What Is a Geotechnical Report?
A geotechnical report documents subsurface soil, rock and groundwater conditions at a site and gives recommendations for foundations, earthwork and pavements. It is prepared by a geotechnical engineer after drilling borings or digging test pits.
What is in a geotechnical report
A typical report includes a site description, the field exploration (number, location and depth of borings or test pits), boring logs, laboratory test results, groundwater observations and engineering recommendations. Logs often show Standard Penetration Test blow counts (N-values, per ASTM D1586), which indicate soil density or consistency.
Recommendations usually cover the foundation type, allowable bearing pressure, estimated settlement, frost depth, slab-on-grade support, lateral earth pressures for walls, seismic site class, excavation and dewatering, fill placement and compaction, and pavement design parameters.
When one is required
The International Building Code (Section 1803) requires geotechnical investigations for most commercial buildings, with some exceptions where the building official accepts presumptive soil values. The International Residential Code allows presumptive bearing values for most houses but lets the building official require a soil test where expansive, compressible, shifting or unknown soils are likely.
Lenders, structural engineers and public agencies frequently require reports even when the code does not, especially for hillside lots, fill sites, sites near water, large retaining walls and anything heavy.
What it costs and how long it takes
Cost depends on the number and depth of borings, drill rig access and lab testing. A small residential investigation with a couple of shallow borings is commonly a few thousand dollars, while commercial investigations cost more. Field work usually takes a day or two; lab testing and the report often take a few weeks.
Give the geotechnical engineer your site plan and building loads so the borings are placed where the structure will be and deep enough to evaluate it.
Frequently asked questions
Is a geotechnical report the same as a perc test?
No. A perc test measures how fast water infiltrates for septic or stormwater design. A geotechnical report evaluates soil strength and behavior for foundations and earthwork, though one firm may do both.
What is an N-value?
The number of hammer blows needed to drive a standard sampler 12 in in the Standard Penetration Test. Higher numbers mean denser or stiffer soil.
Can I build without a soils report?
For many houses the IRC presumptive values are accepted. The building official can require a report where soils are questionable.
Sources
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