This in-house development has been successful in reducing the friction between the CPT rods and soil during testing in very cohesive materials. It’s enabled us to push to much greater depths, avoiding the need to use more expensive and time-consuming drilling techniques.
Pressurised water is pumped through the rods to an outlet opening located approximately 0.5m behind the cone tip and the friction reducer. The water is pumped up the rod string at hydrostatic pressure to form a lubricating layer that reduces the friction between the rods and the ground. The design of the module means the water is distributed behind the cone and therefore has no effect on the cone readings and the CPT data quality remains consistent.
Using the lubricating module means CPTs can be carried out at much greater depths, eliminating the likelihood of early refusals in stiff, cohesive clay; this technique has proven effective when pushing in London Clay. Typically a maximum penetration depth of 18-20m in London Clay would result in refusal whilst pushing 18T on the rig. With the application of the lubricating module depths of over 30m can be achieved with only 5T of push force.