The Finite Element Analysis Of The Composite Uplift Pile

Composite uplift pile uses the technology of compound pile combined with prestressing force. Compared with traditional uplift pile, the composition of composite uplift pile is very different, the main structure is consisted of concrete inner-core, precast durable pile crest and cement-soil in outer pater, with built-in prestressed steel strand. When the pile end is under uplift loading, the load is transmitted from prestressed steel strand to concrete inner-core directly, so the concrete inner-core bears the main load, which transfers the traditional tension state of pile material to compression state, and the concrete cracking problem of traditional uplift pile body is solved effectively. With the advantage of composite pile, the load is transmitted to external cement-soil pile through side friction of concrete inner-core, and then transmitted to the surrounding soil through external cement-soil pile, which forms a multiple load transfer model, not only saving material but also improving the bearing capacity of the pile.In this paper, the structure and construction technology of composite uplift pile was introduced firstly, then the finite element simulation analysis was carried out in two stages—load transfer mechanism and load–bearing characteristics. According to the simulation results, we put forward the further optimization and improvement measures. Finally, the construction process of composite uplift pile was analyzed and discussed with cylindrical cavity expansion theory.Composite uplift pile was a friction pile, which load transfer had administrative levels, the impact on bearing capacity of core rate, pile diameter and pile length were analyzed by changing the parameters of the model and the corresponding rules were as follows:1. The load transfer of composite uplift pile was a multi-level diffusion model: from pile crest to bottom, from concrete inner-core to external pile and then to the surrounding soil.2. The bearing capacity increased with the increase of the ratio of the inner-core before the innercore and the peripheral cement-soil worked collaboratively, and after it increasing the ratio of the inner-core had no significant effect on improving bearing capacity. The key to ensure the load transfer according to the ideal model was that the interface friction between inner and external core of pile was bigger than that between pile and the surrounding soil, which made load transfer diffused hierarchical.3. With the collaborative work of the inner-core and external-core, the bearing capacity increased linearly with the increase of pile length and pile diameter.4. The analysis of the Poisson effect and force point showed that the uplift coefficient λ of composite uplift pile was bigger than that of traditional uplift pile.The structure of composite uplift pile was optimized and improved according to the phenomenon in simulation, mainly included the change of inner-core length, the volume and strength of cement-soil in pile bottom, the surrounding soil. The rules were as follows:1. The effects of the length of core pile on uplift bearing capacity were divided into three categories: When core pile was too short, the side friction shared small uplift load, uplift force was mainly born by cement-soil of core pile top, which resulted in the occurrence of piercing damage. Within the scope of the certain length, ultimate bearing capacity increased with the increase of the length of inner core. As the length of core pile continued to increase, the side friction shared larger uplift load, then the core pile crest had a compaction effect on cement-soil but not destroyed it, which made the whole pile suddenly be pulled out, and with the compaction effect, the side friction between cement-soil and surrounding soil increased, which finally made the ultimate bearing capacity increase compared with the pile without optimization.2. After expanding the cement-soil volume of the pile bottom, with the increase of C/h, the ultimate bearing capacity increased, when C/h increased to a certain extent, the further increase had little effect on the cutting area of cement-soil in the pile bottom, so C/h had an optimal value.(c is the distance between core pile crest and the top of expanding bottom; h is the length of core pile)3. Through continuous optimization and improvement, the section form of composite uplift pile was shown in figure 3-18 finally, the size of expanding bottom part and the length of core pile were combined with the practical engineering, and used the optimal ratio of c/h. If the condition allowed, the strength of cement-soil in expanding bottom part could be increased by increasing the proportion of cement or taking other measures, or even changed the cement-soil to cement mortar for further improving the punching shear capacity of materials of core pile crest.