A common mistake we see in London is skipping the subgrade stiffness assessment before designing the pavement layers. Contractors assume the London Clay will behave uniformly across the site. That assumption often leads to differential settlement and early fatigue cracking within the first three years. A proper flexible pavement design must start with in-situ California Bearing Ratio tests and dynamic cone penetrometer soundings. Without those data points, the asphalt thickness is a guess. We also recommend running a placa de carga test on the finished subgrade to confirm the modulus of reaction before placing the granular base. That single step prevents most of the structural failures we witness across London developments.
The London Clay subgrade can lose up to 50% of its stiffness when soaked — design for the wet season, not the dry spell.
Scope of work
Area-specific notes
London sits primarily on the London Clay Formation, a highly plastic overconsolidated clay that shrinks and swells with seasonal moisture changes. During dry summers the clay cracks and loses confinement. When winter rains return, the softened subgrade cannot support the pavement structure. We have documented cases in Camden and Southwark where rutting exceeded 25 mm within 18 months because the design assumed a constant CBR of 4%. The actual soaked CBR dropped to 1.8%. A flexible pavement design that does not account for this seasonal variation will fail. We always run soaked CBR tests and a consolidacion test to quantify the swelling potential before finalizing the layer thicknesses.
Standards used
BS 5930:2015 (Site investigation code of practice), PD 6691:2022 (Guidance on the design of flexible pavements), AASHTO Guide for Design of Pavement Structures 1993, BS 1377-4 (CBR test method)
Linked services
In-Situ CBR & DCP Testing
On-site California Bearing Ratio tests using a hydraulic jack and proving ring, plus dynamic cone penetrometer soundings for continuous strength profiles. Results are correlated to subgrade modulus for design input.
Falling Weight Deflectometer Survey
FWD testing on existing pavements or trial sections. We measure deflection basins and back-calculate layer moduli using ELMOD software. Useful for overlay design and pavement evaluation.
Laboratory Characterisation
Soaked CBR, compaction (Proctor), Atterberg limits, and particle size distribution on subgrade samples. All tests follow UKAS-accredited procedures under BS 1377 standards.
Typical parameters
Q&A
What is the difference between flexible and rigid pavement design?
Flexible pavements distribute loads through layered granular and asphalt materials. The subgrade carries the load at the bottom. Rigid pavements use a concrete slab with higher flexural strength. In London, flexible designs are common for residential streets and car parks because they accommodate settlement better and are cheaper to repair.
How long does a flexible pavement design study take in London?
A typical study takes four to six working days from site visit to final report. If laboratory CBR tests are required, add three to five days for curing and soaking. We schedule the field work to avoid wet weather delays.
Why is the London Clay problematic for pavement design?
London Clay is a high-plasticity clay that loses stiffness when wet and shrinks when dry. The soaked CBR can drop to 2% or lower. If the design assumes a dry CBR of 4-5%, the pavement will rut and crack within two to three winters. We always test at soaked conditions.
What standards do you follow for flexible pavement design?
We follow PD 6691:2022 for UK practice, the AASHTO 1993 Guide for structural number and layer thickness, and BS 1377 for laboratory testing. For heavily loaded industrial pavements we also reference TRL Report 87.
How much does a flexible pavement design study cost in London?
The cost ranges between £1,310 and £4,770 depending on the number of test locations, access conditions, and whether laboratory soaking is required. A typical residential street with four CBR tests and a design report falls in the middle of that range.