Soil reports and foundations: what a geotechnical report tells you

A geotechnical (soil) report comes from a geotechnical engineer who drills borings on the lot, logs and samples the soil, tests it in a lab and writes recommendations. Its job, in a Houston engineers' recommended practice, is to give the structural engineer and builder enough to design a foundation that suits the site and meets local code. [2]

It is written for one lot and one house position. The same practice says a report done for a new subdivision's streets and utilities is not a substitute for one done for the foundation. [2]

Some clays swell when they get wet and shrink as they dry. USDA describes these soils, called Vertisols, as ones that shrink and swell markedly with changes in moisture, and says the very high shrink-swell potential of Houston Black clay, the Texas state soil, commonly limits building site development. [6] Despite the name, Houston Black is a Blackland Prairie soil, found from north of Dallas south to San Antonio. [6]

Around Houston the usual clay is a different series that behaves the same way. The Lake Charles series, a Vertisol formed in clays of the Beaumont Formation, is extensive in Southeast Texas between the Colorado and Trinity Rivers, and when dry it cracks 1/2 to 2 inches wide. [5] A foundation on soil like that has to be designed for ground that moves.

Texas law reflects this. The exemption that lets anyone draw a private dwelling does not cover engineering design for a residential slab on expansive soil unless the construction meets the applicable International Residential Code requirements. [7]

The International Residential Code (IRC), which most US jurisdictions adopt with local changes, says that where soil science data shows expansive, compressible, shifting or other questionable soils are likely, the building official decides whether to require a soil test. [1]

It classifies soil as expansive by lab tests, among them a plasticity index (PI) of 15 or more or an expansion index above 20, and sends foundations on expansive soil to the International Building Code's design rules. [1] Those rules call for post-tensioned slabs on expansive soil to be designed to the Post-Tensioning Institute's standard, PTI DC10.5, and for conventionally reinforced slabs to follow the WRI/CRSI slab-on-ground method. [3]

Your local code is the one that applies. Ask the permit office whether it requires a soil report for a new house, and expect your engineer to want one regardless.

The Foundation Performance Association in Houston publishes a recommended practice written by structural engineers in Southeast Texas after a large number of residential foundation problems. It is guidance for that region, not code, but it shows what a thorough report covers. [2]

Borings: for a single lot, one per 2,500 square feet of ground floor slab and at least two, inside or close to the house outline, at least 20 feet deep, and 25 feet on wooded lots with clay soils. With trees within 25 feet of the house, one boring goes near the largest tree. [2]

Lab tests: moisture content on every sample, and in clay, Atterberg limits (liquid limit, plastic limit and plasticity index) on at least a third of the samples, along with clay content and soil suction. [2]

Recommendations: the water table if found, ways to deal with expansive soil, earthwork to prepare the site, the foundation maintenance the design assumes, and the probable movement of the ground surface, both up and down. For slabs it asks for design values under both the WRI and the PTI methods, and for drilled piers, depths that account for the soil pushing up as well as the house pushing down. [2]

Ask the geotechnical engineer to walk you through the numbers that drive your design. You do not need to compute anything, but you should know whether your lot is mild or severe.

At a high level, a slab on ground sits on the soil and is designed to ride its movement. Post-tensioned slabs on expansive soil have their own design standard, PTI DC10.5, and PTI says its residential design method has been used on millions of foundations since 1980. [3]

Pier based systems carry the house down to deeper soil. The Houston practice covers slabs on piers, fully suspended structural slabs over a void space, and framed floors on piers with a crawl space, and asks the soil report for the values each one needs. [2]

Which one fits your lot is the engineer's call, made from the report. Every plan on this site lists its foundation options on the plan page, and the plans for sale offer a post tension slab, pier and beam, or a crawlspace. Those are starting points, not a design for your soil: plans may need a foundation designed for your soil report and lot elevation.

USDA's Natural Resources Conservation Service runs Web Soil Survey, free soil maps and data for more than 95 percent of US counties. Draw your lot on it and you can see which soil series are mapped there and how they are rated. [4]

It is a planning tool, and the site itself says onsite investigation is needed for certain engineering applications. [4] It maps soils across a landscape, not under your footprint, so it does not replace borings.

The engineer needs to know where the house sits. The Houston practice expects the client to supply the foundation outline and its location on a site plan, with the trees to be removed, and places borings inside the planned house outline. [2] So the report comes once you have a plan and know roughly where it sits on the lot, and before the foundation is designed and the permit is filed.

If the lot already has fill on it, tell the geotechnical engineer: the practice asks for closer sampling in fill, and for fill placed after the borings to be checked separately. [2]

Does the permit office here require a soil report for a new house, and does my lender?

Will the borings be taken inside the planned house outline, how many, and how deep?

Does the report give the design values my engineer needs for the foundation types we are considering?

What maintenance does the foundation design assume, such as drainage, watering near the slab and the trees we keep?

Who designs the foundation from this report, and are they a licensed engineer in this state?