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C 601 S l l &CE 601: Soil Exploration & Geophysical methods

Subsurface Exploration Techniques

Test Pits Boring Geophysical Methods

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Exploration Techniques

Test Pits: Unlike boring, soil can be visually observed from the sides of the test pit Pit is made by excavating ground (typical size =1 2mx1 2m) the test pit. Pit is made by excavating ground (typical size 1.2mx1.2m) considering sufficient working space.

Trenches: Trenches are long shallow pits. They are more suitable for exploration on slopes than pits.

Suggestions:

Test pits suggested if required exploration depth = 2-4m Test pits suggested if required exploration depth 2 4m

Trenches suggested for slopes (small)

Boring suggested for exploration depth > 4m

Boring Techniques

Auger Boring Use depends ong g Wash Boring Rotary Boring Percussion Boring

Nature of soil

Water table Depth

Sample Disturbance

Accuracy of soil exploration

Auger Boring for soils which can stay open without casing or drilling mud. It is not possible for sands below water table. Good for Highways, railways projects where small depth of soil exploration is needed.

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Auger Boring

Push and rotate the auger until annular space of auger fills up

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3. Sand Bailer H d i i h i Withdraw the auger and clean it

Repeat the processPro

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1. Hand Auger – for shallow depth (3 - 5 m)

– Heavy duty pipe with cutting edge

– Lifted and then left to fall freelyunder self weight. Additional weight (sinker) may be added for ease of sinking

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2. Power Driven Auger – for larger depth

4. Hollow Stem Auger

Auger Boring

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Wash Boring

A casing pipe of 2-3 m length is driven into the soil by a heavy drop soil by a heavy drop hammer.

The soil inside the casing is removed by means of a chopping bit attached to a drill rod which forces water at high pressure.

Soil mixed with water moves up in annular

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moves up in annular gap between drill rod and casing.

Samples are obtained at certain depth by removing drill rod and pushing a sampler instead.

Rotary Boring

Design similar to wash boring

Useful when soil is resistant to auguring or wash b i

Boring is done by rapidly rotating drilling bits attached to bottom of drilling rod.

Soil/rock cuttings removed by circulating drilling fluid

Samples are taken a certain depths by removing drill rod and placing

boring

g p gsampler.

Mud Rotary Drilling: Hollow drilling rods are used to flow mud slurry (Bentonite) to check caving in of the material (soil) at bottom.

Core Drilling: Core barrels with diamond bit are used.

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Percussion Boring

Dry boring or water circulated to remove y gloose soil

Heavy drilling bit or chisel is dropped while inside the casing to chop the hard soil.

Percussion drilling rods may be replaced by cables.cables.

Bore Hole Stabilization

Drilling Mud Use of Casing

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Ground Water Observation

High Permeability Soils

Bore hole/Observation wells (Observation time = 24 to 48 Hrs)

Low Permeability Soils

Casagrande Piezometer (when water level in bore hole does not get stabilize in Piezometer is recommended) Piezometer is recommended)

Piezometers may be installed in bore hole for seasonal variations in High permeability soils. Chemical analysis of ground water may be performed if its constituents can be damaging to foundation.

Soil Sampling

Disturbed Samples: Natural soil structure is modified or destroyed during samplingdestroyed during sampling Representative Samples:

Natural water content and mineral constituents of particular soil layer are preserved

Good for soil identification and water content Non-representative Samples:

Water content altered and soil layers mixed up Of no use.

Undisturbed Samples: Soil structure and the other mineral properties are preserved to an extent. Some disturbance is always there, e.g. due to stress release.

However it should be minimized in order to have suitable sample for our analysis.

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Standard Split Spoon Samplers

Thick wall (0.25in) cylinderSampling tube (dia 51 mm) is split along the lengthRepresentative Disturbed soil samples

Shelby Tube (Thin-wall) Sampler

Thin wall (1/16in = 0.0625 in) ( / )sampling tubeSampler pushed into the ground hydraulicallySample extruded from tube and “Undisturbed” soil sample is obtained

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Standard Split-Spoon Samplerp p p

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Thin Walled Sampler (Shelby Tube)

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Sealing Sampling Tube

Laboratory Test: Index Properties

Index Properties of soil:

Basic soil properties such as (a) Specific gravity (Gs)(b) Grain size distribution (dry/wet Sieve test, Hydrometer test), (c) Liquid Limit (LL), Plastic limit (PL) (d) OMC, Maximum Dry density(Compaction/Proctor test)(e) Permeability (Constant head/Falling head)(f) Relative Density (Minimum & Maximum density for cohesionless soils)

More tests for Problem soils: (a) Shrinkage Limit, Free swell, Swell pressure for Expansive soils(b) Pinhole test, Crumb test for Dispersive soils(c) Chemical Test (PH, Sulphite, Chloride, Iron etc) for soils (may affected with industrial waste or some other waste)(d) Furnace test for Organic Soils (peats etc)

“Representative Disturbed “soil samples are used to perform these tests.

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Laboratory Test: Engineering Properties Engineering Properties of soil:

C lid i P i Consolidation Properties (Oedometer setup)(i) Must to perform for Clayey soils; (ii) Soil parameters obtained: Cc,Cv,Cr, OCR, k

Shear Strength Properties(i) Direct Shear test (for cohesionless soil)(ii) Unconfined Compression test (for cohesive soil)(iii) Triaxial test (for all soil types; cohesive, cohesionless)

Dynamic Properties(i) Cyclic Triaxial test(ii) Cyclic Simple Shear test(iii) Resonant Column test(iv) Bender Element test

“Undisturbed” soil samples are used to perform these tests.

Depth of Exploration

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Economics

Field Exploration

Site Boring Layoutg yTest Borings or Test Pits?It depends on the type of materials, and

what you want to know.

Number and Frequency of BoringsDepth, Sampling Methods and Field

Testing

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No. and Depth of Boreholes

Number of Boreholes Depends on Site Conditions Regulations .. Minimum # of boreholes

Borehole Depth If there are no special concerns then

b h l d h 50f b h h b f borehole depth<50ft beneath the base of the facility

Minimum No. of Boreholes (NJDEP)

Acreage # borings #deep boringsborings

less than 10 4 110-49 8 250-99 14 4100-200 20 5more than 200 24+1boring for 6+1 boring for

each addl 10 acres each addl10 acres

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Guidelines by Bagchi(1994)

Borings should cover an area that is 25% larger than proposed waste limitslarger than proposed waste limits

5 borings/first hectare + 2 borings/each add’l hectare

Borings should extend at least 25 ft below proposed base elevation

Selected borings should extend at least 6ft Selected borings should extend at least 6ft into bedrock/aquifer

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Geophysical Methods

Seismic Reflection Method Seismic Reflection Method Seismic Refraction Method Cross-Hole Test Down Hole Test & Up-Hole Test Spectral Analysis of Surface Wave (SASW)p y ( ) Seismic Cone Penetration Test (SCPT)

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Seismic Reflection Method

Depths greater than f~50 feet

Seismic reflection is particularly suited to marine applications (e.g. lakes, rivers, oceans, etc.)

The inability of water to transmit shear waves 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

Depths less than p~100 feet

Cost Effective as compared to Reflection method (<3to5 times)

Used for

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

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 elevation in one or more boring. Source is triggered in another boring at the same elevation.

S wave travels horizontally from source to receivinghole, 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

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.

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

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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Seismic Cone Penetration Test (SCPT)

Seismic cone is pushed into the groundg

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

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

Computer in the SCPT rig collects and processes all the data & shear wave velocity is measured

Seismic Cone Penetration Test (SCPT)

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Seismic Cone Penetration Test (SCPT)

Seismic Cone Penetration Test (SCPT)

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Seismic Cone Penetration Test (SCPT)

Seismic Cone Penetration Test (SCPT)

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SASW Test (based on Seismic Refraction concept)

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

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SASW Test

Thank You