What Happens During a Compaction Test? A Homeowner's Guide

Most homeowners have never seen a compaction test

If your project involves grading, filling, or new foundation construction, you're going to have a geotechnical technician show up at your site at various points during construction. They'll set up equipment, take some measurements, and either give the contractor a thumbs-up or tell them to rework an area.

The whole thing usually takes 15–30 minutes per test location. Here's what's actually happening.

What the technician brings: the nuclear density gauge

The main instrument is a nuclear density gauge — a yellow or orange device about the size of a large lunchbox with a rod that extends into the ground. It's called "nuclear" because it contains a small radioactive source (Cesium-137 or Americium-241) that emits radiation to measure soil density.

Don't be alarmed by "radioactive." The radiation exposure from standing near a nuclear gauge during a test is less than what you'd get from a dental X-ray. The technician is licensed to transport and operate it under NRC regulations, and the device is designed to be completely safe in normal use. You'll sometimes see the tech's vehicle with a yellow radioactive materials placard — this is standard and required by DOT, not a sign of anything unusual.

The gauge works by measuring how much radiation passes through the compacted soil. Denser soil scatters more radiation, giving the gauge a reading it converts to dry density and moisture content. The whole measurement takes about a minute once the gauge is positioned.

Step by step: what happens during a test

Here's the sequence from the moment the technician arrives at a test location:

1. Inspect the lift. The tech looks at the recently compacted soil — checking that the lift thickness is appropriate (typically 8–12 inches for residential fill), that the surface is uniform, and that there's no obvious problem (soft spots, segregated material, standing water).

2. Prepare the test location. The surface is scraped or bladed to remove any loose material or tire ruts, so the gauge sits on representative compacted soil.

3. Drive the guide rod. A steel guide rod is driven into the soil to create a channel for the gauge's measurement rod. This is done with a manual or pneumatic drill rod driver.

4. Position the gauge and take the reading. The gauge is placed over the guide rod and the measurement rod is lowered to the test depth. The gauge counts radiation for 1–4 minutes and displays density and moisture content.

5. Calculate relative compaction. The tech divides the measured dry density by the maximum dry density from the laboratory Proctor test (done before grading starts on the project's soil samples). The result is the relative compaction — expressed as a percentage.

6. Pass or fail call. If relative compaction meets the specification (typically 90% or 95%), the lift passes and the contractor can continue. If not, the contractor must rework that area before placing the next lift.

Working on a project in Southern California? We can handle the engineering.

What 90% compaction actually means

The Proctor test (ASTM D1557 for Modified Proctor, or D698 for Standard Proctor) establishes the maximum dry density for a specific soil at a specific moisture content. This is done in the lab before grading starts, using representative samples of the fill material.

90% relative compaction means the in-place dry density is at least 90% of that laboratory maximum. In practical terms, it means the soil is dense enough that it won't settle significantly under the loads your foundation will impose.

• Different parts of your project may have different specifications:
• Structural fill under foundations and slabs: usually 90–95%
• Fill under driveways and hardscape: typically 95%
• General site fill away from structures: often 90%
• Backfill in utility trenches: usually 90% (95% under pavement)

The soils report specifies the required compaction for each area. The geotechnical engineer sets these numbers based on the load conditions and the properties of the fill material.

When a test fails

Failed tests happen. They're not a crisis — they're the system working as intended.

• When a lift fails, the tech records the result and tells the grading contractor. The contractor then:
• Adds water if the soil is too dry (moisture content affects compaction ability significantly)
• Removes water through aeration if the soil is too wet
• Makes additional compaction passes with the roller or plate compactor
• In some cases, removes and replaces the fill if the material itself is problematic

Once the contractor has reworked the area, they call the tech back for a retest. No additional fill can be placed in that area until the failed lift passes.

You pay for retests if they're needed. This is one reason it's worth hiring a good grading contractor — experienced contractors know how to manage moisture content and compaction equipment to pass tests consistently. Contractors who are sloppy about moisture or use the wrong equipment fail more tests, and the retesting cost adds up.

The final compaction report

At the end of the grading operation, the geotechnical engineer of record compiles all the field test results into a compaction report (sometimes called a grading certification or final compaction report). This document includes:

• A summary of all tests performed, with location, depth, date, and result for each
• A site plan showing test locations
• Laboratory test results (Proctor tests) for all fill materials used
• Certification by the geotechnical engineer that grading was performed in general accordance with the soils report recommendations and applicable specifications
• Any areas of non-compliance noted and how they were resolved

The building department requires this report before issuing final grading approval. Without it, your project can't get a Certificate of Occupancy. We submit the report directly to the building department once grading is complete and all tests have been reviewed.

Tips for a smooth compaction testing process

Coordinate scheduling in advance. Compaction testing is time-sensitive — tests must be performed on each lift before the next lift is placed. Give us as much advance notice as possible when grading is starting so we can schedule the technician.

Don't place fill on weekends without notifying us. Testing must be performed by a licensed technician. If the contractor is grading on Saturday and no tech is present, that fill can't be accepted by the building department.

Have your fill material tested before grading starts. The Proctor test on the fill material must be completed before field testing begins. If you're importing fill, get us a sample before the first truckload arrives.

Ask your contractor about moisture management. The single biggest cause of failed compaction tests is improper moisture content. A contractor who monitors and adjusts moisture throughout the grading operation will pass tests more consistently.

Related Service

Compaction Testing & Observation

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