Methods of improving the performance of drilled piers in weak rock

Investigations were made of two methods to improve the load-displacement performance of concrete piers socketed into very weak rock. Results of load tests on six full-size piers are reported.One method involved cutting grooves into the socket wall to roughen the pier-rock interface and thus increase the shaft resistance component of load support. Two types of piers were tested: piers with a void at the base (shaft resistance only) and piers having both shaft resistance and end-bearing resistance. In the latter case, flatjack load cells were installed to measure base loads. The test results indicated that increasing the roughness of the socket wall can cause important increases in shaft resistance.The second method involved the application of preload to the socket base to increase the end-bearing component of load support at small displacements. The test results showed that preloading the socket base resulted in a stiffer load-displacement behaviour of the pier-socket system.The initial portions of the load-displacement curves from all the tests were linear, reflecting elastic behaviour of the pier-socket system. This elastic behaviour did not appear to rely on socket roughness. Beyond the limit of proportionality, the load-displacement behaviour of each pier departed from that of an elastic system, and this departure was more rapid for the piers with smooth sockets than for those with roughened socket walls. Preloading the socket base caused the elastic range of loading to be extended.Two design approaches, limit-state analysis and elastic-state analysis, are discussed. Keywords: drilled piers and caissons, large-bored piles, shaft resistance, grooved shaft, load transfer, preload, shale, weak rock.