Awaiting expert review. This page covers safety or regulatory rules that change over time. A qualified reviewer has not yet checked it; the sources below are dated so you can verify each point.
Quick answer
A geotechnical soil investigation is a study of the ground at a site, done by a geotechnical engineer through site reconnaissance, borings, sampling, field tests and laboratory tests, and written up in a report of what the soil layers, water and behavior mean for the foundation.
The report supplies data and recommendations. It does not replace the structural engineer who designs the foundation or repair. This page is held for review: a licensed engineer has not yet signed it.
Quick facts
The key facts about Geotechnical Soil Investigation, with sources:
- Done by
- A geotechnical engineer, often for a structural engineer 2
- Main steps
- Reconnaissance, borings and sampling, lab testing, report 3
- Typical tests
- Moisture content, Atterberg limits, hydrometer, penetration tests, soil suction 2
- Model code trigger
- A building official may require a soil test where expansive, compressible or shifting soils are likely 1
- Without a full study
- The model code lets presumptive values stand in, with limits 1
- Review status
- Held: no licensed reviewer has signed this page
- Price
- Not quoted here; see the cost drivers section
Key takeaways
- A geotechnical investigation measures what is in the ground and how it behaves, then gives an engineer the data to design a foundation or repair.
- It usually has four parts: study the site, drill and sample, test in a lab and write a report 3.
- Soil behavior depends on minerals, moisture, stress and drainage. One soil type is not proof of a problem, and one boring does not describe a whole lot.
- A report gives recommendations for a specific site and structure. It does not diagnose cracks, and it does not replace a structural evaluation 6.
- Review pending: this page is held until a licensed engineer reviews it, and nobody has signed it yet.
What a geotechnical soil investigation is
General information, not a diagnosis of your house. The Foundation Performance Association (FPA) describes a geotechnical report prepared specifically for foundation design and construction, whose job is to give the structural engineer the soil information needed to design a foundation 2.
A manufacturer’s design manual describes the primary purpose of a site investigation as identifying the subsurface layers and the key soil properties for design 3.
The data come from the site itself. A truck-mounted or portable rig drills test holes, called borings, and takes samples. Field tests and lab tests turn the samples into numbers. The engineer then writes what the layers are, where groundwater sits, and what the findings mean for footings, slabs or piers.
It is related to, but not the same as, a foundation inspection, which looks at the building. The investigation looks at the ground. A structural assessment uses both.
Two panels. Left: wet clay soil has swollen and lifts the edge of a slab. Right: dry clay soil has shrunk and left a gap under the slab edge. Arrows show swelling and shrinking.
Illustrative drawing, not to scale and not a design. It shows why moisture matters to soil behavior. Which soils swell, how much and how deep is found by testing a site, not by a drawing.
Review status: held, not yet signed
This page is held for review. Until a licensed geotechnical or structural engineer reviews and signs it, the layout keeps it out of search results. We name the standard and edition behind each rule. We do not give design values, and we never name a reviewer who has not signed.
How an investigation proceeds, from walk-through to report
A manufacturer’s design manual lays out four phases. FPA’s recommended practice fills in what goes into each 32.
- Reconnaissance and planning. The engineer reviews the project and the geology, then walks the site to look at topography, drainage, fill, rubble, slopes and trees. FPA lists streams, ponds, fill, seeps, erosion and large trees as items to note 32.
- Borings and sampling. Holes are drilled at planned locations. Samples are taken and logged as they come out. Roots are noted, and groundwater is measured 2.
- Field tests. Common ones are a hand penetrometer on clay samples and the Standard Penetration Test, ASTM D1586, on sandy soils. The manual calls the SPT the most common basis for soil parameters 23.
- Laboratory tests. Typical items are moisture content, Atterberg limits, clay percentage by hydrometer and, for clay lots, soil suction 2.
- The report. It describes the site, shows boring logs and results, gives groundwater depth and recommends measures for foundation design and site work 2.
Do not dig or drill next to a foundation yourself
Test holes can cut buried utilities and undermine a footing. Borings are done by trained crews after utilities are marked. If you see changing cracks, bowing walls or doors that stick with new cracks, call a licensed structural engineer.
Common tests in a residential investigation and what each shows
| Test or step | What it measures | Why it matters |
|---|---|---|
| Visual classification | Soil type and layering as samples come up 2 | Sets the soil profile |
| Hand penetrometer | Resistance of clay samples 2 | A quick index of stiffness |
| Standard Penetration Test (ASTM D1586) | Resistance of soil to a driven sampler, as blow counts 3 | A common basis for soil parameters |
| Moisture content | Water in each sample 2 | Shows wet and dry zones |
| Atterberg limits and plasticity index | How a clay behaves as moisture changes 2 | Higher plasticity can mean more shrink and swell |
| Hydrometer test | Percentage of clay-size particles 2 | Helps judge expansive behavior |
| Soil suction | How strongly soil holds or pulls water 25 | Used to estimate moisture active depth in clay |
| Groundwater measurement | Water level in the boring 2 | Affects piers, drainage and waterproofing |
Example: what FPA says a foundation report should contain
The report example below reads the reporting section of FPA’s recommended practice. It shows what to look for in a report you receive. It is not a requirement in any code 2.
| Report part | What it contains | Why a homeowner cares |
|---|---|---|
| Site description | Surroundings, anomalies, photos, trees and where borings were taken | Tells you what part of the lot the data represent |
| Boring logs | Field and lab results for each boring | The raw evidence behind every recommendation |
| Material descriptions | Classification of the soils found | Shows layers and fill |
| Water table | Level in the boring, or a statement that holes were dry | Affects piers and drainage |
| Expansive soil measures | Provisions to mitigate expansive soils | The engineer’s answer to shrink and swell |
| Earthwork recommendations | Site preparation before the foundation is built | Fill and compaction steps |
| Maintenance discussion | Foundation maintenance needed to keep the design working | What you must do after construction |
| Trees | Discussion of trees to be removed or kept, if known | Roots pull moisture from clay |
- ReconnaissanceStudy site and geology
- Borings and samplingDrill, sample, log
- Laboratory testsMoisture, plasticity, clay
- ReportData and recommendations
The phases follow a manufacturer’s design manual; FPA’s recommended practice fills in the details. Source: [3] [2]
FPA also says the report should be written on the assumption that the structural engineer will never see the site, which is why the logs and photos matter 2.
When a geotechnical investigation is used
Investigations are used before a new foundation is designed, and before a repair that depends on soil, such as piers.
A manufacturer’s manual says borings should be made at each site where helical piles or resistance piers are to be installed, whether the project is new construction or repair 3. See push piers, helical piers and drilled piers.
Investigations also help when a lot has fill, trees, a slope or a history of movement.
FPA says it wrote its recommended practice because of a large number of residential and light commercial foundation problems, and says trees were known or suspected to be the main contributor in the majority of foundation problems in its local market 2.
For an existing house with cracks, the first step is usually a structural evaluation, which may call for soil data. The engineer guide explains the order.
What an investigation gives you and where it stops
What it provides
Limits to keep in mind
- It describes only the places that were drilled. Soil between borings is inferred.
- Quality varies, so FPA wrote a recommended practice to set minimum content 2.
- Added testing adds cost, which FPA acknowledges 2.
- It reports soil, not building condition; a structural engineer must interpret it for the structure 6.
What a geotechnical report cannot tell you
A report cannot diagnose a crack, name the cause of a particular movement or say a repair will stop all movement. Soil behavior depends on mineral content, moisture, stress and drainage, so one soil type is not proof of a problem. A report gives data and recommendations, not a prescription for your house.
It also does not give you a number that applies across the lot. FPA’s practice says borings should be placed where the foundation will sit, and adds that its own practice was written for Houston and the general southeast Texas area and should be used with caution elsewhere 2.
North Carolina’s engineering board says the analysis, judgment and design of a foundation repair belong to a licensed engineer in responsible charge, and that a statement a foundation has failed should be made by an engineer who evaluated it 6. A soil report feeds that work. It does not replace it.
What drives the cost of a soil investigation
We do not give prices. A fair figure needs dated local quotes for a defined scope. The drivers are how many borings and how deep, access for the drill rig, whether special tests such as soil suction or swell are added, groundwater, lab work, the report’s detail and the engineer’s time.
FPA notes that requesting its full recommended practice would most likely raise the cost of the geotechnical work, and says the extra cost is necessary for the structural engineer to understand the site 2.
A manufacturer’s manual makes a related point: the lack of information about subsoil conditions must be made up by a liberal factor of safety, and a thorough investigation is usually small compared to the cost of failure from wrong assumptions 3.
See the cost guide for how repair quotes are built, and the cost calculator for planning.
How a homeowner can handle a soil investigation
- Ask the structural engineer whether one is needed. The engineer decides what data the design needs. See inspection service.
- Hire a licensed geotechnical or engineering firm, not a salesperson who offers a free soil test as part of a sale. Verify the license with your state board.
- Share what you know. Tell the firm about tree removals, past fill, leaks and earlier repairs, since FPA asks for tree locations and species 2.
- Mark utilities before drilling. The firm or you should call the local locate service.
- Read the report, then give it to your structural engineer.
- Keep it. A future repair or sale may need the same data.
Keeping and using the report
- Store the report with your house records, including boring plan and photos.
- Share it with any engineer or contractor who works on the foundation.
- Compare future conditions with the reported groundwater and moisture.
- Follow the maintenance discussion in the report, such as keeping trees and moisture under control.
- Do not reuse a report for a different lot or a much later project without asking the engineer whether it still applies.
Risks and gaps to watch for
- Borings in the wrong places, or too few for the lot 2.
- Missing groundwater data, or a statement that holes were dry with no time stamp 2.
- No moisture or plasticity data in a clay area 2.
- A report prepared for a street or utilities and used as a foundation report. FPA says the two are not interchangeable 2.
- Recommendations that depend on unrelated sales claims rather than on the logs.
When to call a licensed structural engineer
Call one when cracks widen, walls bow, doors and windows stick together with new cracks, a corner is separating or you have any safety doubt. A soil test does not replace that evaluation.
Code rules and standards behind soil testing
2018 International Residential Code, Section R401.4 Soil Tests. Where quantifiable data created by accepted soil science methodologies indicate expansive, compressible, shifting or other questionable soils are likely, the building official determines whether to require a soil test. The test is done by an approved agency using an approved method 1.
Section R401.4.1. In lieu of a complete geotechnical evaluation, the code assumes presumptive load-bearing values by material class. Where the official determines soils below a stated bearing level are likely, a soils investigation sets the allowable bearing capacity 1. We do not print the values. Check the code your jurisdiction adopted.
Section R401.4.2. Instead of a complete geotechnical evaluation, compressible or shifting soils are removed or stabilized within each active zone 1.
ASTM D1586 is the Standard Penetration Test the manual names as a common field method 3. FPA-SC-04-0 (2001) is a recommended practice, not a code 2.
The Federal Highway Administration’s drilled shaft manual (FHWA-NHI-10-016, 2010) describes site investigation for bridge-scale shafts and says information needed is stratigraphy and groundwater, index properties and strength and deformation properties 4.
Editions and local amendments vary. Rules checked 2026-10-14. Who may sign a geotechnical report also varies by state; verify with the licensing board.
Geotechnical investigation next to related work
| Work | What it examines | Who usually does it | What it does not do |
|---|---|---|---|
| Geotechnical soil investigation | The ground: layers, strength, moisture, groundwater 2 | Geotechnical engineer | Design the foundation or diagnose cracks |
| Soil bearing capacity value | How much load soil can carry, as a number | Engineer, from the report | Apply beyond the tested site |
| Structural evaluation | The building and the cause of movement 6 | Licensed structural engineer | Replace soil data |
| Home inspection | Visible condition; refers structural concerns to an engineer 6 | Home inspector | Test the soil |
| Presumptive values in code | A code stand-in when no full study is made 1 | Used by the designer, where allowed | Describe your specific soil |
Geotechnical investigation myths
- Myth A soil test tells you what is wrong with your foundation.
- Reality It describes the ground. A structural evaluation interprets cracks and movement 6.
- Myth Clay soil means a bad foundation.
- Reality Soil behavior depends on moisture, stress and drainage. Clay alone is not proof of a problem.
- Myth One boring covers the whole lot.
- Reality It describes the point drilled. FPA calls for multiple borings on residential lots 2.
- Myth A street or subdivision report is enough.
- Reality FPA says a report that addresses streets and utilities is not an acceptable substitute for foundation design work 2.
- Myth A free soil test from a repair company is independent.
- Reality Independence matters. Prefer a firm that does not sell the repair.
Regional soil, trees and water that change what is tested
The Federal Highway Administration names Texas and the Gulf Coast, the Appalachian states, the Southwest and the Great Plains as regions where expansive soils are most prevalent, with about twenty states where expansive clays are a problem to some degree 4.
In clay country, moisture tests such as plasticity and suction take on weight 2. In wooded lots on clay, tree roots can matter, and FPA asks for tree location and species 2.
In cold regions, frost depth and frost-susceptible soils matter. Where water tables are high, groundwater data shape drainage and pier decisions.
Do you need a soil investigation?
You probably need one when a new foundation is designed on questionable soil, when piers or other deep support are proposed, or when an engineer asks for it. You may not need one for a small, simple repair that an engineer can design from local experience. Whether to test is the engineer’s call.
Use the guide below to prepare. See engineer or contractor to decide whom to call, and the repair method tool to prepare questions.
- If A new foundation is planned on fill or clay
- Then Ask the structural engineer whether a geotechnical report is needed 12
- If Piers are proposed under an existing house
- Then Ask for borings or probes first 3
- If Cracks or sticking doors are the only concern
- Then Start with a structural evaluation, not a soil test
- If A report was written for a different purpose
- Then Ask whether it applies to the foundation 2
Soil data first, then the repair design
If a company proposes piers without soil information, ask how the depth and count were set. A repair based on assumptions can be wrong in either direction.
Questions about Geotechnical Soil Investigation
What is a geotechnical soil investigation?
It is a study of the ground at a site by a geotechnical engineer. The work includes a site walk, drilled test holes called borings, sampling, field tests and lab tests, and a written report. The report describes soil layers, groundwater and what they mean for the foundation. It supports the structural engineer’s design.
What is included in a geotechnical report?
FPA’s recommended practice lists a site description, boring logs with test results, soil descriptions, groundwater level, measures for expansive soils, earthwork recommendations and a maintenance discussion. A good report also shows boring locations and photos. The exact content is set by the engineer and the site.
Is a soil test the same as a geotechnical report?
A soil test is one part of it. The report gathers site observations, borings, field tests, lab results and recommendations. A single lab test, such as a plasticity test, does not tell the engineer how the whole site behaves, and it does not replace the report.
When does the building code require a soil test?
The 2018 model IRC says the building official determines whether to require one where accepted soil science data indicate expansive, compressible, shifting or other questionable soils are likely. Adoption varies by state and city, so check your building department. A structural engineer may also request one.
How many borings are needed?
It depends on the lot, the building and the soil, and it is set by the engineer. FPA’s practice ties boring quantity to lot type and floor area and adds more near large trees on clay. Its practice was written for southeast Texas, so do not apply its numbers elsewhere.
Can a soil report tell me why my foundation cracked?
Not by itself. A report describes the ground. A licensed structural engineer interprets movement, cracks and the building together with that data. Crack patterns are clues, not diagnoses, and no soil test turns a photo into a cause.
What do plasticity and soil suction mean in a report?
Plasticity, from Atterberg limits, shows how a clay changes with moisture, and higher plasticity can mean more shrink and swell. Soil suction shows how strongly the soil holds water and helps estimate how deep moisture changes reach. The engineer interprets both for the design.
Does a geotechnical report give a bearing capacity?
Reports may give allowable bearing values for a specific site and design. We do not print numbers, because they do not transfer to other lots. The model code also lists presumptive values to use in place of a full study in some cases, with limits. Ask the engineer which applies.
How much does a soil investigation cost?
We do not publish prices. The drivers are the number and depth of borings, access, special tests, lab work, report detail and engineer time. FPA notes that fuller testing raises cost but gives the structural engineer information needed to design the foundation.
Sources
- International Code Council, 2018 International Residential Code, Chapter 4 Foundations, sections R401.4 to R401.4.2 Soil Tests, as reproduced by UpCodes (model text; adopted editions and amendments vary), retrieved .
- Foundation Performance Association, FPA-SC-04-0: Recommended Practice for Geotechnical Explorations and Reports (11 April 2001; written for Houston and southeast Texas), retrieved .
- Hubbell Power Systems (CHANCE), Technical Design Manual, Edition 3 (2014), Section 2 Soil Mechanics, site investigations. Manufacturer document, retrieved .
- Federal Highway Administration, Drilled Shafts: Construction Procedures and LRFD Design Methods, GEC 10, FHWA-NHI-10-016 (May 2010), site investigation and expansive soil sections, retrieved .
- Foundation Performance Association, FPA-SC-16-0: Design Procedure for Drilled Concrete Piers in Expansive Soil (2017), retrieved .
- North Carolina Board of Examiners for Engineers and Surveyors, Foundation Repair Guidelines (revised 10/22/2024), retrieved .
Expert review
Written by the HyreFoundation Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: HELD, G-REV: review is pending. A licensed geotechnical or structural engineer has not reviewed or signed this page, and no reviewer is named.
Code text is from the 2018 model IRC as reproduced by UpCodes (the ICC site did not load); the soil investigation phases and field test names come from one manufacturer manual, labeled as such.
Reporting content and test lists come from a Foundation Performance Association recommended practice written for southeast Texas; expansive soil regions come from the Federal Highway Administration.
No design values, boring counts, depths or prices are given. Rules checked 2026-10-14. General information, not engineering advice.
Suggest a correction. We fix errors and say what changed.