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CIVE 554/650Site Investigation Techniques
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Geotechnical Engineering
Soil
Water
Rock
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Key Soil Engineering PropertiesCompressibility
Settlement
StrengthAbility to carry load
Permeability (hydraulic conductivity)Flow of water through soils
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Settlement Failure
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Serviceability Requirements
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Shear Failure
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Palace of Fine Arts, MexicoA strange case of Palace of Fine Arts in the Alameda area of Mexico City. Built sometime between 1900 and 1934, it was a magnificent and strongly built structure. It was built on grade, level with the square and other buildings nearby. But because of loose sand permeated with water in the subsurface, the massive structure sunk 6 ft into the ground! (Luckily, it settled evenly minimizing structural damage.) Believe it or not, in the 1960's the building moved again. This time it moved 12 ft up! The weight of skyscrapers being built around the Palace had pushed the subsurface water and soil around sufficiently to raise the building.(Source: Why Buildings Fall Down, M. Levy and M. Salvadori, WW Norton & Company, 1992)
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Permeability
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Site Investigation (SI) ObjectiveObtain sufficient information to enable a safe and economic design to be made and to avoid any difficulties during construction
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SI RequirementsSequence, thickness and lateral extent of soil strata and where appropriate level of bedrockObtain representative samples of soils (and rock) for identification and classification and relevant soil parametersIdentify groundwater conditionsInsitu tests to access appropriate soil and/or rock characteristics
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How Much Site Investigation?
More extensive site investigation & testing program
Cost
($)
Cost of site investigation & testing
Cost of construction
Total project cost
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How Much Site Investigation?
More extensive site investigation & testing program
Cost
($)
Cost of site investigation & testing
Total project cost
Minimum project cost
Optimum SI cost
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Risks and ConsequencesWhat is the consequence of failure or changing ground conditions?What will it cost to fix? Are people life in danger?Economic and social costs?
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Soil SamplingCohesionless Soils
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Test Pits
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Hand tools(a) posthole auger;
(b) helical auger
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Drilling with continuous-flight augers(courtesy of Danny R. Anderson Consultants, El Paso, TX)
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Carbide-tipped cutting head on auger flight attached with bolt (courtesy of William B. Ellis, El Paso Engineering and Testing, Inc., El Paso, TX)
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Hollow-Stem Auger Components
(after ASTM,2001)
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Wash boring
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(a) Standard split-spoon sampler; (b) spring core catcher
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Configuration of Standard Penetration Test (SPT) hammers
(a) safety hammer;(b) donut hammer
(after Seed et al., 1985)
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Nvalue = number of blows (hits) it takes to drive a standard split spoon 300mm with a 623N hammer dropped a distance of 762mm
(ASTM D1586)
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N- valueNumber of blows to drive a SPT sample 300mm.
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N60
6060RSBH nnnNnN =
N60 = Field corrected Standard Penetration number
N = Measured penetration number
nH = Hammer efficiency
nB = Correction for bore diameter
nS = Sampler correction
nR = Correction for rod length
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Schmertmann’s(1975) correlation
betweenN60, σ0, and φ’
for granular soils
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Canadian Foundation Manual
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Allowable Bearing Pressureqallow ~ 10 N (kPa)
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Cohesionless Soils
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Cohesive Soils (TILL)
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Soil SamplingCohesive Soils
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Sampling devices (b) thin-walled tube; (c) and (d) piston sampler
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© 2004 Brooks/Cole Publishing / Thomson Learning™
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Soil TestsCohesionless Soils
Grain size distributionSpecific gravityDirect shearPermeabilityRelative density
Cohesive SoilsAtterberg LimitsHydrometerSpecific gravityConsolidation (oedometer)Unconfined compressionTriaxial test
Strengthpermeability
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Limitation of Field Collected SamplesSample disturbanceRemoval of insitu field stress
Sample unloaded
especially important in very soft (weak) soils that have a high insitu void ratio
NEED INSITU SOIL PROPERTIES as this is what structure will feel…..in the field
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Measurement of Insitu Properties
Ground Water Table Location
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Casagrande-type piezometer
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Groundwater Table Measurement
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Casagrande-type piezometer(courtesy of N. Sivakugan, James Cook University, Australia)
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Groundwater Table MeasurementOpen stand pipe with sand packStand pipe must be large enough to prevent meniscus effectsGround water table is where the water pressure is equal to the atmospheric pressure Pwater = Patmoshpere
GWT will vary over seasons and changes in atmospheric pressure. GWT measure wrt a bench mark – this must be stable..
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Measurement of Insitu Properties
Soft Cohesive Soils
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Shear Circular Cylinder of Soil
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Geometry of field vane
(after ASTM, 2001)
Measures Undrained Strength (Cu or Su)
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Field Vane Procedure
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Vanes and Rotation RateThe vanes have a rectangular shape and a height double their diameter, according to the recommendation included in the EUROCODE 7 (1977) and ASTM Standard Code (D 2573); in the latter one also vanes having a tapered end are allowed.The above mentioned codes prescribe that the rotation must be carried out at a rate of 0.1- 0.2 degrees/sec., that is (6 –12 degrees/min).
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Rotation RateRequired rate never happens in the field unless it is mechanized.It is difficult to get driller to take one minute to do the testMake sure it is done consistently…
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Determination of SuUndrained shear resistance at failure (Su) is calculated by the maximum torque required to cut the soil included into the cylinder obtained rotating the vane blades.The general formula, referred to rectangular vanes having height (H) and diameter (D), is:
Su = T /((π D3/2) (H/D + a/2)) (1)where:
T = maximum applied torque (deducted any friction).a = factor depending by the assumed shear stress distribution at the ends of the cylinder obtained rotating the vane blades and amounting to 2/3 for uniform shear stress.
For rectangular vanes having H/D = 2, equation (1) is reduced to:Su = 6T / 7π D3 = 0.273T / D3 (2)
The remoulded shear strength value is calculated using the above mentioned formula (2) introducing the value of the torque, free from any friction, measured after some vane rotation turns, that is when the soil offers a fast constant resistance.
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Undrained Strength & Effective Stress
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Field Vane Corrections
)()( fielducorrectedu SS λ=
λ =correction factor
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Figure 2.18 Variation of λ with cu(VST)/σ’0 [see Eq. (2.31)]
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Variation of preconsolidation pressure with field vane shear strength(after Mayne and Mitchell, 1988)
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[ ] 83.0)(04.7' fielduc S=σ
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Variation of β with plasticity index(after Mayne and Mitchell, 1988)
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™o
fielduSOCR
')(
σβ=
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Measurement of Insitu Properties
SANDS and CLAY
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Mechanical friction-cone penetrometer
(after ASTM, 2001)
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Electric friction-cone penetrometer (after ASTM, 2001)
© 2004 Brooks/Cole Publishing / Thomson Learning™
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Friction Ratio (Fr)
c
c
qf
ceresisconeceresisfrictionalFr ==
tan tan
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Penetrometer tests with friction measurement(after Ruiter, 1971)
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Variation of qc, σ’0, and Dr for
normally consolidated quartz sand
(based on Baldi et al., 1982and Robertson and Campanella, 1983)
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Variation of qc with σ’0 and φ’ in
normally consolidated quartz
sand(after Robertson and Campanella, 1983)
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Correlation between qc, Fr, and the type of soil
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General range of variation of qc/N60 for various types of soil(after Robertson and Campanella, 1983)
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Measurement of Insitu Properties
Pressure Meters
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(a) Pressuremeter; (b) plot of pressure versus total cavity volume
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Dilatometer and other equipment
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a) Schematic diagram of a flat-plate dilatometer;(b) dilatometer probe inserted into ground
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Dillatometer test result conducted at Porto Tolle, Italy (after Marchetti, 1980)
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Chart for determination of soil description and
unit weight(after Schmertmann, 1986)
Note: 1 t/m3 = 9.81 kN/m3
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Rock Sampling
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Rock coring(a) single-tube core barrel;
(b) double-tube core barrel
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Casing and Core Sizes
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Rock Drilling
100run drill oflength
recovered core covRe xlengtheryCore =
run drill oflength longer or 100mm pieces core of lengthTotalRQD =
Rock Quality Designation (RQD)
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RQD
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Expect the Unexpected
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Peck, Hanson and Thornburn
Foundation Engineering (Second Edition)1974 Wiley & Son
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Expect the UnexpectedWaste fillClaySandGravel/cobble seamsBouldersChanging ground conditionsOrganics
DESIGN BASED ONLIMITED BOREHOLE DATA
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Expect the Unexpected
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Expect the Unexpected
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Expect the UnexpectedSand deposits discovered during hand carving slabs of clay during construction of Chicago subway systemOrigin of formation unknown
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Expect the UnexpectedPermeability profile glacial deposit Chicopee, Mass USA (Terzaghi, 1929)Similar problem Oakridge moraine Markham, ON
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Expect the Unexpected
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Expect the Unexpected
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Expect the Unexpected
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New York City
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UW Field Test Site
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Test Pit Locations
BA
A
B180 ft
8 in dia. HDPE pipe
excavation locations
Site trailer
Cross Section
Plan View
West face
East face
Pipe ExitPipe Entrance
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Clay and Sand
Bore limit
Slurry
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Typical Soil Stratigraphy
Clayey SILT
SILTClay lens
SAND
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Subsurface SoilsGrain Size Distribution
0
10
20
30
40
50
60
70
80
90
100
0.001 0.01 0.1 1 10 100Particle Size (mm)
Perc
ent P
assi
ng
SILT with varying sand content trace gravel and clay
SAND with trace gravel, silt and clay
clay lens
Clayey SILT with some sand