part2_soilexploration.pdf

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Part 2

Soil Exploration

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The process of identifying the layers of deposits that underlie a proposed

structure and their physical characteristics is generally referred to as

subsurface exploration.

The purpose of the subsurface exploration:

1. Selecting the type and depth of foundation suitable for a given structure.

2. Evaluating the load-bearing capacity of the foundation.

3. Estimating the probable settlement of a structure.

4. Determining potential foundation problems (e.g., expansive soil, collapsible

soil, sanitary landfill, and so on).

5. Determining the location of the water table.

6. Predicting the lateral earth pressure for earth retaining structures.

7. Establishing construction methods for changing subsoil conditions.

Purposes of Soil Investigation

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Soil Exploration Methods

Ground penetration radar Seismic surveys

Nondestructive techniques used to provide spatial information on soils, rocks, and

hydrological and environmental conditions. Popular methods are:

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Destructive techniques such as test pits, drilling and augering are used for soil

investigation. The holes produced by augering and drilling are called auger holes

and boreholes respectively.

Soil Exploration Methods

Soil Sampling

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The objective of soil sampling is to obtain soils of satisfactory size with minimum

disturbance for observations and laboratory tests. Soil samples are usually obtained

by attaching an open-ended, thin-walled tube called a Shelby tube or, simply, a

sampling tube to drill rods and forcing it down into the soil.

thin-walled tube

(undisturbed samples)

Shelby tube

(disturbed samples)

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Borehole log

Borehole records are recorded in a borehole log
which provides a highly efficient means to
determine the character and thickness of the
different geologic materials that are penetrated

Soils Laboratory Tests

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Samples are normally taken from the field for laboratory tests to characterize the

physical and mechanical (strength and deformation) properties. These parameters are

used to design foundations and to determine the use of soils as a construction material.

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Number and Depths of Boreholes

You have to make judgments on the number, location, and depths of borings to provide sufficient

information for design and construction. The number and depths of borings should cover the zone of

soil that would be affected by the structural loads. There is no fixed rule to follow. In most cases, the

number and depths of borings are governed by experience based on the geological character of the

ground, the importance of the structure, the structural loads, and the availability of equipment

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Number and Depths of Boreholes

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Over the years, several in situ testing devices have emerged to characterize the soil

and to measure strength and deformation properties. The most popular devices are:

1. Standard penetration test (SPT)

2. Vane shear test (VST)

3. Cone penetrometer test (CPT)

4. Flat plate dilatometer (DMT) & Pressure meter test (PMT)

Types of In Situ or Field Tests

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Standard Penetration Test (SPT)

SPT tests are unreliable for coarse

gravel, boulders, soft clays.

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The standard practice now in the U.S. is to express the

N-value to an average energy ratio of 60% (N60). Thus,

correcting for field procedures and on the basis of field

observations, it appears reasonable to standardize the

field penetration number as a function of the input

driving energy and its dissipation around the sampler

into the surrounding soil

Correction of SPT N-values to N60

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Correction of SPT N60 to N1,60

N1,60 = N60 value corrected for overburden pressure or

1,60 60NN C N

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Correlation of Soil Properties with N-Values

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SPT-DR-1

SPT-DR-2

SPT-DR-3

SPT-DR-4

Correlation of Soil Dr with N-Values

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Correlation of Soil f with N-Values

SPT-f -1

SPT-f -2

SPT-f -3

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Correlation of soil elastic modulus E with N-Values

SPT-E-1

SPT-E-2

SPT-E-3

SPT-E-4

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Estimating Soil Profile

Soil profiles can be estimated from layer descriptions and

classifications found in borelogs

Three boreholes (BH) along a proposed

road intersection are shown below. The

soils in each borehole were classified using

ASTM-CS. Sketch a soil profile along the

center line.

Example

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Cone Penetration Test (CPT)

The results of a cone test are displayed as cone

resistance versus depth.

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There is no standard procedure to divide the soil profile into layers. One procedure is to divide the soil profile into
layers such that each layer has different average resistance. The minimum size of the layer should be about 10
cone diameters (Dcone = 36 mm or 44 mm). You need to be careful with excessively large or very low cone
resistance. Excessive large resistance over a depth of about 5 cone diameters may be due to buried object such as
a rock fragment and should be neglected. Low cone values are indicative of soft soils or pockets of loose material
that you need to carefully consider in design and construction.

Interpreting tip resistance variation with depth from CPT

Equivalent composite
distribution of qc

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Correlation of Soil f with qc

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Correlation of soil elastic modulus E with qc for granular soils

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Vane Shear Test (VST)

rapid and economical and are
used extensively field soil-
exploration programs. The test
gives good results in soft and
medium-stiff clays and gives
excellent results in determining
the properties of sensitive clays.

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Flat Dilatometer Test (DMT)