Deformation of Combined Retaining Structure in Geological Environment

As a new type of sliding support structure, composite support structure has excellent performance. It is a structure in which two rows of reinforced concrete piles are placed in an appropriate position and the front pile head is connected with the back pile body by a rigid beam. It will form a double row spatial structure, similar to the seating. h-type anti-slide pile can save (or even do not need) anchoring components, form a double-row pile supporting structure system, and achieve good supporting performance. By adjusting the position of tie beam, the new type anti slide pile has wider universality and applicability. We can make assumptions in the actual calculation.h type anti slide pile has strong anti overturning ability. The compressive strength of the front pile and the tensile strength of the back pile can form a pair of effective anti-overturning bending moments, and the rigid joints also improve the anti-overturning ability.


Introduction
In Our country, the mountainous area is widely distributed, the road traffic construction and other infrastructure projects often encounter this geological situation. Especially in the western region, geological conditions are complex. Such landslides are frequently encountered in our projects, and the thrust force of the landslide is greater than 2000kN/m. We mainly use large pile and prestressed pile to resist such large thrust landslide. However, both of these measures have some disadvantages.
The bending resistance of common pile is limited, the horizontal displacement is difficult to control, and the construction surface is small. Therefore, for some large landslides, ordinary piles are difficult to meet the requirements [1] [2] . Ordinary large pile section (e.g., 3m×5m) will interfere with the slope during the excavation process, which is not conducive to the stability of the slope and easily leads to the extension of the construction period. In addition, its skid resistance capacity is limited, so the pile spacing should not be too large to resist the large landslide thrust, which will lead to the increase of cost.
As the pressure pile above anchor cable, the tension of anchor pile can improve the stress condition of pile body and reduce the cross section size of pile body appropriately. However, engineering practice found that the structure was somewhat at risk during the year. Its main disadvantages are anchor cable corrosion and relaxation. The long service life of the bolt has not been verified, and its durability and reliability are the important defects of the bolt structure, which makes the service area of the bolt structure smaller and smaller. With this in mind, a more durable, more reliable, and suitably sized anti-slide pile structure should be considered to resist high-thrust landslide.

Analysis of mechanism and feature for h type of anti-sliding piles
In China, as a new type of anti-sliding support structure, the door-to-door anti-sliding pile has the advantages of large lateral stiffness, strong anti-sliding force, good construction stability and simple construction. We have applied it preliminarily and obtained good results. The portal type anti-slide pile does not have internal support (anchor), so the anti-slide pile construction can be carried out by taking into account the interaction between pile and soil, through the overall stiffness and three-dimensional effect, so as to maintain the stability of the slope and control the deformation. (See Figure1) Schematic diagram of portal anti-sliding piles According to the basic principle and practical engineering requirements, a new type of anti-slide pile structure, h-type anti-slide pile, is proposed based on the previous research. It is a structure in which two rows of reinforced concrete piles are placed in an appropriate position and the front pile head is connected with the back pile body by a rigid beam. It will form a double row spatial structure, similar to the seating.
The essence is a kind of door type anti-slide pile based on innovative form. Due to the existence of reinforced concrete tie beam, H-type anti-slide pile can save (or even do not need) anchorage members, forming a double-row pile supporting structure system, so as to achieve good supporting performance. Because the position of tie beam can be adjusted, the new type anti-slide pile has wider universality and applicability (As shown in Figure 2). In the actual calculation, we make the following assumptions. h type of anti-sliding piles 1) In the design of tie beam, the front pile and the back pile are connected as nodes, and the Angle of connection point remains unchanged.
(2) The distance between piles is far less than the length of piles, so it can be considered that the stiffness of the front pile and the back pile is far less than the tie beam, which can be approximate to the absolute rigid body.
(3) There is no tensile deformation or compression deformation of the beam. The horizontal displacement of both ends of the tie beam is equal.
On this basis, the deformation pattern of H-type antislide pile can be analyzed (as shown in FIG. 3). The structure is controlled by the soil pressure between piles and the binding beam soil and elastic bearing force under the sliding surface. Once the parameters of soil pressure and foundation stiffness are determined, the structural internal forces can be calculated by using the elastic foundation beam theory and structural mechanics method [3][4] (As shown in Figure 4). Deformation of h type of anti-sliding piles h-type of anti-sliding pile Moment diagram Through the above analysis, it can be concluded that h-type anti-slide pile has the following characteristics: (1) As the tie beam cannot rotate freely between the front pile and the back pile, the bending resistance of the structure is greatly improved. At the same time, because of the rigid connection between the tie beam and the pile, the structure has greater horizontal stiffness.
(2) h-type anti-slide pile can automatically adjust the bending moment change according to the bottom foundation support. This structure allows the material to play its full role, reduces the pile diameter, and saves the amount of steel, while the common single pile and anchor pile do not have these characteristics.
(3) h-type anti-slide pile has strong capsizing ability. The compressive strength of the front pile and the tensile strength of the back pile can form a pair of effective antioverturning bending moments, and the rigid joints also improve the anti-overturning ability.
(4) h-type anti-slide pile significantly reduces the maximum negative bending moment of the front pile relative to the gate type anti-slide pile. The reason for this phenomenon is that the construction of Pile h shortens the length of the front load of the pile. Therefore, the moment of bolt section is under active control.

Influence of cantilever length variation of h-type anti-slide pile
The mechanical mechanism of H-type anti-slide pile and door type anti-slide pile are basically the same, the main difference lies in the cantilever structure of h-type antislide pile, which makes the force force different from door type anti-slide pile. The cantilever beam can not only bear the thrust of soil, but also play the role of slope and retaining. The function mentioned above should be considered in the design of cantilever beam, and its reasonable size should be calculated to control the bending moment of pile. The calculation results show that the cantilever length has an influence on the displacement of H-type pile, especially on the displacement of h-type pile top. Obviously, with the change of cantilever length, the displacement trend of the top of the front pile and the back pile is opposite.In the structural design, it is suggested that the cantilever length should not exceed 0.5h, otherwise it is difficult to control the position shift of Pile H. FIG. 1 shows the comparison diagram of displacement of front and rear pile tops under different cantilever lengths. H-type anti-slide pile structure, the control of the absolute value of positive and negative bending moments is very important. According to the above laws, when the cantilever length is 0.5h-0.67h, the maximum bending moment is relatively small, which is conducive to the optimization of structural design.

Conclusion
h-type anti-slide pile can automatically adjust the structural force to adapt to complex load conditions. Ordinary pile is a statically determinate structure without this characteristic. For h-type anti-slide pile, because the rigid tie-beam connects the front pile and the back pile as a whole, it becomes a kind of statically indecisive structure, which makes the displacement, internal force, bending moment and shear force obviously decrease By replacing anchor pile with h-type anti-slide pile, overlapping construction can be avoided and the cost can be reduced by 5%. The application of h-type anti-slide pile in some engineering parts where the bolt can not be used will show its superiority. Preliminary estimate: using the structure to resist the same landslide thrust, the maximum bending moment is reduced by 60% compared with ordinary anti-slide pile. At the same time, the stress performance is greatly improved. Therefore, h-type antislide pile has high application value and development potential.