Study On Interaction Mechanism Of Cap-Pile Groups-Soil For Isolated Foundation

Cap-Pile Groups-Soil Interaction (CPGSI) is a focal point in the study of pile foundation. In the new code for the design of building foundation, it has been required clearly that CPGSI should be taken into account in the design of pile foundation with local area experience. Because of the regional of geotechnical engineering and the variabilty of the soil properties, it is necessary to study the problem about CPGSI based on the actual condition of the region. It is of importance and has a realistic meaning to quest for the load transfer mechanism and settlement characteristic, excavate the potential of the pile foundation bearing capacity, enhance the design level, change from traditional method to new concept considering of interaction, accelerate the development of the working together theory.Static loading tests on natural soil, single pile and four-piles groups with prototype size were carried out in order to study the behaviour of the CPGSI in the pile foundation in Zhangshan Power Plant Project. The soil resistance under the cap, the shaft resistance and the tip resistanceof pile were measured by installing various measuring gauges. The experiment results are reliable and effective. The static loading tests on four-piles groups with prototype size were first carried in Shanxi province, also is unwonted in China. In the later of the power plant construction, the pressure cell, pile force gauge and vibrating wire strain guages were installed to have a real project measurement. They provied the testing data which are very important to study the CPGSI characteristic.Through analyzing the experimental data and combing with the results of limit element analysis, following conclusions can be drawn out:(1) Based on these test results, the behavious and working properties of the cap-pile groups-soil in the pile foundation were analyzed by considering the CPGSI. The distribution of soil resistance under the cap, shared load ratio and the load transfer mechanism were discussed with comparison to the results of a single pile, and the pile groups efficiency is 1.227 in the condition of the test. The soil under the cap could share partially load and the shared load increase with the settlement of the cap. The distributions of the soil resistance under the cap showed an parabola shape, ie, high value in inner area and low value in external area before the load reaches bearing capacity, after that showed an saddle shape, ie, low value in inner area and high value in external area.(2) The soil resistance for the real project measuring was obviously larger than that for static loading tests, the shared load of soil reached 20% at last. Based on the contrast, it was discovered that increasing the external area and reducing the pile space can increase the shared load of soil. The shared load ratio of soil was variable with development of the project, but its tendency was to increase gradually along with the increase of load. The biggest value of pile shaft resistance occurred along the lower part of the pile under the working load.(3) By comparing the shapes of soil resistance distribution under the cap in static test and in real project measurement, it is show that the depth of foundation influence greatly on the soil resistance distribution under the cap. It was first to put forward that the bearing capacity of composite pile foundation should consider the effect of foundation depth and a calculating formula was provided for composite foundation bearing capacity considering foundation depth. After calculating the bearing capacity by considering depth effection for several pile groups foundations, it is seen that the soil loading sharing ratio increased to 13%~18%.(4) Based on the stiffness distributing and readjusting of composite pile foundation, a new pile arragement method was put forward in this paper, that is, in a certain region, to reduce pile space and extend the external area of thecap, which could make efficiently use of the soil resistance under the cap, and increase the shared load ratio of the soil, in turn to reduce the total amount of piles and to reach the economical goal in the condition of reliability.(5) The working characteristics of natural soil, single pile, non-cap pile foundation and pile foundation were analyzed by using finite element method from the view of the deformation. The cap and piles clamp the soil between piles on the upper part along the piles. It is this kind of clamping action that makes such part of soil move together with the cap and piles, which is first time to proposed as a half-integral working mode of pile groups foundation.(6) Composite pile foundations with different foundation depths were imitated by finite element method and the calculating results show that bearing capacity curve was hyperbola shape. It suggested that the modification about depth of bearing capacity should be limited in a certain depth, which was about 6.0m in this case.(7) The bearing capacity formula of the half-integral working mode was deduced based on above analysis. The ultimate bearing capacity and the design bearing capacity were calculated by using proposed formula, and the testing results prove the formula out.