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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
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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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Application of resistivity survey to
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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Application of resistivity survey to
determine water boundaries
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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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Determination of subsurface profile
using seismic refraction method
Determination of subsurface profile using seismic refraction method
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Determination of subsurface profile
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Determination of subsurface profile
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.