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( II ) Allowable pile bearing capacity
Allowable pile bearing capacity is the sum of the skin resistance and the tip-bearing resistance of a pile. The mechanism of the load transfer from the piles to the soil layer is illustrated as Fig. 2.5(b). The pile bearing capacity depends on several geotechnical and structural factors, such as the ground conditions around the piles, the pile diameter, the pile length, the inclination of the piles and the material of the pile.
The two requirements mentioned hereinbefore control the maximum load capacity per unit plan area of the pile foundation. Therefore, one can increase the total capacity of the pile foundation by: i) increasing the plan area of the pile group so that more piles can be provided, but still satisfying the requirement of minimum pile spacing; ii) increasing the allowable capacity of each pile, the methods of which include ground improvements, longer pile lengths and changes of the pile material.
However, design constraints exist in selecting one or both of the above options to increase the total capacity of the pile foundation.
( I ) Limited increase in plan area
The maximum plan area is controlled by the spacing between loaded elements, which are columns and walls, and their separation distance from the adjacent structures or lot boundaries. If the pile cap is enlarged, the depth of the cap may need to be increased to resist the increased bending stress, which will increase the cost of the pile foundation, and more importantly, the cost of shoring works for constructing the pile cap.
( II ) Limited increase in pile bearing capacity
There is a practical limit in increasing the capacity of a pile. Very high capacity piles involving Grade 55C steel H-piles and 3m diameter bored piles are already frequently used in Hong Kong. There are difficulties in increasing the pile capacity further..
As buildings become taller, the subjected building loads become larger and a higher total capacity of the pile foundation is hence demanded. To achieve this, besides the above two options, the pile group efficiency could be enhanced so that the loads can be distributed to and shared by all the piles efficiently, by the rigid cap assumptions in the pile cap design.
2.3 DIFFERENCES IN STRUCTURAL BEHAVIOUR
2.3.1 Subjected to vertical loading without bending moment
In normal circumstances, vertical loads acting on a pile group produce a vertical displacement as well as out-of-plane rotation of the pile cap. For simplicity, the resultant vertical load is assumed in this section to act at the centroid of the pile group such that no bending moment is generated by the subjected resultant vertical load. The torque produced by the lateral load on the supporting building, as well, is not considered in this section. Finite element models of a 6x6 pile group, one with a rigid cap and another with a flexible cap, are constructed by ABAQUS (2002) for analysis as shown in Fig. 2.6, with bending moments in both directions omitted.
A rigid cap, when subjected to a resultant vertical load at the centroid of the pile group, keeps a nearly planar shape by redistributing the load throughout the whole cap. In the case with identical pile lengths, shown in Fig. 2.6, the resulting deformed rigid cap stays horizontal yet at a lowered position as shown in Fig. 2.7(a). The deformation of each pile is the same, resulting in identical pile loads for the whole pile group, as plotted in Fig. 2.8.
With loss of rigidity, however, a flexible cap bends and twists besides the rigid-body displacement and rotation, when subjected to a resultant vertical load at the centroid of the pile group. It leads to a curved deformed shape as shown in Fig. 2.7(b), with the maximum vertical deflection at the centroid of the pile group where the resultant vertical load acts. The pile loads are not uniform among the piles. Piles located around the centroid support most of the subjected vertical loads while the surrounding piles are only lightly loaded, as shown in Fig. 2.9.