soil classification and description in engineering geology

soil classification and description in engineering geology In civil engineering the term ‘soil’ is used for any cemented or weakly cemented accumulation of detrital mineral particles and rock fragments formed by the erosion and weathering of rock. It can generally be excavated easily and has very different properties from the underlying rock head (the surface between the bedrock and unconsolidated material). A soil may typically contain some organic matter, especially in near-surface layers, and may also have void spaces between the particles which will contain water, or air, or both.

Soils are formed by many processes. Physical weathering processes include the action of wind, water, and glaciers, or disintegration by alternate freezing and thawing; chemical weathering processes include the leaching of soluble materials and the accumulation of insoluble residues; other processes include the incorporation of organic matter; the down-ward movement of fine particles; and disturbance by root penetration, animal burrowing, and desiccation.

If the products of weathering remain at their original location, they constitute a residual soil; alternatively, if the products are transported by gravity, wind, water, or glaciers and are deposited in a different location, they constitute a transported soil. Soil may be termed after their mode of transport and deposition; for example, drift is a glacial or fluvioglacial deposit left after the retreat of glaciers and can vary considerably in composition and thickness; colluvium is slope debris that has moved downslope, largely by gravity alone, and includes head and scree.

The soil profile, shape, size and arrangement of the particles, soil strength, water content, the degree of cementation, the chemical composition, and potential changes in composition are frequently recorded by the engineering geologist as they present a range of problems when considering the design of foundations, the settlement of buildings, the stability of slopes, or the quarrying of material.

Soil description must be distinguished from soil classification. The description indicates the characteristics of both the soil material and the soil mass in situ. Such a description should include reference to the particle size distribution of the soil, plasticity, colour, texture, and mineral composition. Description of the soil mass is best done in the field or on undisturbed samples and would refer to the soil strength in situ with any details of bedding, weathering, and discontinuities. Soil description is particularly important when the soil is to be used in an undisturbed condition, perhaps for supporting foundations. Soil classification is the allocation of a soil to one of a limited number of soil groups using parameters that are independent of its condition in situ and provides a common language for the exchange of information. Soil classification is therefore more useful when the soil is to be disturbed prior to use, perhaps in a dam or embankment, resulting in changes in its water content, fabric, and texture.

Soils having properties that are influenced mainly by clay- and silt-sized particles are referred to as fine-grained soils. Those with properties that are influenced mainly by sand- and gravel-sized particles are referred to as coarse-grained soils.

Engineering soils are characteristically divided into cohesive and non-cohesive types. Cohesion between particles is caused by the presence of clay minerals which, even when they are present in relatively small quantities, usually exert a considerable influence on the properties of a soil, an influence which is sometimes disproportionate to their percentage by weight. In general terms, a soil is considered to be cohesive if the particles adhere after the soil is wetted and subsequently dried, and if significant force is then required to crumble the soil: this definition excludes those soils in which particles adhere when wet due to surface tension.

A convenient way of separating the cohesive and non-cohesive soils is the plastic limit test introduced by Atterberg in 1911. Very simple index tests have been shown to be invaluable in indicating both the proportions of cohesive/non-cohesive materials and the nature of the clay minerals. According to its water content, a soil may exist in the liquid, plastic, semi-solid, or solid state. For a soil to exist in the plastic state, the water content must be sufficient for the particles to slide freely past each other but the magnitude of the net interparticle forces must be enough for cohesion or an overall attraction to be maintained between them. Thus, the plastic limit is defined as the minimum moisture content at which a soil may be rolled by hand on a glass plate into a cylinder 3 mm in diameter; non-cohesive soils will crumble before reaching this stage.

Most fine-grained soils exist naturally in the plastic state. The upper and lower water contents at which plastic behaviour is observed are the liquid limit (WL) and plastic limit (WP), respectively, and these limits are widely used in soil classification. The liquid limit is also determined by arbitrary but standardized test procedures. The liquid limit is defined as the minimum moisture content of the soil at which it will flow under its own weight. It is assessed by dropping a standardized steel cone into a creamy paste of the clay sample; the moisture content at which the cone penetrates the sample by 20 mm is defined as the liquid limit. Similarly, the shrinkage limit is the water content at which further loss of water from the soil will not cause a further reduction in volume.

The plasticity index is defined as WL – WP and represents the range of moisture contents over which the soil behaves in a plastic manner. Soils with a high plasticity index tend to have a high clay fraction and be prone to swelling on the uptake of water. Conversely, soils with a low plasticity index may be susceptible to heave owing to the formation of ice beneath the surface.

Non-cohesive soils include silty, sandy, and gravelly soils in which there is no cohesion except that derived from any minor amount of clay matrix or from water tension between particles. The strength of these soils is derived from any partial cementation and the internal friction provided by the interlocking nature of the grains. Locked sands are a particular type of non-cohesive soil in which the particles have undergone pressure solution at their point contacts, giving a highly interlocking grain structure and a high strength until they are disturbed and the structure is lost.

The properties of non-cohesive soils can be assessed in situ by the standard penetration test (SPT). The test is performed at the required depth in a site investigation borehole by driving a 50 mm split tube sampler 150 mm into the soil. A 65-kg hammer is then used to drive the sampler a further 300 mm and the number of blows required is referred to as the standard penetration resistance (N). The test is very simple and is widely carried out in coarse soils as the value of N relates to the properties of the soil, particularly the density and bearing capacity. It must, however, be used with caution in clayey soils. The split barrel enables samples of the soil under test to be recovered, but in very coarse soils a conical shoe may be fitted to the sampler without affecting the results obtained.

Another test frequently undertaken in situ on non-cohesive soils is the cone penetration test (CPT). This entails driving a conical probe into the soil at 20 mm s⁻¹ followed by a concentric outer sleeve. The ratio of the resistance against the sleeve to the resistance against the cone provides information on soil type and packing arrangement and is used to estimate bearing capacity and settlement.

Soils are classified according to grain size and index limits, and standard terminology is used for effective communication between engineering geologists and other engineers (Fig. 1). If the plasticity index and liquid limit have been determined in the laboratory, then the soil is represented by a point on the chart. Soils plotting below the A-line are silts (M) and those above the A-line are clays (C). In turn, the plasticity of the soil may be described as low, intermediate, high, very high, or extremely high by reference to the liquid limit.

The boundary between fine soils and coarse soils is placed at the point where 35 per cent of material is finer than 0.06 mm. The choice of this dividing point reflects the overriding engineering importance of the finer grains and the particular properties of the clay minerals.

Soils are described according to the dominant particle-size fraction, which may be qualified by the presence of other size fractions or may give an indication of the particle-size distribution, e.g. a gravelly sand or a sandy silty clay. Additional information may also be given about the presence of cobbles and boulders or organic matter. Well-graded soils have no excess of particles in any one size range. Conversely, poorly graded soils either have a high proportion of particles in one size range in a uniform soil, or have large and small particles present with a low proportion of intermediate sizes in a gap-graded soil.

Christopher Mcdonald