chapter 7 - site investigation and geophysics

8/13/2019 Chapter 7 - Site Investigation and Geophysics

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Chapter 7

Site Investigation (S.I)

and

Geophysics


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SI comprises

a) Planning

b) Desk Study

c) Investigation on Natural rock or Man Madeoutcrop

d) Drilling Exploration

e) Observation into borehole


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Planning

The successful design andconstructionreally needprediction datalike soilandrock characteristics, andgroundwaterlevel& knowledge on geology structure.

To obtain that information, engineers andgeologistsacquires MAPandCROSS SECTION SUBSURFACEwhich are having the kind of information such as:

Topography contour for pre and post construction.

Top layer of rocks contour Weathered rocks layer contour

Contour between rock and soil boundaries


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Desk Study

Study on:-

MAP and REPORTS

Aerial Photo and Remote Sensing

Photographs on color or black and white


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Investigation on Natural rock

or Man Made outcrop

1) Investigation on surface:

Test pits and trenches

Adits and Shaft

2) Observation on rock outcrops:

Geological mapping on rock exposed

Sampling on jointed rocks

3) Seismic activities andFaulted:

4) Using geophysics methods inSI


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Drilling Exploration

Rock core drilling

Core orientation

Supervision and logging


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Observation into borehole

Camera (TV)

Packer Test

Geophysics

Dilatometer, Pressuremeter


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Rotary Wash Boring (Borehole)


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Wash boring

The foremost S.Iused around the world.

The soil androck characteristicswererecorded into BORELOG(Figure 7.3 (a) &(b))

Soil samples were taken using spilt barrelmeanwhile rock samplesobtained usingcore barrel.


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Boring Record

Boring logs: Information on subsurface conditionsobtainedfrom the boring operationis typically presentedin the form of a boring log (boring record).

A continuous recordof the various stratafound at theboring is developed. The contents are:

Description/classification of soils and rock type

encountered changes in strata

water level

soil consistency

type and depth of sample and field test


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Limitation of Boring Data

Providing infoon subsurface conditions onlyat the actual drillinglocation.

Interpolationbetween borings to determine conditions does involvesome degree of uncertainty.

Some limitationsinherent to the info shown on typical drillers log:

The employed crews are primarily drilling tradesmen: w/ limitedexperience in detail soil classification; have no familiarity w/ theimportance of subsurface conditions on the features of building designand construction.

Some importance items of info can be innocently passed over by drillerwhose major interest is in the rate of drilling progress.

Assign technically trained personnel: to examine and classify recoveredsoils, to direct the depth as which should be taken, to select the drilling

sequence, to document factors relating to surface and subsurfaceconditions that could influence on design or construction.


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Soil Sampling

Disturbed(but representative):

Grain size analysis

Liquid & plastic limit

Specific gravity Organic content

Classification

Undisturbed:

Consolidation

Hydraulic conductivity

Shear strength


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Rock Sampling

6 meterof core rock lengthmust be obtained forgranitic rocksin order to make sure the rock formationis nota BOULDER.

12 meterof core rock lengthof limestonemust becoring to ensure the rock formation isbedrock. (Hinderfrom cavity, pinnacles, sinkholes or others CARSTICformation structures resulting from present of limestone).

RQD, TCR, SCR andFImust be calculatedforgeotechnical interpretation.

Rock strength Tests: Uniaxial Compression Test, TriaxialCompression Test, Point Load Test and Schmidt

hammer (Strength Test)


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Rock Quality Designation (RQD)

The Rock Quality Designation index (RQD)wasdeveloped by Deere (Deere et al 1967) toprovide aquantitative estimate of rock mass quality from drill corelogs.

RQDis defined asthepercentage of intact core pieceslonger than 100 mm (4 inches) in the total length of core.

The core should be at leastNW size(54.7 mm or 2.15

inches in diameter) and should be drilled with a double-tube core barrel.

The correct procedures for measurement of the length ofcore pieces and the calculation of RQD are summarized

in Figure 7.2.


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Procedure for measurement and calculation of RQD (After Deere, 1989)


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Contd

RQD will be referredto Table 7.1.

Table 7.1indicated the rock quality fromcore obtained from sites.

Sometimes, RQD dataobtained, could nottrustedbecause of drilling techniquesimproper.

For example, the drilling machineshouldbe setup in properly manner.


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Measurement identify rock quality

(Source: Deere, 1989)

RQD (%) Descriptions Rock Quality

0-2525-50

50-75

75-90

90-100

Very PoorPoor

Moderate

Good

Very Good


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Figure 7.3 (a) Borelog in soil condition


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Figure 7.3 (b) Borelog shows the core rock logging


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Core Rock Sample of Quartz Mica Schist at Lebuh

Raya Simpang Pulai to Cameron Highland

S h ti di f R k C
http://localhost/var/MOHAMAD%20FAIZAL/My%20Documents/borehole-tunnel.pdf


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Schematic diagram of Rock Core


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Core Logging Calculations

Total Core Recovery (TCR%) = Core Recovered/Length of Core

Solid Core Recovery (SCR%) = Solid core pieces in full diameter/Length of Core

Rock Quality Designation (RQD%) = Solid Core Pieces >100mm/Length of Core

Fracture Index (FI/m run) = Number of Fractures/Length of Core

Examples Calculation:

TCR = 1.4/1.5 = 93%

SCR = 0.18 +0.71 + 0.17/1.5 = 71%

RQD = 0.23 + 0.33 + 0.24 + 0.15/1.5 = 63%


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Geophysics

Resistivity

Seismic Refraction

Seismic Reflection

Gravity


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Resistivity

Resisitivitymeasurementsare made byinjecting a DC current into thegroundthroughtwo electrodesand measuring theresultingvoltageat thesurface at two other electrodes.

The depth of measurementis related toelectrode spacing.

Resisitivitymeasuresbulk electricalresistivitywhich is a function of thesoilandrock matrix, percentage of saturation andtype of pore fluids.


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Equipments used during carried out the resistivity survey


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Resistivity Sounding


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Resistivity Measurement & Field arrangement


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Resistivity Sounding


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Resistivity

Uses:

Resistivity measurementsare primary used forsoundings to determine depthand thicknessofgeologic strata.

Also can be applied to profiling measurementsforlocating anomalous geologic conditions, detectingand mapping contaminant plumes, locating buriedwastes and mineral exploration.

Can be used for azimuthal measurementsto determinefracture orientation.


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Contd

Advantages:

Good vertical resolution(sounding)

May also be used for profiling

Measurements can be easily madeto depthsoffew hundred feet or more

Various electrode configurationsare availablefor different applications

C td


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Contd

Disadvantages:

Requires intrusive contact with the ground

Station measurements only

Electrode array can be quite long, with outermostelectrode spacing from 9 to 18 times the depth of interest

Susceptible to interferencefrom nearby metal fences,buried pipes, cables, etc

Generally, cannot be used over asphalt or concrete

Effectivenessdecreasesat very low resisitivity values(use electromagnetic measurements)


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Table 7.3 List of resistivity value for several rocks and soils. (Keller and

chknecht, 1966, Daniels and Alberty, 1966)

1m 1mResistivityConductivity

Material Resistivity Conductivity

Igneous& Metamorf

Granite

Basalt

Slate

MarbleQuarzite

Sedimentary Rock

Sandstone

Shale

Limestone

Soil and Water

Clay

Alluvium

Groundwater (Clean)

Marine water

5x103106

103-106

6x102-4x107

102-2.5x108

102-2x108

8-4x103

20-2x103

50-4x102

1-100

10-800

10-100

0.15

10-6-2x10-4

10-6-10-3

2.5x10-8-1.7x10-3

4x10-9

-10-2

5x10-9-10-2

2.5x10-4-0.125

5x10-4-0.05

2.5x10-3-0.02

0.01-1

1.25x10-3-0.1

0.01-0.1

6.7


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Application of resistivity survey to

determine weathering profiles


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determine weathering profiles

Application of resistivity survey to determine sinkholes or cavity of limestone


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Application of resistivity survey to determine sinkholes or cavity of limestone


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determine water boundaries
http://e/CONSULTANCY/RESISTIVITY%20AT%20POLITEKNIK%20BEHRANG/IKRAM_SR_resistivityEDIT.pdf


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Seismic Refraction

Seismic refraction measurementsare madeby measuring the travel timeof a refracted seismic waveas it travels from the surface through onelayer to another and is refractedback to the surfacewhere it is picked upby geophones.

Shockorimpactis made at a point, seismic wavesthrough the

surrounding soil & rock.

The wave speedrelating to the densityandbonding characteristics ofthematerial.

The velocityisdetermined.

The magnitude of thevelocityis than utilized to identified the material.

The travel timeof a seismic waveis a function of soil and rock density andhardness.


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Seismic refraction survey equipment

Seismic

cable

Seismograph

Geophone

Seismic

cable

Striker

plate12V AC

battery

Trigger

cable

12lb Sledge

hammer


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Seismic refraction wave movement

into subsurface


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Seismic Refraction Measurement & Field arrangement

SEISMIC REFRACTION


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SEISMIC REFRACTION

SURVEY LINE SETUP

Contd


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Cont d

Uses:

Primary applicationfor seismic refractionis for determination

of depthandthicknessofgeologic strata, structureandanomalous conditions

Depthcan be calculated under each geophoneto produce adetailed two-dimensional top of rock profile

If compressional P-waveand shear S-wavevelocitiesaremeasured, in situ elastic moduli ofsoilandrockcan bedetermined

Can be used for azimuthal measurementsto determinefracture orientation

Also has application for evaluation of man-made structures

Contd


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Cont d

Advantages:

Typical measurements are less than 100 feetbut caneasily made to greater depths, if necessary

Can resolve up to 3 to 4 layers

Can provide depthunder each geophone

Both P andS waves can be determined

The source of seismic energycan be as simple as 10pound sledge hammer

Contd


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Disadvantages:

The survey line length(source to farthest geophone) may be4 to 5 times the desired depth of investigation

Requires intrusive contact with theground

Stationmeasurement only

Sensitiveto acoustic noiseandvibrations

Seismic velocityof layers must increase with depth

Will not detectthin layers orlayers with inverted velocities

Deepermeasurementswill require explosivesas an energysource

Subsurface profile generated (2D image): Fault detection


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p g ( g )

227.52 m/s

1852.67 m/s

3006.53 m/s

4452.92 m/s

828.82 m/s

Possible

Fault


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Seismic Reflection

The seismic reflectiontechniquemeasuresthe travel time ofseismic wavesfrom theground surface downward to a geologiccontact where part of the seismic energy is

reflectedback to geophones at the surfacewhile the rest of the energy continues to thenext interface.

The travel timeof the seismic waveisafunction of soil androck density andhardness.

Schematic diagram of seismic reflection


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Schematic diagram of seismic reflection

Contd


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Cont d

Uses:

Primary applicationis for determination of

depthandthicknessofgeologic strata,structuralandanomalous conditions.

Contd


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Advantages:

Provides a high resolution cross section(as compared to

refraction) of soil/rock along profile line

The high resolution methoduses frequencies of up to a few 100Hz

Measurementscan be madefrom about 50 feet to a few 1,000feet deep

Measurementsto these depthscan often be made withoutexplosives, often using a 10 pound sledge hammer as a seismicsource

The survey line length(source to farthest geophone) is usually 1to 2 times the desired depth of investigation(much less than thatrequired for refraction measurements)

Both P and S wavescan be measured.

Contd


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Cont d

Disadvantages:

Requires intrusive contactwith the ground

Stationmeasurement only

Sensitiveto acoustic noise andvibration

Can require extensive processing


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Wave velocity in various soils & rock

Type of soil/rock P-wave velocity m/sec

Soil:

Sand, dry silt, fine grained top soil 2001 000

Alluvium 5002 000

Compacted clays, clayey gravel, dense clayey sand 1 0002 500

Loess 250 - 750

Rock:

Slate and shale 2 5005 000

Sandstone 1 5005 000

Granite 4 000

6 000

Sound limestone 5 00010 000


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P-Wave velocities of common soil materials

Material P-Wave Velocities (m/ s)

Air 330

Water 1450-1530

Petroleum 1300-1400

Loess 300-600

Soil 100-500

Snow 350-3000

Solid Glacial Ice 3000-4000

Sand (loose) 200-2000

Sand (dry, loose) 200-1000

Sand (Water Saturated, loose) 1500-2000

Glacial Moraine 1500-2700

Sand and Gravel (near surface) 400-2300

Sand and Gravel (2 km depth) 3000-3500

Clay 1000-2500

Estuarine Muds/ Clay 300-1800

Floodplain Alluvium 1800-2200

Permafrost (Quartenary sediment) 1500-4900

Sandstone 1400-4500

Limestone (soft) 1700-4200

Limestone (hard) 2800-7000

Dolomites 2500-6500

Anhydrite 3500-5500

Rock salt 4000-5500

Gypsum 2000-3500

Shales 2000-4100

Granites 4600-6200

Basalts 5500-6500

Gabbro 6400-7000

Peridotite 7800-8400

Serpentinite 5500-6500

Gneiss 3500-7600

Marbles 3780-7000

Sulphide ores 3950-6700

Pulverised fuel ash 600-1000

Made Ground 160-600

Land fill refuse 400-750

Concrete 3000-3500

Disturbed soil 180-335

Clay landfill cap (compacted) 335-380

Determination of s bs rface profile sing seismic refraction method


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Determination of subsurface profile using seismic refraction method

D t i ti f b f fil


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Determination of subsurface profile

using seismic reflection method

D t i ti f b f fil


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using seismic refraction method

Determination of subsurface profile using seismic refraction method


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using seismic refraction method

Determination of subsurface profile and geological


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Determination of subsurface profile and geological

structure using seismic refraction method


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Gravity

Gravity measurementsdetect changes in

the earth's gravitational fieldcaused by

local changes in thedensity of thesoil androck orengineered structures.

Sk t h f it it


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Sketch of gravity survey over cavity


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Gravity survey

Contd

U


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Uses:

Standard gravity measurementsare primarilyapplied to characterizing geologic structureusing widely spaced stations(100's to 1,000's offeet apart).

Microgravity measurementscan be used tocharacterize detailed localized geologicconditions(such as bedrock channels, caves,and abandoned tunnels and mines) usuallywithin the upper few 100 feet.

Microgravity uses closely spaced stations(a fewfeet to about 50 feet) and a micro gravimeter(capable of reading to a few microgals).

Contd


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Advantages:

Provides a meansto characterize conditions ingeologicandcultural environments, whereothergeophysical methods may fail

Does not require intrusive ground contact

Data can be interpretedto provide estimates of

depth size andthe nature of theanomaly

Can be used inside buildingsandstructures

Contd


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Disadvantages:

Stationmeasurements only

Requires base stationfor drift corrections

Requires accurateelevation measurements

The processof making microgravitymeasurementsis a relatively slowandtedious

in thefield andrequires extensive processingandcorrections

Susceptible to culturalandnatural vibrations


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Q & A

End of the Chapter 7.