The Benkelman beam and the lightweight deflectometer (LWD) are the first instruments on site in Leicester when characterising the subgrade reaction modulus. The beam’s aluminium frame sits across lane closures on the A50 or within the industrial estates of Hamilton, while the LWD’s load cell applies a 10 kg drop weight to simulate a single wheel pass. Data from these devices feeds directly into the structural number calculations, where the resilient modulus (Mr) of the Mercia Mudstone-derived weathered clay becomes the critical input. Without a measured Mr value tied to Leicester’s seasonal moisture variation, any flexible pavement design based on generic CBR tables risks premature rutting in the binder course. Our team runs the full suite of falling weight deflectometer (FWD) impact tests to back-calculate in-situ layer moduli, ensuring the asphalt thickness design responds to actual deflections under a 40 kN pulse load, not textbook assumptions.
The thickness design of a flexible pavement in Leicester collapses to a single equation: keep the tensile strain at the asphalt base below 70 microstrain for 50 million standard axles.
Common questions
What is the minimum CBR value required for flexible pavement design in Leicester?
The UK design standard DMRB CD 225 requires a formation CBR of at least 5% for long-life flexible pavements without a capping layer. If the in-situ CBR falls to 2.5% or below, a capping layer of crushed rock (6F5) or a cement-stabilised soil layer must be placed to raise the equivalent CBR at the sub-base level. We measure CBR using in-situ tests at the formation depth, calibrated with laboratory soaked CBR on Shelby tube samples to capture the worst-case wet condition typical of Leicester’s winter months.
How long does a full flexible pavement design and site investigation take for a Leicester commercial development?
A standard programme runs three to four weeks from mobilisation to final report. Week one covers the dynamic plate load tests and FWD survey on the exposed formation. Weeks two and three focus on laboratory evaluation: classification, Proctor, soaked CBR, and resilient modulus on the subgrade and granular materials. The fourth week compiles the analytical model, runs the strain criteria, and delivers the layer thickness schedule. Projects requiring long-term groundwater monitoring or sulphate testing for Mercia Mudstone sites may extend by an additional week.
How much does a flexible pavement design package cost for a road or car park in Leicester?
The design package, including the FWD survey, site investigation with dynamic plate testing, laboratory resilient modulus and CBR, and the full mechanistic-empirical analysis report, typically ranges from £1,420 to £4,460 depending on the number of test locations and the length of the alignment. A small car park with three test points falls at the lower end, while a multi-lane industrial access road requiring full DMRB compliance and a capping layer verification programme reaches the upper range.
What is the difference between the mechanistic-empirical method and the traditional CBR method for flexible pavement design?
The traditional CBR method uses an empirical chart that links subgrade CBR directly to a single total pavement thickness, without distinguishing between material contributions or failure modes. The mechanistic-empirical method, as per TRL Report 615, models the pavement as a multi-layer elastic system and calculates the tensile strain at the bottom of the asphalt (controlling fatigue cracking) and the vertical compressive strain at the top of the subgrade (controlling structural rutting). This allows us to optimise each layer thickness for the specific traffic load in Leicester, rather than relying on a single-thickness solution that overdesigns or underdesigns for the actual axle spectrum.
