Soil mechanics governs how foundations and earthworks interact with the ground. Terzaghi's effective stress principle establishes the foundational relationship σ = σ' + u, where σ is total stress applied to a soil mass, u is the pore water pressure, and σ' is the effective stress carried by the soil skeleton. Effective stress, not total stress, controls the shear strength and volume change behavior of the soil, making this equation central to every subsequent analysis.
Fine-grained soils are classified by their consistency at varying water contents, defined by the Atterberg limits. The liquid limit is the water content at which soil transitions from a plastic to a liquid state, typically measured using a Casagrande device or fall cone. The plasticity index, PI = LL − PL, defines the range of water content over which the soil behaves plastically and is a primary indicator of clay activity. These limits, along with grain-size distribution, underpin the Unified Soil Classification System, which designates soils by two-letter symbols such as GW for well-graded gravel or CL for low-plasticity clay.
Under sustained load, saturated fine-grained soils undergo consolidation, a time-dependent volume decrease driven by the gradual expulsion of pore water. The coefficient of consolidation, Cv, quantifies how quickly this process occurs; clays with high Cv consolidate faster than those with low Cv, a fact that directly influences settlement predictions and the design of surcharge or wick-drain acceleration schemes. Compression behavior contrasts with shear behavior, which is probed by the Standard Penetration Test, in which a split-spoon sampler is driven 450 mm by a 63.5 kg hammer falling 760 mm, with the recorded blow count N serving as an empirical index of soil density or consistency. The Cone Penetration Test pushes an instrumented cone into the ground at a constant rate, continuously recording tip resistance and sleeve friction to produce a refined stratigraphic profile and engineering properties. For very soft clays, the vane shear test measures undrained shear strength in situ by rotating a four-bladed vane and recording the torque at failure. Field results are synthesized into a soil profile, a graphical cross-section showing the succession of subsurface layers, thicknesses, groundwater location, and characteristic properties used to inform foundation and earthwork design.
Foundation design requires two central quantities: bearing capacity and settlement. The bearing capacity is the maximum pressure the soil can sustain beneath a footing without shear failure or excessive settlement. Terzaghi's equation for a strip footing, q_u = cN_c + γD_f N_q + 0.5γBN_γ, combines the contributions of cohesion, surcharge from foundation depth, and soil weight through bearing capacity factors that depend on the friction angle. Saturated loose granular soils are vulnerable to liquefaction, in which cyclic loading from earthquakes raises pore pressure until effective stress drops to zero and the soil momentarily behaves like a liquid. Deep foundations transfer loads through weak surface soils to stronger strata below. End-bearing piles develop most of their capacity by bearing on a hard stratum at the tip, while friction piles rely on skin friction developed along the embedded shaft. Both are modeled as slender columns transmitting axial loads, often with additional lateral capacity checked against structural and geotechnical criteria.
Soils also exert lateral pressures on retaining structures. Rankine's earth pressure theory calculates active and passive pressures assuming a cohesionless backfill and a planar failure surface, giving K_a = tan²(45° − φ/2) for the active case and K_p = tan²(45° + φ/2) for the passive case. Stability of natural and constructed slopes is assessed by comparing the available shear strength along a potential failure surface to the shear stress required for equilibrium; the factor of safety, typically required above about 1.5 for permanent slopes, is the ratio of these two quantities. Geosynthetics, including geotextiles, geomembranes, geogrids, and geocomposites, enhance soil performance through separation, filtration, drainage, and reinforcement. Engineered fills are compacted to densify the soil matrix by reducing air voids, increasing strength and reducing compressibility and permeability. The Proctor compaction test defines, in the laboratory, the optimum moisture content at which a given compactive effort yields the maximum dry density, providing the reference for field specifications and quality control.
Groundwater flow through porous media is described by Darcy's law, Q = kiA, in which Q is volumetric discharge, k is the hydraulic conductivity, i is the hydraulic gradient, and A is the cross-sectional area. Sands typically exhibit conductivities around 10⁻² cm/s, while clays lie near 10⁻⁷ cm/s, a difference of five orders of magnitude that explains why clay layers act as aquitards. Engineers visualize flow through earth structures using flow nets of intersecting flow lines and equipotential lines, from which seepage quantities, pore pressures, and uplift forces beneath cofferdams, dams, or excavations can be estimated. When construction requires excavation below the water table, dewatering systems of wells, wellpoints, or sumps are installed to lower the groundwater temporarily so that excavation and foundation work can proceed in stable, dry conditions.