Capacity and failure criteria of bored piles in soils and rocks

The pile loading test programme for the KCRC West Rail project provided an unusual opportunity to measure load carrying capacity of large diameter bored piles founded in soils and rocks as well as the load-settlement relationships. Large diameter bored piles founded in soils were reviewed and studied in detail, in particular the degree of friction mobilization using a "mobilisation rating" factor. The behaviour of bored piles constructed under water with temporary casing and under bentonite are different. For the former case, over 90% of the ultimate shaft resistance is mobilised at an average local pile displacement of 2% the pile diameter. For the latter case, friction capacity is achieved at a local pile displacement of less than 1% of the pile diameter. The average shaft resistance normalized with N̄ and β̄ under bentonite are only 0.6 (kPa) and 0.1, respectively i.e. 50% and 70% less than the average values for the piles constructed under water. The design of shaft resistance capacity of piles founded in soils is normally based on either effective stress approach or SPT N approach, however, assuming a constant relationships between shaft resistance and SPT N or effective overburden stress are misleading. Clear trends have been identified in this research by three alternative approaches, τ̄/N̄ against N̄, β̄ against N̄ and β̄ versus σnu'̄. Side resistance of rock socket piles is analysed with regard to the rock type and the analysed results is compared with the findings and correlations from the literature. Specific attention is given to the degree of mobilization in addition to the capacity of side resistance. With regard to the capacity of side resistance, empirical correlations with the unconfined compressive strength (rock or concrete whichever is the lowest) are suggested for granitic rocks in Hong Kong and Singapore separately. The empirical correlation for granitic rock socket in Hong Kong is consistent with the lower bound correlation suggested by Horvath et al (1983). The failure load criteria adopted in Hong Kong were not developed with slow maintained load tests on large diameter long bored piles. Large diameter bored piles have considerable load resistance capacity and it is often impractical to reach the failure load as defined by the various criteria. From the results of this study, a new non-subjective semi-empirical method is proposed for estimating the lower bound interpreted failure loads for piles founded in weathered rocks and saprolitic soils. The method is based on a lower bound estimation of the movement required to mobilize toe resistance and incorporates observations of shaft shortening from pile loading tests. The newly proposed method may allow more effective and consistent designs for large diameter piles in weathered materials