Many construction teams in Derby underestimate how much the local clay subgrade influences pavement performance. Specifying a standard asphalt thickness without verifying the California Bearing Ratio of the underlying soil often leads to premature cracking or rutting within the first few winters. A proper flexible pavement design starts with field exploration and lab testing to match the structural layers to actual ground conditions. We combine in-situ CBR tests with clasificación de suelos to classify the foundation material, then run repeated-load triaxial tests when the traffic category demands a mechanistic-empirical approach. That way the pavement cross-section resists Derby's freeze-thaw cycles without excessive deformation.
Soaked CBR values below 2.5% in Derby's mudstone subgrades demand a capping layer before any granular sub-base is placed.
Process overview
Local context
The falling-weight deflectometer we mobilise across Derby uses a 300 mm diameter load plate and delivers a peak force of 50 kN, simulating a single heavy-axle pass. The device drops a mass from a controlled height onto a buffer system, and the geophones record deflection basins at seven radial offsets. We run at least six drops per test point to check repeatability. Interpreting those deflection bowls through back-calculation reveals the in-situ stiffness of each pavement layer. If the results show a weak subgrade or debonding between asphalt and base, the pavement will fail within two to three years under Derby's dairy and aggregate truck traffic. Early detection lets us adjust the design before reconstruction becomes unavoidable.
Reference standards
BS 1377:1990 (Methods of test for soils), BS EN 13286-47:2004 (CBR test for unbound mixtures), Design Manual for Roads and Bridges (DMRB) CD 225, Eurocode 7 (EN 1997-2:2007) – Ground investigation
Additional services
Subgrade Investigation & CBR Testing
In-situ DCP and laboratory soaked CBR tests on undisturbed tube samples from the Mercia Mudstone. We classify the soil and measure plasticity to predict volume change risk.
Mechanistic-Empirical Pavement Design
Using BISAR or KENLAYER software we compute critical strains in the asphalt and subgrade. Traffic data from Derby's A-road network is incorporated to define the design spectrum.
Pavement Condition Survey & Overlay Design
FWD deflection testing combined with core sampling to assess existing pavements. We recommend overlay thicknesses and material options that extend service life by 10–15 years.
Typical parameters
Common questions
What CBR value is typical for Derby subgrades?
Most Mercia Mudstone subgrades in Derby give soaked CBR values between 2% and 5%, though some sites with reworked clay drop below 1.5%. We always verify with laboratory tests.
How much does flexible pavement design cost in Derby?
For a standard industrial access road the design and testing package ranges between £1,470 and £4,440 depending on the number of boreholes, CBR tests, and traffic analysis required.
Do you follow DMRB or Eurocode 7 for pavement design in Derby?
We apply DMRB CD 225 for highway projects and Eurocode 7 for foundation verification. Both are complemented by BS 1377 laboratory procedures.
What happens if the subgrade CBR is too low for the design traffic?
We recommend lime or cement stabilisation of the upper 300 mm, or a granular capping layer, to raise the effective CBR above the minimum threshold for the selected pavement structure.
Can you design flexible pavement for residential roads in Derby?
Yes. For low-traffic residential streets we use the TRL Road Note 29 method with a 20-year design life, adjusting layer thicknesses to match the local subgrade strength.