Methods of site characterization

• Drilling & Sampling– For visual identification and characterization through laboratory

test results • Soil Borings• Rock Coring

• In-Situ Tests– Characterization through parameters of in-situ tests

• Standard Penetration Test (SPT)• Cone Penetration Test (CPT + CPTu)• Flat Plate Dilatometer (DMT)• Pressuremeter (PMT)• Vane Shear (VST) – suitable when undrained shear strength is up

to about 75 kPa (Geoguide 2)

• Geophysical Methods– Characterization through parameters of geophysical methods

• Mechanical Waves (P-, S-, R-waves)• Electromagnetic (radar, resistivity, dielectric)

Geological Model

Basic Principles

• Cross-sections and Fence diagrams

• Interpretation:

– Layers (like pancakes)

– Youngest at the top

– Oldest at the bottom

• Top-down Interpretation

• Bottom-up interpretation

GEOLOGICAL MODEL - FENCE DIAGRAM

GEOLOGICAL MODEL TOP-DOWN APPROACH

GEOLOGICAL MODEL – BOTTOM-UP APPROACH

Soil Sample Quality Classification

SPT sampler

Causes of Soil Disturbance

In-situ Geotechnical Tests for soils (Mayne, 2003)

Standard Penetration Test (Mayne, 2003)

SPT test procedure

Split Barrel Sampler

Test Termination and Refusal

• The SPT can be halted when 100 blows has been achieved or if the number of blows exceeds 50 in any given 150-mm increment, or if the sampler fails to advance during 10 consecutive blows.

• Refusal:– SPT refusal is defined by penetration resistances

exceeding 100 blows per 51 mm (100/2"). – ASTM D 1586 (Standard Test Method for Penetration

Test and Split-Barrel Sampling of Soils) has re-defined this limit at 50 blows per 25 mm (50/1").

Corrections to SPT ‘N’

• Hammer efficiency correction (N60)

• Overburden Pressure correction

• Dilatancy correction

• Drill rod length correction

• Borehole diameter correction

• Sampler lining correction

Hammer Efficiency (Energy) Correction

• Various types of hammers (drop hammers, automatic trip hammers, etc.) are in use all over the world

• Hammer Efficiency varies from 35%-85% for various hammers

• In practice, blow count is generally adjusted for 60% energy efficiency, if no measurements are available

• If energy efficiency is measured, then Ncorrected=He/60*Nmeasured

Overburden Correction

N60 is Energy corrected standard penetration number, and CN is the overburden correction,

corresponds to a standard effective overburden pressure of 96 kPa.

Liao and Whitman (1986)

'

178.9

oNC

Skempton (1986)

'01.01

2

oNC

For granular soils, the SPT ‘N’ is highly dependent on the effective overburden pressure

(N1)60=CN*N60

Dilatancy correction• In saturated fine dense or very dense sand deposits, the

observed ‘N’ value may be greater than the actual value, due to the tendency of such materials to dilate during undrained shear conditions.– Terzaghi and Peck recommended that N’=15+1/2(No-15)

• N’ and No are corrected and observed SPT values

• The above correction is based on that critical void ratio occurs at approximately N=15, and in fine grained cohesionless soils, the coefficient of permeability is so low that the excess pore water pressure developed by the driving impedes the penetration of the split sampler, thus increasing the SPT ‘N’ value.

Correlations with shear strength parameters

For clayey soils

72.058.0 Np

q

a

u

Kulhawy and Mayne (1990)

For granular soils

2' 00054.03.01.27deg corcor NN

Wolff (1989)

34.0

'

1'

3.202.12

tan

a

o

F

p

N

Kulhawy and Mayne (1990)

qu=unconfined compressive strength

pa=Atmospheric pressure

Parameter Determination (Sand & Silts)

State N*

(blows/ 300mm)

Friction angle, deg

Relative

Density (%)

Very Loose <4 <30 <15

Loose 4 - 10 30 – 32 15 - 35

Medium Dense 10 - 30 32 – 35 35 - 65

Dense 30 - 50 35 – 38 65 - 85

Very dense >50 >38 85 - 100

Parameter Determination (Clays)

Consistency N qu (kPa) AS1726

Very soft <2 <25

Soft 2 – 4 25 –50

Firm 4 –8 50 – 100

Stiff 8 –15 100 – 200

Very stiff 15 –30 200 – 400

Hard >30 >400

su = qu/2 Based on Peck et al (1974)

In-situ Geotechnical Tests for soils (Mayne, 2003)

Cone penetration test (CPT)

A CPT sounding is made by pushing a 3.6-centimeter-diameter probe (cone) into the ground at 2 cm/s, to depths ranging from 15 to 30 meters.

Hydraulic rams are used to apply the full 23-ton weight of the CPT truck to push the probe rods to depth.

Continuous measurements of resistance to penetration of the tip and the frictional sliding resistance of the sleeve of the cone are made, digitized even at 5-mm depth intervals,

Permits detailed inferences about stratigraphy and lithology, using charts that compares these two measurements with the known physical properties of various soils.

CPT

Very effective in-situ test for soil investigation to measure the strength soils and soft rocks and to get a continuous soil profile

Widely accepted for determining geotechnical engineering properties and delineating soil stratigraphy in unconsolidated soil materials.

The resolution of the CPT in delineating stratigraphic layers is related to the diameter of the cone tip, with typical cone tips having a diameter of either 10 or 15 cm².

Cone penetrometer

Cone penetrometer

CPT test rigs

Typical CPT measurementsTip resistance, qc (MPa) Sleeve friction, fs (kPa) Pore water pressure, ub (MPa)

Standard correlations

avo

acTC

p

pq

/

/log0.116.17 10

100c

sf q

fR