Geotechnical engineering models and applications
Soil behavior
Geotechnical questions test effective stress, consolidation, shear strength, and bearing capacity. Always separate total stress from pore pressure: effective stress is total stress minus pore pressure. Use effective unit weight below the water table; a frequent trap is applying saturated unit weight to the full depth and forgetting hydrostatic uplift. Shear strength of saturated fine-grained soil is expressed with effective-stress parameters c′ and φ′; for long-term conditions use drained parameters, and for rapid loading in low-permeability soil consider undrained strength.
Classification and compaction
Know the USCS sieve boundaries and plasticity criteria: gravel >4.75 mm, sand 0.075–4.75 mm, fines <0.075 mm. Liquid limit, plastic limit, and plasticity index drive CL/CH/ML/MH distinctions. Compaction curves show optimum moisture content and maximum dry density; a question may ask whether adding water increases or decreases density on the dry or wet side of optimum. Relative density and standard Proctor versus modified Proctor energy are common traps.
Worked example
A 10 ft sand layer has γ = 120 pcf above the water table, which is 5 ft deep; below it γsat = 125 pcf. Total vertical stress at 10 ft = 120(5) + 125(5) = 1,225 psf. Pore pressure u = 62.4(5) = 312 psf. Effective stress = 1,225 − 312 = 913 psf. Using 62.4 for water and keeping the water table depth explicit avoids the usual error.
Walls and slopes
For retaining walls, active pressure is lower than at-rest, and passive resistance is higher; do not mix them. Slope stability questions usually ask for factor of safety against shear strength, and liquefaction screening considers loose saturated sands and seismic loading.
Ground models, foundations and retaining systems
A borehole describes one sampled location, not an automatically continuous layer across the whole site. Combine geological interpretation, samples, laboratory results and in-situ observations into a ground model, then test alternative credible conditions. Soil parameters require a suitable drainage condition, stress path and scale. An apparent correlation between a penetration index and strength is an empirical model that needs its actual calibration, rather than a universal conversion.
Pile resistance includes compatible shaft and base mechanisms and an appropriate deformation model. Negative skin friction develops when surrounding ground settles relative to a pile; it is not additional beneficial upward resistance. Group settlement can differ from a single-pile response because the stressed ground volume and load distribution change. A capacity estimate does not by itself establish settlement acceptability, structural integrity or installation quality.
Retaining and excavation analyses must include water pressures, surcharge, construction sequence and the movement needed to mobilize resistance. Active pressure is not automatically appropriate for an unyielding wall. A drain can lose performance, so its reliability and maintenance matter. Identify global stability, bearing, sliding, structural resistance, basal heave and seepage as separate possible mechanisms. An empirical safety factor or monitoring trigger must come from the stated assessment framework, not an invented universal safe value.