Application of a new q_c averaging approach for end bearing of driven piles in sand

CPT-based methods to estimate axial pile capacity generally relate the pile base resistance with a q_c value averaged in the vicinity of the pile tip (q_c,avg) and hence implicitly acknowledge the greater zone of influence of a full scale pile compared to a cone. To account for layered deposits (e.g. loose over dense sand), most common designs methods average the cone tip resistance over 1.5 diameters (D) above and below the cone tip or use the Dutch methodology where the averaging is conducted over a zone of 0.7D to 4D below the pile tip and 6D to 8D above the pile tip. This paper examines an alternative averaging approach based on an algorithm which allows estimation of the steady state end bearing resistance of penetrometer with the same diameter as a pile (q_p). End bearing stresses determined at a base displacement of 10% of the pile diameter (q_b0.1) in a database of instrumented static load tests on driven piles are compared with the corresponding q_p values. It is shown that the q_b0.1 /q_p ratio varies with the effective area ratio of driven piles and is independent of the pile diameter. The best-fit equation of the database of end bearing measurements provide a rational and improved means of determining end bearing of driven piles.