Introduction to Subsurface Exploration Introduction to Subsurface ...

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Design subsurface exploration program. ▫ Boring # and depth. ▫ Sampling # and depth. ▫ In-situ testing methods and #. ▫ Characterize Soil and Rock.
Introduction to Subsurface Exploration

Introduction to Subsurface Exploration „ „ „ „ „ „

Objectives Planning Test Pits Soil Borings Soil/Rock Sampling In Situ Tests

Site Characterization „ „ „

Define objectives of exploration Background study Design subsurface exploration program „ „ „

„ „ „

Boring # and depth Sampling # and depth In-situ testing methods and #

Characterize Soil and Rock Develop idealized soil profile Perform monitoring instrumentation

Objectives

Objectives „ „

Get Stratigraphy and G.W.T Determine engineering properties „ „ „

In Situ Tests Disturbed samples, index tests Undisturbed samples, Lab tests

Background study

Background study

Planning „

Boring # and Depth „

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Related to (a) knowledge of site conditions, (b) Type of foundation In general, clay deposit produce more well defined strata and sand can be more locally variable Allow for cross sections

Planning (cont’d) „

# of borings „

Rule of thumb: 1 boring per 2500 ft2 of building area Approximate spacing of boreholes

Planning (cont’d) „

Depth of borings

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

建築技術規則 建築技術規則65條規定:地基鑽探孔應均勻分佈 於基地內600m2鑽一孔,但每孔基地至少一孔, 如基地面積超過5000m2時,當地主管建築機關得 視實際情形規定孔數。鑽探深度如用版基時,應 為建築物最大基礎版寬之兩倍以上,或建築物寬 度之1.5~2倍;如為樁基或墩機時至少應達預計 樁長加3m。各鑽孔至少應有一孔之鑽探深度為前 項鑽孔深度之1.5~2倍。

Test Pits „ „ „ „

„ „

Examine soil strata Ground Water Table Seepage Condition Retrieve disturbed and undisturbed samples Perform density and strength tests in situ Identify organic soil, bedrock ripability, potential borrow soils, etc.

古地震研究-車籠埔斷層

新城斷層

Test Pits „

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

Depth limit (10’~20’) Stability of Walls G. W. T.

Soil Borings „ „ „ „ „ „

Hand Augur, Power Augur Continuous Flight Augur Hollow Stem Augur Rotary Drilling (Rotary Wash) Percussion Drilling Wireline System

Hand Augur, Power Augur „ „ „ „

Remove disturbed samples Determine soil profile Locate G.W.T. Limitations: „ „ „

Above G.W.T. in granular soils Below G.W.T. must be med-stiff clay Difficult 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 samples In soils with some cohesion Hole will collapse in granular and soft soils

Hollow Stem Augur „

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Hollow stem serve as a casing to keep hole open Can get SPT test results Cannot penetrate very strong soil or rock. Problem when sampling below G.W.T.

Rotary Drilling (Rotary Wash) „

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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 Drilling Except circular water, there are two ways to keep stabilization „ Casing „

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

Mud Slurry „

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Slurry may be from natural soils or slurry by adding Bentonite Can’t determine G.W.T.

Percussion drilling „ „ „

Fast No samples Gravel

Wireline System

Soil/Rock Sampling „

Disturbed samples „ „ „

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Undisturbed Samples „ „

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Split spoon sampler Standard penetration test Soil sampler (sand, silt, peat) Shelby tube (Thin wall tube) Piston sampler

Rock Cores

Split Spoon Sampler „

Standard Penetration Test „

Split-Spoon Sampler „ „ „ „ „

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~ 2’ in length 2” O.D. Area ratio < 10% 1.5” I.D. w/o liner considered undisturbed 1.375” I.D. w/ liner Area ratio = (Do2-Di2)/Di2 ~ 110%.

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

Soil sampler

Undistrubed Sampler „

Shelby Tube „ „ „

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

Piston Sampler „ „ „

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Thin wall tube with a piston, 50 mm~120 mm For sensitive soils, this is better The presence of the piston prevent distortion by not admitting excess soil Use piston tube to achieve vacuum in sampler for extraction of sample

Shelby Tube

Piston Sampler

Sample disturbance

Undisturbed sampling of sands „

Freezing Piston with circulation tubes with nitrogen gas.

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Impregnation A substance that would harden (gel) with little to no expansion.

Coring of rock „

Core barrel + Coring bit „ „

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Single tube Double tube

Recovery Ratio (length of core recovered/theoretical length of rock cored)

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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 SPT-N (bpf) 0

20

40

60

Soil Profile 80

100

0

Depth (meters)

4 8 12 16 20

1982 B1

Fill

1982-B3

Silty Sand

1982-B5 Sandy Silt Gravelly Sand Desiccated OC Clay

24

Clayey Sand

28

OC Clay Gravelly Sand

SPT „ „

Method Standardization N values are very dependent on „ Equipment used (Em) „ Size of hole (Cb) „ Type of spoon – lined or unlined (Cs) „ Length of rods (Cr) „ Operator „ To standardized find N60 – 60% of theoretical energy N60=Em Cb Cs Cr N/0.6

SPT „

Correction for overburden stress „ „

„

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

SPT „

Correction for ground water e.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 Δu. „ 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 properties Granular soil: Dr, φ, E, Liquefaction „ Cohesive 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. „

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Settlement and bearing capacity of granular soils

SPT „

Advantage „ „ „ „ „

• Disadvantage

Inexpensive Availability Sample obtained Huge database Able to penetrate local hazard

cu = undrained strength

– – – – –

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

Is One Number Enough???

γT = unit weight

DR = relative density

IR = rigidity index

γT = unit weight

φ' = friction angle

LI = liquefaction index

OCR = overconsolidation

φ' = friction angle

K0 = lateral stress state

c' = cohesion intercept

eo = void ratio

eo = void ratio

Vs = shear wave

qa = bearing capacity

E' = Young's modulus Cc = compression index

qb = pile end bearing

fs = pile skin friction

k = permeability

qa = bearing stress

CLAY

σp' = preconsolidation

SAND

N

Vs = shear wave

E' = Young's modulus Ψ = dilatancy angle qb = pile end bearing

fs = pile skin friction

Cone Penetrometers

ƒ ƒ ƒ ƒ ƒ

Electronic Steel Probes with 60° Apex Tip ASTM D 5778 Procedures Hydraulic Push at 20 mm/s No Boring, No Samples, No Cuttings, No Spoil Continuous readings of stress, friction, pressure

CPT

Cone Penetration Tests (CPT) Cone Trucks

Mobile 25-tonne rigs with enclosed cabins to allow testing under all weather conditions

CPT Profile qt (MPa)

Depth (meters)

0

fs ub qt

20

40

u b (kPa)

fs (kPa) 60

0

500

1000

-200

0

0

0

4

4

4

8

8

8

12

12

12

16

16

16

20

20

20

24

24

24

28

28

28

0

200 400 600 800

Comparison CPT and SPT Downtown Memphis SPT-N (bpf) and qc (MPa) 0

20

40

60

80

Soil Profile 100

0

4

1982 B1 1982-B3

Depth (meters)

8

CPT-qc (MPa) 12

16

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Desiccated OC Clay

28

OC Clay Gravelly Sand

Mechanical Dutch cone Electric cone

Measurements „

Gravelly Sand

Clayey Sand

Method „

Sandy Silt

24

CPT „

Silty Sand

1982-B5

20

„

Fill

Tip resistance Sleeve friction Water pressure Others

CPT „

Applications „ „ „

Soil identification Granular soil: Dr, φ, E, Liquefaction Cohesive soil: Su, OCR

SPT-N vs. CPT-qc Robertson and Campanellas correlation (1983) between qc, Fr, and soil type

qc-σ0'-Dr For NC quartz sand

qc-σ0'-ψ For NC quartz sand

CPT „

Advantage „ „ „ „ „

Continuous profile Accurate Pore water pressure Inexpensive Fast

• Disadvantage – Doesn't work in gravel – No sample – Limited penetration depth

Seismic Piezocone Test Obtains Four Independent Measurements with Depth: ‰ Cone Tip Stress, qt ‰ Penetration Porewater Pressure, u ‰ Sleeve Friction, fs ‰ Arrival Time of Downhole Shear Wave, ts

Vs fs u2 u1

60o

qc

Downhole Shear Wave Velocity

‰ Anchoring System ‰ Automated Source ‰ Polarized Wave ‰ Downhole Vs

Vane Shear Test

Vane Shear Test

Vane Shear Test

Vane Shear Test „

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Used primarily to access the undrained strength of soft clay. Method „ „ „

Borehole, pipe, Push and rotate Relate peak strength to undrained strength, Su Rotate continued for 10~25 revolution to remold soil and then the residual strength is measured

Vane Shear Test „

Assumptions in evaluation „ „ „ „

Undrained Isotropic No disturbance due to insertion No progressive failure (perfect plasticity)

Su = k T k: correction factor

Vane Shear Test „

Advantage „ „ „ „

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Fast and economical Reproducible in homogeneous deposits Significant data base Very good for estimating sensitvity

Disadvange „

Su is the only application

Pressuremeter Test (PMT) „

Method „ „

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Pre-bore PMT Self-boring PMT

Measurement „

Pressure-deformation relationship

Pressuremeter Test (PMT)

Pressuremeter Test (PMT)

Pressuremeter Test (PMT)

Evaluation of Pressuremeter test

Pressuremeter Test (PMT) „

Applications „ „

Esitmating soil strength parameters A better approach is to use the PMT results directly for foundation design

Pressuremeter Test (PMT) „

Advantage „ „ „

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

Disadvantage „ „ „ „ „

Soil stratigraphy must be known in advance Excess pore water pressure not known Dependent on borehole disturbance More time consuming and expensive Misleading if soil is highly anisotropic

Dilatometer Test (DMT)

Dilatometer Test (DMT) „ „

Method Measurements „ „ „ „ „

Thrust A-pressure (→0.05 mm) B-pressure (→ 1.1 mm) C-pressure (0.05 mm ←) Corrections for readings

Dilatometer Test (DMT)

Dilatometer Test (DMT) „

Applications „

„

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

Determination of soil description and unit weight

(Schmertmann,1986)

Dilatometer Test (DMT) „

Advantage „ „ „

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Simple and rapid, rugged, less disturbed Good for horizontal stress, OCR Nearly continuous profile

Disadvantage „ „ „ „ „

Limited field exposure Availability Difficult in hard soil Thrust measurement complicates the system No sample obtained

Plate Load Test (PLT)

Evaluation of PLT (Sand)

Evaluation of PLT (Clay)

Screw plate test