In remedial applications as the pile is not positioned directly below the wall the connection will produce amoment. The total moment is the (design load) x (the distance between the centre of the wall and the centre of the shaft). IT IS VERY IMPORTANT TO KEEP THE SHAFT AS CLOSE TO THE WALL AS POSSIBLE TO REDUCE THIS MOMENT. In practice the shafts are set a minimum of around 120mm from the face of the wall by the dimensions of the Post Driver. Obstructions may make it necessary to exceed this. It is sometimes necessary to break out the foundation to allow the shaft to be fixed close to the building wall.
This moment can be resisted by the building structure, the pile or a combination of both. The building is stiffer than the pile and as much moment as possible will always run into the building. This is restricted by the moment capacity of the structure and the ability to transfer the moment from the pile connection into the structure.
Unless the wall is freestanding it will always have a moment of resistance equal to the stability moment at the base of the wall. Generally, this moment is assumed to run up into the brickwork and the remaining moment taken on the pile. At corners the structure is stronger and can take a larger proportion of the load providing a suitable connection can be achieved.
The moment capacity of the different shaft sections is given below (this is taken from bending tests):
42mm + grout + 12mm bar = 3.5 kN m
60mm + grout + 12mm bar = 6.5 kN m
42mm + 60mm + grout + 12mm bar = 9.3 kN m
The need for head strengthening depends on the load and the distance between the shaft and the face of the wall. Recommendations are given on the Building Connection Details contained in the Standard Details Manual.
The moment acting on the head of the support quickly reduces due to soil pressure around the shaft. The depth of the support that requires strengthening can be estimated from the analysis in section 21. Generally high moments were found to reduce to the capacity of the minimum shaft (3.5kN.m) within one length of tube (1.25m). Soft clays and similar materials give least restraint to the support head. Consideration should be given to strengthening the head or using a larger diameter shaft
Soft clays and similar materials give least restraint to the support head. Consideration should be given to strengthening the head or using a larger diameter shaft.