Behavior of a piled cap foundation under a vertical loading

Abstract

Hong Kong is a small city with a dense population. Due to limited land resources

and dense population, skyscrapers are built to meet the commercial and residential

needs. To fulfill the functional requirements, the superstructure is usually

supported by piles through a thick cap. A bored pile foundation is one of the most

common deep foundation systems with a high capacity adopted in Hong Kong.

Most of those piles are founded on Grade III or better rock. However, as lack of

field experience and high quality database, the behavior of such pile foundation

system is still unclear. These bored piles are designed as the end bearing piles. In

addition, the load distribution among the piles and the load shared by the pile cap

are also not properly addressed in the local design. In order to properly address the

above-mentioned issues related to the design of pile foundation, in particular for

large diameter bored piles, a comprehensive field study was launched.

Furthermore, the numerical simulations were also carried out.

A comprehensive field study was carried out on the behavior of a building

supported by large diameter bored piles. The building (40-storey public housing)

is Y-shaped with three wings (Wing A, Wing B, and Wing C) and a central core

(Central Core). The superstructure is resting on a Y-shaped foundation cap (2.3m

in thickness) which is supported by 18 bored piles (2.2m in diameter). The piles

terminated at Grade III or better rocks. The field test results indicated that the pile

group took up most of the applied load, while the cap had an insignificant

contribution to the capacity of the foundation system. The axial force in the pile

increased from the outer edge to the inner area of the cap. The shaft resistances

mobilized in the soil layer and the rock layer were significant. As a result, the

axial force reduced to a not significant level at the toe.

The finite element model could generally reflect the bored pile deformation and

the load distribution along the pile. The numerical simulation results could

generally reveal the load distribution among the piles. Based on the numerical

analyses, the load distribution among the piles was strongly dependent on the cap

stiffness. Moreover, when some pile stiffness reduced, the applied load will

redistribute among the piles. More loads will transfer to the nearby stiff piles. The

effected region of redistribution in axial force increased with the cap stiffness.