How soil is tested for construction, and why

Why soil is tested before building

Soil carries the weight of every building. Soft clay can settle unevenly for years, expansive clay can lift and crack a slab, loose fill can compress, a high water table floods basements, and steep slopes can creep downhill. The investigation finds these problems while they are cheap to design around, rather than after the walls crack.

Large buildings always get one. For houses, it depends on the place: building departments in areas with expansive clay, fill or steep lots commonly ask for a soils report, and lenders or engineers may ask for one on any unusual site.

How a soil investigation works

It starts at a desk: the engineer reviews the site plan, geology maps and published soil surveys, including the USDA data behind this site, to know what to expect. Then a drill rig visits the site and bores holes at selected locations to build a profile of the layers.

The most common field test is the standard penetration test (SPT). As a Florida A&M University engineering course describes it, a 140-pound hammer is dropped 30 inches to drive a split-spoon sampler 18 inches into the ground; the blows needed for the last 12 inches give the "N-value", a measure of how dense or stiff the soil is. The spoon also brings up a sample. Thin-walled tubes collect undisturbed samples of clay for strength and settlement tests, and the crew notes where groundwater appears.

How deep the holes go depends on the building. The same course gives a general rule of drilling through any unsuitable layers (fill, organic or compressible soil) to soil that can carry the load, and at least 1.5 to 2 times the width of the loaded area; heavy structures go to bedrock.

Lab tests

Back at the lab, samples are classified and tested. Typical tests include moisture content, grain size (how much sand, silt and clay), plasticity limits that show how a clay behaves as it gets wetter, organic content, and for clay, strength and consolidation tests that predict how much and how fast a foundation will settle. Swell tests check expansive clay.

What the geotechnical report contains

The report includes a site map with boring locations, a log for each boring (soil descriptions, sample depths, SPT blow counts and the water level) and a soil profile across the site. Its recommendations are what the structural engineer and builder use: foundation type and depth, allowable bearing pressure, expected settlement, groundwater and drainage advice, retaining wall pressures, and how to prepare and compact the ground.

Septic perc tests are a different test

A percolation test for a septic system measures how fast water soaks into the soil, not how much weight it can carry. It is usually required by the county health department before a septic permit. See where soil is best for septic systems for how soil survey ratings compare across the country.

What a soil survey can and cannot tell you

USDA soil survey data, the source behind this site, maps soil properties at a scale of fields and hillsides. It is a good first look at a lot: drainage, flooding, slope, clay content and septic limitations. It cannot replace borings on the actual site, because a soil map unit includes patches of different soil and construction often changes what was there. Use the ZIP lookup for the first look, and a geotechnical engineer for the answer.