Derby sits on a mix of river terrace deposits from the Derwent and glacial tills left by the last ice age. In our experience, that combination creates a tricky subgrade. You get stiff clays in the west near Allestree, then loose sands and gravels closer to the city centre. That variability is exactly why road subgrade design in Derby needs site-specific testing. Without it, you risk differential settlement under the pavement. We always start with trial pits to log the soil profile visually, then take disturbed and undisturbed samples for lab classification. The plasticity index of those clays can swing from low to high within 200 metres. That alone changes the CBR design value significantly. For low-volume roads we often use the ensayo CBR directly on recompacted samples, while for trunk roads we run repeated load triaxial tests to capture resilient modulus.
A saturated subgrade in Derby can lose 60% of its bearing capacity. That's why we always check the water table before finalising the design CBR.
Process overview
Local context
The biggest risk we see in Derby is the transition between the river terrace gravels under the city centre and the stiff till under the suburbs. If a road alignment crosses that boundary, the subgrade stiffness can change by a factor of three. That creates differential movement at the pavement interface. Cracking starts within two years if it is not addressed. We mitigate this by specifying a capping layer of granular material across the transition zone, typically 300 to 450 mm thick. The design CBR is taken at the 5th percentile of all test results, not the average, so the pavement works on the weakest spot.
Reference standards
BS 1377-9 (in-situ tests), BS EN 1997-2 (Eurocode 7 – ground investigation), Design Manual for Roads and Bridges (DMRB) CD 225, BS 6031 (earthworks)
Additional services
Site Investigation & Profiling
Trial pits, window sampling, and DCP logging to map soil variability across the site. We log every layer to BS 5930 and produce a geotechnical model for the design engineer.
Laboratory Testing & CBR Design
Classification tests, compaction curves, soaked CBR, and resilient modulus. All testing is UKAS-accredited. We deliver a design CBR value with a statistical confidence interval.
Stabilisation Recommendations
If the subgrade is too weak, we assess lime or cement stabilisation. We run pH tests and Eades & Grim tests to determine the percentage of stabiliser needed for a 7-day strength target.
This service complements our laboratory testing work for a complete project analysis.
Typical parameters
Common questions
How do you determine the design CBR for a road in Derby?
We take samples from trial pits at formation level, compact them to the target density, soak them for four days, and then run the CBR penetration test. The design value is usually the 5th percentile of all results, as per DMRB CD 225.
What is the typical subgrade strength in Derby?
The glacial tills in areas like Allestree and Littleover give soaked CBR values between 2% and 5%. The river terrace deposits nearer the city centre can reach 8% to 10% when well-drained, but they drop quickly if the water table rises.
How long does a road subgrade investigation take?
A standard investigation for a 500-metre stretch takes about two to three weeks. That includes site work, sample transport, lab testing, and the final report. We can expedite the CBR tests to seven days if the programme is tight.
What is the cost range for road subgrade design in Derby?
The cost typically ranges between £710 and £2,160 depending on the number of test pits, the extent of lab testing, and whether stabilisation trials are needed. We provide a fixed quote once we see the site layout.
Do you also design pavement layers?
We do not design the asphalt or concrete layers. Our work stops at the subgrade and capping layer. We give the design engineer a certified CBR value and a modulus of subgrade reaction so they can size the pavement above.