Site investigation

Site investigation finds out what lies beneath a site before anything is designed. Most foundation failures, claims and cost overruns trace back to ground that was different from what was assumed. A good investigation is cheap insurance: it typically costs a small fraction of a percent of the project but controls the design of foundations, retaining structures, excavation and drainage.

It proceeds in stages. A desk study reviews geological maps, old borelogs, satellite images, flood and groundwater records. A walkover survey looks for springs, cracks in nearby buildings, fill, rock outcrops and signs of landslides. The main investigation then uses boreholes with standard penetration tests (SPT) and undisturbed tube samples in clays; static cone penetration tests (CPT) for continuous profiles; trial pits for shallow strata; and rock coring with RQD where rock is met. Groundwater levels are recorded and sometimes monitored with piezometers. Geophysical methods such as seismic refraction and electrical resistivity fill gaps between boreholes.

Depth and spacing depend on the structure: boreholes should go below the depth where the added stress becomes small, typically around one and a half to two times the footing or raft width, and into rock for piles. Samples go to laboratory tests for index properties, strength, consolidation, chemistry and permeability. The output is a geotechnical report with borelogs, a soil profile, design parameters and recommendations.