CIE 409 기초공학 1
lecture 02
subsoil exploration
CIE 409 기초공학 2
Subsoil explorationDefinition of subsurface exploration
The process of identifying the layers of deposits that underlie a proposed structure and their physical characteristics
Purpose of subsurface explorationSelecting the type and depth of foundationEvaluating the load-bearing capacity of the foundationEstimating probable settlementWater table location
….Phase of exploration
Collection of preliminary informationObtaining information regarding the type of structure to be builtObtaining general idea of the topography and the type of soil to be encountered
ReconnaissanceVisual inspection of the site
Site investigationPlanning, making test boreholes, and collecting soil samples at desired intervals for subsequent observation and laboratory tests
e.g. depth of the boring a) depth at which the effective stress increase is equal to (1/10)q
CIE 409 기초공학 3
Measurement of Soil Properties
Seismic Refraction
Seismic Reflection
Rayleigh Wave, SASW
Surface Seismic Surveys
Suspension Logging
Seismic Cross-Hole
Seismic Down (up)-Hole
Seismic Cone
Borehole Methods
Low Strain Tests
SPT
Becker Hammer
Cone
Dilatometer
Pressuremeter
High Strain Tests
Field Methods
Resonant Column
Ultrasonic Pulse
Bender Element
Low Strain Tests
Cyclic Triaxial
Cyclic DSS
Cyclic Torsional Shear
High Strain Tests
Laboratory Methods
Shaking Table
Centrifuge
Model Tests
Field ObservationsInstrumented Sites
Measurement Methods
CIE 409 기초공학 4
In Situ Testing Devices
http://www.ce.gatech.edu/~geosys/Faculty/Mayne/Research/index.html
CIE 409 기초공학 5
Standard Penetration Test , SPT
http://www.ce.gatech.edu/~geosys/Faculty/Mayne/Research/index.html
CIE 409 기초공학 6
SPT
SPTNumber of blow counts required for the sampler having an external diameter of 50.8 mm and a length of 76 cm to penetrate three successive penetrations of 15 cm using a drop hammer weighing 63.5 kg dropped from a height of 76 cm. The number of blows for the first 15 cm is disregarded because of the likely disturbance at the bottom of the drill hole. The numbers for the second and third 15 cm increments (total 30 cm) are added and designated the standard penetration resistance, N.Performed every 1.5 - 2m.The efficiency, representing the ratio of the energy that reaches the rod to the available potential energy of the hammer, ranges from about 45 – 78%. In general, most correlations are based on N values corresponding to the efficiency of 60%, and termed N60.
CIE 409 기초공학 7
Corrections to SPT N-value
(N1)60 = CN N60 = Energy-Corrected SPT Value normalized to an effective overburden stress of one atmosphere : (N1)60 = (N60) / (vo’)0.5 with stress given in atm. (Note: 1 atm = 1 bar = 100 kPa).
N1/60=NmCnEm/(0.6Eff)
6060RSBHN
N
CIE 409 기초공학 8
Corrections to SPT N-value
Effect Variable Term Value
Overburden Stress
svo' CN(Pa/svo')0.5 but < 2
Energy Ratio1
· Safety Hammer· Donut Hammer· Automatic Hammer
H 0.6 to 0.850.3 to 0.60.85 to 1.0
Borehole Diameter
· 65 to 115 mm· 150 mm· 200 mm
B1.001.051.15
Sampling Method
· Standard sampler· Sampler without liner
S1.0
1.1 to 1.3
Rod Length
· 10 m to 30 m· 6 to 10 m· 4 to 6 m· 3 to 4 m
R1.00.950.850.75
Particle Size
Median Grain Size (D50) of Sand in mm
P60 + 25 log D50
Aging Time (t) in years since deposition
A1.2 + 0.05 log (t/100)
Overconsolidation OCR OCROCR0.2
1 Obtain by energy measurement per ASTM D4633
http://www.ce.gatech.edu/~geosys/Faculty/Mayne/Research/index.html
CIE 409 기초공학 9
SPT - Correlations
Correlation with Dr (equations in page 87-88)Correlation with ’ of sands (equations in p. 88-89)Correlation with clay properties Don’t use!Correlation with consistency and unconfined compression
CIE 409 기초공학 10
Method of Exploration - Sampling
Split spoon samplingThe extracted soil is first inspected by the inspector, classifies the material, places a portion of the sample in a glass jar that is covered tightly and shipped to the engineer for visual inspectionClay: can retain a part of the characteristics of the undisturbed soilSand: compacted through sampling, hence the relative density cannot be determinedAble to obtain information on the stiffness of the soil in situ through use of the standardized procedure in driving the split spoon sampler (SPT)
.
2
220(%)
i
ir D
DDA
CIE 409 기초공학 11
Method of Exploration - Sampling
Thin wall tube (Shelby tube)Made of seamless steelUsed to extract undisturbed clayey soilsOutside diameters of 50.8mm (2 in.) and 76.2mm (3 in.)Used for laboratory test (consolidation and shear tests)
CIE 409 기초공학 12
CPT (Cone Penetration Test)
A CPT device consists in a cylindrical probe with a cone-shaped tip with different sensors that allow a real time continuous measurement of soil strength and characteristics by pushing it into the ground at a speed of 2 cm/s. The typical CPT probe measures the stress on the tip, the sleeve friction and the pore water pressure. Some of them are equipped with a geophone in order to be able to perform shear wave velocity measurements.The data is normally read by a field computer that displays it real-time and stores it at regular depth intervals. Measurements can be taken at any intervals desired.
http://www.ce.gatech.edu/~geosys/Faculty/Mayne/Research/index.html
CIE 409 기초공학 13
Cone Penetration Testing (CPT)
Real-Time readings in computer screenPenetration at 2 cm/s
Sand
Clay
Buried Crust
Clay
http://www.ce.gatech.edu/~geosys/Faculty/Mayne/Research/index.html
CIE 409 기초공학 14
Cone Penetrometer Configuration
http://www.ce.gatech.edu/~geosys/Faculty/Mayne/Research/index.html
CIE 409 기초공학 15
CPTCone penetration test (CPT)
Originally known as the Dutch cone penetration test60 cone with a base area of 1000mm2 and diameter of 35.7mm attached to the lower end of a 15mm rod surrounded by a 20mm pipe. The cone is pushed 0.5 m into the ground at a rate of 1 - 2 cm/s. The pressure exerted on the rod is plotted against depth. Measures a) cone resistance (qc) and b) frictional resistance (fc)Modified CPT: Electronic cone and Mechanical cone
Correlation with N60 for sands
CIE 409 기초공학 16
Method of Exploration - CPTCorrelation with undrained shear strength
Nk = bearing capacity factor (Nk = 15 for an electric cone)(Nk = 10 - 20)vo = total vertical stress’vo = effective vertical stress
kv
vc
v
u
Nqs 1
'' 0
0
0
• Correlation with ’ of sands• Correlation with Dr• Correlation with ’p, OCR• Type of soil (Fig. 2.23) Fr = fc/qc