Soil, rock, and the ground that everything is built upon.
Geotechnical engineering is the branch of civil engineering concerned with the behavior of soil and rock and how they interact with foundations, slopes, and earth structures — making it the discipline that determines what the ground beneath a project can safely support.
Geotechnical engineering studies the mechanics of soil and rock to make sure structures can be founded safely on or in the ground. It begins with a subsurface investigation — borings, sampling, and laboratory and in-situ testing (SPT, CPT, Atterberg limits, consolidation, and shear-strength tests) — to characterize the site. From that data the engineer classifies the soils (often by the Unified Soil Classification System) and develops strength, compressibility, and groundwater parameters.
With those parameters the engineer evaluates bearing capacity and settlement for foundations, the stability of slopes and excavations, and the earth pressures acting on retaining walls. In seismic regions the work also includes evaluating liquefaction potential and ground motion. The geotechnical report — recommendations for foundation type, allowable bearing pressure, settlement, lateral earth pressures, and construction considerations — is the basis the structural and civil teams design from.
Subsurface exploration, sampling, and lab/in-situ testing (SPT, CPT, Atterberg limits, consolidation) to characterize and classify soil and rock.
Bearing capacity, settlement, and the selection and design of shallow footings, mats, and deep foundations (piles, drilled shafts).
Analysis of natural and engineered slopes, embankments, excavations, and shoring to guard against landslides and failures.
Lateral earth pressures and the design of retaining walls, sheet piles, and braced or anchored excavation support.
Liquefaction and seismic site-response evaluation, plus ground-improvement methods such as compaction, grouting, and soil mixing.
A geotechnical engineer evaluates the soil and rock at a site to determine how to found a structure safely. They plan subsurface investigations, interpret boring and lab data, calculate bearing capacity and settlement, analyze slope and excavation stability, design retaining walls, and assess seismic hazards. The result is a geotechnical report that the structural and civil engineers use to design foundations and earthworks.
Geotechnical engineers focus on the ground — soil and rock behavior, bearing capacity, settlement, and slope stability — and recommend what the soil can support. Structural engineers design the building or bridge itself: the foundations, columns, beams, and load paths. The two work closely together, because the geotechnical recommendations (allowable bearing pressure, settlement limits, lateral earth pressures) define the loads and constraints the structural design must satisfy.
A geotechnical report documents the subsurface conditions found during investigation and gives engineering recommendations — foundation type, allowable bearing pressure, expected settlement, lateral earth pressures, groundwater, and construction considerations. Building codes (IBC Chapter 18) require it for most significant structures because foundation design cannot proceed safely without knowing the strength and compressibility of the supporting soil.
The typical path is a civil engineering degree (often with a geotechnical or soils emphasis or a graduate degree), passing the FE Civil exam, gaining experience under a licensed engineer, and then passing the PE Civil exam with the Geotechnical depth module to become a licensed Professional Engineer. Licensure is required to stamp geotechnical reports and foundation recommendations.