A geotechnical report is a licensed engineer’s evaluation of the soil, rock, and groundwater beneath a piece of land, used to determine whether the ground can safely support a proposed structure and what kind of foundation it needs. Licensed geotechnical engineers or certified engineering geologists produce these reports by drilling into the ground, testing samples in a laboratory, and translating the results into design recommendations for architects and structural engineers. Most building departments require the report before issuing a construction permit, and commercial lenders often demand one as part of their due diligence before financing a project.
When You Need One
The International Building Code, which forms the basis for local building codes across most of the country, requires geotechnical investigations under a broad set of conditions. Section 1803 requires that soil be classified, groundwater evaluated, and foundation-bearing conditions confirmed before construction begins. The code also calls for an investigation whenever deep foundations like driven piles are planned, when a building will sit on compacted fill thicker than 12 inches, or when foundations will bear on rock where depth and quality may vary.
Specific hazards trigger additional testing. In areas prone to expansive soils, the building official must require testing to confirm whether those soils are present. An investigation is also required when groundwater sits within five feet below the lowest proposed floor. Structures in higher seismic design categories (C through F) need an investigation that evaluates slope instability, liquefaction, settlement, and surface displacement from faulting.
There is one narrow exception. The building official can waive the investigation if satisfactory data from nearby properties already shows that none of the triggering conditions apply.1ICC Digital Codes. International Building Code 2021 Chapter 18 Soils and Foundations In practice, this waiver is uncommon for anything beyond simple, low-risk sites where adjacent construction has already characterized the ground.
Building permits are not the only reason these reports get ordered. Commercial property lenders routinely require them to protect their collateral against foundation problems. Properties that need on-site wastewater systems require soil testing so a health department can confirm the ground will filter effluent, set setback distances, and protect groundwater before issuing a septic permit.2U.S. Environmental Protection Agency. Septic Systems Reports, Regulations, Guidance, and Manuals Steep hillside lots, sites near known landslides, and parcels flagged on municipal hazard maps for liquefaction or soil expansion are common triggers as well.
What Is Inside the Report
A geotechnical report turns raw subsurface data into recommendations a design team can act on. The IBC dictates certain required content, and most reports follow a similar structure regardless of project size.
Soil and Rock Classification
Every report classifies soil encountered during drilling using the Unified Soil Classification System (ASTM D2487), with rock classified under ASTM D5878.1ICC Digital Codes. International Building Code 2021 Chapter 18 Soils and Foundations These classifications tell the structural engineer what sits beneath the building at each depth: loose sand versus dense gravel versus stiff clay versus weathered bedrock. They drive nearly every other recommendation in the report.
Groundwater
The report documents groundwater levels observed during and after drilling. High groundwater complicates excavation, requires dewatering during construction, and dictates waterproofing for any below-grade spaces. A basement or underground parking level built where the water table is shallow needs very different engineering than one built above it.
Bearing Capacity and Foundation Recommendations
The report gives an allowable bearing pressure, the maximum load per square foot the soil can carry without excessive settlement. Based on that value and the soil profile, the engineer recommends a foundation type. Strong bearing soil near the surface may call for conventional spread footings. Weak upper soils overlying deeper competent material may require drilled piers or driven piles. The report also provides lateral earth pressure values for retaining walls and any below-grade walls that must resist soil pushing against them.
Seismic Site Class
In seismically active regions, the report assigns a seismic site class from A through F based on shear wave velocity, penetration resistance, or shear strength in the upper 100 feet of soil. Site Class A is hard rock, where earthquake waves pass through with minimal amplification. Site Class E is soft clay that can dramatically amplify ground shaking. Site Class F covers the worst conditions, including liquefiable soils, and requires a site-specific seismic study rather than standard code provisions.3ICC Digital Codes. International Building Code 2018 Chapter 18 Soils and Foundations When no subsurface data is available, most codes default to Site Class D, which often produces more conservative and expensive structural design than actual ground conditions would require. That default alone is a practical reason to invest in the investigation.
How the Field Investigation Works
The crew’s job is to characterize what’s underground without being able to see it, which means drilling holes and measuring how the ground resists penetration. Two methods dominate.
Standard Penetration Test Borings
The standard penetration test (SPT) is the workhorse of residential and commercial investigations. A hollow-stem auger or rotary drill advances a borehole, and at regular intervals (typically every five feet) a 140-pound hammer is dropped 30 inches to drive a split-barrel sampler into the soil at the bottom of the hole. The number of hammer blows needed to advance the sampler over a specific interval produces the “N-value,” a direct measurement of soil density and resistance.4ASTM International. Standard Test Method for Standard Penetration Test SPT and Split-Barrel Sampling of Soils The sampler also retrieves a physical soil sample for classification in the field and later lab testing.
Cone Penetration Testing
Cone penetration testing (CPT) takes a different approach. An instrumented cone is pushed into the ground at a steady rate of about two centimeters per second while sensors continuously measure tip resistance, sleeve friction, and pore water pressure. The result is a continuous profile of soil conditions instead of the point-by-point snapshots that SPT borings provide. CPT is faster, produces more repeatable results, and costs less per data point, but it does not retrieve a physical soil sample. Engineers often pair a few CPT soundings with a smaller number of traditional borings to get both continuous data and actual samples for the lab.
Depth and Number of Borings
How deep and how many borings depend on the structure. For typical residential projects, two to four borings drilled 15 to 25 feet deep is common, though the engineer adjusts based on what the drill encounters. Major structures require considerably more: the U.S. Army Corps of Engineers standard is that borings extend to the greater of six meters or twice the building height, with minor structures requiring test pits at least three meters deep.5U.S. Army Corps of Engineers Transatlantic Division. AED Design Requirements Geotechnical Investigations
Karst and Sinkhole Sites
Sites underlain by limestone or other carbonate rocks need extra work to look for underground voids. Standard borings alone can miss a cavity sitting five feet to the side of the drill hole. Geophysical methods like electromagnetic terrain conductivity surveys scan broader areas to flag anomalies that warrant targeted borings. Where surveys identify suspect zones, borings are placed at the anomaly locations and must extend at least 20 feet below ground surface or the proposed grade, whichever is deeper. Borings in karst terrain log water saturation both at the time of drilling and again 24 hours later, because water movement through fractured rock behaves differently than in ordinary soil.
What to Do Before the Drill Rig Arrives
Give the geotechnical firm your site plans showing the proposed building footprint, property boundaries, and planned grading. The engineer uses this to decide where to place borings. A boring under a planned column footing yields more useful data than one drilled in the middle of a future parking lot.
Physical access matters more than most owners expect. The drill rig and support truck need a path to each boring location, which may require clearing vegetation, removing temporary fencing, or grading a rough access road. On tight urban lots, access constraints can force compromises in boring locations. The U.S. Army Corps of Engineers acknowledges that heavily urbanized areas present particular difficulties, and that obstacles or right-of-entry problems can prevent borings from being placed directly where structures are planned.6U.S. Army Corps of Engineers. Geotechnical Investigations EM 1110-1-1804
Federal law requires anyone planning excavation or drilling to contact the national 811 one-call notification system before penetrating the ground. Under 49 U.S.C. ยง 60114, a person intending to engage in excavation, tunneling, demolition, or construction must use the one-call system to establish whether underground facilities exist in the work area. Utility operators then mark the locations of buried gas, electric, water, and communication lines. Skipping this step violates federal law and can produce civil penalties.7Office of the Law Revision Counsel. United States Code Title 49 Section 60114 One-Call Notification Systems
Provide any historical information about the site as well: previous grading work, old foundation plans, records of fill placement, or known environmental issues. A site that was once a gas station or industrial facility may need environmental screening before geotechnical drilling begins.
Lab Work and How Long It Takes
Soil samples collected in the field go to a geotechnical laboratory. Common tests measure moisture content, grain size distribution, plasticity (how much a clay soil deforms before cracking), and shear strength (how much force the soil can resist before failing). Where expansive soils are a concern, the lab measures the expansion index. The IBC defines expansive soil using specific thresholds: a plasticity index of 15 or greater, particular grain size distributions, or an expansion index above 20.1ICC Digital Codes. International Building Code 2021 Chapter 18 Soils and Foundations
The engineer correlates lab results with field observations and penetration data to build a subsurface model. That model becomes the basis for the recommendations in the report: foundation type, allowable bearing pressure, expected settlement, drainage requirements, and construction procedures. From start to finish the process typically takes two to six weeks. Simple residential sites with cooperative soil come in faster; complex commercial projects or sites needing specialized testing for corrosion potential or chemical analysis of contaminated ground take longer.
What It Costs
A basic residential investigation with two to three borings and a standard lab program generally runs between $1,000 and $5,000. The national average sits around $2,700. The biggest cost drivers are the number of borings, the depth drilled, and whether the site needs specialized testing beyond the standard soil mechanics suite. Utility locating through 811 adds $200 to $500 if the owner doesn’t handle it independently.
Commercial projects cost more because they demand more borings, deeper drilling, and a broader range of lab tests. Each additional boring typically adds $300 to $900 depending on depth and soil conditions. Sites with unusual hazards, like sinkhole risk, steep slopes needing slope stability analysis, or liquefaction potential in seismic zones, require specialized studies that can push total costs well above $5,000. Building departments also charge plan review fees to have the submitted report evaluated, though those fees vary widely by jurisdiction.
How Long the Report Stays Valid
Geotechnical reports do not stay valid forever. Most building departments treat reports as current for roughly two to five years from the date of publication, though specific timeframes vary. After that window, the department typically requires either a full new investigation or an update letter from the original firm confirming that conditions have not changed.
Even within the validity period, certain changes make a report obsolete. If the building footprint shifts significantly from what was studied, the existing borings may no longer represent the soil beneath the planned structure. The Federal Highway Administration notes that when a project alignment changes, additional subsurface exploration must be conducted along the new alignment.8Federal Highway Administration. Checklist and Guidelines for Review of Geotechnical Reports and Preliminary Plans and Specifications The same logic applies to private work. If you commissioned a report for a two-story house and later redesign it as a three-story building with a basement, the original recommendations likely no longer apply. Major grading, adjacent construction that changed drainage patterns, or new information about environmental contamination can all trigger a requirement for supplemental investigation.
If the Report Turns Out to Be Wrong
Geotechnical engineers are held to the same general standard as other professionals: they must exercise the ordinary skill and competence of members of their profession practicing in the same area. Nobody expects perfection. Soil is inherently variable, and a boring captures conditions at a single point rather than across the entire site. But failing to follow well-established investigation principles, ignoring obvious warning signs, or making recommendations without adequate data crosses into negligence.
Geotechnical firms limit their financial exposure through contract provisions. Limitation-of-liability clauses are standard, and they often cap the firm’s total exposure at a fixed dollar amount or a multiple of the professional fee. When a foundation fails and the report is questioned, the dispute usually turns on whether the engineer gathered enough data, interpreted it correctly, and communicated the risks clearly.
If you’re on the receiving end of a report, read the limitations section. Every report contains one, and it spells out what the engineer did and did not evaluate. Foundation problems outside the stated scope are much harder to pin on the geotechnical firm. If the report recommends additional testing or monitoring during construction and you skip it, that decision may shift liability from the engineer to you.