Chapter 2 – Identification and Classification of Soil and Rock

For a complete version of this document click here.

2.1. Introduction

To perform properly, a structure must interact favorably with the soil on which it rests. The modern foundation engineer, who often must build in areas that were considered too poor to build upon a few years past, must be well versed in the fundamentals of soil mechanics. This knowledge will be used in the design of structural foundations and earthworks to answer questions such as:

  • Will settlements be excessive?
  • Can the structure tolerate settlements?
  • Will the proposed cut or fill slopes have adequate stability?

It is imperative, therefore, that the engineer have adequate knowledge of the soil conditions at a site before he or she can attempt to answer these questions. Investing a few thousand dollars into an adequate boring and testing program may prevent costly failures or overconservative design, resulting in design and construction savings of hundreds of thousands of dollars.

This section presents criteria for soil and rock identification and classification plus information on their physical engineering properties. Common soils and rock are discussed as well as special materials such as submarine soils and coral, saprolitic soils, lateritic soils, expansive and collapsing soils, cavernous limestone, quick clay, permafrost and hydraulically placed fills.

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.

2.2. Soils

2.2.1. Overview of Soil Types

See Table 2-1 for principal soil deposits grouped in terms of origin (e.g., residual, colluvial, etc.) and mode of occurrence (e.g., fluvial, lacustrine, etc.).

Table 2-1 Principal Soil Deposits

Major Division Principal Soil Deposits Pertinent Engineering Characteristics
Material formed by disintegration of underlying parent rock or partially indurated material. Residual sands and fragments of gravel size formed by solution and leaching of cementing material, leaving the more resistant particles; commonly quartz.

Residual clays formed by decomposition of silicate rocks, disintegration of shales, and solution of carbonates in limestone. With few exceptions becomes more compact, rockier, and less weathered with increasing depth. At intermediate stage may reflect composition, structure, and stratification of parent rock.

Generally favorable foundation conditions.

Variable properties requiring detailed investigation. Deposits present favorable foundation conditions except in humid and tropical climates, where depth and rate of weathering are very great.


Organic Accumulation of highly organic material formed in place by the growth and subsequent decay of plant life. Peat. A somewhat fibrous aggregate of decayed and decaying vegetation matter having a dark color and odor of decay.

Muck. Peat deposits which have advanced in stage of decomposition to such extent that the botanical character is no longer evident.


Very compressible. Entirely unsuitable for supporting building foundations.
Material transported and deposited by running water. Floodplain deposits. Deposits laid down by a stream within that portion of its valley subject to inundation by floodwaters.

Point bar. Alternating deposits of arcuate ridges and swales (lows formed on the inside or convex bank of mitigating river bends.) Ridge deposits consist primarily of silt and sand, swales are clay-filled.

Channel fill. Deposits laid down in abandoned meander loops isolated when rivers shorten their courses. Composed primarily of clay; however, silty and sandy soils are found at the upstream and downstream ends.

Backswamp. The prolonged accumulation of floodwater sediments in flood basins bordering a river. Materials are generally clays but tend become siltier near riverbank.

Alluvial Terrace deposits. Relatively narrow, flat- surfaced, river-flanking remnants of floodplain deposits formed by entrenchment of rivers and associated processes.

Estuarine deposits. Mixed deposits of marine and alluvial origin laid down in widened channels at mouths of rivers and influenced by tide of body of water into which they are deposited.

Alluvial-Lacustrine deposits. Material deposited within lakes (other than those associated with glaciation by waves, currents, and organo-chemical processes. Deposits consist of unstratified organic clay or clay in central portions of the lake and typically grade to stratified silts and sands in peripheral zones.

Generally favorable foundation conditions; however, detailed investigations are necessary to locate discontinuities. Flow slides may be a problem along riverbanks. Soils are quite pervious.

Fine-grained soils are usually compressible. Portions may be very heterogeneous. Silty soils generally present favorable foundation conditions.

Relatively uniform in a horizontal direction. Clays are usually subjected to seasonal volume changes.

Usually drained, oxidized. Generally favorable foundation conditions.

Generally fine-grained and compressible. Many local variations in soil conditions.

Usually very uniform in horizontal direction. Fine-grained soils generally compressible.


Material transported and deposited by glaciers, or by meltwater from the glacier. Glacial till. An accumulation of debris, deposited beneath, at the side (lateral moraines, or at the lower limit of a glacier (terminal moraine. Material lowered to ground surface in an irregular sheet by a melting glacier is known as a ground moraine.

Glacio-Fluvial deposits. Coarse and fine-grained materials deposited by streams of melt water from glaciers. Material deposited on ground surface beyond terminal of glacier is known as an outwash plain. Gravel ridges known as kames and eskers.

Glacio-Lacustrine deposits. Materials deposited Very uniform in a horizontal within lakes by melt water from glaciers. Consisting direction.
of clay in central portions of lake and alternate
layers of silty clay or silt and clay (varved clay in peripheral zones.

Consists of material of all sizes in various proportions from boulder and gravel to clay. Deposits are unstratified. Generally present favorable foundation conditions; however, rapid changes in

Many local variations. Generally, these present favorable foundation conditions.


Material transported and deposited by ocean waves and currents in shore and offshore areas. Shore deposits. Deposits of sands and/or gravels formed by the transporting, destructive, and sorting action of waves on the shoreline.

Marine clays. Organic and inorganic deposits of Generally very uniform in fine-grained material.


Relatively uniform and of moderate to high density.


Material transported and deposited by gravity. Talus. Deposits created by gradual accumulation of unsorted rock fragments and debris at base of cliffs.

Hillwash. Fine colluvium consisting of clayey sand, sand silt, or clay.

Landslide deposits. Considerable masses of soil or rock that have slipped down, more or less as units, from their former position on steep slopes.


Generally very uniform in composition. Compressible and usually very sensitive to remolding.

Previous movement indicates possible future difficulties. Generally unstable foundation conditions. 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:

  1. Excellent foundation material for supporting structures and roads.
  2. The best embankment material.
  3. The best backfill material for retaining walls.
  4. Might settle under vibratory loads or blasts.
  5. Dewatering can be difficult due to high permeability.
  6. If free draining not frost susceptible. 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. Categories are illustrated on the plasticity chart in Figure 11-6, where the classification procedure is discussed in more detail.

In general, the 1. 2. 3. 4. 5. 6. 7. 8. 9.

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 are as follows:

  1. Relatively low shear strength
  2. High Capillarity and frost susceptibility
  3. Relatively low permeability
  4. Difficult to compact

Compared to clay, silts exhibit the following characteristics:

  1. Better load sustaining qualities
  2. Less compressible
  3. More permeable
  4. Exhibit less volume change 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 subrounded, with an average dimension between 3 and 12 inches.
  • Gravel4 – 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.5
  • 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.6
  • Fines – The portion of a soil passing a Number 200 U.S. standard sieve.
  • Marl – Unconsolidated white or dark gray calcium carbonate deposit.
  • Muck – Finely divided organic material containing various amounts of mineral soil.
  • Pest – 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.

For a complete version of this document click here.