Bournemouth's expansion from a coastal heathland settlement into a major conurbation has placed increasing demands on its transport infrastructure. The town sits on a complex geology of Eocene sands, clays and gravels of the Branksome Sand and Boscombe Sand formations, with several valleys carved into the plateau. These terrace gravels and variable head deposits create challenges for pavement engineers who need consistent subgrade support. A laboratory CBR test quantifies the bearing capacity of compacted soil under controlled moisture and density conditions, giving designers the California Bearing Ratio value needed to determine pavement layer thicknesses. We run soaked and unsoaked CBR tests in our Bournemouth laboratory, following BS 1377-4:1990 and the Specification for Highway Works Clause 600 series, with moulds prepared at Proctor optimum moisture content to simulate worst-case field conditions.
A soaked CBR below 2% on a Bournemouth subgrade means the pavement design must switch from a capping layer to a full stabilisation strategy — the cost difference is substantial.
Service characteristics in Bournemouth
Typical technical challenges in Bournemouth
The CBR loading frame in our Bournemouth laboratory uses a motorised drive with a calibrated 50 kN load cell and a penetration piston of 49.6 mm diameter. Preparing specimens from the Poole Road area, where the Branksome Sand contains lenses of silt, demands careful moisture conditioning. If the soaking water has a different ionic concentration from the site groundwater, the clay minerals can swell anomalously and produce a falsely low CBR. We use de-aired water at Bournemouth tap-water chemistry as a baseline, but for highly plastic clays — rare but present in the Reading Formation outcrops to the north — we request site-specific water. The failure mechanism in a CBR test is shear punching beneath the piston, and the result is heavily influenced by the surcharge weight, which simulates the confining pressure from overlying pavement layers. An incorrect surcharge can shift the CBR by 5 to 10 points.
Our services
Our Bournemouth laboratory delivers a comprehensive CBR testing service for pavement and earthworks projects throughout the South Coast region.
Soaked CBR for pavement design
Three-point CBR determination at target density and moisture content, with 96-hour soak and swell monitoring. We test subgrade, capping and sub-base materials per Series 600, delivering the CBR value at 2.5 mm and 5.0 mm penetration with full load-penetration curves.
Unsoaked CBR for earthworks control
Immediate CBR testing on as-compacted specimens for embankment and fill acceptance where the design assumes unsaturated conditions. Useful for granular fills and free-draining materials where soaked testing is overly conservative.
Common questions
What does a laboratory CBR test cost in Bournemouth?
A single-point CBR test — one mould compacted and tested at a specified moisture and density — is priced between £100 and £190 depending on whether soaking is included and the number of surcharge rings required. A three-point CBR family for a full design curve is typically between £300 and £450.
What is the difference between soaked and unsoaked CBR?
A soaked CBR measures the bearing capacity after the soil has been saturated for 96 hours, simulating long-term wet conditions beneath an impermeable pavement. An unsoaked CBR tests the material at its as-compacted moisture content. For UK highways, the soaked CBR is the standard design input because it represents the weakest condition the subgrade will experience.
How is the CBR value used in pavement thickness design?
The soaked CBR of the subgrade determines the required thickness of capping, sub-base and base layers using the design charts in Highways England CD 225 or IAN 73/06. A CBR below 2% requires a stabilised capping layer or a granular replacement. Between 2% and 5% a capping layer is needed. Above 5% the sub-base can be placed directly, and above 15% the pavement thickness reduces significantly.
Can you test aggregate materials for sub-base CBR?
Yes, we test Type 1 and Type 2 sub-base aggregates, crushed concrete and slag-bound materials in the CBR mould. For granular materials with particles larger than 20 millimetres, we follow the scalping procedure in BS 1377-4 to replace oversized material with an equivalent mass of 20 mm to 37.5 mm material.