What Does a Geotechnical Report Tell You About Your Site?

A geotechnical report tells you what your building will sit on and what the ground will do about it: soil types and strengths at depth, groundwater levels, bearing capacities, expected settlement, seismic site class, and the foundation systems the geotechnical engineer recommends. Your structural engineer designs the foundation from these recommendations — without them, they're guessing at the single most consequential assumption in the building. The report has two halves: the factual half (what was found — boring logs, lab results, groundwater observations) and the interpretive half (what it means — bearing values, settlement predictions, foundation recommendations, earthwork and drainage guidance). Both halves matter, but the recommendations are what your structural engineer builds from. Read it with one caution in mind: a report is an engineering interpretation of a handful of holes in the ground, not an X-ray of the whole site. Conditions between borings are inferred, and every competent report says so. That is not a weakness to skim past — it is the reason the report also specifies who should observe the excavation, and what should happen if the ground that shows up differs from the ground that was drilled.

Skip to the recommendations first — that's the actionable core. It typically covers allowable bearing capacity for shallow foundations, deep foundation options with capacities if shallow won't work, estimated total and differential settlement, lateral earth pressures for retaining walls, seismic site class and design parameters, and earthwork requirements like compaction and over-excavation. Then read the numbers the way your structural engineer will. Bearing capacity comes with conditions attached: a stated value at a stated minimum footing depth and width, sometimes with an allowed increase for depth or width. Settlement estimates split into total settlement (usually tolerable if uniform) and differential settlement (the kind that cracks finishes and racks doors). Lateral design values appear as active, at-rest, and passive pressures plus a coefficient of friction for sliding — the retaining wall designer needs all four. Earthwork language matters just as much: compaction is usually specified as a percentage of the maximum dry density from ASTM D1557 (90–95% is typical for structural fill), and over-excavation depths below footings and slabs are prescriptions, not suggestions. Then read the boring logs with the site plan in mind: where were the holes, how deep, and what did they hit? A report with two shallow borings on a three-acre commercial site is thin coverage, and your structural engineer should say so. Groundwater depth deserves special attention — it affects excavation, waterproofing, buoyancy, and corrosion, and seasonal variation means the observed level isn't necessarily the design level.

The boring log is a depth-by-depth diary of the site. For each stratum it records the soil description and classification, the samples taken, and the Standard Penetration Test results performed under ASTM D1586 — the number of hammer blows (the N-value) needed to drive a standard sampler one foot. N-values are the report's shorthand for consistency and density: very low counts (roughly 0–4) mean very loose sand or very soft clay, mid-range counts mean medium, workable material, and counts above about 30–50 mean dense or hard ground, with 'refusal' where the sampler effectively stops advancing. A footing recommendation that looks generous is often just the arithmetic of the N-values underneath it. Soil names follow the Unified Soil Classification System (ASTM D2487), and the two-letter symbols repay learning. GW and SW are well-graded gravels and sands — generally friendly founding soils. SM and SC are sands carrying silt or clay fines — workable, but moisture-sensitive. ML is silt, CL is lean clay, and CH is fat clay, the high-plasticity material behind most expansive-soil problems. The lab results explain which behavior to expect: Atterberg limits and the plasticity index describe how much a clay swells and shrinks with moisture change, the Expansion Index test (ASTM D4829) grades that potential from very low through very high, moisture-versus-density testing sets the compaction target, and sulfate and resistivity results tell the structural engineer how aggressive the soil is toward buried concrete and steel. When a recommendation seems expensive — deepened footings, a stiffened slab, imported select fill — trace it back to one of these numbers. It is almost always there.

The International Building Code puts geotechnical investigation in Chapter 18. Section 1803 requires investigations in defined circumstances — including sites with questionable, expansive, or compressible soils, certain seismic design situations, and foundations the building official has reason to doubt — and many jurisdictions go further and require a report for commercial work as a matter of local policy. Where an investigation genuinely isn't required, the code still only lets a designer fall back on presumptive load-bearing values from IBC Table 1806.2, and those are deliberately conservative: clay's presumptive value is just 1,500 psf, so 'skipping the report' usually means bigger footings designed blind, not money saved. On cost, plan on roughly $1,500–$4,000 for a limited residential investigation, $4,000–$12,000 for a small commercial site with several borings and a standard lab program, and $15,000–$40,000 or more for large sites, multi-story buildings, deep foundations, or rock coring. The drivers are boring count and depth, access (a limited-access rig working through a building or a tight yard costs more than a truck rig on open ground), laboratory scope such as shear-strength and consolidation testing, and how much engineering the findings demand. Two costs owners forget to budget: the geotechnical engineer's review of the foundation plans their report recommended, and their observation during grading and footing excavation. Both are small, and both close the loop the report opens.

Take a typical finding from a small commercial site. Boring B-2, drilled near a proposed building corner, shows asphalt and topsoil to about one foot; loose, undocumented fill from one to three feet, with SPT N-values of 2–3; then native lean clay (CL) from three to ten feet with N-values of 8–14, a plasticity index in the low 20s, and an Expansion Index of 55 — the medium expansion range. Groundwater wasn't encountered to fifteen feet, but rust-colored mottling at about eight feet suggests seasonal perched water. Watch how the recommendation writes itself from those facts. Footings cannot bear on the loose fill, so the report requires it removed and footings founded on the native clay at a minimum of 30 inches below finished grade, with an allowable bearing capacity of 2,000 psf, estimated total settlement of about three-quarters of an inch, and differential settlement of roughly half that. The medium-expansion clay drives the rest: the clay under slabs and footings is to be moisture-conditioned before concrete, the slab gets specific jointing and reinforcement (or the project elects a stiffened foundation), and site drainage must keep roof and surface water away from the building edge — because the mottling says water visits this soil seasonally even though drilling day was dry. Now the counter-example. If the design team had ignored the report, used an assumed bearing value, and poured a standard slab on the fill, the predictable first-year outcome is differential movement: the fill consolidates unevenly, the slab edge lifts and settles with the clay's moisture cycle, and the building shows it as cracked finishes, sticking doors, and a slab the tenant blames on the builder. None of that is bad luck. Every element of it was written down, in numbers, in a report somebody chose not to design from.

Not all geotechnical reports are equally useful. The difference is usually in the scoping conversation before drilling starts. Here's how I make sure the report answers the questions the design team will ask.

• Share the structural concept early: building size, loads, and basement depth drive boring depth and spacing
• Ask for specific recommendations with numbers — bearing values, settlement estimates, lateral pressures
• Confirm seismic site class and design parameters are included, not left for the structural engineer to assume
• Require groundwater discussion with seasonal context, not just the level on drilling day
• Get earthwork and drainage recommendations — most foundation failures are water problems, not soil problems
• Keep the geotechnical engineer engaged through design: have them review the foundation plans they recommended