The geotechnical report is the single most important document in site civil design. It tells you what is under the ground, what the ground can support, how it will behave when you load it, excavate it, or add water to it. Every foundation, every retaining wall, every pavement section, every slope on your project is designed using parameters from the geotech report. If the report is incomplete, your design is based on assumptions. If the assumptions are wrong, things move, crack, or fail.

This article covers what a geotechnical report contains, what civil engineers need to extract from it, and the questions to ask when key information is missing.

What Is in a Standard Geotechnical Report

Site Exploration

The field investigation, typically consisting of hollow-stem auger borings, cone penetrometer tests (CPTs), and/or test pits. The report should include:

• Boring location map — shows where on the site each boring was drilled. Critical for understanding which borings are representative of which areas.
• Boring logs — detailed descriptions of each soil layer encountered, including soil classification (USCS system), color, moisture content, consistency or density, and the depth of each layer change.
• SPT blow counts — the Standard Penetration Test (SPT) N-value at regular intervals (typically every 2.5 or 5 feet). The N-value is a measure of soil density/consistency. N less than 4 is very soft/loose. N of 10 to 30 is medium dense. N greater than 50 is very dense or refusal (rock).
• Groundwater observations — the depth at which groundwater was encountered during drilling and the stabilized water level measured 24 to 48 hours after drilling. Seasonal high groundwater may be different from what is measured at the time of drilling.

Laboratory Testing

Samples collected during the field investigation are tested in the laboratory. Common tests include:

• Atterberg limits — liquid limit, plastic limit, and plasticity index. These characterize the fine-grained soil's behavior and its potential for expansion (shrink-swell).
• Expansion index — a direct measure of how much the soil swells when wetted. An expansion index above 90 is "very high" and requires special foundation design considerations.
• Grain size analysis — the percentage of gravel, sand, and fines. Important for drainage design and soil classification.
• Shear strength — direct shear or triaxial compression tests to determine cohesion and friction angle. These parameters are used in retaining wall design and slope stability analysis.
• Consolidation — measures how much the soil compresses under load and how long it takes. Critical for settlement predictions on compressible soils.
• R-value or CBR — resistance value or California Bearing Ratio of the subgrade soil. Used directly in pavement section design.
• Corrosivity — pH, chloride, sulfate, and resistivity of the soil. Determines whether concrete or metal pipes need special protection.

Engineering Analysis and Recommendations

The heart of the report. This section provides the design parameters that civil and structural engineers use:

• Allowable bearing pressure — how much load the soil can support under foundations, typically in pounds per square foot (psf). A value of 2,000 psf is typical for medium-dense sand or stiff clay. Values of 500 to 1,000 psf for soft soils may require deep foundations.
• Lateral earth pressure coefficients — for retaining wall design. Active, passive, and at-rest pressure coefficients, plus the equivalent fluid density for simplified design.
• Pavement section recommendations — the recommended asphalt and aggregate base thickness for parking lots and roadways, based on the subgrade R-value and the expected traffic loading.
• Seismic design parameters — site class (A through F per ASCE 7), mapped spectral accelerations, and seismic design category. These drive the structural design and the seismic earth pressure on retaining walls.
• Settlement estimates — predicted total and differential settlement under the proposed loads. Differential settlement (uneven settling) is usually more damaging than total settlement.
• Grading recommendations — minimum compaction requirements (typically 90 percent relative compaction), moisture conditioning requirements, and subgrade preparation procedures.

What to Look for as the Civil Engineer

When you receive a geotech report, here is your checklist:

1. R-value or CBR for pavement design. If it is not in the report, you cannot design the pavement section. Ask the geotech to test it. Do not assume a value.
2. Expansion index. Expansive soils affect flatwork, building slabs, retaining walls, and utility trenches. If the EI is above 50, you need to account for it in your design. Above 90, you may need moisture-conditioned pads, post-tensioned slabs, or deepened foundations.
3. Groundwater depth. If groundwater is within 5 feet of the proposed utility trench depths, you need a dewatering plan. If it is within the excavation depth for foundations or basements, it drives the shoring and waterproofing design.
4. Corrosivity data. If soil sulfate levels exceed 1,500 ppm, Type V cement or sulfate-resistant concrete is needed for any concrete in contact with soil. If soil resistivity is low (high corrosivity), ductile iron pipe may need polyethylene encasement.
5. Lateral earth pressure for walls. If the report does not include equivalent fluid densities or lateral pressure coefficients for the specific wall heights and surcharges on your project, request a supplemental letter.
6. Slope stability analysis. If the project involves slopes over 10 feet tall or building setbacks from existing slopes, the report should include a slope stability analysis with a minimum factor of safety of 1.5 static and 1.1 to 1.25 seismic.

When the Report Is Not Enough

A geotech report prepared for a previous project on the same parcel is not automatically valid for your project. Different building locations, different loading conditions, and different excavation depths can render an old report inadequate. Most jurisdictions require a current report (within 2 to 5 years) or a letter from the geotechnical engineer confirming that the previous report's recommendations still apply.

If the report was prepared at the due diligence or entitlement stage, it may be based on a preliminary site plan. If the building location, size, or configuration changes during design, the geotech recommendations may need to be updated. Always have the geotechnical engineer review the final grading plan and building plans to confirm that the original recommendations are still valid.

Some reports are thorough. Some are minimal. If your report gives you a bearing capacity and a pavement section but nothing on lateral earth pressure, expansion potential, or corrosivity, you have a report that was written for a simple project and you may be designing a complex one. Ask for what you need. The supplemental testing costs a fraction of the rework that comes from discovering a problem during construction.