Research On The Settlement Characteristics Of Post Grouting Pile Group Foundation

Pile foundation post grouting includes both pile end pressure grouting and pile side pressure grouting. This technique is an organic combination of the soil reinforcement technique and the pile foundation engineering techniques, which overcomes the inherent faults of pile foundation hole-making and reinforces the soil mass nearby pile end and pile side so as to substantially raise the pile bearing capacity. A great number of project practices show that this technique is featured by its high bearing capacity, wide application scope, flexible construction methods, significant benefits and easy popularization. It has been proved in practice that it is an effective technique to raise the vertical bearing capacity of bored pile and reduce the pile group differential settlement.The research of this paper is conducted under the Experimental Study of Bridge Pile Foundation Post Grouting Techniques of Beijing-Shanghai High-speed Railway, which is a major subject of science and technology R&D program of China's Ministry of Railways (2008G032-3). Most areas along the Beijing-Shanghai High-speed Railway are of soft soil; the Quaternary System soil layer at the surface is 20 to 75m thick, with quite complicated geological conditions. It is a controlling factor for guaranteeing the entire project quality to control the settlement after construction and settlement characteristics of pile foundation and to guarantee the pile foundation bearing capacity. Therefore, it is necessary to conduct the research and study of pile foundation post grouting techniques to reduce the pile foundation settlement and shorten the settlement stability time. The mechanism of pile foundation post grouting techniques is still unclear, and the study of the grouting for reducing pile group settlement, in particular, is rare. Therefore, it is of important theoretical values as well as important project application values to conduct the research and study of pile foundation post grouting techniques to reduce the pile group foundation settlement and shorten the settlement stability time. Therefore, researches of the subject of post grouting pile group foundation settlement characteristics are conducted for this paper.The main research work and conclusions of this paper are as follows:a large-scale post grouting field test of long and large pile group has been conducted; abundant measurement data and documents have been accumulated, and measurement data have been analyzed and researched to come to the conclusion that the pile side grouting and pile end grouting have obviously reduction and controlling effects on the settlement of pile group foundation; the effect of pile end pressure grouting on shortening the consolidation settlement is more stable. Based on the field test data, the mechanism of the influence of post grouting on the pile group settlement characteristics is explored; adopting the Vesic theory, the influence of post grouting pile grout on the deep soft soil layer is expounded; it is proposed that the effect of pile side grouting on the soil around pile in deep soft soil layer is small, and the pile end grouting may more effectively reinforce the soil around pile; the effect of pile end grouting in controlling the settlement of long and large pile group is good, even better than the pile side grouting. With consideration given to the field test, it is proposed than soil parameters may be inverted from the results of single pile static load test after grouting, and then the settlement amount of pile group foundation after grouting may be calculated with the shear displacement method, which is verified according to the prevailing technical code. With the FLAC3D program, the research of the three-dimensional numerical simulation static load test of post grouting pile group foundation is conducted; the load-settlement regularities of the pile group foundation in the deep soft soil layer after grouting within the design load, and the influence of the changes of pile side and pile end conditions caused by grouting on the pile group foundation settlement are studied, and it is concluded that, as for the long and large pile group foundation in the deep soft soil layer, the pile side grouting and the pile end grouting may reduce its settlement during normal use conditions, and the pile end grouting is with better effects. The reduction of the pile group settlement caused by the pile end grouting is due to the effective reinforcement of pile end substratum by the grouting strengthening body formed at the pile end; moreover, the upward flow of high pressure grouting along the interface between pile and soil at the pile side improves the pile side interface conditions within a certain scope above the pile end and somewhat solidifies the pile side soil. Time series modeling and settlement prediction are conducted for the settlement-time data of the post grouting and non-grouting pile group foundation in the deep soft soil layer, based on nonparamatic time series model and multi-variate kernel function estimation. Applicability of time-series models is also analyzed. The analysis and results show that:the nonparametric time series model can achieve good prediction results, and is more practicable in engineering by contrast with the conventional parametric time series models.