| Of all slope failure types, landslide is the widest distribution and the most severe geological hazard. Our nation is one of the countries suffering the most serious landslide hazard loss in the world. With the fast development of our national economic construction, the Three Gorges Reservoir project, Qinghai-Tibet railway project, West-East natural gas transferring project, West-East electricity transmission project, Wenchuan earthquake reconstruction, etc, a great number of key projects related to national welfare and the people's livelihood are successively carried out. During the enforcement of the projects, many of them are suffered severe threaten or potential threaten. In order to protect the safety of national welfare and the people's livelihood, the administration department paid much attention to the landslide hazard and took effective steps to control the landslides. Just in the Three Gorges Reservoir Area, of all the 4429 geological hazard points, there are 2490 large-scale landslides or collapses in the submerged area and immigration removal area. And the nation has spent about 12 billion RMB on controlling the landslides as well as other geological hazards. As a result, it is urgent to systematic study the way of controlling the landslides to provide scientific basis for the engineering practice.Anti-slide pile is one of the most significant measures for controlling the landslide, and it is widely used in the landslide control project. However, due to the gap between the theory study and the engineering practice of anti-slide pile, the determination of design parameters of anti-slide pile always depend on engineering experience, lacking of reasonable standard and scientific theory basis. As a result, the design scheme is difficult to achieve both safety and economy. If the design scheme is too conservative, a great deal of investment will be wasted. On the contrary, it will cause engineering hidden trouble and the cost is even more. Therefore, there is pressure on carrying out research on anti-slide pile optimization study on the base of studying on the interaction mechanism of anti-slide pile and landslide.On the basis of the study on the characteristics and formation mechanism of typical accumulation landslide, the pushing force patterns of accumulation landslide are analyzed in detailed. With the design principle and internal force calculation of anti-slide pile, the geological model for anti-slide pile and landslide interaction is established. According to the analysis on the process of anti-slide pile and landslide interaction, the interaction mechanism of anti-slide pile and landslide is studied deeply. In addition, the minimum pile interval model and the maximum pile interval model are proposed based on the soil arching effect. And the three-stage load sharing ratio model is given, including the soil arching behind the piles, the soil arching between the piles and the sliding mass in front of the pile. Combined with the related study results in the aspects of pile interval, section dimension and anchoring ratio optimization research, the optimization study on the anti-slide pile is carried out, and the way of comprehensive optimization study is pointed out as well. The main study results are as follows:(1) Concerning the model of anti-slide pile and landslide interaction is similar with Terzaghi's mobile door test, on the basis of improved mobile door test by Chevalier, the process of anti-slide pile and landslide interaction can be also divided into three stages, which are initial stage, transition stage and final stage. From the point of anti-slide pile and landslide interaction is a three-stage process, the anti-slide pile and landslide interaction model which is based on the interaction process is proposed. The stage when the soil arching behind the piles plays a dominant role is called "transition stage", while the stage when the soil arching between the piles plays a dominant role is called "final stage".(2) According to the model based on the process of anti-slide pile and landslide interaction, the pile interval calculation model is established. With the analysis of the process of anti-slide pile and landslide interaction, the minimum pile interval model and the maximum pile interval model are proposed based on the soil arching effect respectively, and they can provide scientific basis for the pile determination for engineering practice.(3) The differential pile interval distribution principle is proposed based on the theory analysis. Under the condition that the parameters of section dimension and sliding mass are determinate, the product of pushing force of sliding mass and the clear distance is a constant, therefore, the differential pile interval distribution principle is proposed based on the characteristics of pushing force of sliding mass. For the landslide that the section dimension parameters have been determined yet, the pile interval should be smaller where the pushing force of sliding mass is larger, while the pile interval should increase when the pushing force of sliding mass becomes smaller. The final pile distribution on the plane should present the characteristics of dense in the middle and sparse in the both sides.(4) According to the characteristics of anti-slide pile engineering, the pile-soil load sharing ratio based on the anti-slide pile and landslide is proposed. It is defined to describe the extent of load transfer from the sliding mass to the anti-slide pile, and it can weigh the effectiveness of anti-sliding. According to the difference of load transfer process between anti-slide pile and landslide, the soil arching behind the piles can be called end-bearing soil arching, and the soil arching between the piles can be called friction soil arching. Instead of the traditional two-stage load transfer pattern, a new three-stage load transfer pattern including end-bearing soil arching, friction soil arching and the sliding mass in front of the pile is proposed. With the new load transfer pattern, the change law of load sharing ratio during the process of anti-slide pile and landslide interaction can be studied in detailed.(5) The systematic numerical modeling study is carried out to research the anti-slide pile and landslide interaction. An improved model for anti-slide pile and landslide is proposed instead of the traditional one. In the aspect of pile pattern choice, in order to overcome the disadvantage of the circular pile and square pile, a new rectangle section dimension with the length of the section height is 1.5 times of the section width. In the aspect of study range, in order to prevent the impact by the boundary condition, there are four piles chosen as the study range, and the emphasis is on the middle two piles. In the aspect of constraint condition, according to the steep back and gentle front characteristics of the accumulation landslide in the Three Gorges Reservoir Area, the boundary of the sliding mass in front of the piles should be free but not constraint condition.(6) In order to study different parameters conditions the change law of anti-slide pile and landslide interaction soil arching effect and the pile-soil load sharing ratio, the anti-slide pile and landslide interaction soil arching effect and the pile-soil load sharing ratio are studied systematically under different pile interval, different shear strength parameters, different pile-soil interface parameters, different pushing force of sliding mass. The main conclusions are as follows:①According to the numerical simulation under different ratio between pile interval and section width, it can be found that the maximum principal stress arching is the arching existing between the piles, and its arch springing is the interface between the pile and the sliding mass. With the increase of the ratio between pile interval and section width, the effect of maximum principal stress arching becomes weak. The minimum principal stress arching is the arching existing behind the piles, and its arch springing is the back of the piles. With the increase of the ratio between pile interval and section width, the minimum principal stress arching will be destroyed by the pushing force.②When the section width of the anti-slide pile is determinate, with the increase of the pile interval, the load sharing ratio of arching existing behind the piles decreases, and the load sharing ratio of arching existing between the piles increases. It shows that with the increase of the ratio between pile interval and section width, more loads are transferred to the arching existing between the piles increases. Due to the bearing capacity depend on the friction between the piles and the sliding mass, the bearing capacity of the arching existing between the piles is limited to a certain range, and its load sharing ratio line becomes gentle.③When the section width of the anti-slide pile and the pile interval are determinate, with the increase of sliding mass cohesion (from zero to a certain threshold limiting), the load sharing ratio of the arching existing behind the piles decreases, while the load sharing ratio of the arching existing between the piles increases. It shows that the increase of sliding mass cohesion can enhance the bearing capacity of the arching existing between the piles. When the cohesion increases to a certain threshold limiting, the load sharing ratio of arching existing behind the piles, arching existing between the piles and the sliding mass in front of the pile remain the same. It shows that when the cohesion increases to a certain threshold limiting, the bearing capacity of both the arching existing behind the piles and the arching existing between the piles have enough capacity against the pushing force, and even larger cohesion has little effect on soil arching.④When the section width of the anti-slide pile and the pile interval are determinate, with the increase of sliding mass internal frictional angle, the load sharing ratio of the arching existing behind the piles has a slight increase, and the load sharing ratio of the arching existing between the piles has a slight decrease. It shows that the increase of sliding mass internal frictional angle can enhance the bearing capacity of the arching existing behind the piles. When the internal frictional angle increases to a certain threshold limiting, the load sharing ratio of arching existing behind the piles, arching existing between the piles and the sliding mass in front of the pile remain the same. It shows that when the internal frictional angle increases to a certain threshold limiting, the bearing capacity of both the arching existing behind the piles and the arching existing between the piles have enough capacity against the pushing force, and even larger internal frictional angle has little effect on soil arching.⑤When the section width of the anti-slide pile and the pile interval are determinate, with the increase of pile-soil interface frictional angle, the load sharing ratio of the arching existing behind the piles has a slight decrease, and the load sharing ratio of the arching existing between the piles has a slight increase. It shows that the increase of pile-soil interface frictional angle can enhance the bearing capacity of the arching existing behind the piles.⑥When the section width of the anti-slide pile and the pile interval are determinate, with the increase of sliding mass pushing force, the load sharing ratio of the arching existing behind the piles has a significant decrease, and the load sharing ratio of the arching existing between the piles has a significant increase. It shows that the increase of sliding mass pushing force increases the load sharing ratio of the arching existing behind the piles. When the sliding mass pushing force reach a certain threshold limiting value, the arching existing behind the piles is ruptured, and the load is mainly supported by the existing between the piles.(7) With concerning the three-dimensional soil arching effect in aspect of the direction along the siding mass depth, under the pushing force with triangle distribution, the maximum principal stress has a significant change after and before the piles due to the soil arching effect. In addition, the impact range of the maximum principal stress becomes smaller with the increase of the depth of siding mass. It shows the impact range of the soil arching effect becomes smaller with the increase of the depth of siding mass under this situation. Therefore, during the anti-slide engineering practice, the law of soil arching effect becomes smaller with the increase of the depth should be concerned, and the reasonable anti-slide pile should be carried out based on the impact range of soil arching effect.(8) In the aspect of single design parameter optimization of anti-slide pile, under the condition both satisfying the engineering safety and the check of bending resistance, shear resistant, lateral stress and construction reinforcement, the main optimization aim is to make the cost of the anti-slide pile lowest. By using the golden section optimization method, the optimization study on anchoring ratio and the section dimension can be carried out.(9) Based on the study on the single design parameter optimization and the relationship between the pile pattern parameters and the distribution parameters, the comprehensive optimization study method for anti-slide pile is proposed. Firstly, the initial pile interval is given according to the law that under the condition that the parameters of section dimension and sliding mass are determinate, the product of pushing force of sliding mass and the clear distance is a constant. Secondly, the optimization study of section dimension and anchoring ratio are carried out based on the initial pile interval. Also, the section dimension and anchoring ratio can check whether the pile interval is proper. According to the loop iteration, the final comprehensive optimization scheme can be determinate based on the study on the pile interval model, section dimension and anchoring ratio optimization study.(10) The premise of anti-slide pile optimization study is the interaction mechanism between anti-slide pile and landslide, and the relationship between the external factors of anti-slide pile (such as pushing force, shear strengthen parameters, the thickness of sliding mass, the unit weight of sliding mass, etc.) and internal factors of anti-slide pile (such as pile interval, anchoring ratio, section dimension, etc.) should be studied in detailed. With concerning the standard both the engineering safety and the investment economy, the reasonable pile interval should assure the arching existing behind the piles and the arching existing between the piles work together. Meanwhile, under the premise of single anti-slide pile investment optimization, the comprehensive distribution of anti-slide pile on the plane should be concerned. According to the differential pile interval distribution principle, it is will make the anti-slide pile and landslide synergistic work together still further. The study method proposed above can provide scientific theory basis to determine the pile pattern and distribution parameters for anti-slide pile engineering practice.
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