Soil Mechanics Fundamentals
Soil mechanics is the branch of civil engineering that studies the behavior of soils under loads and environmental conditions. The classification of soils is based on particle size distribution, with the USCS system dividing soils into coarse-grained soils such as gravels and sands, and fine-grained soils such as silts and clays. The particle size boundaries are defined by sieve openings, with the No. 200 sieve at 0.075 millimeters separating coarse from fine soils. The Atterberg limits describe the moisture content boundaries between the liquid, plastic, and solid states of fine-grained soils. The plasticity index, calculated as the difference between the liquid limit and the plastic limit, indicates the range of moisture content over which the soil behaves plastically.
The effective stress principle is the most important concept in soil mechanics. The total stress at any point in a soil mass is carried partly by the soil skeleton as effective stress and partly by the pore water as pore water pressure. Only changes in effective stress cause volume changes and strength changes in soils. When a load is applied to saturated clay, the initial increase in pore water pressure gradually dissipates as water drains from the soil, a process called consolidation. The rate of consolidation depends on the soil permeability and the drainage path length. Primary consolidation can take years to complete in thick clay deposits with low permeability.
Shear Strength of Soils
The shear strength of soil determines its ability to resist sliding and bearing failures. The Mohr-Coulomb failure criterion describes the shear strength as a function of cohesion, normal stress, and friction angle. The cohesion intercept represents the shear strength at zero normal stress, which is significant for clay soils but negligible for sands. The friction angle ranges from 30 to 45 degrees for sands and gravels and from 15 to 30 degrees for clays. wind load calculation methods for low rise buildings. seismic force resisting system design options. steel column buckling design according to AISC specification. The effective stress shear strength parameters are measured in the laboratory using direct shear tests or triaxial compression tests on undisturbed soil samples.
The drained and undrained shear strength of soils are fundamentally different and depend on whether pore water can drain during loading. In drained loading, pore water dissipates and the soil volume changes during shearing. In undrained loading, no drainage occurs and the soil shears at constant volume. The undrained shear strength of saturated clay is equal to the cohesion in terms of total stress. The undrained strength is used for short-term stability analyses of foundations on clay, while the drained strength is used for long-term stability analyses.
Earth Pressure and Retaining Structures
The lateral earth pressure exerted by soil against retaining structures depends on the wall movement relative to the soil. At-rest earth pressure exists when the wall does not move relative to the soil. Active earth pressure develops when the wall moves away from the soil, reducing the lateral pressure. Passive earth pressure develops when the wall moves into the soil, increasing resistance. The Rankine and Coulomb theories provide methods for calculating these pressures based on soil properties and wall geometry.
The design of retaining walls must consider the lateral earth pressure, surcharge loads from adjacent structures or traffic, groundwater pressure, and the wall weight. The wall must be stable against sliding, overturning, and bearing capacity failure. The factor of safety against sliding should be at least 1.5. The factor of safety against overturning should be at least 2.0. The maximum bearing pressure under the wall base must not exceed the allowable bearing capacity of the foundation soil. Drainage behind the wall is essential to prevent hydrostatic pressure buildup that can cause wall failure.