Clay seams below a pile tip - How much do they affect pile performance?

This paper reports the results of a numerical investigation, of the behaviour of an axially loaded pile resting on rock which has a clay seam at some depth below the tip. The following aspects are examined: 1. The ultimate bearing capacity of the tip; 2. The axial stiffness (settlement per unit load) of the tip; 3. The axial stiffness of the pile head; In each case, the effects of the clay seam in reducing capacity and stiffness are examined. These effects are shown to be dependent on the depth and thickness of the clay seam, relative to the pile tip diameter. The findings of the numerical analyses are compared with recommendations made in two series of design recommendations. The effects of a clay seam on the settlement interaction between two piles, and the effects on the performance of an axially loaded group of piles, are then examined.