soil investigations.pdf

7/29/2019 Soil Investigations.pdf

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Soil investigations for assessing the design

Prof G L Sivakumar Babu

Department of Civil Engineering

Indian Institute of Science

Email: [email protected];

[email protected]

Mobile No 9448480671


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Ob ectives

to ground? How to use them in the analysis of civil

engineering structures


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Geophysical Methods

Mechanical Wave Measurements Crosshole Tests (CHT)

Downhole Tests (DHT) Spectral Analysis of Surface Waves

Seismic Refraction

Suspension Logging

Electromagnetic Wave Techniques Ground Penetrating Radar (GPR)

Electromagnetic Conductivity (EM)

Surface Resistivity (SR)

Magnetometer Surveys (MT)


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Mechanical Wave Geophysics Nondestructive measurements (s < 10-4%)

-

types (conducted across surface).

Measurements of wave dispersion:

velocity, frequency, amplitude, attenuation.

Determine layering, elastic properties,

, , Four basic wave types: Compression (P),

S ear S , ay e g , an ove .


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Mechanical Wave Geophysics Compression (P-) wave is fastest wave;

.

Shear (S-) wave is second fastest wave.

Is irectiona an po arize . Most

fundamental wave to geotechnique. Rayleigh (R-) or surface wave is very close

to S-wave velocit 90 to 94% . H brid

P-S wave at ground surface boundary.

-


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Mechanical Body Waves

P-wave

S-wave


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Mechanical Body Waves

S RTime

Oscilloscope

SourceReceiver (Geophone)


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Mechanical Waves (Compression)

P - Wave Velocities

Weathered Rocks

Intact Rocks

Steel

Till

Ice

Sea Water

Clay

Sand

0 1000 2000 3000 4000 5000 6000 7000 8000

Fresh Water

Compression Wave Velocity, Vp (m/s)


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Mechanical Waves (Shear)

S - Wave Velocit ie

Weathered Rocks

Intact Rocks

Steel

Till

Ice

Sea Water

Clay

Sand

0 1000 2000 3000 4000

Fresh Water s =

Shear Wave Velocity, VS (m/s)


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Geophysical Equipment

Seismograph Spectrum Analyzer

Portable Analyzer Velocity Recorder


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Seismic Refraction

oscilloscope

Note: Vp1 < Vp2

t1t2

Determine depthto rock la er z

Source

(Plate)

t3t4

il: V

x1x2 xx4zR

Rock: Vp2


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Seismic Refraction

0.020

)

xV Vc p2 p1

0.015

econd

1

2

V V

c

p2 p1

0.010

Time(s p2

m/s

0.005

Travel

1 Depth to Rock:

c = .

0.000

0 10 20 30 40 50

p1 =m/sc = .

alues

Distance From Source (meters)

x values

t


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Shear Wave Velocity, Vs

Fundamental measurement in all solids

, , , ,

Initial small-strain stiffness represented

by shear modulus: G0 = Vs2alias G = G = G

Applies to all static & dynamic problems at

- s Applicable to both undrained & drained

loading cases in geotechnical engineering.


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Seismic

Equipment


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Crosshole TestingOscilloscope

ASTM D 4428

Pump

t x = fctn(z)from incl inometers

Downhole

Vs = x/tHammer(Source) Velocity

TransducerTest

Receiver)xpackerSlo e

PVC-casedBorehole

PVC-casedBorehole

Note: Verticality of casing

must be established by

slope inclinometers to correctdistances x with depth.

InclinometerInclinometer


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Testing Equipment


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Downhole TestingOscilloscopePump

with normal load

tx

Hammer

packerz2

Test

Depth

Interval

HorizontalVelocity

Transducers

eop oneReceivers)

Shear Wave Velocit : R12 = z12 + x2

Cased

Borehole

Vs = R/t 2 = z2 x


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In-Situ Surface Wave Testing

Signal

Analyzer

SensorsSource

Accelerometer

L a y e r 1

RayleighSurface

L a y e r 2

aves a y e r L a y e r 4


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Shear Wave Measurements


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Seismic Piezocone Test (SCPTu)


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Seismic Piezocone Test

Obtains Four Inde endent

VsMeasurements with Depth:Hybrid of Penetrometer

one ress, qt Penetration Porewater Pressure,u Sleeve Friction, fs

60ou12 Arrival Time of Downhole Shear

Wave, ts

qc


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Automated Seismic Source

Electronically-actuated Self-contained Left and rightl i tiolarization Modified beam uses finto enhance shear wavegeneration Successfully tested todepths of 20mepths of 20m Capable of being usedwith traditional impulsehammerammer


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Sounding Memphis, Shelby County, TN

0 10 20 30 40

q t (MPa)

0 100 200 300

fs (kPa)0 1000 2000 3000

u2 (kPa)

0 100 200 300 400

Vs (m/sec)d = 35.7 mm

5 5 5

0

5Vs

10

15

h(m

)

10

15

10

15

10

15 fs

20

25

Dept

20

25

20

25

20

25

u2

30

35

30

35

30

35

30

35

qt


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Seismic Flat Dilatometer (SDMT)


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Geophysical Methods

Electromagnetic Wave


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Electromagnetic Wave Geophysics

Nondestructive methods

Non-invasive; conducted across surface. Measurements of electrical & ma netic

properties of the ground: resistivity

, , ,

and magnetic fields.

over w e spec rum n requenc esHz < f < 1022 Hz).


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Electromagnetic Wave Geophysics

Surface Mapping Techniques:

Ground Penetrating Radar (GPR)

Electrical Resistivity (ER) Surveys


Downhole Techniques

Resistivity probes, MIPs, RCPTu 2-d and 3-d Tomography


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GPR surveys conducted on gridded areas

Pair of transmitting and receiver antennae Short im ulses of hi h-fre EM wave

Relative changes in dielectric properties

.

Depth of exploration is soil dependent (up

to 30 m in dry sands; only 3 m in wetsaturated clay)


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Xadar Sensors & Software GeoRadar


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Penetrating Radar (GPR)

Crossing an underground utility corridor


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Penetrating Radar (GPR)


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Ground Penetrating

Radar GPR

Geostratigraphy


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Examples of Ground Penetrating Radar (GPR)

Use u in Locating Un ergroun Uti ities


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Results from Ground Penetrating Radar (GPR)


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Results from Ground Penetrating Radar (GPR)


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Electrical Resisitivity Measurements


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Electrical Resistivity (ER) Surveys

Resisitivity R (ohm-m) is an electrical.

Arrays of electrodes used to measure

c anges in potentia .

Evaluate changes in soil types and variationsin pore fluids

, ,

sinkholes), stratigraphy, contaminant plumes.


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Resisitivity

easuremen s

What will be gained by

changing electrode

spac ng

Depth of ER survey:i.e., greater spacingn uences eepe


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Resistivity Values (ConeTec & GeoProbe, 1997)

Glacial Till

Weathered Rocks

Loose Sands

Sands & Gravels

Loam

1 10 100 1000 10000

ay

Bulk Resistivity, (ohm-meters)


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lect ical Resistivit


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Magnetometer Surveys (MS)

Measure relative changesn e ear s magne c

field across a site.


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Applicability of In-Situ Tests

SPT

CLAYS SILTS SANDS GRAVELS Cobbles/Bould ers

ethod

CPT

u

Test DMT

In-Si

VST

0.0001 0.001 0.01 0.1 1 10 100 1000

Geophysics

ra n ze mm


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Equivalent Elastic Modulus


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Equivalent Modulus with Strain Level


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Equivalent Modulus for Monotonic Loading

1

o

0.8

o,G

/G

Cyclic

0.4

0.6

usRat

Monotonic

0.2Modul

Loading

0

1E-06 1E-05 0.0001 0.001 0.01 0.1

Shear Strain


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Conclusions

Knowledge of state of ground and stateof structure is enhanced usin extensive

instrumentation

instrumentation is are useful to design

,

structures under complex stress and

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