Week 2

Learning Outcomes

Student should be able to:

Describe the various methods of Soil Investigation Methods.(CO1-PO10)

Derive information from these investigations. (CO1-CO10)

Discuss the limitations and advantages of each methods. (CO1-PO10)

Learning Outcomes

Student should be able to:

Explain how SPT are done (CO1-PO10) Derive information from the bore-log. (CO1-

PO10) Discuss sampling method and the laboratory tests to determine the soil properties.(CO1-

PO10)

Geophysical InvestigationsGeophysical Investigations

Initial Site Exploration/Preliminary Surveys• Assist with Placement of Borings/In-Situ Tests

Difficult Locations• Gravels, Cobbles, Boulders, Debris• Difficult Terrain• Contaminated Sites

Supplementary Exploration• Observe Variations Between

Borings/Soundings/Outcrop, etc.• Locate Irregularity

Example of Geophysical Investigation:

1) Seismic refraction survey2) Resistivity survey

Geophysical InvestigationsGeophysical Investigations

Advantages geophysical method:Advantages geophysical method:

1 - Continuous subsurface profile

2 - Quick to conduct enable the coverage of a large area relatively

quickly

3 - Non-intrusive methods: geophysical methods do not involve

destruction and excavation. Most of the methods are silent and

environmentally friendly, and can therefore be implemented at all

hours of the day, including in densely populated areas. 

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Applications of Seismic Reflection & Applications of Seismic Reflection & RefractionRefraction

Seismic reflection and refraction have numerous potential applications to a variety of environmental and geotechnical problems, including:

Depth and characterisation of bedrock surface

Buried channel definition

Depth of water table

Mapping of faults and other structural features

Location of karst features

Seismic Refraction Survey (summaries)

- Conduct by impact the ground and observing the arrival of the waves at various point. Recorded by geophones. Velocity of the waves is related to the properties of various layers.

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Seismic ReflectionSeismic ReflectionReflections of sound waves (compression waves) from the subsurface arrive at the geophones some measurable time after the source pulse. If we know the speed of sound in the earth and the geometry of the wave path, we can convert that seismic travel time to depth. By measuring the arrival time at successive surface locations we can produce a profile, or cross-section, of seismic travel times. A simple concept.

In practice, the speed of sound in the earth varies enormously. Dry, sand might carry sound waves at 250 m/s or less. At the other extreme, unfractured granite might have a velocity in excess of 6,000 m/s.

The more layers between the surface and the layer of interest, the more complicated the velocity picture. Various methods are used to estimate subsurface velocities including refraction analysis, borehole geophysical measurements, estimates from known lithologic properties, and analysis of reflection times at increasing offsets. Generally, a combination of velocity estimation methods will give the best results.

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Seismic ReflectionSeismic Reflection

Source: www.geosphereinc.com

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Seismic RefractionSeismic RefractionWhen a sound wave crosses an interface between layers of two different velocities, the wave is refracted. That is, the angle of the wave leaving the interface will be altered from the incident angle, depending on the relative velocities. Going from a low-velocity layer to a high-velocity layer, a wave at a particular incident angle (the "critical angle") will be refracted along the upper surface of the lower layer. As it travels, the refracted wave spawns upgoing waves in the upper layer, which impinge on the surface geophones.

Sound moves faster in the lower layer than the upper, so at some point, the wave refracted along that surface will overtake the direct wave. This refracted wave is then the first arrival at all subsequent geophones, at least until it is in turn overtaken by a deeper, faster refraction. The difference in travel time of this wave arrival between geophones depends on the velocity of the lower layer. If that layer is plane and level, the refraction arrivals form a straight line whose slope corresponds directly to that velocity. The point at which the refraction overtakes the direct arrival is known as the "critical distance", and can be used to estimate the depth to the refracting surface.

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Seismic RefractionSeismic Refraction

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Seismic RefractionSeismic Refraction

Vertical GeophonesSource(Plate)

Rock: Vp2

ASTM D 5777

Soil: Vp1

oscilloscope

x1x2x3x4

t1 t2 t3 t4

Note: Vp1 < Vp2

zR

Determine depthto rock layer, zR

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Seismic RefractionSeismic Refraction

Vertical GeophonesSource(Plate)

Rock: Vp2

Soil: Vp1

oscilloscope

“Shortest path not necessarily the fastest one”

Fastest path is the shortest one

Fastest path is NOT the shortest one

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Geophysical Properties

0 1000 2000 3000 4000 5000 6000 7000 8000

Compression Wave Velocity, Vp (m/s)

Fresh Water

Sea Water

Clay

Sand

Till

I ce

Weathered Rocks

Intact Rocks

Steel

P - Wave Velocities

0 1000 2000 3000 4000

Shear Wave Velocity, VS (m/s)

Fresh Water

Sea Water

Clay

Sand

Till

I ce

Weathered Rocks

I ntact Rocks

Steel

S - Wave Velocities

Resistivity, (ohm- meters)

Resistivity Values ( ICE, London, 1976)

} V s = 0

1 10 100 1000 10000

Bulk Resistivity, (ohm- meters)

Clay

Loam

Loose Sands

Sands & Gravels

Glacial Till

Weathered Rocks

Resistivity Values (ConeTec & GeoProbe, 1997)

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Results from Seismic Refraction

Resistivity Survey (summaries)

- Electrical current is input into the ground and the resistivity of the various layers is measured. Different soil materials has different resisitivity.

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Surface Wave TestsSurface Wave TestsNon invasive method based on the geometric dispersion of Rayleigh waves, which are waves that travel along the ground surface resulting from a vertical impact or continuous vibration source (like waves in the sea).

The relationship between velocity of propagation of Rayleigh waves and frequency can be determined experimentally analysing the particle motion induced on the ground surface by the propagation.

Seismic waves are generated using either impact sources or vibrators and are detected using vertical velocity transducers (Geophones). The recorded ground motion is then analysed in the frequency domain to estimate the experimental dispersion curve (the relationship between frequency and velocity).

The experimental dispersion curve is finally used in an inversion process to estimate the variation with depth of the velocity of propagation of shear wave, which is linked to the small strain stiffness of the soil.

The inversion process is based on the numerical propagation of Rayleigh wave propagation in layered linear elastic media.

2so VG

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Electrical MethodsElectrical MethodsElectrical properties are among the most useful geophysical parameters in characterizing earth materials. Variations in electrical conductivity (or its inverse, resistivity) typically correlate with variations in water saturation, fluid conductivity, porosity, permeability, and the presence of metal. Depending on the particular site, these variations may be used to locate contaminant plumes, salt water intrusion, stratigraphic units, sinkholes, fractures, buried drums and tanks, and any other feature whose electrical properties contrast with the surrounding earth.

Ground conductivity can be measured either directly, using the galvanic resistivity method, or inductively, using electromagnetic induction (EM). Because EM requires no direct contact with the ground surface, data can be acquired more quickly than with resistivity. Resistivity, however, can provide better vertical resolution and is generally less sensitive to sources of "noise" such as fences, buildings and overhead powerlines.

Concept of Electrical Resistivity Method used to determine the sub surface profile:

The electrical resistivity use the concept that electrical resistance varies significantly enough among different types of soil and each materials, to allow identification of specific types once their resistivities in the field are measured. In the field electrodes are used and electrical current are supplied. The voltage drop in the soil material within the zone created by the electrodes electric field is measured by voltmeter. The soil electrical resisitivity is computed:

P = Resistivity of soil material, ohm.mD = electrode spacing, mR = Resistance, ohmsV = Voltage drop between two electrodes

The resistivity obtained is then correlated with known values to determine the

types of material.

DRI

DVP 22

Electrical Resisitivity Measurements

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ElectroMagnetic InductionElectroMagnetic Induction

Combined 3-D Plot/Contour Map of EM Induction Data

Electromagnetic Electromagnetic Conductivity (EM)Conductivity (EM)

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Applications of Electrical MethodsApplications of Electrical MethodsEM and Resistivity can be applied to a wide variety of problems

encountered in environmental, groundwater, geotechnical, and archaeological work, including:

Location of buried drums, tanks, trenches, and utilities

Location of landfills and bulk buried materials

Delineation of contaminant plumes

Depth of water table and aquifer identification and mapping

Continuity of stratigraphic interfaces such as clay layers

Mapping of faults and fractures

Location of karst features