Stydy On Failure Mechanism And Grouting Reinforcement Of Composite Foundation With Plain Concrete Piles Under Embankment

Composite foundation technique of low-intensity rigid pile, including plain concrete pile and CFG pile, has been extensively introduced in dealing with soft ground used to support the embankment in of Beijing-Shanghai high-speed railway, Wuhan-Guangzhou, Zhengzhou-Xi’an passenger dedicated lines, etc. However, the theory of composite ground with rigid piles under embankment is lagged far behind practical engineering applications. The working behaviors and calculation theory of composite ground with rigid piles under embankment based on super-sub structure interaction is not mature. There are fewer researches on failure patterns and mechanism of composite ground with rigid piles under embankment. The railway and highway specifications are lack of the stability calculation methods for composite foundation of rigid pile under embankment, and the engineering experience support is not enough. Besides, there is a lack of the strengthening technologies for composite foundation of rigid pile under embankment during operating at present. In view of this, by taking the composite ground with plain concrete piles as the research object in this paper, to study working behaviors, failure patterns and mechanism, and grouting reinforcement technology of composite ground with plain concrete piles under embankment based on mathematical analysis, model test, numerical calculation, field test and monitor test on the field are of great theoretical significance and practical value.First, according to the load transfer mechanism and deformation law of composite ground with plain concrete piles under embankment, the composite ground with plain concrete piles under embankment is divided into five parts, including embankment fill upper layer, comprehensive cushion, composite foundation, upper layer and lower layer of underlying layer, which are regarded as an interactive system. Based on the deformation mode of composite foundation proposed by Alamgir and Mjura, an analytical method for composite ground with plain concrete piles under embankment is derived according to the compatibility of stress and deformation of the interactive system under the condition of considering the relative movement between piles and soils, and the different settlements of foundation soil around piles at the same level plane. The analytical method is verified with simulation results, including embankment surface settlement, upper and lower penetration amount of pile, deformation pattern, the friction in the contact surface between pile and soil, and pile-soil stress ratio. The results show that the analytical method can better reflect working behaviors of composite ground with plain concrete piles.Second, a series of centrifuge model tests are performed to investigate the deformation and failure pattern of composite ground with plain concrete piles under embankment. The effects of pile spacing and diameter ratio, compression modulus of soil around piles, and upper loads on deformation mode, upper and lower penetration amount of the pile, pile strain, pile-soil stress ratio, and failure patterns of the plain concrete piles under embankment have been analyzed. Based on the failure patterns of the plain concrete pile located in different position and pile-soil relative displacement of the composite foundation, the pile location in the ground can be divided into four zones:tension-bending, bending-shear, compression bending, and pressure bearing. The plain concrete pile in these zones show tension-bending failure, bending-shear failure, compression bending failure, penetration failure, lateral displacement and inclination of pile toward the toe of the embankment slope, and soil between piles flow around the piles. This indicates that the piles located in different zones have different stress states and anti-sliding mechanisms. These conditions may change with different upper loads, compression moduli of soil layer around piles, and pile spacing and diameter ratios.The one by one exit mechanism of functions of the plain concrete piles is introduced into numerical analysis, when rupture breaking takes place in the piles. A series of numerical simulated calculations on deformation and failure characteristics and mechanism of composite ground with plain concrete piles under embankment are performed. The numerical calculation results show that the closer to the embankment toe the pile is, the larger the bending moment and shear is. The bending failure of the pile located in composite foundation under embankment is more possible to occur than the shear failure of that. The piles closest to the embankment toe fail first. Then the piles get closer to the embankment center fail gradually. This indicates that the plain concrete piles located in the ground under embankment fail one by one in bending failure mode, but not fail simultaneously as an assumption for design. The piles at some distance from embankment toe are mainly in pressured state and do not fail in bending failure mode. The numerical calculation results demonstrate correctness of the forced state division zones of the plain concrete pile under embankment proposed by the centrifuge model test. After identifying state of the pile located in composite foundation under embankment, a whole stability analysis method for composite ground with plain concrete piles under embankment is proposed by using finite element analysis method based on strength reduction method. The whole stability analysis method and strength reduction method without considering state of the pile are applied to calculate safety coefficients of the basic model C-1, in which pile spacing and diameter ratio of plain concrete piles is 3.0. The safety coefficients of the basic model C-1 are 0.92 and 2.14, respectively. It is clear that the stress states and anti-sliding mechanisms of the pile located in the composite foundation can be well considered by the whole stability analysis method, which can make the calculation result more reasonable.Then, an oil-water isolator type grout pressure inspecting device is successfully designed and achieved. A grouting simulation test device, which can supply controllable grouting pressure from 0.0 MPa to 1.0 MPa and simulate grouting process in soil, is developed in this paper. A series of experiments on grouting reinforcement of saturated soft clay are performed by using the grouting simulation test device. The experiment results show that diffusion mechanism of grouting in the saturated soft clay can be divided into four stages:grout ball diffusion, first fracture surface diffusion, second fracture surface diffusion under passive earth pressure, and following fracture surface diffusion. A calculation formula of grouting pressure turning from the compaction grouting to the fracture grouting is presented for the saturated soft clay on the basis of the correlation between diffusion characters of the grout in the saturated soft clay and grouting pressure, water-cement ratio of the grout and simulation grouting depth studied. In addition, from the comprehensive effect of grout diffusion in the clay, reducing porosities and improving the compression modulus and shear strength of the clay reinforced by grouting, it is found that the grouting pressure is the bigger, the better, but cannot be too big. There is an optimal value for water-cement ratio of the grout. The experiments lead to the conclusion that the optimal water-cement ratio of the grout is 0.7 for grouting to reinforce the clay.Furthermore, the supplementary geological exploration is performed for a station embankment of passenger dedicated line in state of continuing settlement. The reasons brought about too large settlement of the embankment are presented by the geological exploration. Then, the comprehensive technical schemes and key parameters to quickly reinforce the established composite foundation under the station embankment underlying concealed karst cavity are proposed according to the correlation between diffusion characters of the grout in the saturated soft clay and grouting pressure, water-cement ratio of the grout and simulation grouting depth, as well as the calculation formula of grouting pressure, and deformation and failure patterns of composite ground with plain concrete piles under embankment and stress states of the pile located in different zones. Total settlement of the station embankment after improvement is calculated by using the analytical method for composite ground with plain concrete piles under embankment. Stability of the station embankment after improvement is analyzed based on the whole stability analysis method for composite ground with plain concrete piles under embankment. The calculation and analysis results are compared with the in-situ monitoring results and settlement value of the station embankment calculated accordance with the specification. It indicates that the results certify the validity and availability of the comprehensive technical schemes combined with several kinds of grouting techniques for quick reinforcement the station embankment.