Edge piles take up more loads than central piles in rigid cap

Because of the effect of interaction of individual piles, central piles tend to settle more than the edge piles when the pile cap is under a uniform load. For the pile cap to be rigid, local deformation of central piles would not take place. In its place, stiff pile cap would transfer the loads from central piles and redistribute them to the outer piles. Consequently raking piles at the edge take up a higher fraction of the total loads and are subjected to higher axial and bending loads in case the pile cap is stiff. In the extreme case, side piles can take up as much as about two to three times the loads in the central piles and this can lead to the failure of these raking edge piles.

There are numerous choices regarding the design to tackle the uneven distribution of loads. The first one includes the lengthening of side piles to stabilize the piles under high loads. Though the increased length of outer piles tends to attract more loads and this seems not to be a good solution. Other way out is to lengthen the central piles aiming at getting more loads and this evens out the load distribution among the piles. 

Fig: Stress bulb effect on load distribution in piles