In Peterborough, the variability of the underlying Oxford Clay Formation catches out contractors who rely solely on visual classification. You dig a trial pit in Fengate and find stiff grey clay; half a mile toward Hampton the same stratum is weathered to a soft, fissured consistency. The laboratory CBR test cuts through that ambiguity. We run soaked and unsoaked California Bearing Ratio determinations on remoulded or undisturbed samples, giving you a direct measure of subgrade strength that feeds straight into pavement thickness design per the Design Manual for Roads and Bridges. For sites near the Nene floodplain where groundwater fluctuates seasonally, we often pair the CBR programme with a grain size analysis to confirm fines content, and with Atterberg limits to pin down the plasticity characteristics that govern long-term swelling behaviour.
A soaked CBR of 2% on Oxford Clay versus 15% on river gravel changes the pavement thickness by a factor of three—that is the cost of guessing.
Our approach and scope
Site-specific factors
The Oxford Clay around Peterborough has a typical undrained shear strength of 60-150 kPa in its intact state, but remoulding during earthworks and subsequent saturation can drop the soaked CBR below 2%. That is a subgrade class S4 in DMRB terms—the weakest category. On the Eye Green expansion or the Great Haddon development, where bulk earthworks move thousands of cubic metres of clay fill, the risk is not just pavement failure but differential heave. If the formation is left exposed over winter, frost action and wetting fronts degrade the upper 300 mm rapidly. A laboratory CBR test on specimens moulded at the target density and moisture content provides a defensible design value. We also flag when the CBR results point toward the need for slope stability analysis on approach embankments, particularly where the A15 or A47 corridors cross soft ground.
Regulatory framework
BS 1377-4:1990 — Soaked CBR determination, BS 5930:2015+A1:2020 — Code of practice for ground investigations, Eurocode 7 (BS EN 1997-2:2007) — Ground investigation and testing, DMRB CD 225 — Design for new pavement foundations, Specification for Highway Works (SHW) Series 600 — Earthworks
Linked services
Soaked CBR Determination
Full BS 1377-4 procedure with 96-hour soaking, surcharge application, and force-penetration curve analysis at both 2.5 mm and 5.0 mm piston travel.
Moisture-Density Relationship
Standard Proctor compaction curves to establish optimum moisture content and maximum dry density before CBR specimen preparation.
Multi-Point CBR Programmes
Three-point or five-point CBR families for sites with variable geology, providing a design CBR profile rather than a single value.
Swelling Potential Assessment
Swell measurement during the 96-hour soaking phase, critical for Oxford Clay subgrades where volume change can crack overlying bound layers.
Typical parameters
Q&A
How much does a laboratory CBR test cost in Peterborough?
A standard soaked CBR test with compaction curve costs between £100 and £170 per specimen, depending on whether the sample is remoulded or undisturbed and the number of compaction points required.
What is the difference between soaked and unsoaked CBR?
Unsoaked CBR tests the material at its moulded moisture content. Soaked CBR simulates long-term field conditions after groundwater rise or infiltration. For UK highway design, the soaked value is the design parameter; it is almost always lower and governs pavement thickness.
How long does a CBR test take from sample receipt to results?
Allow 7 to 10 working days. The compaction curve takes one day, specimen preparation and soaking requires 4 days, and the penetration test plus reporting takes a further 2 days. Larger programmes may extend the turnaround slightly.
Can you test CBR on cohesive and granular soils?
Yes. The procedure works for both, though granular materials require careful specimen preparation. For clean sands and gravels that do not hold a moulded shape, we often recommend complementing the CBR with in-situ plate load testing to obtain a stiffness modulus directly on the compacted layer.
What CBR value is considered acceptable for a road subgrade?
Under DMRB CD 225, a soaked CBR of 2% or higher is the minimum for a subgrade without stabilisation, but values below 5% typically require a capping layer. Industrial pavements in Peterborough's distribution parks often target 15% CBR at formation to minimise the granular sub-base thickness.