Research On Bearing Behavior Of Large Diameter And Super Long Pile Group In Large Thickness Collapsible Loess Site

With the increase of high-rise and super high-rise buildings, the application of large diameter long piles and super long piles increases sharply. Although large diameter and super long piles are used in large quantities, the design theory of super long pile on the basis of the calculation theory of the conventional pile, it is difficult to achieve economic and reasonable,there is a contradiction between theory and practice. Collapsible numerical experience on negative friction of building codes in loess region is obtained through three field tests, Many scholars have questioned the rationality of the value, and the theoretical research is far behind the engineering practice. Due to the economic and technical difficulties, the static load test of pile group in collapsible loess site has not been carried out, and the indoor test is relatively rare. Therefore,it is urgent to study the bearing characteristics of super long piles and optimize the design of pile foundation in collapsible loess site.The bearing capacity of long pile group under vertical load is studied deeply' by the model test of super-long pile group foundation, and the axial force, lateral resistance and negative friction resistance of pile foundation are given. The settlement of the long pile foundation and the load transfer characteristics are analyzed in depth. On the basis of this, the influence range and trend of the pile on the pile and pile soil are analyzed by the micro-angle angle under the vertical load. The results can provide the basis for further study on the sedimentation characteristics of long pile foundation,including:In this paper, through the study of the similarity of the collapsible loess model, the indoor collapsible loess material is used in the indoor model test, and the large diameter long pile group model is worked out. The main contents and research results are as follows:(1) The natural loess has a very strong structure, in the process of sampling,transportation and sample preparation,it will inevitably cause disturbance to the sample.Based on the similarity theory of model test and Monte Carlo principle, the artificial collapsible loess was prepared by using the free drop method of quartz powder, sand,bentonite, gypsum and industrial salt. Soil tests, energy spectrometer element analysis and electron microscopy scanning test were carried out for soil samples with different water content and compaction degree. The results show that the composition of the artificial loess is consistent with that of the natural collapsible loess. The collapsibility of the artificial loess increases with the increase of the initial water content, and then decreases with the increase of the initial water content. The results show that the artificial loess has the structural characteristics that are susceptible to stress and water immersion. With the increase of compaction and water content, the anisotropy, area ratio and equivalent diameter of the particles increase, while the flatness and filling ratio decrease. There is a good correlation between the soil vorticity coefficient and the microstructure parameters such as anisotropy,roundness and area ratio. The collapsibility mechanism of the artificial loess similar material is explained from the microscopic point of view,and it has a good fit with the natural loess collapse law, and can be used in the indoor model test.(2) In Geotechnical Engineering Laboratory of infiltration under the two conditions of large diameter and super long pile group model test and conventional simulated rainfall, soil layers and stratified sampling before and after the experiment of model test for indoor soil test,microstructure and scanning X ray spectrometer element test the physical and mechanical parameters, measured soil micro structure images and elements, and analyses the vertical load and the effect of rainfall infiltration in different depths of soil mechanics model from macro and micro perspective.(3) Through the model test, the settlement law of large-diameter and long-group piles in loess area was studied,and the influence of the increase of water content on the foundation settlement of the bearing layer and pile soil was analyzed. The load transfer characteristics of large diameter and long pile foundation are studied, and the distribution law of axial force,end resistance and side friction resistance of pile group foundation is obtained and the influence of water infiltration on load transfer property is analyzed.This paper analyzes the bearing characteristics of super long piles on collapsible loess fields,and provides technical reference for optimizing the design and construction of pile foundation in the future.