Engineering - PDI

Chapter 2 – Section 1: Soil Formation, Physical Properties, Moisture

From ‘FM 5-472 NAVFAC MO 330 AFJMAN 32-1221(I)’ by Department of the Army

The soil in an area is an important consideration in selecting the exact location of a structure. Military engineers, construction supervisors, and members of engineer reconnaissance parties must be capable of properly identifying soils in the field to determine their engineering characteristics. Because a military engineer must be economical with time, equipment, material, and money, site selection for a project must be made with these factors in mind.


The word soil has numerous meanings and connotations to different groups of professionals who deal with this material. To most soil engineers (and for the purpose of this text), soil is the entire unconsolidated earthen material that overlies and excludes bedrock. It is composed of loosely-bound mineral grains of various sizes and shapes. Due to its nature of being loosely bound, it contains many voids of varying sizes. These voids may contain air, water, organic matter, or different combinations of these materials. Therefore, an engineer must be concerned not only with the sizes of the particles but also with the voids between them and particularly what these voids enclose (water, air, or organic materials).

Soil Formation

Soil formation is a continuous process and is still in action today. The great number of original rocks, the variety of soil-forming forces, and the length of time that these forces have acted all produce many different soils. For engineering purposes, soils are evaluated by the following basic physical properties:

    • Gradation of sizes of the different particles.
    • Bearing capacity as reflected by soil density.
    • Particle shapes.

An engineer extends soil evaluation by considering the effect of water action on the soils. With a complete evaluation, an engineer can determine whether or not the soil is adequate for the project.


Soil forms when rocks that are exposed to the atmosphere disintegrate and decompose, either by mechanical action (wind, water, ice, and vegetation), chemical action, or both. The resulting material may remain where it is formed or it may be transported by water, glaciers, wind, or gravity and deposited at a distance from the parent rock.

Geologists classify rocks into three basic groups:

    • Igneous (formed by cooling from a molten state).
    • Sedimentary (formed by the accumulation and cementation of existing particles and remains of plants and animals).
    • Metamorphic (formed from existing rocks subjected to heat and pressure).


At a particular location are usually several layers (strata), one above the other, each composed of a different kind of soil. Strata may be a fraction of an inch or many feet thick. The upper layer is called the topsoil or agricultural soil since it supports plant growth. For an adequate soil evaluation for engineering uses, identify all strata to whatever depth may be affected by the construction. A vertical cross section through the earth, with the depths and types of soil indicated, is called a soil profile.

Physical Properties

A soil’s physical properties help determine the engineering characteristics. The following properties are the basis for the soil-classification system used in engineering identification of soil types. The discussion of the physical properties of soil focuses on the soil particles themselves. The terms particle and grain are used interchangeably.

    • Grain size.
    • Particle shape.
    • Gradation.
    • Density.
    • Specific gravity.
    • Moisture.
    • Consistency.
    • Organic soil.

Physical characteristics of soil particles include size and shape. The proportions of different-sized particles determine an aggregate’s gradation. Density or compactness refers to the closeness of packing of soil particles; the closer the packing, the greater the compactness and the larger the soil weight per unit of volume. Plasticity characteristics of fine-grained soil components include the liquid limit (LL) and the plastic limit (PL); shrinkage ratios; dry strength; and unconfined, compressive strength. Specific gravity of soil particles aids in their identification. The presence of organic matter is important to the engineering use of soils. Color, texture, odor, structure, and consistency are readily observed factors that aid in soil description.

Grain or Particular Size

Soils are divided into groups based on the size of the particle grains in the soil mass. Common practice is to distinguish the sizes by using sieves. A sieve is a screen attached across the end of a shallow, cylindrical frame. The screen permits smaller particles to fall through and retains the larger particles on the sieve. Sieves with screen openings of different sizes (the largest on the top and the smallest at the bottom) separate the soil into particle groups based on size. The amount remaining on each sieve is measured and described as a percentage by weight of the entire sample. The size groups that are designated by the USCS are cobbles, gravels, sands, and fines (silt or clay), as shown in Table 2-1. Further discussion on these size groups can be found later in this chapter and in Appendix B.

Grain or Particular Shape

The shape of the particles influences a soil’s strength and stability. Two general shapes are normally recognized—bulky and platy.


The bulky shapes include particles that are relatively equal in all three dimensions. In platy shapes, one dimension is very small compared to the other two. For example, a thick book would be considered bulky, but a page of the book would be platy. Bulky shapes are subdivided into four groups: angular, subangular, subrounded, and well-rounded (see Figure 2-1, page 2-4). These four subdivisions are dependent on the amount of weathering that has occurred. Cobbles, gravel, sand, and silt usually fall into this bulky-shape group. These groups are discussed in the order of desirability for construction.

Angular-shaped particles are those that have recently broken up. They are characterized by jagged projections, sharp ridges, and flat surfaces. The interlocking characteristics of angular gravels and sands generally make them the best materials for construction. These particles are seldom found in nature because weathering processes normally wear them down in a relatively short time. Angular material may be produced artificially by crushing, but because of the time and equipment required for such an operation, natural materials with other grain shapes are frequently used.

Subangular-shaped particles have been weathered to a point that the sharper points and ridges of their original angular shape have been worn off. These particles are still very irregular in shape with some flat surfaces and are excellent for construction.

Subrounded particles are those on which weathering has progressed even further. While they are still somewhat irregular in shape, they have no sharp corners and few flat areas. These particles are frequently found in streambeds. They may be composed of hard, durable particles that are adequate for most construction needs.

Rounded particles are those in which all projections have been removed and few irregularities in shape remain. The particles approach spheres of varying sizes. Rounded particles are usually found in or near streambeds, beaches, or dunes. Possibly the most extensive deposits exist at the beaches where repeated wave action produces almost perfectly rounded particles that may be uniform in size. They may also be found in arid environments due to wind action and the resulting abrasion between particles. They are not desirable for use in asphalt or concrete construction until the rounded shape is altered by crushing.


The platy shapes have one dimension relatively small compared to the other two. They have the general shape of a flake of mica or a sheet of paper. Particles of clay soil exhibit this shape, although they are too small to be seen with the naked eye. Coarse-grained soil particles are individually discernible to the naked eye; fine-grained particles with platy or bulky shapes are not.


Gradation describes the distribution of the different size groups within a soil sample. The soil may be well-graded or poorly graded.

Well-Graded Soils

Well-graded soils must have a good range of all representative particle sizes between the largest and the smallest. All sizes are represented, and no one size is either overabundant or missing (see Figure 2-2).

Poorly Graded Soils

Poorly graded soils can be classified as either uniformly graded or gap graded. A uniformly graded soil consists primarily of particles of nearly the same size. A gap-graded soil contains both large and small particles, but the gradation continuity is broken by the absence of some particle sizes (see Figure 2-2).


The structure of the aggregate of soil particles may be dense (closely packed) or loose (lacking compactness). A dense structure provides interlocking of particles with smaller grains filling the voids between the larger particles. When each particle is closely surrounded by other particles, the grain-to-grain contacts are increased, the tendency for displacement of individual grains under a load is lessened, and the soil is capable of supporting heavier loads. Coarse materials that are well-graded usually are dense and have strength and stability under a load. Loose, open structures have large voids and will compact under a load, leading to settlement or disintegration under foundation or traffic loads.

Specific Gravity

The specific gravity is the ratio between the weight-per-unit volume of the material and the weight-per-unit volume of water at a stated temperature. There are three ways of determining and expressing specific gravity:

    • Specific gravity of solids.
    • Apparent specific gravity.
    • Bulk specific gravity.

The specific gravity of solids is the method most widely used when testing soils. The apparent and bulk specific-gravity methods are used in testing fine and coarse aggregates. The specific gravity of solids is explained further in Section IV of this chapter, along with the test procedure.


The term moisture content (w) is used to define the amount of water present in a soil sample. It is the proportion of the weight of water to the weight of the solid mineral grains (weight of dry soil) expressed as a percentage.

The moisture content of a soil mass is often the most important factor affecting the engineering behavior of the soil. Water may enter from the surface or may move through the subsurface layers either by gravitational pull, capillary action, or hygroscopic action. This moisture influences various soils differently and usually has its greatest effect on the behavior of fine- grained soils. The fine grains and their small voids retard the movement of water and also tend to hold the water by surface tension.

Many fine-grained soils made from certain minerals exhibit plasticity (putty- like properties) within a range of moisture contents. These soils are called clays, and their properties may vary from essentially liquid to almost brick hard with different amounts of moisture. Further, clays are basically impervious to the passage of free or capillary moisture. Coarse-grained soils with larger voids permit easy drainage of water. They are less susceptible to capillary action. The amount of water held in these soils is less than in fine- grained soils, since the surface area is smaller and excess water tends to drain off.

Surface Water

Surface water from precipitation or runoff enters the soil through the openings between the particles. This moisture may adhere to the different particles or it may penetrate the soil to some lower layer.

Subsurface Water

Subsurface water is collected or held in pools or layers beneath the surface by a restricting layer of soil or rock. This water is constantly acted on by one or more external forces.

Gravitational Pull

Water controlled by gravity (free or gravitational water) seeks a lower layer and moves through the voids until it reaches some restriction. This restriction may be bedrock or an impervious soil layer with openings or voids so small that they prevent water passage.

Capillary Action

Voids in soil may form continuous tunnels or tubes and cause the water to rise in the tubes by capillary action (capillary moisture). Since the smaller the tube, the stronger the capillary action, the water rises higher in the finer soils that have smaller interconnected voids. This area of moisture above the free water layer or pool is called the capillary fringe.

Adsorbed Water and Hygroscopic Moisture

In general terms, adsorbed water is water that may be present as thin films surrounding separate soil particles. When soil is in an air-dried condition, the adsorbed water present is called hygroscopic moisture. Adsorbed water is present because soil particles carry a negative electrical charge. Water is dipolar; it is attracted to the surface of a particle and bound to it. The water films are affected by the soil particle’s chemical and physical structures and its relative surface area. The relative surface area of a particle of fine-grained soil, particularly if it has a platy shape, is much greater than for coarse soils composed of bulky grains. The electrical forces that bind adsorbed water to a soil particle also are much greater.

In coarse soils, the adsorbed layer of water on a particle is quite thin in comparison to the overall particle size. This, coupled with the fact that the contact area with adjacent grains is quite small, leads to the conclusion that the presence of the adsorbed water has little effect on the physical properties of coarse-grained soils. By contrast, for finer soils and particularly in clays, the adsorbed water film is thick in comparison to the particle size. The effect is very pronounced when the particles are of colloidal size.

Plasticity and Cohesion

Two important aspects of the engineering behavior of fine-grained soils are directly associated with the existence of adsorbed water films. These aspects are plasticity and cohesion.

Plasticity is a soil’s ability to deform without cracking or breaking. Soils in which the adsorbed films are relatively thick compared to particle size (such as clays) are plastic over a wide range of moisture contents. This is presumably because the particles themselves are not in direct contact with one another. Coarse soils (such as clean sands and gravels) are nonplastic. Silts also are essentially nonplastic materials, since they are usually composed predominantly of bulky grains; if platy grains are present, they may be slightly plastic.

A plasticity index (PI) is used to determine whether soil is cohesive. Not all plastic soils are cohesive. Soil is considered cohesive if its PI is greater than 5. That is, it possesses some cohesion or resistance to deformation because of the surface tension present in the water films. Thus, wet clays can be molded into various shapes without breaking and will retain these shapes. Gravels, sands, and most silts are not cohesive and are called cohesionless soils.

In engineering practice, soil plasticity is determined by observing the different physical states that a plastic soil passes through as the moisture conditions change. The boundaries between the different states, as described by the moisture content at the time of changes, are called consistency limits or Atterberg limits, named after the Swedish scientist who defined them years ago.

From ‘FM 5-472 NAVFAC MO 330 AFJMAN 32-1221(I)’ by Department of the Army