Bearing Properties and Stability Analysis of the Slope Protection Framework Using Recycled Railway Sleepers

Abstract

The slope protection framework developed using recycled railway sleepers offers a novel sustainable solution for slope protection. However, this has been inadequately reported, and its force and deformation, its protective effect, and the bonding characteristics between sleepers are still unclear. The slope protection framework project of a recycled railway sleeper embankment slope on the Beijing–Tongliao railway was numerically analyzed using three typical recycled railway sleeper slope protection structures. The bearing properties and the slope stability of rectangular, rhombic, and herringbone framework structures were determined. The results show that the stress state, stress level, and failure mode of the three types of slope protection structures are similar on average. The slope protection skeleton’s stress concentration position and failure area are all concentrated at the sleeper connection node at the slope base. The rectangular and rhombic framework structures have better stability than the herringbone framework. This study proposes applying a slope protection framework constructed entirely using recycled railway sleepers. Furthermore, it allows for proper disposal of recycled railway sleepers and a reduction in stone mining.

1. Introduction

Chinese railways replace several recycled railway sleepers every year owing to routine maintenance and construction in railways. According to statistics, the amount of recycled railway sleepers in the entire railway line is more than three million, with an annual growth rate of 1.2 million [1]. Many recycled railway sleepers are centrally stored or directly piled along the railway, occupying the space for land use and indirectly affecting railway transportation and the surrounding environment. Due to China’s low-carbon economic system, protecting the ecological environment and controlling greenhouse gas emissions is crucial for promoting sustainable economic growth [2]. Therefore, the proper disposal of recycled railway sleepers requires immediate attention.

The present clarity of the environmentally friendly method for disposing of recycled railway sleepers involves crushing, heating, and reusing [3,4]. First, the cement sleeper is broken into particles with a diameter of 40 mm and then heated to high temperatures to mix the particles. The aggregate and peripheral adhesion components of cement undergo powder separation. Finally, the separated cement components are used to improve the strength of the foundations, and the aggregate is used to construct general structures or as a roadbed filler. The results show that the recycled aggregate is of relatively lower quality than natural aggregate; consequently, the recycled concrete exhibits low compressive strength [5,6,7]. Li reported that the properties of recycled coarse aggregates are different than those of natural coarse aggregates, mainly characterized as low density, high water absorption, and low soundness [8]. The quality of recycled concrete wildly fluctuates because of the different recycled aggregate sources [9,10,11]. In recycled aggregate concrete with a low and high water–binder ratio, gravel damage and level of attachment the main factors leading to the deterioration of its mechanical properties [12,13,14,15]. Owing to the high cost required for processing and separating recycled railway sleepers, the scope of reusing them is limited [16]. The track quality affects the safe operation of the train [17] and deteriorates its interaction performance with infrastructure [18].

A slope protection framework is common for light slope protection, mainly in rectangular, rhombic, herringbone, and other structures. It is used to prevent shallow slope failure [19,20,21]. Although the slope protection framework is widely used in engineering, it is mainly based on experiential and structural designs, and its protective effect is primarily evaluated qualitatively via field observation. The applicability, protective effect, and construction technology of the slope protection framework have become the focus of engineering research [22,23,24,25,26,27]. Therefore, this study considers the embankment slope protection project of the Beijing–Tongliao railway as the background and utterly recycled railway sleepers as the prefabricated component, forming a slope protection structure after assembling. A numerical simulation was used to analyze the stress and deformation of three typical skeleton structures to study the bearing properties and stability of the slope protection skeleton, thereby providing a theoretical basis and reference for engineering applications. Furthermore, it allows for the proper disposal of recycled railway sleepers and a reduction in stone mining.

2. Project Profile

The recycled railway sleepers were assembled at the project site into three skeleton forms: rectangular, rhombic, and herringbone framework structures, which were successfully used in the Beijing–Nantong Railway embankment slope protection project, as shown in Figure 1 [22]. This study considered a typical working section with a slope ratio and height of 1:1.5 and 5.5 m, respectively. The skeletons and C25 concrete beams were buried 20 cm into the slope and at the base of the hill. A 4 m-long anchor cable was arranged at the rectangular and rhombic frame joints. The slope soil is silty clay, the internal friction angle φ is 40°, and cohesive force c is 10 kPa. The geometric parameters of the slope protection skeleton, anchor cable, and slope toe beam were selected according to the actual project layout to simplify the calculation, not considering the slope groundwater and vegetation protection, and only considering the recycled railway sleeper slope protection structures.

3. Numerical Simulation of Slope Protection Framework Developed Using Recycled Railway Sleepers

Based on the ABAQUS finite element computing platform, a three-dimensional finite element model was established to analyze the slope protection performance of the three skeleton slope protection structures. Considering the boundary effect of the model, the foot of the slope extends 8 m to the existing ground, and the top of the hill extends 8 m backward in the section direction of the mountain. Along the depth direction of the slope, the model depth is 5 m below the ground. Due to the different lattice widths of the three kinds of skeletons, the model’s width is two times the lattice width.

3.1. Finite Element Model and Parameter Selection

The three-dimensional eight-node hexahedron element (C3D8), connecting beam and soil mass, was used for grid division of the slope protection skeleton. The grid encryption processing was performed for the slope protection skeleton of the soil mass near the connecting beam. Considering the regularity and continuity of the mesh, the three-dimensional eight-node wedge element (C3D6) was used to divide the mesh at the sharp corner area. An anchor cable was simulated using a beam element (B31), as shown in Figure 2 [22].

The strength grade of the concrete used for the skeleton and the connecting beam was C50, and the linear elastic constitutive model was adopted for concrete. The anchor cable material was selected according to the HRB400 steel bar based on the actual engineering design. The process of soil deformation was complex, and the choice of the linear elastic constitutive model affected the calculation. The Mohr–Coulomb model can reasonably simulate the failure mode of soil and is convenient for slope stability analysis. Therefore, the Mohr–Coulomb model was adopted in the constitutive soil model, and the model parameters are listed in

Table 1. Model material parameters.

Parameters	Silty Clay	Rebar	Concrete
γ (kN/m3)	21	75	25
E (MPa)	16.2	2 × 105	2.8 × 104
ν	0.3	0.25	0.2
c (kPa)	10	—	—
φ (°)	40	—	—

.

3.2. Boundary Conditions and Simulation Steps

3.2.1. Boundary Conditions

The upper surfaces of the soil, slope protection frame, and coupling beam were free surfaces without any constraints. However, the bottom surface of the soil model was considered sufficiently deep to constrain the displacement of the bottom surface along with three directions. The normal lateral displacements of the soil, slope protection skeleton, and connecting beam were constrained.

3.2.2. Simulation Steps

• Ground stress balance: The ground stress balance calculates the initial ground stress of the soil to ensure that the initial ground stress is consistent with the actual soil condition.
• Add slope protection structure: After balancing the ground stress, the slope protection skeleton and connecting beam at the slope base are excavated. The slope protection skeleton and connecting beam are activated to simulate the construction sequence of the slope protection structure.
• Slope stability analysis: The strength reduction method continuously reduces the strength parameters of the slope soil, calculates the safety factor of the slope under the support condition, and obtains the evolution law of the slope potential sliding surface and the working state of the slope protection skeleton.

4. Resulting Analysis of the Rectangular Framework Structures

4.1. Skeleton Stress and Displacement of Slope Protection

The stress and displacement distribution clouds of the rectangular framework structure are depicted in Figure 3 and Figure 4, respectively. The skeleton stress associated with slope protection gradually increased from the top to the base of the slope, and the stress concentration phenomenon was substantial at the node position of the skeleton and anchor cable. The stress distribution of the horizontal skeleton was relatively uniform and smaller than that of the longitudinal structure. The stress distribution on the two longitudinal frames of the model was the same, and the maximum stress was 190 kPa at the base of the slope. However, the pressure at the top of the hill gradually decreased. The stress mutation phenomenon can be attributed to different stiffnesses of the bond layer at the sleeper node position. The stress of the horizontal skeleton was symmetrically distributed. Furthermore, it was more significant at the middle part of the transverse skeleton, and the node position of the structure and the maximum stress was 50 kPa.

The horizontal displacement at the top and base of the slope was significant, as shown in Figure 4. By contrast, the horizontal displacement at the middle of the hill was relatively small. Furthermore, the horizontal displacement of the longitudinal skeleton was more significant than that of the transverse framing. The flat removal of the longitudinal scaffolding decreased first and then increased from the base to the top of the slope. The sleeper node position had a small numerical mutation. The horizontal displacement of the transverse skeleton was relatively small, whereas those of the row nearest to the base and the top of the slope were the smallest and the largest, respectively. The horizontal displacement of each transverse skeleton row was relatively uniform.

4.2. Bending Moment and Shear Force of Slope Protection

The skeleton’s bending moment and shear force distributions are shown in Figure 5. The longitudinal bending moment of the structure gradually increased from the base to the top of the slope. The transverse bending moment of the skeleton also rose from the base to the top of the hill. The maximum bending moment of the transverse framing was observed at the middle of each frame, which was small at the node position. The sheer force of the skeleton increased from the base to the top of the slope, and those of the transverse and longitudinal frames were larger at the node position and more undersized at the middle part of the skeleton.

4.3. Slope Stability Analysis

The strength reduction method was used to calculate and analyze the slope stability under the skeleton support condition of slope protection. The plastic sliding zone of the slope was obtained, as shown in Figure 6a. When the hill was damaged, the plastic failure was most prominent at the beam position at the base of the slope. The plastic sliding zone extended from the bottom of the hill to approximately 5 m behind the top of the mountain, and the potential sliding zone contained the entire slope protection system. The variation trend of soil displacement during slope failures was obtained by selecting feature points of the sliding region at the base, middle, and top of the slope, as shown in Figure 6b. The soil displacement gradually increased and abruptly changed when it reached the critical state by increasing the reduction coefficient. Then, the removal rapidly increased, the reduction coefficient was unchanged, and the slope was destroyed. The corresponding reduction coefficient of the soil was Fs = 2.23, which is the safety factor of the hill.

5. Resulting Analysis of the Rhombic Framework Structure

5.1. Skeleton Stress and Displacement of Slope Protection

The skeleton’s stress and displacement distribution clouds are shown in Figure 7 and Figure 8. Additionally, the rhombic framework structure had a prominent stress mutation similar to the rectangular framework structure at the sleeper connection node. The maximum stress of the rhombic framework structure was approximately 190 kPa at the base of the slope. Then, the pressure decreased, the skeleton stress associated with each section was close, and there was a prominent stress concentration at the node position.

As shown in Figure 8, the horizontal displacement at the base and top of the slope was significant, whereas the skeleton at the middle of the hill was tiny. The flat removal of the structure tended to increase from the base to the top of the mountain. The displacement value of the middle position of each frame was small, whereas that of the node position was large. However, the overall displacement was small, and the maximum removal was less than 0.21 mm.

5.2. Bending Moment and Shear Force of Slope Protection

The skeleton’s bending moment and shear force distributions are shown in Figure 9. The bending moment of the rhombic framework structure wildly fluctuated, the maximum bending moment appeared in the middle of the skeleton, and the moment of the node was small. Compared with other frame structures, the sheer force distribution law of the rhombic framework structure was more prominent. Moreover, the shear forces at each section’s node position and middle position were more significant and more minor, respectively. In contrast to rectangular framework structures, the bending moment and shear force of the rhombic framework structures appeared to fluctuate in a particular range from the base to the top of the slope; however, the overall trend did not increase or decrease.

5.3. Slope Stability Analysis

The plastic sliding zone of the slope under the support of the rhombic framework structure is shown in Figure 10. The failure mode of the slope was consistent with the working condition of the rectangular framework structure, as shown in Figure 10. Plastic failure was most visible at the position of the connecting beam at the base of the slope, and the plastic sliding zone extended from the bottom to the top of the hill. This sliding zone contained the entire slope protection system. By increasing the reduction coefficient, the soil displacement gradually increased. When the reduction coefficient was F_s = 2.24, the soil displacement rapidly increased. Then, the reduction coefficient further increased. The calculation was terminated when the reduction coefficient was F_s = 2.31. If the slope displacement mutation is used as the criterion for slope failure, the slope safety factor is F_s = 2.24 under the rhombic framework structure support.

6. Resulting Analysis of the Herringbone Framework Structure

6.1. Skeleton Stress and Displacement of Slope Protection

The skeleton stress distribution of the herringbone framework structure is depicted in Figure 11. The stress of the herringbone framework structure gradually increased from the top to the base of the slope, as shown in Figure 11a. The stress distribution of the horizontal skeleton was relatively uniform and smaller than that of the longitudinal structure. Stress was estimated along the longitudinal and horizontal axis of the frame (Figure 11b). The pressure was highest at the base of the longitudinal skeleton slope and gradually decreased toward the top. The stress distribution of the skeleton on the horizontal axis was symmetrical, and the stress mutation was visible at the sleeper connection node.

The horizontal displacement distribution of the herringbone framework structure is shown in Figure 12. The horizontal displacement at the top and base of the slope was more significant than that in the middle of the hill. The flat removal of the longitudinal skeleton decreased first. Then, it increased from the base to the top of the slope, with a small numerical mutation at the sleeper node position. The horizontal displacement of the skeleton was relatively tiny along the transverse axis; however, it was the greatest near the base of the slope. The removal of the horizontal framing near the top of the hill was the smallest.

6.2. Bending Moment and Shear Force of Slope Protection

The bending moment and shear force distributions of the herringbone framework structure are shown in Figure 13. The bending moments of the longitudinal and transverse skeletons gradually increased from the base to the top of the slope. The maximum bending moment of the transverse framing was observed at the middle of the frame, which was small at the node position. The sheer force exhibited an increasing trend from the base to the top of the slope, and those of the transverse and longitudinal skeletons were greater at the node and smaller at the middle of the frame.

6.3. Slope Stability Analysis

The plastic sliding zone of the slope under the support of the herringbone framework structure is shown in Figure 14. When the hill was damaged, the plastic failure of the coupling beam at the base of the slope was the most prominent. The plastic sliding zone extended from the bottom of the hill to approximately 4 m behind the top of the mountain, and the potential sliding zone contained the entire slope protection system. By increasing the reduction coefficient, the soil displacement gradually increased. When the critical state was reached, the soil displacement abruptly changed. Then, the displacement increased rapidly, the reduction coefficient was unchanged, and the slope was damaged. At this point, the corresponding soil reduction coefficient was F_s = 2.17, which is the safety factor of the hill.

7. Conclusions

This study examined a slope protection structure’s mechanical and deformation characteristics developed using a recycled sleeper framework. The strength reduction method analyzed the protective effects of three types of slope protection framework structures. The findings are summarized as follows.

(1) The stress state, stress level, and failure form of rectangular and herringbone framework structures were very similar among the three types of slope protection skeleton forms. Owing to different structural conditions, the stress state of the rhombic framework structure was slightly different than those of the rectangular and herringbone framework structures; however, the overall trend was similar.

(2) All three types of slope protection skeletons had their stress concentration points and failure area at the slope base’s sleeper connection node. Thus, joints near the slope base of this study require considerable attention. The stress concentration position and failure area of the three types of slope protection skeletons are all concentrated at the sleeper connection node of the slope base. Therefore, strengthening the joints near the slope base in this project is crucial.

(3) The rectangular and rhombic framework structures had better strength than the herringbone framework structure in terms of support stability. Additionally, the stability of rectangular and rhombic framework structures was very similar. The difference in stability of the herringbone framework system may be because it did not use an anchor cable combined support under the selected working conditions.