

Depths greater than ~50 feet Seismic reflection is particularly suited to marine applications (e.g. lakes, rivers, oceans, etc.) The inability of water to transmit shear waves makes collection of high quality reflection data possible even at very shallow depths that would be impractical to impossible on land.

Seismic Refraction Method

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

Depths less than ~100 feet Cost Effective as compared to Reflection method (<3to5 times) Used for computation of layer thickness of soil

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Differences in Seismic Reflection and Seismic Refraction Method

http://www.enviroscan.com/html/seismic_refraction_versus_refl.html

Seismic Reflection uses field equipment similar to seismic refraction, but field and data processing procedures are employed to maximize the energy reflected along near vertical ray paths by subsurface density contrasts.

Seismic Refraction involves measuring the travel time of the component of seismic energy which travels down to the top of rock (or other distinct density contrast), is refracted along the top of rock, and returns to the surface.

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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.

Cross-Hole Test

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Down Hole Test

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

Down Hole method: 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

Down Hole Test

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

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


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SASW Test (Spectral Analysis of Surface Waves)

SASW does not require Boring like other tests Sensors are spread along a line on the surface & the source is also located on the surface Sensors receive Rayleigh waves, which are the surface waves Dispersion curve (phase velocity Vs frequency) is created. Then individual dispersion curves from all receivers are combined into a single composite dispersion curve, called field dispersion curve. Forward-modeling procedure is then used to match the field dispersion curve with a one-dimensional layered system of varying soil layer stiffnesses and thicknesses.

The shear wave velocity profile that generates a dispersion curve that most

closely matches the field dispersion curve is then presented as the shear wave velocity

profile for the site.

MASW Test (Multichannel Analysis of Surface Waves)

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MASW does not require Boring like SASW. 24 or more channels (Sensors) are placed over a few to a few meters of distance (eg: 2-200 m) Sensors receive Rayleigh waves, which are the surface waves. Dispersion curve (phase velocity Vs frequency) from each sensor is created. MASW deals with various frequencies range (eg: 3-30 Hz) Active MASW method generates surface waves actively through an impact source like sledge hammer, where as Passive MASW method utilizes surface waves generated by traffic, thunder, tidal motions etc. Investigation depth is usually shallower than 30 m with the active method. MASW utilizes dispersion properties of surface waves for the purpose of shear wave velocity (Vs) profiling in 1D (depth) or 2D (depth and surface location) of soil strata.

MASW Test (Multichannel Analysis of Surface Waves)

Thank You

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