If you’re overseeing a new build or infrastructure project, there’s one test that should be high on your list before any construction starts: CBR testing. Widely used across the industry for civil engineering purposes, the California Bearing Ratio (CBR) test provides essential data on sub grade ground strength, helping you design suitable road pavement structures and avoid costly delays.
The method was originally developed by the California State Highway Department to assess the bearing capacity of soils. Today, it remains a cornerstone of site investigation for new developments.
Here’s what you, as a site manager, need to know.
What Is the California Bearing Ratio Test
CBR testing measures how well the ground can support a load. The CBR test procedure involves using a metal plunger to apply force to a soil sample and comparing the penetration resistance to that of a standard crushed rock material. The pressure required to achieve equal penetration at depths of 0.1 and 0.2 inches is used to calculate the CBR values, expressed as a percentage.
This penetration test provides insight into how subgrade soil or a high quality sub base will perform under stress—vital for pavement design, road construction, and temporary works.
Two common methods are used:
- In situ test – carried out directly on site at surface level
- Laboratory tests – performed under controlled conditions using extracted soil samples
Why You Need CBR Testing on New Sites
Many new sites involve preliminary site investigations, imported fill, or disturbed ground. Without testing, critical project requirements may be based on guesswork.
CBR testing helps you:
- Assess bearing capacity of the subgrade soil
- Determine necessary surface stiffness for roads and slabs
- Inform the use of plate load tests or compaction related tests
- Satisfy design needs based on load values and expected traffic
- Identify areas of very weak soil needing reinforcement
When to Schedule a Test
You should schedule CBR testing early, ideally during your site investigation phase. It’s especially relevant:
- For typical site layouts with car parks, access roads, or pavements
- When conditions suggest variable or inconsistent strength
- After encountering multiple samples or fill layers
- When preparing a proposed construction centreline
- To support design choices for platform and traffic loadings
Site Investigation and Preparation
Accurate results start with detailed site investigation. A visual inspection identifies initial concerns across the test area, and soil samples are collected from multiple locations to capture the full range of soil behaviour.
These investigations may include compaction related tests and chemical analysis to assess moisture content, density, and maximum particle size—all of which affect the CBR values. A well-prepared test location ensures data from both in situ and laboratory tests is consistent and reliable.
How the Test Works
A small pit is excavated, and a metal plunger is pushed into the ground at a steady rate. The pressure required to reach set depths is compared to a standard material, producing a CBR value. These values help determine bearing capacity and are critical to pavement thickness calculations.
You’ll typically perform multiple tests across the site to account for local variation. The dynamic cone penetrometer (DCP test) is often used alongside to measure penetration depth and confirm equivalent CBR values. Some sites may also include an incremental plate load test or plate load assessment to supplement surface performance data.
Laboratory Testing and Analysis
In the lab, CBR test procedures replicate field conditions under controlled settings. Engineers conduct penetration tests, track settlement, and compare findings against reference charts for different soil types.
Other laboratory tests—such as the incremental plate load test—may assess stiffness and compaction. Results are interpreted in the context of penetration resistance, moisture content, and particle grading to determine if the soil meets project requirements.
Field Testing and Equipment
Field tests provide on-the-spot insight into how soil responds to construction loads. Common methods include the plate load test, DCP test, and plate bearing test.
These rely on equipment such as dial gauges, reaction frames, and plunger assemblies to track how the soil settles under applied pressure. Testing is carried out across multiple locations to reflect any variation in ground conditions.
Understanding the Results
CBR values are expressed as percentages and generally fall into these ranges:
- Less than 5% – Very weak soils, often requiring replacement or stabilisation
- 5–10% – Weak subgrades that may need strengthening
- 10–30% – Typically acceptable for standard pavement design
- Above 30% – Strong, stable ground with minimal intervention required
Interpretation depends on load required, measured pressure, and reaction load. Your engineer will recommend base layer thickness or reinforcement based on this data.
Real-World Example: Avoiding Over-Engineering
On a development in the South West, in situ CBR testing showed inconsistent strength due to recent fill. Instead of using the same pavement thickness site-wide, engineers adapted the design by applying greater compaction only where needed. This reduced material use and construction time while still meeting structural requirements.
CBR vs. Plate Load vs. DCP Testing
Each method serves a different purpose:
- CBR testing is best for determining subgrade strength in roads and pavements
- Plate load testing evaluates settlement over wider areas for structures like working platforms or piling rigs
- DCP tests are useful for quick checks or identifying weak zones, but less precise for final design
Choosing the right test depends on your site conditions, structure type, and load demands.
CBR Testing Checklist for Site Managers
Before commissioning a test, ask:
- Is the ground disturbed or compacted?
- Are you building roads, pavements, or access routes?
- Do you need proof of ground strength for design or regulation?
- Are there different zones or fills across the site?
- Are you still in the investigation or design phase?
If the answer is yes to most, CBR testing should be a priority—particularly for UK construction companies working to regulatory standards.
Common Mistakes to Avoid
To ensure accurate data, avoid:
- Testing too early, before ground compaction or fill is complete
- Using a single test to represent the whole site
- Failing to mark and document the exact test location
- Ignoring moisture content, which affects penetration resistance
- Skipping retests after earthworks or weather changes
Common Challenges and Limitations
While the test is reliable, there are some limitations:
- Results reflect current soil conditions only—not long-term changes
- Very weak soil or mixed strata can make values harder to interpret
- The test requires experience and the right tools to perform correctly
That said, CBR testing remains one of the most dependable tools in civil engineering when used appropriately.
How Long Does It Take? What Should You Budget For
An in situ test usually takes under an hour. Reports are typically returned in 1–2 working days. Laboratory testing may take longer depending on sample processing.
In terms of cost, CBR testing is a small investment compared to the cost of structural failure, overdesign, or ground remediation.
FAQs: CBR Testing
How many CBR tests do I need?
One per 250–500m² or per change in ground condition is a typical guideline.
Can the same location be tested more than once?
Yes—especially after compaction, earthworks, or weather-related changes.
Does moisture affect CBR results?
Yes. Saturated ground reduces penetration resistance. Test when conditions match the expected use.
Is CBR testing suitable for platform or crane pad design?
It can assist early planning, but for detailed settlement analysis, use a plate load test.