BEHAVIOR OF LARGE DIAMETER FLOATING BORED PILES IN SAPROLITIC SOILS

C. W. W. Ng, J. H. M. Li and T. L. Y. Yau

ABSTRACT :In Hong Kong large diameter long piles are commonly used to support tall buildings to resist both vertical and horizontal loads. These piles penetrate through and may found in saprolitic soils or rocks. Generally, the design for side shear resistance (often called skin friction) of large diameter bored piles (drilled caissons or shafts) in saprolites involves considerable uncertainty and design parameters must usually be verified by field tests. In this paper, 28 full-scale load tests on large diameter machine bored piles constructed in various saprolitic soils were reviewed in detail, in particular the degree of mobilisation of side shear resistance using a mobilisation rating factor. The diameter of these floating piles ranges from 1.0m to 1.5m and from 22m to 75m in depth. The authors were heavily involved in the recent construction and testing of 12 of these piles. For the bored piles constructed under water, 90% ultimate side shear resistance was mobilized at an average local pile displacement of 1.9% of the pile diameter with a 95% confidence range of 0.9% to 3.0%. For the piles constructed under bentonite, ultimate side shear resistance was achieved at a local pile displacement of 1% of the pile diameter. For non-grouted bored piles constructed under water, the mobilized side shear resistance lies between 0.5 and 2.0 (kPa) and values fall within 0.1 and 0.5, with average values of 1.2 (kPa) and 0.3 respectively. Post-grouting improves the capacity of side shear resistance by a factor of about 2 as compared with the non-grouted piles constructed under water. On the contrary, piles constructed under bentonite show a reduction of capacity of side shear resistance between 50% and 70% depending on the method of analysis.

Keywords: Weathering, saprolites, granitic, volcanic, bored piles, pile test, side shear resistance, degree of mobilization (IGC: E4/K7)