Common Soil Conditions and Their Typical CBR Values

A CBR test on soft clay might return 3%. Run the same test on well-compacted gravel and the result can sit above 40%. Neither figure is an error. Both reflect how differently each soil resists penetration under load, which is exactly what the California Bearing Ratio test is designed to measure.

That variation in CBR values is the point of this article. Once you know what “normal” looks like for the soil type in front of you, a low result stops being a surprise and starts being a design input: capping layer, drainage, stabilisation, or a straightforward pass.

Why Soil Type Dictates CBR Values

The CBR test pushes a standard 50mm diameter plunger into a prepared soil sample at a fixed rate of 1.25mm per minute. It records the load needed to reach penetrations of 2.5mm and 5.0mm, then expresses that load as a percentage of what a standard crushed rock reference material would need to reach the same depth.

Two soils tested identically can still produce very different numbers, because friction, cohesion, and moisture content behave differently in each. A well-compacted granular soil resists penetration mainly through interparticle friction, while a clay resists through cohesion, which weakens sharply as moisture rises. That single distinction explains most of the spread in results across a typical site.

The method was developed by the California Division of Highways, later Caltrans, in the late 1920s and adopted statewide in 1935. In the UK it’s governed by BS 1377-9, which sets out the in-situ test procedure for subgrade and sub-base assessment.

The ranges below are the figures commonly cited across UK geotechnical practice for each soil type. They’re a starting point for interpreting a result, not a guarantee for every sample: moisture, compaction, and disturbance on the day of testing all move the number within that soil type’s typical range.

Clay Soils: The Moisture-Sensitive Variable

Clay is the most common subgrade material encountered on UK sites, and the most sensitive to conditions on the day of testing.

Typical outcome: 2% to 5%. Soft to firm clay usually returns 2% to 3%. Very stiff or dry clay can reach 5%, though that result won’t hold if conditions change.

Why Results Vary

Clay strength is controlled almost entirely by moisture content. Water breaks down interparticle bonding, which lowers subgrade strength and increases deformation under a given load. A clay that tests competent in a dry summer can lose bearing capacity rapidly once winter rainfall raises the moisture content.

Design implication: engineers typically design for worst-case moisture rather than the conditions on test day. A single CBR reading taken in summer may not represent the subgrade’s performance over the life of the pavement. Drainage, capping layers, or stabilisation are the usual responses where clay dominates.

Sandy Soils: Friction-Driven Performance

Sands behave differently from clay because they carry no meaningful cohesion. Strength comes entirely from how tightly the particles are packed together.

Typical outcome: 10% to 20%. Well-compacted sand routinely reaches the upper end of this range, provided the maximum particle size stays below the 20mm limit the CBR test requires.

Key Influences

• Compaction level: loose sand displaces sideways under the plunger instead of resisting it, which can roughly halve the apparent bearing capacity compared with dense sand.
• Moisture: affects packing and apparent cohesion, though less dramatically than in clay.
• Particle grading and confinement: well-graded, confined sand resists penetration more effectively than uniform, unconfined material.

Design implication: compaction before testing isn’t optional. The difference between loose and dense sand can be large enough to change both the required pavement thickness and the volume of imported material.

Gravel and Crushed Rock: High-Strength Materials

Gravelly soils and crushed rock sit at the top of the CBR scale, and it’s worth separating two categories that get blurred together in practice.

Naturally occurring gravel: 30% to 60%+. Typical result for gravelly soils encountered in situ, depending on grading and compaction.

Specified sub-base material, such as Type 1: 80% to 100%. This is processed, well-graded crushed rock compacted to a design specification, which is why it performs closer to the 100% reference value than natural gravel does.

Practical Considerations

Large particles bearing directly under the plunger can distort a single reading, which is why multiple tests across a site are standard practice rather than a precaution. High CBR values here translate directly into thinner, more cost-effective pavement foundations and less imported material overall.

Chalk: Strong Until Disturbed

Chalk is common across southern England and behaves in a way that catches out anyone assuming a single test tells the whole story.

Typical outcome: 5% to 20%. Intact chalk can show respectable bearing capacity. Once broken down into remoulded “putty chalk,” that strength drops sharply and doesn’t recover.

Design implication: limit trafficking by construction plant on exposed chalk, particularly in wet conditions. Repeated passes from site vehicles or load concentrated under crane outriggers can degrade an intact chalk subgrade faster than most engineers expect.

Silt: Low Strength, High Risk

Silt combines poor drainage with low cohesion, which makes it one of the least forgiving soils on a construction site.

Typical outcome: below 2%. Silt deforms easily under load, which also makes it difficult to compact to a stable state.

Design implication: soil replacement or stabilisation is the usual route where silt dominates the subgrade. Expect greater construction depth and design mitigation than a clay or sand subgrade would require.

Peat and Organic Soils: Unsuitable for Construction

Peat is decomposed organic matter holding a high proportion of water, and it offers almost no resistance to the CBR plunger.

Typical outcome: below 1%. The penetration load required is negligible, and settlement risk is severe.

Design implication: peat is removed entirely rather than designed around. Without piling or deep ground improvement, no foundation design should rely on a peat subgrade.

Understanding the CBR Test in Context

CBR testing can be run in the lab or in-situ, on undisturbed or remoulded samples. Lab testing typically compacts soil into a mould at a set density and moisture content, often soaking it for four days to simulate the wettest conditions the subgrade is likely to see in service.

In-situ testing uses vehicle-mounted equipment to provide a reaction load, sometimes alongside a plate bearing test where particle sizes are too large for the standard CBR plunger. Dynamic cone penetrometer and light-weight deflectometer testing supplement these results, particularly on coarse material outside the CBR test’s normal range.

Interpreting Your Results for Pavement Design

A low CBR value isn’t a failed test. It’s a design input that determines subgrade treatment, pavement thickness, and whether soil improvement is needed before construction proceeds.

What counts as a good result depends entirely on what the material needs to do. As a general guide used in UK pavement design:

Layer	Minimum CBR
Subgrade	3%
Capping	15%
Sub-base	30%

These thresholds follow the general design guidance in CD 225, which is periodically revised. Always check the current edition before finalising a specification.

Higher values allow thinner pavement structures and less imported material. Regular testing during construction, not just at design stage, confirms that materials at each depth continue to meet the assumptions the design was built on.

Final Thoughts

The CBR test hasn’t changed much since the 1930s, and it doesn’t need to. Understanding how soil type drives the result is what turns a percentage on a report into a usable design decision.

If you’re working with variable ground conditions and want a result you can build a design around, book a CBR test with our team.