Soil Mechanics

soil mechanics, the study of the physical properties and utilization of soils, especially used in planning foundations for structures and subgrades for highways.

The first scientific study of soil mechanics was undertaken by French physicist Charles-Augustin de Coulomb, who published a theory of earth pressure in 1773. Coulomb’s work and a theory of earth masses published by Scottish engineer William Rankine in 1857 are still primary tools used to quantify earth stresses. These theories have been amended in the 20th century to take into account the influence of cohesion, a more recently discovered property of soils that causes them to behave somewhat differently under stress than Rankine and Coulomb predicted.

Soil is a natural aggregate of mineral particles, sometimes including organic constituents; it has solid, liquid, and gaseous phases. How the soil of a given site will support the stresses put upon it by the weight of structures, or how it will respond to movement in the course of construction, depends upon six properties—internal friction (the resistance of a soil mass to sliding, inversely related to the amount of moisture in the soil and thus greater in sands and gravel than clays) and cohesion (molecular attraction between soil particles, much higher in clays than sands or silt), both of which lessen the tendency of soils to shear, or slide along planes; compressibility (the degree to which soil may be made denser by various means including tamping and vibration, and thus able to support greater loads); elasticity (the ability of soil to reexpand after being compressed); permeability (the degree to which a soil will conduct a flow of water); and capillarity (the degree to which water is drawn upward from the normal water table).

The thoroughness of soil surveys at a given site depends on the size of the project to be carried out. Visual examination of the surface may suffice in some cases. Soil characteristics generally vary more rapidly vertically (with depth) than horizontally. Subsurface examination techniques include trench-digging, boring (to test resistance as well as to obtain samples), and pumping subsurface matter to the surface with water. Seismic testing (measuring the speed with which shock waves generated by explosives are transmitted through the ground) and measurement of the electrical resistance of the soil also yield information helpful in the evaluation of soil. Grain size and plastic properties of samples taken from the site are measured in a laboratory. Occasionally data obtained from previous studies of soils near the site are useful.

Foundations are designed to convey the weight of a structure to the ground underneath and around it. Stress distribution that is not properly matched to the characteristics of the soil may result in structural failure through shearing of the soil or uneven settling. Spread foundations may be either of the spread footing (made with wide bases placed directly beneath the load-bearing beams or walls), mat (consisting of slabs, usually of reinforced concrete, which underlie the entire area of a building), or floating types. A floating foundation consists of boxlike rigid structures set at such a depth below ground that the weight of the soil removed to place it equals the weight of the building; thus, once the building is completed, the soil under it will bear the same weight it bore before excavation was begun. Deep foundations may be end-bearing piles (which convey all the weight put on them end-to-end, from the building above to the bedrock on which they are set), friction piles (which transfer some of the pressure put on them to the soil around them, through friction or adhesion along the surface where pile sides interface with soil), or caissons (extra-large piles cast in place in an excavation, rather than prefabricated and sunk).

Slopes stay in place because the downward pull of gravity is countered by forces of cohesion and friction between particles. Various changes may upset the balance between these forces, precipitating a slide; in particular, an increase in the amount of water borne in the soil of a slope may drastically reduce cohesion and friction. The stability of slopes is graded such that 1.0 indicates forces exactly balanced, 2.0 signifies that the forces of stability are twice as great as those tending toward movement, etc. A slope with a reading of less than 1.0 is collapsing. The banks of dams, highway cuts, and railway cuts are designed to certain standards of stability as measured by this scale. Stability may be increased by draining, gradient leveling, compacting, or reinforcing the slope with injections of cement. In dam construction an impermeable core is used to prevent excess seepage of water from lowering stability, while the slopes consist of permeable material that buffers the weight of water along the dam.

Soil mechanics, by examination of the subgrade of roads and highways, helps to determine which type of pavement (rigid or flexible) will last longer. The study of soil characteristics is also used to decide the most suitable method for excavating underground tunnels.