Compactness refers to how closely a mass of soil particles are packed together; the closer the packing, the greater the compactness and the larger the weight of soil per unit volume. The structure of a total mass of soil particles may be dense. In this case, the particles are closely packed and have a high degree of 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. This lessens the tendency for displacement of the individual grains under load, and the soil is then capable of supporting heavier loads. Well-graded coarse materials usually are dense and have strength and stability under load.

On the other hand, the structure may be loose, in which case the particles are not packed as closely together as possible, thereby lacking compactness. Loose, open structures have large voids, which will lead to settlement or disintegration when foundation or traffic loads are applied.

Specific gravity is designated by the symbol G₂. It is defined as the ratio between the weight per unit volume of the material and the weight per unit volume of water at a stated temperatureusually 20C. If you use the system international (SI) (metric) system, you can determine specific gravity by the following formula:

Test procedures will be discussed in detail later in this chapter. The specific gravity of the solid substance of most inorganic soils varies between 2.60 and 2.80. Tropical iron-rich laterite, as well as some lateritic soils, generally has a specific gravity of 3.0 or more. Sand particles composed of quartz have a specific gravity of about 2.65. Clays can have values as high as 3.50. The solids of soil particles are composed of minerals. Generally, these minerals have a specific gravity greater than 2.60. Values of specific gravity smaller than that are an indication of the possible presence of organic matter.

The moisture content of a soil mass is often the most important factor affecting the engineer-ing characteristics of the soil. The water may enter from the surface or may move through the sub-surface layers by either gravitational pull, capillary action, or absorption. This moisture is present in most cases. It influences various soils differently; it probably has the greatest effect upon the behavior of the soil when the soil is subjected to loading.

Surface water results from precipitation or runoff and 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 is collected or held in pools or layers beneath the surface by a restricting layer of soil or rock. This water is constantly acted upon by one or more external forces. Water controlled by gravity (free or gravitational water) seeks a lower layer and moves through the voids or spaces until it reaches some restriction. This restriction may be a bedrock or an impervious layer of soil whose openings or voids are so small as to prevent water passage. The voids or spaces in a soil may form continuous tunnels or tubes and cause the water to rise in the tubes by capillary action (capillary moisture). The smaller the tube, the stronger the capillary action; therefore, the water rises higher in the finer soils, which have smaller interconnected voids. This area of moisture above the free water layer or pool is called the capillary fringe. Another force acting on soil water is absorption by the atmosphere. As the moisture evaporates from the soil surface, more moisture is drawn from the soil below and is, in turn, also evaporated. This process continues until the soil is in an airdry condition in which the moisture in the soil is in equilibrium with the moisture vapor in the air. In this airdry state, the moisture remaining in the soil is in the form of thin films of water surrounding the individual soil particles and is called the hydroscopic moisture. These moisture films are due to naturally occurring elec-trical forces, which bind the water molecules to the soil particles. Hydroscopic moisture films may be driven off from airdried soil by heating the material in an oven at a controlled temperature for 24 hr or until constant weight is attained. To define the amount of water present in a soil sample, the term moisture content (symbol w) is used. 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 or 

When wet soil is dried in air in the laboratory, the amount of hydroscopic moisture remaining in the airdried soil, expressed as a percentage of the weight of the dry soil, is called the hydroscopic moisture content.