The process of boring a hole for a pile seriously disturbs the clay in the neighborhood of the hole. Placing the concrete causes water to be absorbed by the clay surrounding the pile, leading to softening and reduced strength. The resistance of the pile is calculated by the relations:
Qs = 0.45su‘As ———- (1)
And Qb = 9suwAb ———- (2)
Where w is 0.8 for B less than 1 m and 0.75 for B greater than 1 m. The Working load for the pile is given by:
Qa = Qb + Qs /2, for plain shafts ————- (3)
For belled piles the working load is given by:
Qa = Qb + Qs /2.5, for B less than 2 m ———- (4)
Qa = (Qb/3) + (Qs/1.5) for greater than 2 m ———- (5)
The working load is evaluated from settlement considerations. Finally the settlement for the structured should be checked.
For the case of bored pile in over consolidated clay, the adhesion factor should be taken as 0.3.
For a belled pile the shaft resistance is ignored for a distance of two base diameters above the top of the under ream.