The shape of the deformation pattern varies depending on flexibility of the foundation and type of soil. Figure 1 illustrates the relative distribution of soil contact pressures and displacements on cohesion less and cohesive soil. Linear contact pressure distributions from uniformly applied pressure q are often assumed for settlement analysis, Figure 1-c and 1-d. An applied load Q may cause an unequal linear soil contact pressure distribution, Figure 1-e.
(1) Cohesionless soil
Cohesionless soil is often composed of granular or coarse-grained materials with visually detectable particle sizes and with little cohesion or adhesion between particles. These soils have little or no strength when unconfined and little or no cohesion when submerged. Apparent adhesion between particles in cohesionless soil may occur from capillary tension in pore water. Settlement usually occurs rapidly with little long-term consolidation and secondary compression or creep. Time rate effects may become significant in proportion to the silt content such that the silt content may dominate consolidation characteristics.
(a) Uniformly loaded rigid foundations (footings of limited size or footings on cohesionless soil) may cause less soil contact pressure near the edge than near the center, Figure 1-a, because this soil is pushed aside at the edges due to the reduced confining pressure. This leads to lower strength and lower modulus of elasticity in soil near the edge compared with soil near the center. The parabolic soil contact pressure distribution may be replaced with a saddle-shaped distribution, Figure 1-b, for rigid footings or mats if the soil pressure does not approach the allowable bearing capacity.
(b) The distortion of a uniformly loaded flexible footing, mat, or
embankment on cohesionless soil will be concave downward, Figure 1-c, because the soil near the center is stressed under higher confining pressure such that the modulus of elasticity of the soil is higher than near the edge.
(c) The theory of elasticity is not applicable to cohesionless soil when the stress or loading increment varies significantly throughout the soil such that an equivalent elastic modulus cannot be assigned. Semi-empirical and numerical techniques have been useful to determine equivalent elastic parameters at points in the soil mass based on stress levels that occur in the soil.
Cohesive soil often contains fine-grained materials consisting of silts, clays, and organic material. These soils have significant strength when unconfined and air-dried. Most cohesive soil is relatively impermeable and when loaded deforms similar to gelatine or rubber; i.e., the undrained state. Cohesive soils may include granular materials with bonding agents between particles such as soluble salts or clay aggregates. Wetting of soluble agents bonding granular particles may cause settlement in loose or high void ratio soil
(a) A uniform pressure applied to a rigid foundation on cohesive soil, Figure 1. Relative distribution of soil contact pressures and displacements of rigid and flexible mats or footings on cohesionless and cohesive soils Figure 1-b, can cause the soil contact pressure to be maximum at the edge and decrease toward the center because additional contact pressure is generated to provide stress that shears the soil around the perimeter.
(b) A uniform pressure applied to a flexible foundation on cohesive soil, Figure 1-d, causes greater settlement near the center than near the edge because the cumulative stresses are greater near the center as a result of the pressure bulb stress distribution indicated in Figure 2. Earth pressure measurements from load cells beneath a stiffening beam supporting a large, but flexible, ribbed mat also indicated large perimeter earth pressures resembling a saddle-shaped pressure distribution similar to Figure 1-b.
(c) Elastic theory has been found useful for evaluation of immediate settlement when cohesive soil is subjected to moderate stress increments. The modulus of elasticity is a function of the soil shear strength and often in creases with increasing depth in proportion with the increase in soil shear strength.
(d) Cohesive soil subject to stresses exceeding the maximum past pressure
of the soil may settle substantially from primary consolidation and secondary compression and creep.
Sources of Stress
Sources of stress in soil occur from soil weight, surface loads, and environmental factors such as desiccation from drought, wetting from rainfall, and changes in depth to groundwater.
(1) Soil weight
Soil strata with different unit weights alter the stress distribution. Any change in total stress results in changes in effective stress and pore pressure. In a saturated soil, any sudden increase in applied total stress results in a corresponding pore pressure increase. This increase may cause a flow of water out of the soil deposit, a decrease in pore pressure, and an increase in effective stress. Changes in pore water pressure such as the raising or lowering of water tables also lead to a reduction or increase in effective stress.
(2) Surface loads
Loads applied to the surface of the soil mass increase the stress within the mass. The pressure bulb concept, Figure 2, illustrates the change in vertical stress within the soil mass. Placement of a uniform pressure over a foundation with a minimum width much greater than the depth of the soil layer will cause an increase of vertical stress in the soil approximately equal to the applied pressure.
(3) Rules of thumb for static loads
Preliminary settlement analyses are sometimes performed before the structural engineer and architect are able to furnish the design load conditions.
(a) Some rules of thumb for line and column loads for buildings described
in Table 1 are based on a survey of some engineering firms. Tall multistory structures may have column loads exceeding 1000 tons. Column spacings are often 20 to 25 ft or more. The average pressure applied per story of a building often varies from 0.1 to 0.2 tsf.
(b) Vertical pressures from embankments may be estimated from the unit wet weight times height of the fill.
(c) Vertical pressures from locks, dams, and retaining walls may be estimated by dividing the structure into vertical sections of constant height and evaluating the unit weight times the height of each section.