Land and data-center sites, North Carolina
Guides

Geotechnical reports: what they tell you about the ground

A geotechnical report is a professional engineer's study of the soil, rock, and groundwater under a site, and what they mean for foundations, roads, and fill. It rests on borings and lab tests, not on maps. Order one before you design a building or commit to a price on land where the ground is in doubt.

Start a land search

Quick summary: This guide explains what a geotechnical engineer tests, what the report tells a buyer or builder, when to order one, and how it differs from a soil scientist’s evaluation. It is for land buyers and builders who plan foundations, roads, or fill. The key takeaway: public soil maps flag problems, but only borings and a sealed report tell you what the ground under your building can carry.

What a geotechnical report is

A geotechnical report is a professional engineer’s written study of the ground under a site and what it means for construction. The engineer drills borings, takes samples, tests them, and turns the results into recommendations. Those recommendations cover the foundation type, the soil it can rest on, site grading, fill, and groundwater control.

North Carolina treats this work as engineering. The practice of engineering includes investigation, evaluation, and design of structures and of land and water use, under N.C. Gen. Stat. § 89C-3. Only a licensed professional engineer may offer it, under § 89C-23.

What a geotechnical engineer tests

A geotechnical engineer tests the soil, the rock, and the water in the ground at the places where you plan to build. The common parts of the work are these:

  • Borings. A drill rig bores holes at planned spots and depths. The crew logs each layer by color, moisture, density, and material type.
  • Standard penetration test. A hammer drives a sampler into the soil, and the crew counts the blows. The NCDOT geotechnical manual calls it the most common test on its projects for soil strength and sample collection.
  • Soil classification. A lab sorts the samples by grain size and plasticity, so the engineer knows how each layer behaves under load and when wet.
  • Groundwater. The crew measures the water level when a boring is done and again later. The NCDOT manual measures it at 0 hours and again after about 24 hours to find the static level.
  • Rock. Where the drill meets refusal, the crew can core the rock and record how much intact rock comes back.
  • Problem soils. The engineer looks for soils that swell and shrink with moisture, soft or organic layers, and old buried fill.

The report then gives the bearing capacity, the pressure the soil can carry under a footing without failure or too much settlement. It also gives the expected settlement and any special foundation or grading steps.

The short version: A soil map is a forecast. A geotechnical report is a measurement at the spots where your buildings go, sealed by an engineer who answers for it.

When a buyer or builder orders one

A buyer orders a geotechnical report when the ground can change the price, the design, or the schedule. These are the common cases:

  1. A larger building or a commercial project. The structural engineer needs soil values to design the foundation.
  2. Fill or cut. A pad needs fill, or the grading plan cuts into a slope. The engineer sets compaction targets and tests the fill. See Bringing in fill dirt: permits, floodplain, and compaction and Retaining wall permits in North Carolina.
  3. Problem soils on the map. The soil survey shows shrink-swell clay, wet soils, or shallow rock.
  4. Old use. The land holds old fill, a former building, or a filled pond.
  5. Roads and utilities. A subdivision needs pavement design and trench depths, and rock can change the cost of both.

Order the study inside the due diligence period, so the results arrive before your deadline. The engineer needs time to schedule a drill rig, run lab tests, and write the report. See A land due diligence checklist for North Carolina.

How it differs from a soil scientist’s evaluation

A geotechnical engineer reads soil as a material that carries load. A soil scientist reads it as a medium for water, wastewater, and plants. The North Carolina soil scientist statute says it does not stop licensed engineers from practicing soil mechanics and foundation engineering, under N.C. Gen. Stat. § 89F-8.

Question Who answers
Will the soil carry a foundation, a slab, or a road? Geotechnical engineer
How should fill be placed and compacted? Geotechnical engineer
Can this lot use septic, and where? Licensed soil scientist or the local health department
Are there hydric soils or signs of a wetland? Licensed soil scientist, with the agencies on jurisdiction

A tract with septic and new houses can need both studies. The soil scientist maps where the drainfields fit, and the engineer tests where the houses and roads go. See Licensed soil scientists: what they do for a land purchase.

A geotechnical report is also not an environmental study. It reports what the ground can carry, not whether it holds contamination.

The professional engineer seal

A geotechnical report you rely on should carry a North Carolina professional engineer’s seal. Final drawings, plans, and reports a licensee prepares must be certified and stamped with the seal when issued, under N.C. Gen. Stat. § 89C-16. The seal shows the engineer’s name and license number.

Check the license with the North Carolina Board of Examiners for Engineers and Surveyors, which offers a public license lookup. A drilling crew or a testing lab can collect data, but an engineer interprets it and seals the recommendations.

Take action: Before you sign, find out what the public soil and terrain data show for the tract, and plan borings where they flag doubt. Start with Acreage that holds up after the survey.

What public data shows first

Public data shows likely soil types, slopes, and flood zones before anyone drills. The USDA soil survey maps soil units and reports drainage, depth to rock, and shrink-swell for each one. Those ratings tell you where to put the first borings. See Web Soil Survey: how to read the soils on a parcel.

The NCDOT manual tells its own staff to review existing data before any drilling. That data includes topographic maps, aerial photos, geologic maps, soil surveys, and well logs. The review helps place borings where problems are likely.

Public data has limits. A soil map unit covers a large area and can hold different soils inside it. It cannot show old fill, a buried foundation, or the exact depth of rock under one building pad. Borings answer that. See Gross acres are not usable acres.

Key recap

  • A geotechnical report is a professional engineer’s study of soil, rock, and groundwater for foundations, roads, and fill.
  • The work rests on borings, penetration tests, lab classification, groundwater readings, and rock coring where needed.
  • Order one inside the due diligence period when the ground can change the design, the price, or the schedule.
  • A soil scientist answers septic and wet-soil questions. A geotechnical engineer answers load questions.
  • Rely only on a report that carries a North Carolina professional engineer’s seal.

Questions

What is in a geotechnical report?

A geotechnical report shows where the borings were drilled, the soil and rock layers found, the test results, and the groundwater levels. It then recommends a foundation type, the bearing capacity, the expected settlement, and the steps for grading, fill, and drainage.

Do I need a geotechnical report to build a house?

Not always. The answer depends on the site, the house, and the local building inspections office. A structural engineer or a lender may also ask for one. Ask the county or city building inspections office what it requires for your site before you design the foundation.

Who prepares a geotechnical report in North Carolina?

A licensed professional engineer who practices geotechnical engineering prepares and seals it. The engineer often works with a drilling crew and a testing lab. Check the license with the North Carolina Board of Examiners for Engineers and Surveyors.

References

Primary sources cited on this page, in APA style.

Tell us what the project needs

Load, acreage, counties, and timeline. We reply with how we would run the search and where we would start.

Start a site searchTry the calculators