lecture 02 subsoil explorationcontents.kocw.net/KOCW/document/2015/hanyang/parkdoohee/... · 2016-09-09 · CIE 409 기초공학 10 Method of Exploration - Sampling Split spoon sampling

CIE 409 기초공학 1

lecture 02

subsoil exploration


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


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


In Situ Testing Devices

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


Standard Penetration Test , SPT



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.


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


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



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


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


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)


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.



Cone Penetration Testing (CPT)

Real-Time readings in computer screenPenetration at 2 cm/s

Sand

Clay

Buried Crust

Clay



Cone Penetrometer Configuration



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


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

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lecture 02 subsoil explorationcontents.kocw.net/KOCW/document/2015/hanyang/parkdoohee/... · 2016-09-09 · CIE 409 기초공학 10 Method of Exploration - Sampling Split spoon sampling