Mechanics Characteristics Theoretical Research On Group Piles-Caps Composite Foundation And Application For Large-span Building

To meet the development demand of large-span building and building build in soft clay area, small group of piles with low-cap foundation have gotten extensive development and application. But at present the small group of piles with low-cap foundation designed with interaction principle are mainly considered to meet the capacity, not satisfy the cap settlement, especially uneven settlement requirements. The study on joint-action of piles-soils-caps-structure is not enough now.By collecting information, theory analysis and numerical simulation, this paper systematically studies the interaction mechanism and suffering force characteristics of piles-soils-caps-structure. And on the basis discusses the stress influences of beams,columns caused by pan bottom shape settlement. The methods of decrease uneven settlement on the influences of joint-action of group caps of piles are discussed also.The elastic theory suggested by polous is improved in the work to make it more accuracy. The improved formula is used to study the joint-action of piles-soils-caps-structure. The load-settlement behavior of the small group of piles with high-cap and low-cap is study by the program which be written using FORTRAN software platform language firstly. Then a method is presented to replace low-cap piles group by a equivalent single pile with a low-cap.The pile bearing capacity of piles with high-cap which settlement is bigger than the single pile's is less. The pile bearing capacity of piles with low-cap is bigger than the single pile's, the settlement of piles with low-cap is less than the high-cap's and bigger than the single pile's. As pile length increases, pile spacing increases, pile diameter increases or pile number increases, the pile bearing capacity of piles with low-cap increases, but associated with bigger settlements.A computer program using elastic finite element is made to calculate the inner stress and point displacement of the large-span building. The constructing step and the progress of rigidity increasing can be considered. The program can also work together with the program calculating joint-action of piles-soils-caps, so as to realize the joint-action of piles-soils-caps-structure. The foundation settlement and the structure stress are determined by the programs and contrasted with the result of regardless the joint-action. From the conclude, it can be found that the foundation settlement has a shape similar with the pan bottom. The result of structure stress contrasting shows that the pillar stress increase at the location of corners and fringes, but the pillar stress decrease at the inner place while considering the joint-action. For the structure beams, the joint-action let the biggest stress increasing. The result illustrates that the main reason causing cracks in some beams is the subordinate stress caused by the settlement. The other reason may be in the design which did not consider the safety enough or the quality of the construction is not very good.About 30% load continue to increase after the building structure finish. If piles with low-cap designed according to bearing capacity, large settlement and uneven settlement will be cause by the later load on large-span building.To solve the pan bottom shape settlement, not only different load, but also joint-action of group caps of piles must be considered in the pile design. As much as possible to reduce pan bottom shape settlement, the stiffness ratio of pile to soil at the location of corners and fringes can be de decreased firstly. The method makes the pile cap settlement at the location of corners and fringes larger than the inners'. Secondly, the bearing capacity can be increased and the settlement can be decreased at the inner cap. Thirdly, "settlement reservation" measures can be taken to, structural measures may be taken at the joint of cap and pillar which have less settlement by prediction according to the settlement difference after the load completely imposeing. In summary, the idea of "design by settlement controlling" need be carried out in the pile design in order to reduce the uneven settlement as much as possible.