Optimal Design and Numerical Analysis of Soil Slope Reinforcement by a New Developed Polymer Micro Anti-slide Pile


 As a new material, polyurethane polymer has been widely used in engineering in recent years due to the excellent engineering mechanical properties. Based on the characteristics of this material, a multi pipe grouting micro anti-slide pile is proposed, which is formed by using polymer slurry as grouting material. Compared with traditional anti-slide pile, the polymer micro pile has the advantages of strong applicability, no water reaction, small disturbance, fast construction, economy and durability. As a flexible retaining structure, polymer micro-piles can strengthen the slope by cooperating with the forces. However, there is no report on the reinforcement of slope by polymer micro piles at present. In this paper, a three-dimensional multi row polymer micro piles model for slope reinforcement considering different embedded depth and pile location is established. Safety factor, thrust force of landslide behind pile, length of pile and mises stress are taken as four factors to evaluate reinforcement effect, the optimal reliability of polymer micro anti-slide pile for slope reinforcement is evaluated by giving different weight values to each factor through multi factor comprehensive evaluation method. The safety factor of slope (Fs), landslide thrust behind pile and mises stress of pile are analyzed under different embedded depth (le) and pile position (px). The results show that the best embedded depth is about1/8 − 1/12 of the horizontal length of the landslide behind the pile when multi row polymer micro piles are used to reinforce the slope; the optimum position of pile arrangement is 0.55–0.65 times the slope length from the top of the slope.

reviewed the development history of polyurethane materials, and introduced non-aqueous reactive

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FLAC 3D to coupling analysis of anti-slide pile in slope. According to the shear strength reduction technology, he 61 calculated the safety factor of the slope reinforced by piles. It was found that the safety factor was significantly 62 higher when the pile was located in the middle of the slope.

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Based on the analysis above, polymer materials are mostly used for foundation reinforcement and dam 64 seepage control by grouting. However, there are few studies on the micro anti-slide pile formed by using polyurethane polymer as grouting material. The design scheme and structural stress law of the slope reinforced by slope gradient is i=0.4, the slope height is 20 m, the slope length is 50 m, the ground to underground 2 m is silty 82 clay, below 2 m is granite. In order to study the reinforcement effect of pile group, the pile row number is 5, the 83 pile spacing is 3D = 0.9 m, the pile is arranged in quincunx shape, and the pile section is circular. The ideal 84 elastic-plastic model obeying Mohr Coulomb failure criterion is adopted for slope soil. The interaction mode 85 between pile and soil is normal contact, the contact property is "hard contact", and the friction coefficient is 0.46.

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Combined with the horizontal displacement at the foot of the slope and the vertical displacement at the top of the 87 slope, forming plastic penetration zone as the instability criterion of the slope. The initial stress field is considered 88 as gravity field, and all models adopt unified boundary conditions. The horizontal displacement of the left and 89 right sides of the slope is constrained in Z direction, the front and back sides are constrained in X direction, and the 90 bottom side is fully constrained in X, Y and Z directions, the three-dimensional stress c3d8 attribute is used for 91 mesh generation, and the mesh generation is shown in Fig.1. The finite element software is used to calculate, and 92 the stability factor is Fs=1.205 when there is no support to reinforce the slope. The relevant parameters of the 93 calculation model are shown in Table 1, and the schematic diagram of the slope model strengthened by polymer 94 micro piles is shown in Fig.2. 4 safety factor of the slope is not less than the safety factor required by the landslide treatment design, and it should 101 not be too large to avoid waste. Therefore, the safety factor after reinforcement of the slope is taken as an 102 optimization factor affecting the design of anti-slide pile. With the change of pile position, the thrust of landslide 103 behind pile will also change. Smaller horizontal resistance means less engineering materials and quantities to meet 104 the economic requirements. Therefore, the thrust of landslide behind pile is selected as an optimization factor 105 affecting the design of anti-slide pile. In the process of anti-slide pile design, the design of pile length is also very 106 important. If the length of the pile is small and the critical sliding surface is too deep, the expected reinforcement 107 effect will not be achieved. If the pile is too long, the construction difficulty will increase and materials will be 108 wasted, which will lead to local cracking of the pile (Emirler et al. 2020). The design pile length is regarded as an 109 optimization factor affecting the design of anti-slide pile.

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Because the slenderness ratio of the micro anti-slide pile is greater than 30 and the diameter is generally less 111 than 400 mm, when the polymer slurry is used as the pile material, considering that the formed polymer micro pile (1) 120 Where a1, a2 and a3 refer to the first, second and third principal stresses respectively. When the shape change 121 ratio reaches a certain degree, the material begins to yield. Mises stress uses stress contour to represent the stress 122 distribution in the model, which can clearly describe the change of a result in the whole model, so that analysts can 123 quickly determine the most dangerous area in the model. Therefore, mises stress is selected as one of the 124 evaluation indexes.

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In this paper, safety factor, landslide thrust behind pile, pile length and mises stress are selected as the four 126 optimization objectives which affect the design of anti-slide pile. In the multi-factor comprehensive evaluation method, we regard the research object as a system and make     Table. 2.

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When the anti-slide pile is used to reinforce the slope, a part of the pile needs to be driven into the embedded 167 layer, so that the anti-slide pile can provide the anti-slide force against the sliding of the weak layer, but the deeper 168 the pile is driven into the embedded layer, the better. When the anti-sliding piles can provide sufficient anti-sliding 169 force, if the length of piles is increased blindly, the difficulty and the cost of construction will be increased, lead to 170 half the battle. Therefore, the selection of reasonable embedded depth is an important part of anti-slide pile design.

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In the process of numerical simulation of slope reinforced by anti-slide piles, the simulation of embedded depth of

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With the change of pile position, the thrust force of landslide behind the pile will also change. Lower 207 horizontal resistance means less engineering material and quantity, and also can meet the economic requirements.       used as the basis for judging the stability of the slope. A smaller safety factor is required. Therefore, when px=30 m 260 and 35 m, the displacement of the pile body is smaller than that when px=27.5 m. Table 4 shows the relationship 261 between safety factor, horizontal displacement of foot and vertical displacement of top of slope.
9 the thrust of landslide behind pile and mises stress of pile also affect the design of anti-slide pile. Fig.12. describes 264 the relationship curve between mises stress and pile length at different pile positions. It can be seen from the 265 diagram that when the length of the pile reaches the depth of the embedded layer, the mises force acting on the pile 266 body will reach an extreme value, but not the maximum value; it will reach the maximum value at the bottom of 267 the pile. When the pile position is selected to be arranged above the middle of the slope, the mises force of the fifth 268 row of piles is the largest among the five rows. The fourth row of piles is subjected to the maximum mises force

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As shown in Table 5, it is calculated that the maximum reliability value of slope reinforcement with multi-row 291 high polymer micro anti-slide piles is k=0.483 when the location of pile laying is px=30 m. Therefore, the optimum 292 position of multi-row high polymer micro anti-slide piles is 0.55-0.65L from the top of the slope, L is the 293 horizontal distance between the top and the foot of the slope. piles is discussed, the safety factor of slope, landslide thrust behind pile, mises stress on pile and pile length are 297 considered when anti-slide pile is embedded in different depth and pile position, based on the finite element 298 method and multi factor comprehensive evaluation, the optimal reliability of slope reinforced by polymer micro 299 anti-slide pile is studied. The main conclusions are as follows:

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(1) With the increase of embedded depth, the safety factor of slope gradually increases and then remains 301 stable, the mises stress of pile body gradually increases, and the vulnerability of pile body increases; the change of 302 embedded depth has little influence on the thrust force of landslide behind pile.

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(2) When using multi-row high polymer micro anti-slide piles to reinforce slope, the best embedded depth 304 H is about 1/8-1/12 of the horizontal length of the landslide mass behind the piles, which is obtained by 305 multi-factor comprehensive evaluation method.

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(3) As the pile position is gradually away from the top of the slope, the safety factor of the slope reaches 307 its maximum value in the middle and lower part of the slope; it is inferred that the fragile state of the pile body 308 changes from the front pile to the rear pile; the thrust value of the landslide behind the pile changes parabolically 309 and reaches its maximum value near the middle of the slope.

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(4) When using multi-row high polymer micro anti-slide piles to reinforce the slope, the optimum position 311 of pile arrangement is 0.55-0.65L from the top of the slope, which is obtained by multi-factor comprehensive 312 evaluation method, L is the horizontal distance between the top and the foot of the slope.

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Due to its strong adaptability, durability, fast forming speed, anhydrous reaction and small disturbance in       Safety factor at different embedded depth