Side shear resistance of large diameter bored piles in geomaterials

Recently, construction began on the Kowloon-Canton Railway Corporation West Rail Phase I project in Hong Kong. A Full-Scale Deep Foundation Load Testing Programme was implemented prior to the main works beginning, in which 12 vertical pile and 2 barrette load tests were performed. This provided a rare opportunity to study the behaviour of large diameter (greater than 600mm) bored piles in shaft friction and rock socket friction, which were measured in 11 of the tests. The piles were installed in a variety of saprolites and weathered rocks and extensive instrumentation was installed in each to measure movement, strain and shortening. Shaft resistance was back analysed from the raw test data and the results were combined with others from literature to form a database for the study of shaft resistance in saprolites and socket resistance in weathered rocks. By examining the initial shaft resistance-movement relationships of the piles in saprolites, trends in the slope of the linear region of the relationships could be observed. The mobilized saprolite shear modulus G was back-calculated to be approximately 0.35 N̄ where N̄ is the mean uncorrected SPT value. At greater movements, the piles constructed under water, mobilized 90% ultimate friction capacity at an average local movement of 1.9% of the pile diameter whereas for the piles constructed under bentonite, 100% frictional capacity was achieved at a movement of only 1% of the pile diameter. The friction capacities of the piles constructed under bentonite were 50% to 70% less than the values for the piles constructed under water. The differences in behaviour may be associated with the formation of a bentonite cake layer. Trends in rock socket friction capacity could be identified for the piles in granitic rocks in Hong Kong and Singapore based on the unconfined compressive strength. The observed correlations are lower than for piles in sedimentary rocks in other countries. For example, the mean correlation suggested by Horvath et al (1983) results in approximately 25% greater socket resistance than the relationship for sockets in granite in Hong Kong. In contrast to the piles in saprolites, the use of bentonite for the construction of rock sockets does not appear to have a detrimental effect on performance. The question of the suitability of a design method for shaft resistance based on an estimation of wet concrete pressure remains unanswered. For the piles in the database, the analysis of shaft resistance by considering wet concrete pressure was inconclusive