# What is a geotechnical report, and why does a project need one?

**Short answer:** A geotechnical report is a licensed soils engineer's study of what is under a site and what it can carry. It documents borings, soil classification, bearing capacity, expansion, groundwater and seismic behavior, then recommends a foundation. A project needs one because the plan checker, the structural engineer and the budget all depend on its conclusions.

## Key facts

| Fact | Value | Source |
| --- | --- | --- |
| Who writes it | A licensed geotechnical or soils engineer, stamped and signed |  |
| Core field work | Borings or test pits, sampling, and laboratory testing |  |
| What it decides | Foundation type, bearing values, and the design of any retention |  |
| Right time to order it | During schematic design, before the structure is engineered |  |
| Common late-order penalty | A structural redesign and a return to the plan check queue |  |

## What a soils engineer is actually hired to answer

Every building transfers its weight into the ground, and the ground is the only
part of the project nobody designed. A geotechnical report is the document that
replaces assumption with measurement: what the soil is, how much load it will
take, how it behaves when wet, how it behaves when shaken, and what kind of
foundation is appropriate as a result.

The engineer is answering a narrow question with wide consequences. Not "is this
site good" but "given this proposed structure at this location, what
foundation system, what bearing values, what drainage and what construction
conditions are required for it to perform." The answer is specific to the parcel
and to the building, which is why the report has to name both.

It is also a regulatory document. A plan checker reviewing a project in Los
Angeles County will expect the structural drawings to reference a stamped report
and to follow its recommendations. Without it, the submittal is incomplete
before anyone reads the architecture.

## How the investigation is done

### In the field

The engineer drills borings or excavates test pits at points chosen to
represent the building footprint and any planned cuts. Samples come out at
intervals, penetration resistance is logged as the drilling proceeds, and the
depths are recorded against elevation. On sloped or difficult sites the location
of a boring is partly an access decision, which is one reason the field day
needs planning rather than a phone call.

### In the laboratory

Samples get tested for the properties the design depends on: classification,
moisture and density, shear strength, consolidation, expansion potential and
sometimes corrosivity, which matters for buried concrete and metal. Laboratory
work is the part of the schedule owners forget, because nothing visible is
happening while it runs.

## What the report actually contains

| Section | What it tells you | What it decides |
| --- | --- | --- |
| Boring logs | The layers below the site and their depths | Where competent bearing material starts |
| Soil classification | What the material is, layer by layer | How the soil behaves when loaded and wetted |
| Bearing capacity | Allowable pressures for footings or piers | Footing sizes and foundation cost |
| Expansion potential | How much the soil swells and shrinks | Slab design, moisture barriers, landscape rules |
| Groundwater | Depth encountered and expected variation | Waterproofing, dewatering, buoyancy checks |
| Seismic parameters | Site class and design ground motion values | Structural lateral design |
| Liquefaction and slope stability | Behavior under shaking, stability of cuts | Whether deep foundations or mitigation are required |
| Recommendations | Foundation type, backfill, drainage, observation | Most of the structural and civil drawing set |

The recommendations section is the one to read closely if you only read one. It
is written as instructions to the design team and to the builder, and it often
contains construction requirements that carry cost: over-excavation and
recompaction, specific backfill material, subdrains behind walls, or a
requirement that the engineer observe the excavation before concrete is placed.

## Where it sits in the sequence

The report is an input to structural engineering, not a parallel task. The
structural engineer cannot size footings without bearing values, cannot detail
a pier without a depth, and cannot complete the lateral design without site
seismic parameters. Civil drainage design depends on it as well, and anything
holding back soil, temporary or permanent, is designed from its conclusions.
That dependency is why the report sits at the front of most of the
[site work and excavation](/insights/topics/shoring) questions owners ask.

A reasonable order looks like this.

- Site is under contract or owned, and a rough building footprint exists
- Geotechnical investigation is commissioned and field work is scheduled
- Report is delivered and reviewed by the design team
- Structural and civil design proceed using its values
- Drawings are submitted with the report attached

The one that matters is the third position. Once the report exists, its
conclusions are a design input and the team designs around them. Once the
drawings are finished, the same conclusions arrive as corrections.

## The expensive version of the same decision

The report costs what it costs regardless of when it is ordered. What changes
with timing is everything downstream.

Consider a project that designs a full basement on the assumption of shallow
footings, completes structural drawings, and then commissions the investigation
because the plan checker asked for it. The report comes back with weak surface
soils and groundwater higher than assumed. The foundation becomes piers, the
basement needs a different waterproofing approach, the structural drawings are
redone, the cost estimate moves, and the submittal restarts.

Every one of those consequences was avoidable, and none of them was caused by
bad soil. They were caused by finding out about the soil after the decisions had
been made. A soil condition discovered during design is a design constraint. The
same condition discovered after permit is a redesign, a resubmittal and a
budget conversation nobody planned for.

The second common failure is scope. A report written for a single story addition
does not cover the subterranean garage the owner decided to add six months
later. Reports are written against a described project, and changing the project
can require a supplemental investigation or an addendum letter.

## What to ask when you commission one

**Describe the real project, including the worst case.** If a basement is
possible, say so before the borings are drilled. Adding depth to the scope later
often means going back to the site.

**Ask how many borings and how deep.** The minimum investigation that satisfies
a plan checker is not always the investigation that gives your structural
engineer confidence. On a sloped or irregular site, the difference between two
borings and four is small money against a foundation redesign.

**Ask what construction observation the report will require.** Many
recommendations obligate the engineer to observe excavation, subgrade or pier
drilling. Those visits are a real line item and a real scheduling dependency.

**Ask whether anything in the report will trigger additional studies.** Slope
stability, liquefaction potential or unusual groundwater can lead to
supplemental work, and it is better to hear about that possibility in advance.

**Give the report to everyone who needs it, immediately.** Structural, civil,
architect and builder. It is common to find a design team working from a summary
email while the actual recommendations sit unread in a folder.

## What the report cannot do

It is not a guarantee. Borings sample specific points and the engineer
interpolates between them, so conditions can still differ from what was
predicted, particularly on fill or on hillsides with variable material. A good
report says so plainly and describes what should happen if the field conditions
do not match.

It is also not a feasibility study. It will not tell you whether a site is worth
buying, whether the design pencils, or what the project should cost. It tells
you what the ground requires, and translating that into money and schedule is
the design team's job.

What it does buy is the removal of the largest unknown on a project, early
enough for that knowledge to still be useful. On any job involving excavation,
retention or hillside work, the investigation should be one of the first
professional services commissioned, well before the design is fixed. That
sequencing is the core of how the site work described under
[concrete and shoring](/concrete-shoring) is planned, and it is the difference
between a foundation that was designed and one that was discovered.

## Frequently asked questions

### How long does a geotechnical investigation usually take?

Plan on several weeks rather than several days. Drilling itself may take one day on a small residential site, but scheduling the rig, obtaining any access or encroachment approvals, running laboratory tests and writing the report all take time. Difficult access, deep borings or a required supplemental study extend it further, so it belongs on the schedule as its own line.

### Can a project reuse a geotechnical report from a neighboring property?

Generally no. Soil conditions change over short distances, particularly on hillsides and in areas with old fill, and a report is written for a specific parcel and a specific proposed structure. A neighboring report can be useful background for a feasibility conversation, but a plan checker will expect an investigation performed for the site being permitted.

### Who reads the report after it is written?

More people than most owners expect. The structural engineer takes bearing values and seismic design parameters from it, the civil engineer takes drainage and grading guidance, the plan checker verifies that the drawings follow its recommendations, and the contractor builds to it. It also usually requires the engineer to observe and approve conditions in the field during construction.

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Source: https://www.tricanagroup.com/insights/what-is-a-geotechnical-report-and-why-does-a-project-need-one
Topic: Concrete, Shoring & Site Work
Author: Hagop Sargisian, Tricana Group
Area served: Los Angeles County, CA; Orange County, CA
Published: 2026-06-30T09:00:00-07:00