Anatomy of a soils report
Before anything gets built, an engineer reads the ground. Here's every part of a geotechnical report, dissected one page at a time: what each section says, and why it matters to you.
One project, one address, one scope
Every report opens by naming exactly what it covers: a single proposed structure on a single lot. The transmittal letter behind the cover states what was investigated, for whom, and under whose professional responsibility. The report covers this site and this project, nothing else.
Transmittal letter: the short formal letter that delivers the report and records who it was prepared for.
Check that the address and the described project match yours. A soils report for the lot next door, or for a different structure, isn't your report.
The whole investigation on one page
What we found, whether the site works, and the headline numbers, compressed into a few bullets. One of them is the sentence the entire report exists to deliver: whether the site is suitable for what you want to build.
Geotechnical: the branch of engineering concerned with how soil and rock support what we build on them.
If you read only one page, read this one. One caveat: the report itself states that the full text governs. If the summary and the report ever disagree, the report wins.
Where we looked, and proof of it
A location map places your lot in its terrain. The geotechnical map shows your property line, the existing house, the proposed structure, and a red dot labeled B-1: the exact spot where we drilled into the ground.
Boring: a narrow test hole drilled to sample the soil below the surface.
Data from a boring near your future foundation is relevant data. This map is the evidence of where the report's knowledge actually comes from.
A foot-by-foot record of the ground
Foot by foot, the log records what the drill found: the soil type in engineering shorthand (the USCS code), how dense it is, how wet it is, where samples were taken, and whether groundwater showed up before the hole was terminated.
USCS: the Unified Soil Classification System, the standard shorthand engineers use to name soil types.
This is the ground your project will actually bear on. Every recommendation later in the report is an interpretation of these few lines.
Your soil, put through its paces
Samples go to the lab for testing. The star is the expansion index, a measure of how much your soil swells when it gets wet. Expansive soil is one of the most common causes of cracked slabs and sticking doors in Southern California.
Expansion index (EI): a lab test score for how much a soil swells when wet, from very low to very high.
An EI of 10 is very low. That's good news. An EI over 90 changes your foundation design, and your budget.
Your lot has a backstory
Millions of years before it was a building site, your lot was a riverbed, a seafloor, or an alluvial fan. The report names the geologic formations beneath you, because formations behave in known, predictable ways.
Formation: a named body of rock or soil that geologists have mapped across a region.
A boring samples one point. Geology fills in everything between. It's how engineers anticipate what the drill didn't touch.
The seismic design values
California building code requires every structure to be designed for the shaking expected at its exact location. The report delivers those values: site class, spectral accelerations, and whether any fault zone touches the property.
Site class: a building code category for how the ground at a site responds to earthquake shaking.
Your structural engineer cannot size a single beam or footing without these numbers. They come from this page.
The numbers that end up on your plans
Everything converges here: how much weight the soil can carry (bearing capacity), how wide and deep footings must be, how the slab gets built, and how water must be kept moving away from the foundation.
Bearing capacity: the maximum pressure a foundation is designed to place on the soil, in pounds per square foot.
These numbers go straight onto your foundation plan, and the city plan checker verifies them against this report, line by line.
A license on the line
The engineer's stamp is what turns these pages into a legal document. It means a state-licensed professional engineer accepts professional responsibility for every recommendation inside.
PE: Professional Engineer, a state license that carries personal responsibility for the work it stamps.
Cities won't accept an unstamped report. Neither should you.
ENGINEERING
REPORT
Hillside Lot · Ramona, California
The following is a summary of our geotechnical study, findings, conclusions, and recommendations. In the event of a conflict between this summary and the report, the report shall prevail.
• The proposed ADU is located on a gently sloping lot in Ramona, California.
• One exploratory boring was advanced to 10 feet using a hand auger; samples were collected with a California Sampler.
• Earth materials encountered consisted of stiff, gray, fine sandy silt, moist, to the maximum depth explored.
• Groundwater was not encountered to the maximum explored depth of 10 feet.
• The site is not located within a designated Earthquake Fault Zone.
• Results of our study indicate that the site is suitable from a geotechnical standpoint for the proposed development, provided the recommendations in this report are incorporated into design and construction.
| DRILLING DATE 06/02/26 | TOTAL DEPTH 10 FT | WATER NOT ENCOUNTERED |
| METHOD HAND AUGER | DIA. 8 IN | LOGGED BY FM |
EXPANSION CHARACTERISTICS (ASTM D4829)
MOISTURE–DENSITY (ASTM D2216)
| SAMPLE | DEPTH | MOIST. % | DRY DENSITY |
|---|---|---|---|
| B-1 | 2 FT | 9.3 | 104.7 PCF |
| B-1 | 5 FT | 10.6 | 107.3 PCF |
DIRECT SHEAR (ASTM D3080) — φ = 30°, c = 200 PSF
The subject property lies within the Peninsular Ranges Geologic Province, a structurally active region extending along much of Southern California.
The area is primarily underlain by Eocene-age sedimentary formations collectively known as the Poway Group, including the Stadium Conglomerate and Mission Valley Formation, deposited in fluvial and shallow marine environments roughly 40 million years ago.
No known active faults directly underlie the property, and the immediate area is not designated as a seismic hazard zone.
GENERALIZED PROFILE
Site-specific seismic design parameters were developed in accordance with the 2022 California Building Code (ASCE 7-16):
| PARAMETER | VALUE |
|---|---|
| SITE CLASS | D — STIFF SOIL |
| SDS (SHORT-PERIOD) | 0.823 g |
| SD1 (1-SECOND) | 0.402 g |
| PGAM | 0.45 g |
| RISK CATEGORY | II |
The site is not located within a State of California Earthquake Fault Zone (Alquist-Priolo). No active surface faults are known to project through or toward the site.
Liquefaction potential is considered low due to dense soil conditions and absence of shallow groundwater.
SHALLOW FOUNDATIONS
SLABS-ON-GRADE
SURFACE DRAINAGE
This report has been prepared for the exclusive use of the client for the proposed project described herein. Our services have been performed in accordance with generally accepted geotechnical engineering principles and practice.
It is critical that our firm be contacted to observe footing excavations to verify that subsurface conditions match those described in this report.
Sincerely,
MOMENT ENGINEERING
Principal Geotechnical Engineer
That's the whole report: sixteen pages, one question answered.
Can this ground support what you want to build? Every soils report Moment Engineering prepares answers that question for one specific site: yours. We'll tell you what your project needs, and what it doesn't, before any work begins.
Next in the series: Anatomy of a Grading Plan.
Talk to Moment about your project