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
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
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
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
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)
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
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
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
Undisturbed Samples
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
~ 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
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
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.
Impregnation A substance that would harden (gel) with little to no expansion.
Coring of rock
Core barrel + Coring bit
Single tube Double 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 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.
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
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
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
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
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
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
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
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