Research Of Pile Raft Foundation Soil Reaction Model And Raft Design

To pile-raft foundations design, it is extremely difficult to estimate the true value of soil reaction for supporting building, so it is common in early piled-raft foundation design that the entire building vertical loads are resisted only by the piles and soil reaction is ignored. This approach is quite safer for piles, but unsafe for raft, even extravagant. Being devoid of warranty for load of raft, consequently, design in the field of experiential lead to crack and water leakage of the basement's ground floor (raft), even destroy. Soil reaction model is created in this paper for the research of interaction of pile-raft foundation accurately. The responses of the variety of soil reaction distributing, soil reaction bearing proportion for vertical load of building and internal force of raft are studied.In analytical methods for pile-raft system, it is more tractable using Mindlin's solution for soil flexibility matrix, however Mindlin's solution is determined based on semi-infinite elastic body, on the assumption that tensile modulus and compressive are equal. For calculation of settlement of pile foundation, the Mindlin's solution tends to yield a lower value, because of soil tensile modulus overestimated, and equivalent pier method based on Boussinesq's solution, catch a higher value because of it underestimated. In order to help for evaluating properties of soil in interaction more precisely, the paper applies elastic theory solution on different modulus in tension and compression for vertical load into semi-infinite elastic body. Through quoting Heaviside function, the solution of elasticity theory with different modulus in tension and compression is established founded on the solution of elasticity theory for the material incapable in tension and Mindlin'solution for equal modulus. While modulus in tension is less than modulus in compression, the result is between the two ones. Thereafter, the solutions for different modulus in tension and compression are computed with combination of both. These theories embedded in interaction of pile-raft and soil have been developed for pile foundation displacement field. The soil behavior of response of individual piles nonlinearity is modeled using load-transfer function described by Heydinger. The method is derived to simulate the complicated nonlinear contact on consideration of various factors, including effective stress changes due to pile installation, effective stress changes due to soil reconsolidation and effects of pile loading. For solution of shear locking, the quadrilateral thin-thick plate bending element TMQ based on the concept of generalized confirming element for raft is used.Based on these theories, this paper presents a nonlinear analysis model on pile-soil-raft nonlinear interaction and a large-scale program named DMTCPR has been worked out in MATLAB7.0. This program founded on hybrid approach is not only of low computation cost evidently, but also it economize a great deal of time. The method having accuracy and efficiency better, can analysis real engineering. The method provides solutions for contact of both raft and soil, and plastic deformation of soil. In order that construction on different modulus in tension and compression is defined to contribute a pile-soil-raft nonlinear finite element analysis, By using of user interface subroutines HYPELA of MARC, a nonlinear finite element analysis program, HYPELA is provided in FORTRAN, based on input and out parameter. Elasticity theory with different modulus in tension and compression for the solution of pile-raft foundation is introduced into MARC, as a new approach on research of MARC, accordingly, MARC is further developed.Good agreement between the present DMTCPR and all over the world researches. It shows that it is accurate that the analytical result and a case engineering measurement data are compared. It indicates that the model is reasonable and the program is reliable. By the analysis of examples, the paper further studies that the responses of foundation average settlement, foundation difference settlement, load bearing proportion, soil reaction distributing and raft internal force with the ratio of tensile modulus to compressive modulus. The calculated result indicates that the soil reaction on Mindlin'solution tends to obtain a lower value, and internal force of raft are inexact.Based on the program DMTCPR, three types of case engineering: tall building, medium tall building and multistory building, has been analysed. Range of soil load bearing proportion and raft internal force change rule are present, and suggestions for engineering design are achieved. Numerical results indicate that, for a tall building of more than 20 story, the load bearing proportion is generally less than 10%, and raft of the building is common thicker, soil reaction could be ignored; and that for a medium tall building of 8 to more than 10 story is about 15%. Its full utilization in the design reduces the construction cost of pile-raft foundation and the soil reaction need be considered in raft design for safety of construction,especially thinner raft of axes pile. For multistory building, settlement of foundation being in law, soil load bearing proportion is a higher value, in general, more than 25%. When it is applied fully, obvious economic benefit was achieved with particular consideration of soil reaction to raft. Soil reaction being computed accurately is the premise of raft design reasonably. Some interesting conclusions above, which may be useful for application of pile-raft foundation and revision of criterion, are obtained.