A geotechnical engineering report is a formal evaluation of the soil, rock, and groundwater at a building site, prepared and sealed by a licensed Professional Engineer, that tells you whether the ground can support what you plan to build and what kind of foundation will work. The International Building Code requires one before most foundation and grading permits can be issued, and building departments will hold up a permit until the report is in hand and reviewed. Everything downstream — footing size, foundation type, excavation depth, retaining wall design — flows from its findings.
When You Need One
IBC Section 1803 lists specific conditions that trigger a mandatory investigation. These are enumerated triggers, not discretionary suggestions, and hitting any one of them means the report has to happen before permit approval.1International Code Council. IBC Chapter 18 – Soils and Foundations
- Any project using deep foundations such as drilled shafts or driven piles, regardless of site conditions.
- Shallow foundations bearing on more than 12 inches of compacted fill.
- A water table sitting above, or within five feet below, the lowest floor level of a below-grade space.
- Soil whose classification, strength, or compressibility is uncertain enough that the building official wants it tested before accepting any bearing claims.
- Areas likely to contain expansive clays.
- Sites where borings reveal inconsistent rock structure beneath the proposed foundation.
- Structures in Seismic Design Categories C through F, which must have slope stability, liquefaction, settlement, and surface displacement from faulting evaluated.
Local jurisdictions often add their own requirements on top of these. If you aren’t sure whether your project qualifies, the building department will tell you during the pre-application conversation, and in practice they’ll refuse to approve grading or foundation work until they see a report.
Extra Scrutiny in Higher Seismic Zones
Projects in Seismic Design Categories D through F face the strictest requirements. The investigation has to evaluate liquefaction potential using site peak ground acceleration and earthquake magnitude consistent with maximum considered earthquake ground motions, and if liquefaction risk exists, the report must assess total and differential settlement, lateral soil movement, reduced bearing capacity, increased pressures on retaining walls, and even flotation of buried structures like underground tanks.1International Code Council. IBC Chapter 18 – Soils and Foundations
The report also has to assign a seismic site class from A (hard rock) through F (soils needing site-specific evaluation), based on shear wave velocity, standard penetration resistance, or undrained shear strength measured in the upper 100 feet of the soil profile. The site class cascades through every seismic design calculation for the structure, so building officials look at this closely.
How Long a Report Stays Valid
Reports don’t last forever. Most building departments treat a report as stale after a set period, commonly around three years, or whenever site conditions change significantly from what the report describes. Adjacent excavation, altered drainage, new fill, or a change in the building’s footprint or loading can all invalidate the findings. When conditions change or the report ages out, the original engineer typically issues an update letter or addendum confirming whether the recommendations still apply, rather than redoing the whole investigation.
What’s Inside the Report
The document moves from raw field data to lab results to engineering recommendations that a structural designer can use directly. If you’re paying for one, these are the sections you should expect to see.
Boring Logs and Soil Classification
Boring logs are the backbone. Each log records the depth and description of every soil and rock layer the drill encountered, sampling resistance (blow counts), sample recovery percentages, groundwater levels observed during and after drilling, and the type of drilling equipment used.2Federal Highway Administration. Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications Soils are classified under the Unified Soil Classification System per ASTM D2487, which uses particle size, liquid limit, and plasticity index to assign standardized labels like SW (well-graded sand), CL (lean clay), or MH (elastic silt).3ASTM International. ASTM D2487 – Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) Any engineer reading those labels knows how the soil will behave without having set foot on the site.
Bearing Capacity and Settlement
The report states the maximum allowable bearing pressure, which tells the structural engineer how much weight the soil can safely carry per square foot. That number determines footing sizes and whether shallow foundations will work or whether the project needs piles or drilled shafts. Settlement estimates predict how much the building will sink under its own weight, both total settlement and differential settlement between parts of the structure. Differential settlement causes most of the real-world trouble: cracked walls, doors that stop closing, structural distress.
Lateral Earth Pressures and Groundwater
For basement walls, retaining walls, or anything below grade, the report provides lateral earth pressure values so the wall can be designed to resist horizontal soil forces without deflecting or failing. Groundwater data includes the level observed during drilling, the level measured 24 hours later (the more reliable reading), and any sign of seasonal fluctuation or artesian pressure. Where groundwater will interfere with construction, the report recommends temporary dewatering. Well-graded sands and gravels are particularly prone to infiltration during excavation, and the report flags where dewatering will be needed to keep the hole stable.2Federal Highway Administration. Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications
Seismic Design Parameters
In seismically active areas the report hands the structural engineer the seismic site class, spectral response acceleration values, and an evaluation of site-specific hazards like liquefaction or slope instability. For structures in Seismic Design Categories D through F, it also has to recommend mitigation, which can include deeper foundations, ground stabilization, or structural systems that tolerate the anticipated displacements.1International Code Council. IBC Chapter 18 – Soils and Foundations
Soil Corrosivity
If the project has buried steel or concrete, the report usually includes corrosivity testing. Lab analysis measures pH, electrical resistivity, and concentrations of chlorides and sulfates. Acidic soils and soils with elevated chlorides or sulfates attack concrete and corrode steel reinforcement, buried pipes, and foundation anchors. When corrosive conditions show up, the report recommends sulfate-resistant concrete mixes, protective coatings, or cathodic protection.
What Happens on Site
Field work is the visible part of the process. Drill rigs show up and advance borings to predetermined depths, collecting soil and rock samples at regular intervals. The number, depth, and spacing of borings depend on the project. A small residential lot might need two or three borings to 15 or 20 feet; a commercial building with a below-grade parking structure could require a dozen borings 50 feet deep or more.
Before any drill touches the ground, the team contacts 811 to have underground gas, electric, water, sewer, and telecommunications lines marked. Calling 811 before excavation provides roughly a 99 percent chance of avoiding a strike that could cause injury, service disruption, or environmental damage, and skipping it creates serious liability under state excavation damage prevention laws.4U.S. Department of Transportation. Call 811 Before You Dig
The workhorse test in the borehole is the Standard Penetration Test. A 140-pound hammer drops 30 inches onto a drill rod, driving a two-inch split-barrel sampler into the soil at the bottom of the hole.5ASTM International. ASTM D1586/D1586M-18e1 – Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling of Soils The technician records the blows needed to drive the sampler through three successive six-inch intervals; the first six inches are discarded as seating, and the blow count for the remaining 12 inches is the N-value. An N-value below 10 means loose or soft material. Above 50 means very dense soil or rock, and the test usually stops there. Engineers use the N-value directly to estimate bearing capacity, predict settlement, assess liquefaction risk, and feed the seismic site classification.
Alongside the SPT, workers pull relatively undisturbed samples using thin-walled Shelby tubes that preserve the soil’s natural moisture and structure. Where borings hit bedrock, the work shifts to diamond-tipped core drilling, and the engineer looks at both core recovery percentage and Rock Quality Designation to judge how fractured the rock is. Samples are sealed, labeled, and shipped to the lab, and open boreholes are backfilled with grout before the crew leaves the site.
Laboratory Testing
In the lab, samples go through grain-size analysis (sieving and hydrometer testing), Atterberg limits (liquid and plastic limit tests that describe how a clay behaves at different moisture contents), unconfined compression tests, and consolidation tests that predict how much and how fast the soil will compress under sustained load. The numbers feed directly into foundation design. A single consolidation test can decide whether a mat foundation will do or whether the project needs piles driven to a deeper bearing stratum.
Who Can Sign It
Only a licensed Professional Engineer can sign and seal a geotechnical report. Every state regulates the practice of engineering, and the seal carries the engineer’s personal legal responsibility for the accuracy and adequacy of the findings.6National Society of Professional Engineers. What Is a PE? Without that seal, building departments won’t accept the document, so verifying the engineer’s license status in your state is a reasonable step before you hire the firm.
The engineer’s duty isn’t limited to the client who paid for the report. Under the NSPE Code of Ethics, an engineer whose professional judgment is overruled in a way that endangers life or property must notify the client and other appropriate authorities, and must withdraw from a project where the client insists on plans that don’t meet applicable engineering standards.7National Society of Professional Engineers. Code of Ethics for Engineers That matters in geotechnical work, where ignoring unfavorable soil data can be catastrophic years later.
If the Report Finds Bad Ground
Sometimes the report delivers unwelcome news: the soil is too weak, too compressible, or too unstable for the planned structure with conventional foundations. This doesn’t necessarily kill the project, but it changes the approach and raises costs. Typical responses:
- Over-excavate the unsuitable material and replace it with engineered compacted fill, which works when the problem layer is shallow.
- Switch to deep foundations — piles or drilled shafts — that bypass the weak soil and bear on a competent stratum below.
- Improve the ground in place with techniques like deep dynamic compaction, stone columns, soil mixing, or grouting. The Federal Highway Administration identifies more than a dozen ground modification technologies suited to different soils and project needs.8Federal Highway Administration. Ground Modification Methods Reference Manual – Volume I
- Design the structure to tolerate the anticipated movement, using flexible connections or a mat foundation that distributes loads more broadly.
- Shift the building footprint to a better part of the site, or reconsider the site entirely.
The report typically recommends an approach or ranks several options by feasibility and cost. The worst outcome isn’t bad soil. It’s bad soil nobody tested for, discovered when the foundation starts to fail and the fix costs many times what the investigation would have.
Cost and Timeline
Costs vary with the number and depth of borings, site access, the lab testing program, and regional labor rates. A straightforward residential investigation with two or three shallow borings typically runs $1,500 to $5,000. Complex commercial or multi-story projects that need deeper borings, extensive lab testing, and seismic analysis can run $10,000 to $25,000 or more. Government fees for drilling permits add to the total and vary by jurisdiction.
From the day drilling starts to delivery of the signed report, the process usually takes two to six weeks. Field work on a small residential site might wrap in a day, but lab testing, analysis, and report preparation take the bulk of the time. Projects needing specialized work like seismic site response or extensive consolidation testing run toward the longer end. Plan to deliver the report to the building department at least 60 days before your target permit approval date, and work backward from your construction schedule so a slow lab or a weather delay doesn’t push the whole project.
If You’re Buying a Property With an Existing Report
Buyers often inherit a geotechnical report prepared for the seller or a previous owner, and reliance on someone else’s report is where liability gets tricky. In roughly half of states, the economic loss doctrine prevents a third party with no direct contract with the engineer from suing for monetary damages if the report turns out to be wrong. In the other half, courts may find the engineer owed a duty of care to anyone who foreseeably relied on the report. Disclaimers in the report limiting reliance to the named client don’t reliably tip the outcome in jurisdictions that reject the economic loss doctrine.
Most geotechnical firms carry professional liability insurance, with the vast majority maintaining at least $1 million in coverage per claim, and many contracts cap the firm’s total exposure at the fee amount or a multiple of it. Courts in many jurisdictions enforce those caps, though they generally won’t protect a firm that willfully or intentionally failed to carry out its professional duties. If you’re relying on a report you didn’t commission, ask the original engineer for a reliance letter naming you, and check whether the report is still within its validity window given any changes to the site since it was issued.