Chapter 3 – Overview of Soil Mechanics

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3.1. Introduction

This section presents an overview of basic soil mechanics, especially as they relate to retaining walls in general and sheet pile walls in particular. For engineering purposes, we shall consider the earth to be made up of rock and soil.

Soil will be defined as naturally occurring mineral particles which are readily separated into relatively small pieces, and in which the mass may contain air, water, or organic materials (derived from decay of vegetation).
Rock is that naturally occurring material composed of mineral particles so firmly bonded together that relatively great effort is required to separate the particles (i.e., blasting or heavy crushing forces).

The mineral particles of the soil mass are formed from decomposition of the rock by weathering (by air, ice, wind, and water) and chemical processes. Because they generally do not enter into sheet piling design, we will not discuss rocks further.

3.2. Soils

3.2.1. Overview of Soil Types

Beyond these general geologic classifications, geotechnical engineers further classify soils to enable quantification of their engineering properties. Two systems are discussed in this book: the Unified Soil Classification System (USCS) and the Modified Unified System (MUD). The laboratory and field tests used with both of these systems, such as tests for grain size, Atterberg limits, etc., are discussed elsewhere in this book.

3.2.2. Unified Soil Classification System (USCS)

This system is used primarily for engineering purposes and is particularly useful to the geotechnical engineer. Therefore, they should be used for all structural-related projects; such as bridges, retaining walls, buildings, etc. Precise classification requires that a grain size analysis and Atterberg Limits tests be performed on the sample. The method is discussed in detail in ASTM D 2487.

One of the main uses of soil classification systems is the determination of various soil properties based on the classification of the soil. Soils can be classified by the Unified System using visual inspection, Atterberg Limits and sieve analysis, tests which a) are relatively easy to perform and b) make greater allowance for sample disturbance than, say, consolidation or triaxial tests. Many correlations exist that enable soil properties to be estimated using the soil classification and in some cases other relatively simple tests. These correlations enable the geotechnical engineer to make estimates – in some cases the only estimates possible – of soil properties for preliminary design purposes.

Many of the “typical properties” of soils are based on SPT results; these are discussed with the SPT. Those discussed in this section are derived from other tests. It should be noted that these “typical” properties and correlations are of varying quality, expressed by standard deviation, which is the range above and below the average trend, within which about two-thirds of all values occur. These relationships are useful in preliminary analyses but must not supplant careful tests of structural properties. The relationships should never be applied in final analyses without verification by tests of the particular material concerned.

3.2.2.1. Coarse-Grained (Cohesionless or Granular) Soils

Coarse-grained soils are those soils where more than half of particles finer than 3” size can be distinguished by the naked eye. The smallest particle that is large enough to be visible corresponds approximately to the size of the opening of No. 200 sieve used for laboratory identification. Sands are divided from gravels on the No. 4 sieve size, and gravels from cobbles on the 3” size. The division between fine and medium sands is at the No. 40 sieve, and between medium and coarse sand at the No. 10 sieve. Generally, the engineering properties of cohesionless or granular soils are as follows:

Excellent foundation material for supporting structures and roads.
The best embankment material.
The best backfill material for retaining walls.
Might settle under vibratory loads or blasts.
Dewatering can be difficult due to high permeability.
If free draining not frost susceptible.

3.2.2.2. Fine-Grained (Cohesive or Organic) Soils

Soils are identified as fine-grained when more than half of the particles are finer than No. 200 sieve (as a field guide, such particles cannot be seen by the naked eye). Fine-grained soils are classified according to plasticity characteristics determined in Atterberg limit tests.

In general, the engineering properties of cohesive soils are as follows:

Very often, possess low shear strength.
Plastic and compressible.
Loses part of shear strength upon wetting.
Loses part of shear strength upon disturbance.
Shrinks upon drying and expands upon wetting.
Very poor material for backfill.
Poor material for embankments.
Practically impervious.
Clay slopes are prone to landslides

Differing from clays are silts; some characteristics of silts areas follows:

Relatively low shear strength.
High Capillarity and frost susceptibility
Relatively low permeability
Difficult to compact

Compared to clay, silts exhibit the following characteristics:

Better load sustaining qualities
Less compressible
More permeable
Exhibit less volume change

3.2.2.3. Examples of Sample Descriptions

Granular soils:

Medium dense, grey coarse to fine SAND, trace silt, trace fine gravel (SW).
Dry, dense, light brown coarse to fine SAND, some silt (SM).

Fine grained soils:

Very stiff brown silty CLAY (CL), wet
Stiff brown clayey SILT (ML), moist
Soft dark brown organic CLAY (OH), wet.

3.2.3. Modified Unified System (MUD)

For many years, soils engineers have successfully used the Unified Soil Classification System to categorize soil samples. The major advantage of this system is the easily understood word picture used to describe the soil samples after classification. The major disadvantage is the number of time-consuming classification tests that must be done to develop the word picture.

At present, numerous private firms and State agencies are using the nomenclature of the Unified System but without the classification testing. This process of visually identifying soil samples as known as the Modified Unified Description (MUD).

The procedure involves visually and manually examining soil samples with respect to texture, plasticity and colour. A method is presented for preparing a “word picture” of a sample for entering on a subsurface exploration log or other appropriate data sheet. The procedure applies to soil descriptions made in the field or laboratory.

It should be understood that the soil descriptions are based upon the judgment of the individual making the description. Classification tests are not intended to be used to verify the description, but to provide further information for analysis of soil design problems or for possible use of the soil as a construction material.

It is the intent of this system to describe only the constituent soil sizes that have a significant influence on the visual appearance and behaviour of the soil. This description system is intended to provide the best word description of the sample to those involved in the planning, design, construction, and maintenance processes.

3.2.3.1. Definition of Terms

Boulder – A rock fragment, usually rounded by weathering or abrasion, with an average dimension of 12 inches or more.
Cobble – A rock fragment, usually rounded or surrounded, with an average dimension between 3 and 12 inches.
Gravel – Rounded, subrounded, or angular particles of rock that will pass a 3 inch square opening sieve (76.2 mm) and be retained on a Number 10 U.S. standard sieve (2.0 mm).
Sand – Particles that will pass the Number 10 U.S. standard sieve and be retained on the Number 200 U.S. standard sieve (0.074 mm).
Silt – Material passing the Number 200 U.S. standard sieve that is nonplastic and exhibits little or no strength when dried.12
Clay – Material passing the Number 200 U.S. standard sieve that can be made to exhibit plasticity (putty like property) within a wide range of water contents and exhibits considerable dry strength
Fines – The portion of a soil passing a Number 200 U.S. standard sieve.
Marl – Unconsolidated white or dark grey calcium carbonate deposit.
Muck – Finely divided organic material containing various amounts of mineral soil.
Peat – Organic material in various stages of decomposition.
Organic Clay – Clay containing microscopic size organic matter. May contain shells and/or fibres.
Organic Silt – Silt containing microscopic size organic matter. May contain shells and/or fibres.
Coarse-Grained Soil – Soil having a predominance of gravel and/or sand.
Fine-Grained Soil – Soil having a predominance of silt and/or clay.
Mixed-Grained Soil – Soil having significant proportions of both fine-grained and coarse-grained sizes.

3.2.3.2. Visual – Manual Identification

Constituents are identified considering grain size distribution and the results of the manual tests. In addition to the principal constituent, other constituents that may affect the engineering properties of the soil should be identified. Secondary constituents are generally indicated as modifiers to the principal constituent (i.e., sandy clay or silty gravel). Other constituents can be included in the description through the use of terms such as with, some and trace.

Other terms that might be used include the following:

Marl – A white or grey calcium carbonate paste. May contain granular spheres, shells, organic material or inorganic soils. Reacts with weak hydrochloric acid.
Muck – Black or dark brown finely divided organic material mixed with various proportions of sand, silt, and clay.
May contain minor amounts of fibrous material such as roots, leaves, and sedges.
Peat – Black of dark brown plant remains. The visible plan remains range from coarse fibres to finely divided organic
material.
Organic Clay – Dark grey clay with microscopic size organic material dispersed throughout. May contain shells and/or fibres. Has weak structure that exhibits little resistance to kneading.
Organic Silt – Dark grey silt with microscopic size organic material dispersed throughout. May contain shells and/or fibres. Has weak structure that exhibits little resistance to kneading.
Fill – Man-made deposits of natural soils and/or waste materials. Document the components carefully since presence and depth of fill are important engineering considerations.

3.2.3.3. Soil Sample Identification Procedure

First Decision –

Is sample coarse-grained, fine-grained, mixed-grained or organic?
If mixed-grained, decide whether coarse-grained or fine-grained predominates.

2nd Decision –

What is principal component?
Use as noun in soil description. Example: Silty Sand

3rd Decision –

What is secondary component?
Use as adjective in soil description. Example: Silty Sand

4th Decision –

Are there additional components?
Use as additional adjective. Example: Silty Sand, Gravelly

Examples Of Descriptions Of The Soil Components

Sand – Describes a sample that consists of both fine and coarse sand particles.
Gravel – Describes a sample that consists of both fine and coarse gravel particles.
Silty fine Sand – Major component fine sand, with non plastic fines.
Sandy Gravel – Major component gravel size, with fine and coarse sand. May contain small amount of fines.
Gravelly Sand – Major component sand, with gravel. May contain small amount of fines.
Gravelly Sand, Silty – Major component sand, with gravel and nonplastic fines.
Gravelly Sand, Clayey – Major component sand, with gravel and plastic fines.
Sandy Gravel, Silty – Major component gravel size, with sand and nonplastic fines.
Gravelly Sand, Clayey – Major component gravel size, with sand and plastic fines.
Silty Gravel – Major component gravel size, with non plastic fines. May contain sand.
Clayey Gravel – Major component gravel size, with plastic fines. May contain sand and silt.
Clayey Silt – Major component silt size, with sufficient clay to impart plasticity and considerable strength when dry.

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