introduction to subsurface exploration - chihping lin...

Introduction to Subsurface Exploration

Introduction to Subsurface Exploration

ObjectivesPlanningTest PitsSoil BoringsSoil/Rock SamplingIn Situ Tests

Site CharacterizationDefine objectives of explorationBackground studyDesign subsurface exploration program

Boring # and depthSampling # and depthIn-situ testing methods and #

Characterize Soil and RockDevelop idealized soil profilePerform monitoring instrumentation

Objectives

Objectives

Get Stratigraphy and G.W.TDetermine engineering properties

In Situ TestsDisturbed samples, index testsUndisturbed samples, Lab tests

Background study

Background study

Planning

Boring # and DepthRelated to (a) knowledge of site conditions, (b) Type of foundationIn general, clay deposit produce more well defined strata and sand can be more locally variableAllow for cross sections

Planning (contd)# of borings

Rule of thumb: 1 boring per 2500 ft2 of building area

Approximate spacing of boreholes

Planning (contd)Depth of borings

Depth > 2B (Strength concern)Depth D1 at (v / q) < 0.1 Depth D2 at (v / v0) < 0.05 Minimum Depth = min(D1, D2)In deep excavations, depth > 1.5 depth of excavation

65600m25000m21.523m1.5~2

Test PitsExamine soil strataGround Water TableSeepage ConditionRetrieve disturbed and undisturbed samplesPerform density and strength tests in situIdentify organic soil, bedrock ripability, potential borrow soils, etc.

Test Pits

Examine fault zone for evidence of recent movement.Identify slickensides evidence of slope movementLimitations:

Depth limit (10~20)Stability of WallsG. W. T.

Soil Borings

Hand Augur, Power AugurContinuous Flight AugurHollow Stem AugurRotary Drilling (Rotary Wash)Percussion DrillingWireline System

Hand Augur, Power Augur

Remove disturbed samplesDetermine soil profileLocate G.W.T.Limitations:

Above G.W.T. in granular soilsBelow G.W.T. must be med-stiff clayDifficult to penetrate dens sand and stiff-hard clay. Practical limit ~ 10

http://www.mastrad.com/mackit.htm

Hand Augur and Core Sampler

http://ewr.cee.vt.edu/environmental/teach/smprimer/core/coresmp.mov?

Continuous Flight Augur

Rapid drilling and disturbed samplesIn soils with some cohesionHole will collapse in granular and soft soils

Hollow Stem Augur

Hollow stem serve as a casing to keephole openCan get SPT test resultsCannot penetrate very strong soil orrock.Problem when sampling below G.W.T.

Rotary Drilling (Rotary Wash)

Can obtain all types of samples in soil & rock, undisturbed, disturbed, cores.Require relatively large expensive equipment.Require pumps for circulating fluid (mud)Holes requires stabilization

Rotary DrillingExcept circular water, there are two ways to keep

stabilizationCasing

Used in sands and gravels, and soft clay, esp. below G.W.T.Installation very slow, and removed can be very time consuming.

Mud SlurrySlurry may be from natural soils or slurry by adding BentoniteCant determine G.W.T.

Percussion drilling

FastNo samplesGravel

Wireline System

Soil/Rock Sampling

Disturbed samplesSplit spoon samplerStandard penetration testSoil sampler (sand, silt, peat)

Undisturbed SamplesShelby tube (Thin wall tube)Piston sampler

Rock Cores

Split Spoon Sampler

Standard Penetration TestSplit-Spoon Sampler

~ 2 in length2 O.D.1.5 I.D. w/o liner1.375 I.D. w/ linerArea ratio = (Do2-Di2)/Di2 ~ 110%.

140#, 30, (6, 6, 6)Typically 4 in top 10, then every 5

Area ratio < 10%considered undisturbed

Soil sampler

Undistrubed SamplerShelby Tube

2.5 and 3 are most commonArea ratio ~ 10%For sand, put a spring core catcher at the end of shelby tube

Piston SamplerThin wall tube with a piston, 50 mm~120 mmFor sensitive soils, this is betterThe presence of the piston prevent distortion by not admitting excess soilUse piston tube to achieve vacuum in sampler for extraction of sample

Shelby Tube

PistonSampler

Sample disturbance

Undisturbed sampling of sands

FreezingPiston with circulation tubes with nitrogen gas.

ImpregnationA substance that would harden (gel) with little

to no expansion.

Coring of rock

Core barrel + Coring bitSingle tubeDouble tube

Recovery Ratio(length of core recovered/theoretical length of

rock cored)Rock Quality Designation (RQD)Sum(length of recovered pieces >=

4)/(theoretical length of rock cored)

Rock drilling

In-Situ Geotechnical Tests for Soils

SPT

Representative SPT Profile

Downtown Memphis

0

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0 20 40 60 80 100

SPT-N (bpf)

Dep

th (m

eter

s)

1982 B11982-B3

1982-B5

Soil Profile

Silty Sand

Sandy Silt

Gravelly Sand

Desiccated OC Clay

Clayey Sand

OC Clay

Fill

Gravelly Sand

SPTMethodStandardizationN values are very dependent on

Equipment used (Em)Size of hole (Cb)Type of spoon lined or unlined (Cs)Length of rods (Cr)OperatorTo standardized find N60 60% of theoretical energy

N60=Em Cb Cs Cr N/0.6

SPT

Correction for overburden stressto standard of 1 tsf (~100 kPa)Nc = Cn NCn=0.77 log10(20 Pa/v)=f(v)(N60)1 = Cn N60, which is used in estimating many engineering parameters, particular for seismic design work.

SPT

Correction for ground watere.g. above G.W.T., N=30 for medium-dense silty fine sand, below G.W.T. N=45 because the soil is dilative and SPT cause negative u.Terzaghi recommended

(N60)1 =15+((N60)1 15)/2, for (N60)1 >15 and silty sands or find sands below G.W.T.

SPT Applications

Development of engineering propertiesGranular soil: Dr, , E, LiquefactionCohesive Soil: not much

Site specific correlations w/ lab test results is about all that can be done, although Cu and OCR have been related to SPT results.

Settlement and bearing capacity of granular soils

SPT

AdvantageInexpensiveAvailabilitySample obtainedHuge databaseAble to penetrate local hazard

Disadvantage Operate dependent Accuracy is poor Not good for gravel No continuous profile No good correlation for

clay

N

DR = relative densityT = unit weightLI = liquefaction index' = friction anglec' = cohesion intercepteo = void ratioqa = bearing capacityp' = preconsolidationVs = shear waveE' = Young's modulus = dilatancy angleqb = pile end bearingfs = pile skin friction

SAND

cu = undrained strengthT = unit weightIR = rigidity index' = friction angleOCR = overconsolidationK0 = lateral stress stateeo = void ratioVs = shear waveE' = Young's modulusCc = compression indexqb = pile end bearingfs = pile skin frictionk = permeabilityqa = bearing stress CLAY

Is One Number Enough???

Cone Penetrometers

Electronic Steel Probes with 60 Apex TipASTM D 5778 ProceduresHydraulic Push at 20 mm/sNo Boring, No Samples, No Cuttings, No SpoilContinuous readings of stress, friction, pressure

CPT

Cone Penetration Tests (CPT)

Mobile 25-tonne rigs withenclosed cabins to allowtesting under all weather conditions

Cone Trucks

CPT Profile

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28

0 20 40 60

qt (MPa)

Dep

th (m

eter

s)

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0 500 1000

fs (kPa) u b (kPa)

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-200 0 200 400 600 800

qt

ub

fs

Comparison CPT and SPTDowntown Memphis

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0 20 40 60 80 100

SPT-N (bpf) and qc (MPa)

Dep

th (m

eter

s)1982 B11982-B31982-B5CPT-qc (MPa)

Soil Profile

Silty Sand

Sandy Silt

Gravelly Sand

Desiccated OC Clay

Clayey Sand

OC Clay

Fill

Gravelly Sand

CPT

MethodMechanical Dutch coneElectric cone

MeasurementsTip resistanceSleeve frictionWater pressureOthers

CPT

ApplicationsSoil identificationGranular soil: Dr, , E, LiquefactionCohesive soil: Su, OCR

Robertson and Campanellas correlation (1983)between qc, Fr, and soil type

SPT-N vs. CPT-qc

qc-0-DrFor NC quartz sand qc-0-

For NC quartz sand

CPT

AdvantageContinuous profileAccuratePore water pressureInexpensiveFast

Disadvantage Doesn't work in

gravel No sample Limited penetration

depth

60o

fs

qc

Vs

u1

u2

Cone Tip Stress, qtPenetration Porewater Pressure, uSleeve Friction, fsArrival Time of Downhole Shear

Wave, ts

Obtains Four Independent Measurements with Depth:

Seismic Piezocone Test

Downhole Shear Wave Velocity

Anchoring SystemAutomated SourcePolarized WaveDownhole Vs

Vane Shear Test

Vane Shear Test

Vane Shear Test

Vane Shear Test

Used primarily to access the undrainedstrength of soft clay.Method

Borehole, pipe, Push and rotateRelate peak strength to undrained strength, SuRotate continued for 10~25 revolution to remold soil and then the residual strength is measured

Vane Shear Test

Assumptions in evaluationUndrainedIsotropicNo disturbance due to insertionNo progressive failure (perfect plasticity)

Su = k Tk: correction factor

Vane Shear Test

AdvantageFast and economicalReproducible in homogeneous depositsSignificant data baseVery good for estimating sensitvity

DisadvangeSu is the only application

Pressuremeter Test (PMT)

MethodPre-bore PMTSelf-boring PMT

MeasurementPressure-deformationrelationship


Evaluation of Pressuremeter test


ApplicationsEsitmating soil strength parametersA better approach is to use the PMT results directly for foundation design

Pressuremeter Test (PMT)Advantage

Stress-strain response obtainedKo is obtained (SBPMT better in this regard)Excellent tool for pile (esp. lateral load)

DisadvantageSoil stratigraphy must be known in advanceExcess pore water pressure not knownDependent on borehole disturbanceMore time consuming and expensiveMisleading if soil is highly anisotropic

Dilatometer Test (DMT)


MethodMeasurements

ThrustA-pressure (0.05 mm)B-pressure ( 1.1 mm)C-pressure (0.05 mm )Corrections for readings



ApplicationsSAND: Classification, Stratigraphy, Liquefaction, Dr, State parameter, Clay: Su, Kh, Coeff. Of consol., Stress history, M, E, G

Determination ofsoil descriptionand unit weight(Schmertmann,1986)

Dilatometer Test (DMT)Advantage

Simple and rapid, rugged, less disturbedGood for horizontal stress, OCRNearly continuous profile

DisadvantageLimited field exposureAvailability Difficult in hard soilThrust measurement complicates the systemNo sample obtained

Plate Load Test (PLT)

Evaluation of PLT (Sand)

Evaluation of PLT (Clay)

Screw plate test

introduction to subsurface exploration - chihping lin...

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