introduction to subsurface exploration
TRANSCRIPT
Microsoft PowerPoint - 02bSubsurfaceExplorationIntroduction 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
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.
-
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
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
Disturbed samples Split spoon sampler Standard penetration test Soil sampler (sand, silt, peat)
Undisturbed Samples Shelby tube (Thin wall tube) Piston sampler
Rock Cores
Standard Penetration Test Split-Spoon Sampler
~ 2’ in length 2” O.D. 1.5” I.D. w/o liner 1.375” I.D. w/ liner Area ratio = (Do
2-Di 2)/Di
2 ~ 110%.
140#, 30”, (6”, 6”, 6”) Typically 4 in top 10’, then every 5’
Area ratio < 10% considered undisturbed
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
Impregnation A substance that would harden (gel) with little
to no expansion.
Coring of rock
rock cored) Rock Quality Designation (RQD) Sum(length of recovered pieces >=
4”)/(theoretical length of rock cored)
Rock drilling
SPT
SPT-N (bpf)
D e
p th
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
• Disadvantage – Operate dependent
– Accuracy is poor
N
DR = relative density γT = unit weight LI = liquefaction index φ' = friction angle c' = cohesion intercept eo = void ratio qa = bearing capacity σp' = preconsolidation Vs = shear wave E' = Young's modulus Ψ = dilatancy angle qb = pile end bearing fs = pile skin friction
SAND
cu = undrained strength γT = unit weight IR = rigidity index φ' = friction angle OCR = overconsolidation K0 = lateral stress state eo = void ratio Vs = shear wave E' = Young's modulus Cc = compression index qb = pile end bearing fs = pile skin friction k = permeability qa = bearing stress CLAY
Is One Number Enough???
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)
Mobile 25-tonne rigs with enclosed cabins to allow testing under all weather conditions
Cone Trucks
CPT Profile
0
4
8
12
16
20
24
28
qt
ub
fs
0
4
8
12
16
20
24
28
SPT-N (bpf) and qc (MPa)
D e
p th
Measurements Tip resistance Sleeve friction Water pressure Others
CPT
Applications Soil identification Granular soil: Dr, φ, E, Liquefaction Cohesive soil: Su, OCR
Robertson and Campanellas correlation (1983) between qc, Fr, and soil type
SPT-N vs. CPT-qc
For NC quartz sand
• Disadvantage – Doesn't work in
60o
fs
qc
Vs
u1
u2
Cone Tip Stress, qt Penetration Porewater Pressure, u Sleeve Friction, fs Arrival Time of Downhole Shear
Wave, ts
Seismic Piezocone Test
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
Disadvange Su is the only application
Pressuremeter Test (PMT)
Measurement Pressure-deformation relationship
Pressuremeter Test (PMT)
Pressuremeter Test (PMT)
Pressuremeter Test (PMT)
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)
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
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.
-
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
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
Disturbed samples Split spoon sampler Standard penetration test Soil sampler (sand, silt, peat)
Undisturbed Samples Shelby tube (Thin wall tube) Piston sampler
Rock Cores
Standard Penetration Test Split-Spoon Sampler
~ 2’ in length 2” O.D. 1.5” I.D. w/o liner 1.375” I.D. w/ liner Area ratio = (Do
2-Di 2)/Di
2 ~ 110%.
140#, 30”, (6”, 6”, 6”) Typically 4 in top 10’, then every 5’
Area ratio < 10% considered undisturbed
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
Impregnation A substance that would harden (gel) with little
to no expansion.
Coring of rock
rock cored) Rock Quality Designation (RQD) Sum(length of recovered pieces >=
4”)/(theoretical length of rock cored)
Rock drilling
SPT
SPT-N (bpf)
D e
p th
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
• Disadvantage – Operate dependent
– Accuracy is poor
N
DR = relative density γT = unit weight LI = liquefaction index φ' = friction angle c' = cohesion intercept eo = void ratio qa = bearing capacity σp' = preconsolidation Vs = shear wave E' = Young's modulus Ψ = dilatancy angle qb = pile end bearing fs = pile skin friction
SAND
cu = undrained strength γT = unit weight IR = rigidity index φ' = friction angle OCR = overconsolidation K0 = lateral stress state eo = void ratio Vs = shear wave E' = Young's modulus Cc = compression index qb = pile end bearing fs = pile skin friction k = permeability qa = bearing stress CLAY
Is One Number Enough???
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)
Mobile 25-tonne rigs with enclosed cabins to allow testing under all weather conditions
Cone Trucks
CPT Profile
0
4
8
12
16
20
24
28
qt
ub
fs
0
4
8
12
16
20
24
28
SPT-N (bpf) and qc (MPa)
D e
p th
Measurements Tip resistance Sleeve friction Water pressure Others
CPT
Applications Soil identification Granular soil: Dr, φ, E, Liquefaction Cohesive soil: Su, OCR
Robertson and Campanellas correlation (1983) between qc, Fr, and soil type
SPT-N vs. CPT-qc
For NC quartz sand
• Disadvantage – Doesn't work in
60o
fs
qc
Vs
u1
u2
Cone Tip Stress, qt Penetration Porewater Pressure, u Sleeve Friction, fs Arrival Time of Downhole Shear
Wave, ts
Seismic Piezocone Test
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
Disadvange Su is the only application
Pressuremeter Test (PMT)
Measurement Pressure-deformation relationship
Pressuremeter Test (PMT)
Pressuremeter Test (PMT)
Pressuremeter Test (PMT)
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)