Geotechnical considerations on the design of piled rafts on rock

Abstract

This research shows the effect of misestimating the pile spring constant (Kpl) and the modulus of subgrade reaction (Ksg) on design of piled rafts on rock. The high mass stiffness of rock contacting the base of such rafts decreases the pile load share and consequently enhances the sensitivity of their design to Kpl and Ksg values usually utilized by the commercially available software. A crude estimation is commonly made for these two parameters. The research goal was achieved through carrying out a three-dimensional finite element sensitivity study on a representative piled raft model. Ranges of Kpl and Ksg needed for the study were determined through conducting two pile load tests and an extensive survey of plate loading tests in a major construction rocky area. Analysis of the study results showed that pile load share is almost equally sensitive to the frequently made errors in predicting Kpl and Ksg values. Pile load share and raft maximum moment can be underestimated by magnitudes up to 60% if both of Kpl and Ksg are misestimated. Such underestimation increases with increasing pile spacing and practically independent of raft rigidity. Recommendations were introduced to enhance the common practice of designing piled rafts on rock.