Piles as settlement reducers in raft
Kambala Siva Nagi Reddy
Date of Issue2010
School of Civil and Environmental Engineering
Shallow foundation such as rafts is often the most economical and viable solution when the soil near the ground surface is competent and provides sufficient bearing capacity. If the raft settlement is excessive, a limited number of piles can be introduced to reduce the raft settlements. The role of piles in raft settlement reduction is not clearly understood. In this study, model tests were conducted to unravel the effects of various piled raft parameters in reducing raft settlements. The experimental results revealed that the number of piles added was the primary factor affecting raft settlement. For a given pile number, stiffer piles would cause the largest reduction in settlement. The effectiveness of the piles was also influenced by pile spacing. Piles of different length and width but the same capacity and stiffness have similar effect on raft settlement; particularly when the applied load was well below the raft bearing capacity. The model tests also revealed that although short piles could help to reduce raft settlement at low applied load level, they were ineffective in enhancing the raft bearing capacity. The larger pile stiffness caused the piles to carry a large proportion of the applied load, thus reducing the load transmitted through raft bearing pressure, initially. Thus, significant increase in bearing pressure, and hence larger raft settlement, would occur only when pile capacities had been substantially exhausted. The results from the model test indicated that when piles were well spaced, at least 75% of the single pile capacity could be mobilised in the piled raft under typical working load condition; and could reach 100% in many instances. The experimental data showed that the interface contact pressure-displacement relationship or soil spring curve at raft-soil interface was highly non-linear. A unique soil spring relationship appeared to apply to all nine rafts at moderate stress level. 3-D finite element analyses of the model piled rafts yielded load-settlement curves which were in good agreement with the test data. However, significant differences existed between the test results and the finite element output in some key aspects such as the raft-soil interface response and load distribution between raft and piles. Further study is needed to uncover the reasons for these observed discrepancies.