CIE3032 지반공학및실험 Lecture 02 subsurface exploration 1

LECTURE 02

Subsurface exploration

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 2

Lecture outline

What is subsurface exploration?

In-situ test methods and soil samplingHigh strain methodsLow strain methodsSoil sampling

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 3

Definition of subsurface explorationThe 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

Subsurface exploration

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 4

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

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 5

In Situ Testing Devices

http://www.ce.gatech.edu/~geosys/Faculty/Mayne/Research/index.html

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 6

Standard Penetration Test , SPT

http://www.ce.gatech.edu/~geosys/Faculty/Mayne/Research/index.html

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 7

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.

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 8

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

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 9

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

1 Obtain by energy measurement per ASTM D4633

http://www.ce.gatech.edu/~geosys/Faculty/Mayne/Research/index.html

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 10

SPT - Correlations

Correlation with ’ of sands (Fig. 2.14, equations in p. 83-84)Correlation with Dr (Table 2.4, equations in page 83)Correlation with clay propertiesCorrelation with consistency and unconfined compression

CIE3032 지반공학및실험 Lecture 02 subsurface exploration 11

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

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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)