How often do you test during grading?

Typically every 2 feet of fill depth vertically, or every 1,000–2,000 square feet horizontally, whichever comes first. The exact frequency is specified in your soils report.

What does 90% compaction mean?

It means the in-place density is at least 90% of the maximum laboratory density for that soil type. Structural fill areas typically require 90%; areas under slabs or pavement often require 95%.

Can the contractor compact without the inspector present?

No. Tests must be performed before the contractor proceeds to the next lift. If we're not there to verify it, the building department won't accept it.

What happens if a test fails?

The contractor recompacts that area — usually with more moisture and additional compaction passes — and we retest. The area must pass before any additional fill is placed.

Do I pay for failed tests?

You pay for retests if they're needed. That's why it's in your interest to hire a good grading contractor who knows how to compact properly the first time.

Home›Resources›What Is Compaction Testing?

ExplainerGeotechnical

What Is Compaction Testing?

How field testing verifies your earthwork meets engineering specifications — and what happens when it doesn't.

Dirt That Looks Solid Isn't Always Solid

Here's something most people don't realize: the dirt under your foundation isn't just piled up and smoothed over. It's engineered fill — soil that's been placed in controlled layers, compacted to a specific density, and tested to verify it meets the geotechnical engineer's specifications.

Compaction testing is the field work that verifies this. A geotechnical technician comes to your site, takes density tests as the contractor places fill, and confirms that each layer of soil is compacted to at least 90% (or 95%, depending on the application) of its maximum laboratory density. If it passes, construction continues. If it fails, the contractor has to rework that area and retest.

This isn't bureaucratic box-checking. Poorly compacted fill settles unevenly under load, which causes foundation cracks, floor slabs that heave or settle, and walls that separate from frames. Compaction testing prevents these problems by catching weak fill before the foundation goes in.

What Happens During Compaction Testing

Compaction testing is a multi-step process coordinated between the contractor and the testing firm:

📋

Pre-Construction Meeting

The geotechnical engineer, contractor, and inspector meet to review the soils report recommendations, compaction requirements, and testing schedule.

🚜

Fill Placement in Lifts

The contractor places fill soil in horizontal layers ("lifts") typically 8-12 inches thick before compaction. Each lift must be compacted before the next one goes down.

🔬

Field Density Testing

Our technician performs nuclear density gauge or sand cone tests on each compacted lift, measuring the soil's in-place density and moisture content.

📊

Pass/Fail Determination

We compare the field density to the laboratory maximum density. If the fill achieves the specified compaction (usually 90% or 95%), that lift passes and the contractor can proceed.

⚠️

Failed Tests & Rework

If a lift fails, the contractor recompacts that area and we retest. No additional fill can be placed until the failed area passes.

✅

Final Report

At project completion, we issue a final compaction report documenting all tests, locations, and results. The building department requires this for final inspection approval.

Need Compaction Testing & Observation? We serve all of Southern California.

Request a Proposal →or call (619) 374-8677

Why Compaction Testing Protects Your Investment

Compaction testing is your insurance policy against settlement damage. Properly compacted fill doesn't settle. Poorly compacted fill absolutely will — and when it does, you're looking at cracked foundations, floor slabs that drop, doors that won't close, and repair bills that dwarf what proper testing would have cost.

Building departments require compaction testing because they've seen what happens when it's skipped: foundations that settle 2-3 inches in the first year, requiring structural underpinning and tens of thousands in repairs. Contractors who "just eyeball it" and assume the fill is good enough because it looks solid.

We test soil because assumptions kill projects. That's the entire point of field verification — catching problems when they can still be fixed cheaply, not after the house is built and the damage is done.

When You Need Compaction Testing?

Common project types and triggers:

New Foundations

Required for any foundation built on fill — whether it's a few inches or several feet. This includes slabs, footings, and grade beams.

Fill Import

If the contractor is bringing in fill soil from off-site, every lift must be tested as it's placed and compacted.

Utility Trenches

Trenches for water, sewer, or electrical must be backfilled and compacted to prevent settlement under driveways, sidewalks, or structures.

Retaining Walls

Backfill behind retaining walls must be compacted to prevent lateral pressure buildup and wall failure.

Building Pads on Slopes

Cut-and-fill pads on hillside lots require extensive compaction testing to verify the fill portion is stable.

Common Questions

What clients typically ask about compaction testing?:

Ready to Move Forward?

We handle compaction testing? for projects across Southern California.

Tell us about your project and we'll send a proposal with scope, deliverables, and fee.

Request a Proposal (619) 374-8677

Learn more about this service:

Compaction Testing & Observation→