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ENCE 3610 Soil Mechanics

Lecture 3

Site Exploration and Characterisation

Field Exploration Methods

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Reference Standards for Exploration Techniques

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�SPT Test Procedure and Types

The SPT was originally developed in the 1900s and proceeds by driving a thick-walled, split-barrel (a.k.a., “split spoon”) sampler into the ground using incremental blows from a drop hammer. The sampler is driven a total of 18 inches into the ground. The number of blows required to drive the sampler the 12-inch vertical interval between 6 and 18 inches is referred to as an N value or the blow count. The procedure is presented in ASTM D1586, and a schematic of the SPT is shown in the following slide.

The SPT provides a disturbed sample of the tested material and generates useful data that can be used to correlate to many engineering properties. Many factors can affect the SPT results, and there are several vastly superior in situ testing methods. Nevertheless, the test is still almost universally referenced and often required in the United States. One reason for this is the large amount of historical (i.e., legacy) data available. Numerous correlations have been published (see Chapter 8) and their use along with SPT represents the Standard of Practice in many parts of the country. (UFC 3-220-10)

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Advantages and Disadvantages of SPT Test

SPT Tests are generally corrected in two stages:

Mechanical aspects of hammer and boring (borehole size, efficiency of hammer, etc.)
Overburden (effective or intergranular) stress

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Correction of SPT Results

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SPT Efficiency Correction Factors�(without overburden correction)

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SPT Correction Example�(without overburden)

Given

SPT reading 20m below ground surface
Readings: 10 – 15 – 12
Using Automatic Hammer, 80% efficiency
Borehole diameter = 100 mm
Standard Sampler
Rod length at least distance below ground surface

Find

Corrected N₆₀ SPT reading

Solution

N-value = sum of last two 6” blow counts = 15 + 12 = 27
Correction Factors

E_f = 80, for automatic hammer (efficiency of the hammer=80%), so C_E=80/60=1.333

All other correction factors C_R=C_S=C_B=1
N₆₀ = (27)(1.333)(1)(1)(1) = 36 blows/ft (or blows/300 mm)

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Correction for Overburden

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SPT Correction for Overburden

Applied after the other correction factors
Only applied to an N₆₀ SPT result��
Example

N₆₀ = 20
σ’_z = 2.5 ksf

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SPT Correlations for Soil Properties

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Other Field Tests

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Cone Penetrometer Test (CPT)

Direct reading of soil resistance – both lateral and direct – is possible with 280 mm long cone penetrometer
Allows considerable data to be obtained in a short time
Does not allow for sample recovery
Friction Ratio:��R_f = f_c/q_c

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Types of CPT Devices and Correlations

Types

Dutch (Delft) mechanical cone penetrometer
Fugro Penetrometer

Electrical Cone Penetrometer
Piezocone Penetrometer

Dynamic cone penetrometer

Correlations

Modulus of elasticity (sands and clays)
Undrained Shear Strength (sleeve and cone resistance)
SPT
Soil Classification (since no sample recovery, should be used with another technique which allows visual inspection of soil samples
D₅₀

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Soil Classification Using CPT

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CPT Soil Classification Example

Given

CPT Result
q_c = 19.1 MPa
R_f = 1 %

Find

Soil classification solely based on CPT Results

Solution

First: 19.1 MPa = 19,100 kPa

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Menard Pressuremeter

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Marchetti Dilatometer

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Vane Shear Test

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Soil Boring Logs

Probably the most common – and thus the most important – method of communicating the results of soil borings used
Lack of adequate soil borings is the most common reason problems arise in construction of foundations
Gives a profile of the soil and rock types and qualities as a function of depth

Soil Borings Should Include:

Site Location Map
Location of Borings w/respect to proposed structure
Boring Logs
Laboratory Test Results
Other Special presentations

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Sample Log

Description of location of boring and site, and sometimes conditions during test
SPT or other sampling results
Phreatic Surface (water table)
Types of soils, usually with classification

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CPT Boring Results

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Boring Location Plan and Interpreted Subsurface Profile

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Geophysical Methods

Based on methods used for geologic research, especially in the oil and gas industry
Acoustic Methods

Seismic Refraction
Crosshole Tests
Downhole Tests
Spectral Analysis of Surface Waves

Electromagnetic Wave Methods

Electric Resistivity
Ground Penetrating Radar
Magnetic or Electromagnetic Surveys

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Seismic Refraction

Shock is made at one point, usually using a special hammer
Sound waves (specifically “p” compression waves) travelling through the soil are picked up at various distances from the hammer blow point by geophones
Analysing the reflections can indicate the depth and nature of the soil layers

Should not be used where a hard layer overlays a softer layer; will indicate two layers being one
Should not be used in an area covered with asphalt or concrete
Should not be used when a frozen layer is at the surface

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Seismic Refraction Example (Single Layer)

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Example of Spectral Analysis of Surface Waves

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Ground Penetrating Radar

Radio waves are transmitted into the earth and their reflection is recorded
Especially useful in finding rock and other hard formations or objects such as pipes, tanks, etc.
Limited to a depth of 10 m or less in electrically conductive soils (wet, fine grained soil deposits)

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Electric Resistivity

Uses the variations in resistivity of different materials
Electrical currents are induced into the ground through electrodes
Soil or rock resistivity can be determined by measuring the potential difference between known horizontal distances

Methods of conducting the test

Electrical Profiling: electrodes are spaced over an area and lines are run across the area. Practical for prospecting for sand and gravel deposits or ore deposits
Electrical Sounding: central location for the electrodes is selected and series of reading is taken by increasing the electrode spacing from the centre. Can provide information on variation of subsurface conditions and depth of water table