Laboratory CBR Testing for Road & Pavement Design in Ashford

A single-point CBR test—compacted, soaked, and penetrated—runs between £100 and £180 depending on specimen count and whether you need the Proctor compaction curve determined first. A full suite for a small residential site with five test points typically falls in the lower end of that range per sample. We quote fixed-price per specimen so there are no surprises when the invoice lands.

For flexible pavement design under Kent County Council adoption, we recommend one soaked CBR test per distinct soil type encountered, with a minimum of three tests per 500 m of road alignment. If the formation exposes both Weald Clay and river gravels, you need separate determinations for each material. The Design Manual for Roads and Bridges suggests a minimum of one test per 200 m² of formation on variable ground.

Ashford’s Weald Clay contains a significant fraction of smectite and illite clay minerals that absorb water into their crystal lattice. When the specimen is submerged for 96 hours, these minerals swell, reducing the effective inter-particle friction and softening the soil matrix. The plunger then records lower resistance at the standard penetrations. This soaked value represents the pavement’s weakest condition—typically late winter when the water table is high.

We run the laboratory CBR test because UK pavement design is calibrated against BS 1377-4 soaked values, not field CBR. That said, we also perform DCP testing on site and can correlate the penetration index to lab CBR using established conversion charts. The lab test remains the reference method for deriving the subgrade modulus used in capping and sub-base thickness calculations.