Numerical Study On Load-Sharing Ratio Of Piled-Raft Foundation

The population growth and rising of high-rise buildings lead several challenges in foundation design.Piled-raft foundation(PRF),which consists of pile's group and cap/raft above,is widely used for medium-rise and high-rise buildings around the world.Commonly,the design concept of this PRF,which is assumed that piles support all the load and excluded the contribution of soil beneath raft,is over-conservative and uneconomical.However,load sharing by raft has gained positive results since 1960s.The new design concept includes contribution of soil beneath the raft.According to studies based on field measurements,load sharing by raft shares up to 60%of total load(Katzhenbach R et al.,2000[40]).Especially,even the design at small spacing as conventional design,the load sharing by raft still shares up to 30%(Cooke R W 1986[27]).Hence,the number of piles and pile's length which are required in conventional design can be reduced as well as both time and cost of construction.Despite the benefits of PRF,load sharing ratio is hard to predicts due to its complex interactions between components and soil conditions corresponding with settlement and bearing capacity criteria.There are two design stages such as preliminary design by using simplified analytical methods and detail design by using numerical approaches.Unfortunately,there are no accepted rules or standard for analyzing piled-raft foundation.Therefore,observation method is widely used for safety purpose.Influences of several parameters,such as raft's dimensions,piles' dimensions,spacing ratio,slenderness ratio and soil's properties,are examined in order to find major factors in piled-raft foundation.Hence,61 numerical tests were conducted by using 3-dimensional FEM software,Plaxis-3D foundation v1.6,and concentrated on influences and load sharing mechanism of five parameters such as:(1)Three soil parameters:soil's young modulus,soil's cohesion and soil's angle of friction were examined in cohesion-less soil or cohesive soil.(2)Two dimensionless ratios of pile's dimension:slenderness ratio and spacing ratio were addressed in cohesion-less soil or cohesive soil According to the result from the parametric study,slenderness ratio and spacing ratio of pile are major factors toward load sharing mechanism while soil's angle of friction and soil's cohesion have little influences.Modulus of elasticity does not have any effects toward load sharing ratio but it is significant factor toward settlement criteria.In addition,empirical equations for estimating the load sharing by raft were obtained from regression analysis based on numerical tests for homogenous cohesion-less soil and cohesive soil under working state and ultimate state.The empirical equations provide solid agreements with other studies:simplified analytical methods,empirical methods,experimental laboratory tests and 21 study cases from field observations.In addition,a proposed design method is developed for analyzing piled-raft system which consists of empirical equations mentioned above and settlement calculation by using Poulos-Davis-Randolph(PDR)method or modified Poulos-Davis-Randolph(modified PDR)method.These proposed methods provided a better prediction than other few simplified analytical methods which are available for PRF.Moreover,it also provides flexibility in design by using only two major factors to estimate the load sharing ratio in PRF rather than complicated analytical calculation.