Research On Negative Skin Friction Of Post-treated Micro-pile In Soft Road Subgrade According To Construction Process

As a new method that developed in recent years to strengthen soft soil subgrade, post-treated techniques of micro-pile has been successfully practiced in several expressways and achieved satisfying comprehensive benefits. However, there are a few knotty problems such as load transferring mechanism and optimization theory of post-treated composite foundation have not been reasonably solved, and the situation that theory research lagging behind engineering practice seriously restricts the further development of post-treated micro-pile. In this paper, the Negative Skin Friction(NSF) of micro-pile in soft soil roadbed post-treated by micro-pile composite foundation was carefully studied on the basis of theoretical analysis in combination with the structure of soft soil and engineering construction process.Firstly, engineering properties of soft soil were re-understood through the study of structure and disturbance of soft clay. The soft soil was in an underconsolidated state in the formation process, and it was the structure of soft soil that led to soft soil showing some engineering characteristics of superconsolidation soil on the macroscopic. Lamellar structure mainly with larger pores and support frame determined the deformation trend and the engineering characteristics of soft soil, and structural soft soil stress strain curve could be simulated by five straight lines. The disturbance problem of structural soft soil was the bearing capacity damage of Lamellar structure in soft clay, resulting in bearing capacity reduce of soft soil in elastic deformation stage, and the deformation trend of disturbed soil is similar to undisturbed soil. The concept of virtual preconsolidation pressure was put forward to defining the disturbance degree of soft soil, so that the definition of disturbance degree could be connected with engineering practice and e-lgp curve, In which the existing calculating analysis method would be used to calculate soft soil additional deformation due to disturbance. There was an essential difference between remolded soil and soft soil with a disturbance degree of 100%.Secondly, Deformation control and soft soil consolidation problem in post-treated composite foundation of micro-pile were deeply studied on the basis of construction process. In terms of engineering philosophy, the post-treated concept of "guide" combined "plugging", first " guide" and then "plugging" for controlling roadbed settlement was much more effective than pre-treated concept. Learning from bag sand well consolidating theory, a comprehensive consolidation analysis of soft soil foundation post-treated by micro-pile was worked out under the specific circumstances,such as the multiple loading of road subgrade, the consolidation conversion process, Soil permeability coefficient changing, et al.Thirdly, the load transfer process and NSF of post-treated composite foundation of micro-pile were thoroughly researched in this part. On the premise of studying the effect of concrete tray formed by grouting, the load transmission characteristics and the effect of internal factors(such as soil arching effect, tray effect, interaction of soil and pile, et al) were comprehensively analyzed during learning the interrelation of compacted soil, micro-pile and tray, soft clay. A load transfer model of micro-pile post-treated soft roadbed was totally built to analyze the solving process of micro-pile’s NSF and deformation. The model was used in Fu Xiang expressway project to calculate the settlement of abutment subgrade post-treated by micro-piles, which showed that the post-treated model was certainly flexible, reasonable and effective.Finally, load transfer problem of subsequent loading and design ideas of post-treated composite foundation were carefully researched. The subsequent load transfer model of compacted soil and micro-pile, soft clay and micro-pile was built on the basis of the limit analysis method and the load transfer method to analyze and solve engineering problem. Deformation range and characteristics of the inner, outer filling pile were pointed out, and the basic hypothesis of NSF was discussed. A new design idea of composite foundation with post-treated micro-pile was introduced after analyzing the allowable deformation of subgrade and pavement, which take the traffic function requirement as basic goal.In conclusion, the developing course of soft soil structure, micro-pile post-treated technology and pile-side NSF was summarized in this assay after analyzing the engineering application and the developing technical problems of post-treated composite foundation. Post-treated composite foundation bearing mechanism and deformation control mechanism were studied to deeply understand the vertical bearing mechanism of reinforced body, the deformation mechanism of soil around pile, the load transfer mechanism and its variation between pile and soil, and to lay the theoretical foundation for the optimization design of micro-pile post-treated composite foundation.