Stability and Countermeasures for a Deposit Slope with Artificial Scarp: Numerical Analysis and Field Monitoring

.is paper presents the results of the stability analysis of a deposit slope with an artificial scarp in a tunnel exit and an evaluation of the effectiveness of four proposed reinforcement schemes. A typical slope section was used to study the deposit slope stability and retaining mechanisms of the reinforcement systems. A series of two-dimensional (2D) finite element models (FEM), combined with a strength reduction technique, was established using the Phase software. According to field monitoring results, the horizontal displacements of the front, middle, and rear of the slope decreased gradually, and the safety factor increased successively. .e front of the deposit slope was in a state of limit equilibrium as a result of the artificial scarp formed by long-term manual excavation. Anchors and concrete frame beams provided stress compensation and improve the stability of the deposit slope, and front prestressed anchor cables and stability piles strengthened the mechanical properties of the rock and soil masses and provided resistance at the front of the deposit. Rear stability piles prevented the front of the deposit from being pushed and the middle and rear of the deposit from being pulled and provided resistance at the front of the deposit. .e field monitoring also showed that the deformation of the deposit slope was effectively controlled..e study results provide insights into the effectiveness of measures for reinforcing and maintaining the stability of deposit slope with artificial scarps.


Introduction
Over the past few years, transportation facilities (e.g., railroads and highways) have increasingly been extended into mountainous and hilly areas in conjunction with rapid economic and social development [1][2][3][4][5]. During tunnelling in such areas, rock engineers normally encounter the problem of slope stability. Statistics data shows that there are up to 148 landslides and rockfalls along the Sichuan-Tibet railway [6], and along the Dujiangyan City to Siguniang Mountain railway the stability of 32 tunnel slopes needs to be measured and evaluated [7]. Slope instabilities have threatened the safety of tunnel portals. Slope failures cause not only economic losses but also the losses of human life [8,9]. erefore, stability assessment and reinforcement on slope in mountainous regions are the premise for tunnel construction and operation [10]. e analysis of slope stability is a difficult task that involves the evaluation of a large number of factors, including geology, topography, slope materials, engineering activities, and rainfall [11]. In addition, a complete analysis, including geological and geomorphological fieldwork, field monitoring, and numerical modelling, is a necessary step for slope stability assessment [8]. us, this analysis and assessment of slope stability are crucial to the safe design and implementation of mitigation measures. Currently, slope stability can be analyzed by the limit equilibrium method, numerical methods [12,13], and experimental methods [14]. Numerical methods, which have been widely applied to slope stability analyses, have significantly improved the speed and accuracy of slope stability analysis. In addition, the strength reduction method has gradually become a focus of theoretical research [15][16][17]. Various reinforcing measures (e.g., descending slopes, drainage, plugging cracks, piles, and rock bolts) have been applied to slope stabilization [18]. Stability piles and anchor frame beams are typical reinforcing measures used for large-scale slope engineering. Many researchers have studied the reinforcement mechanisms and reinforcement parameters of individual reinforcement countermeasures [19][20][21]. However, given the complexity of geological conditions, environmental factors, geotechnical parameters, and other factors involved in slope engineering, especially large-scale slope engineering, comprehensive reinforcement countermeasures are becoming more commonly applied. Comprehensive reinforcement mechanisms and optimization of reinforcement parameters related to slope stability await further study. In recent decades, much deposit slope engineering had been constructed [22][23][24]. e deposit slope is a kind of mechanical medium with complex characteristics such as discontinuity, heterogeneity, and anisotropy, which is different from common rock slope and soil slope [23]. In view of the complexity of the deposit slope problem, there is still a long way to study and solve practical engineering problems.
is case study focused on evaluating how an artificial scarp can influence the stability of a deposit slope and on evaluating the effectiveness of the four proposed reinforcement schemes. A two-dimensional (2D) finite element model was developed and combined with a shear strength analysis technique to analyze a typical deposit slope section to obtain insight into the pile-anchor-soil interaction mechanism and its contribution to deposit slope stability. e analysis was conducted using the Phase 2 software. e movement of the deposit slope was monitored during construction, and the field measurements were compared to the numerical analysis results. Using a comparison analysis, we were able to perform a comprehensive evaluation of risks associated with the Taihedong tunnel deposit slope with an artificial scarp. Furthermore, we performed a reinforcement system design to ensure the safety of tunnel slope construction and operation.

General Description.
e Taihedong tunnel, which is located in the northern Qingxin district of the city of Qingyuan in Guangdong Province, China, is a six-lane divided expressway tunnel ( Figure 1). A scarp formed by manual excavation is above the tunnel exit. e scarp is 220 m long and 25 to 40 m high, and its slope ranges from 40 to 70°. e front edge of the scarp is a broad area formed by manual excavation. e right tunnel is inside the scarp, and the left tunnel is at the edge of the scarp. e supporting capacity has been seriously weakened by manual excavation at the toe of the deposit slope. e stability of the deposit slope has also diminished as deformation of the slope has occurred and tension cracks have formed as a result of excavation of the toe of the deposit. e slope materials at the tunnel exit are mainly composed of Quaternary deposits. A slope stability analysis of the tunnel exit and reinforcement of the unstable sliding mass are needed to assess the effects of surface water, underground water, and human engineering activities [25,26] and ensure slope safety. e potentially unstable geological body at the tunnel exit is a colluvial deposit with multilayer and multistage characteristics. is deposit can be divided into three subdomains: the existing deformation area (Zone I), a potential deformation area (Zone II), and a paleo deposit area (Zone III). e volume of the deposit is approximately 2.5 million m 3 (including 0.55 million m 3 in Zone I). e results of a geological survey indicate that there are eight tension cracks in the existing deformation area (Zone I). ese cracks are typically 40-70 m long and 20-50 cm wide, with a maximum width of nearly 1 m. e tunnel exit is located at the front of Zone I. Reinforcement was necessary to limit the deformation of the deposit slope before the tunnel excavation.

Ground Conditions.
Borehole exploration and ultrahigh-density resistivity testing were performed to determine the deposit profiles, and soil samples were collected for laboratory tests. Figure 2 shows a representative cross section of the study site, based on the boring tests. e deposit slope is composed of four layers: from top to bottom, gravel soil, silty clay, fully weathered argillaceous siltstone, and strongly weathered argillaceous siltstone. e parent rock of the gravel soil is mainly strongly and moderately weathered argillaceous siltstone. Gravel, silty soil, and silty clay are interspersed with the gravel soil. e parent rock of the silty clay, which was formed by weathering of argillaceous siltstone after colluvium, is mainly strongly and moderately argillaceous siltstone. Fully weathered argillaceous siltstone had completely weathered into hard and plastic silty clay. e original structure of fully weathered argillaceous siltstone, which only retained the appearance of the original rock, was completely destroyed. e strongly weathered argillaceous siltstone, with an argillaceous silty structure, massive structural deterioration, and crack development, was broken, soft, and crushable by hand. e soil and rock properties determined from the site investigation and associated laboratory tests are summarized in Table 1.

Potential Sliding Plane.
e various types of relevant strata at the study site exhibit significant differences in their engineering geological properties and permeability. To better guide the layout of the slope reinforcement system, the results of in situ tests (boring tests, geological surveying and mapping, and ultra-high-density resistivity surveying) were analyzed to estimate the potential slip surface of the deposit slope. Weak contact surface layers have developed in the following ways between the different soil and rock layers and could be potential sliding planes. (1) A weak contact surface layer has developed between layers of silty clay and fully weathered argillaceous siltstone. e gravel soil layer of the colluvial deposit is a permeable stratum. Consequently, relatively impermeable silty clay and fully weathered argillaceous siltstone have been infiltrated by rainwater. A weak layer has formed between them. (2) A weak contact surface layer has formed between fully weathered argillaceous siltstone and strongly weathered argillaceous siltstone. Shallow landslides and surface cracks have caused rainwater infiltration. A deep sliding surface has formed between the fully weathered argillaceous siltstone and the strongly weathered argillaceous siltstone, as confirmed by the results  Advances in Civil Engineering of the boring tests and the ultra-high-density resistivity survey.
On the basis of a comprehensive consideration of the structural conditions and activity characteristics of the deposit slope, sliding surfaces can be divided into existing sliding surfaces and potential sliding surface. e deposit slope can be divided into front, middle, and rear three-stage sliding surfaces. e leading edge of these is located at the toe of the deposit slope (Figures 1 and 2).

Computer Program for Numerical Analysis.
A 2D plane strain numerical model was developed to analyze the slope stability based on the actual geological conditions at cross section 1-1, using the Phase 2 software (version 8.0). One of the major features of Phase 2 is finite element slope stability analysis using the shear strength reduction (SSR) method. e model was developed on the basis of the mechanical properties of the soil and rock in each stratum summarized in Table 1.
Two-dimensional six-node triangular plane strain elements were used to discretize the 1-1 profile section of the deposit slope. e deposit model was uniformly meshed, with 2,183 elements connected with 4,496 nodes. All of the elements were found to be of good quality, on the basis of several trial-and-error tests. e number of bad elements was zero [27][28][29]. e boundary conditions of the slope model were set to constrain movement in both the x and y directions on the lateral sides and at the base of the slope, whereas the upper slope surface was unconstrained (Figure 3). Only gravity loading was applied to the model. e ratio of the horizontal to vertical stress was maintained at 1.0 [30]. e shear strength reduction (SSR) approach, with a tolerance of 0.001, was used to determine the critical strength reduction factor (SRF) [31,32]. is approach involves the determination of the SRF or the factor of safety (SOF) by successive reduction of the cohesion (c) and internal friction angle (φ) of the soil until failure occurs.
An iterative nonconvergence failure criterion was used to determine the critical SRF [33]. e deposit slope material was considered to be an elastic and perfectly plastic substance obeying a Mohr-Coulomb failure criterion. is was controlled by keeping the peak values equal to the residual values [27,34]. e discrete stability piles and concrete frame beams were modeled as standard linear beams with flexural rigidity [35]. e prestressed anchors were modeled as tiebacks. e anchors were modeled as fully bonded. Figure 3 illustrates the finite element model established. e properties of the structural elements are summarized in Table 2.

Stability Analysis of Original Slope.
e critical SRF and displacements for natural slope debris are shown in Figure 4. e numerical simulation results show that critical SRF values of 1.015, 1.017, and 1.029 and maximum displacements of 425 mm, 510 mm, and 855 mm were obtained for the front, middle, and rear of the deposit slope, respectively, along the 1-1 profile section. e stability analysis of the middle of the deposit slope included consideration of the front of the deposit slope. e stability analysis of the rear of the deposit slope included consideration of the front and middle of the deposit slope. e results indicate that the deposit slope is in a state of limit equilibrium. e results show very good agreement with the field measurements. e deformation area is located above the artificial scarp and the tunnel exit, at the front edge of the deformation of the deposit slope. e factors of safety for the middle and rear of the deposit slope are considerably larger than that for the front of the deposit slope. us, the front of the deposit slope must be reinforced before excavation.

Slope Reinforcement Schemes.
To analyze the global stability of the deposit slope with and without a reinforcement system and to further study the effects of different reinforcement schemes, four reinforcement schemes were considered: (A) front prestressed anchor cables and stability piles; (B) front prestressed anchor cables and stability piles and rear stability piles; (C) front prestressed anchor cables and stability piles, anchors, and concrete frame beams; and (D) front prestressed anchor cables and stability piles, rear stability piles, and anchors and concrete frame beams (as adopted in engineering practice). ese four schemes are summarized in Table 3. e symbol "×" indicates a type of reinforcement that was not included in the reinforcement system. e critical SRF values (Table 4) and displacements for the deposit slope for the four different reinforcement schemes are shown in Figure 5. e results indicate that the SRF values are the lowest for the front of the deposit slope and the highest for the rear of the deposit slope for all four reinforcement cases, A, B, C, and D. ese results were found to be in good agreement with the measured results. Figure 5 shows that the slope stability levels associated with schemes A and B were not significantly different. Similar      e reinforcement schemes differ primarily in whether the rear stability of the pile is considered. Schemes A and B were selected to analyze the function of stability piles and their influence on slope stability. Critical SRF values of 1.045 and 1.047 were obtained for schemes A and B, respectively. It appears that rear stability piles can effectively prevent the continued expansion of surface cracks, restrict deformation of the middle and rear of the deposit slope, and prevent the front soil from being pushed.
Critical SRF values of 1.045 and 1.015 were obtained for schemes A and O, respectively. e safety factor for scheme A was 3.0% higher than that for scheme O. is result shows that the front stability piles provided resistance to deformation of the soil at the front of the deposit. e prestressed anchor cables improved the mechanical properties of the rock and soil. e front of the deposit was reinforced by the prestressed anchor cables and stability piles, and its stability was improved.
Critical SRF values of 1.422 and 1.045 were obtained for schemes C and A. e safety factor for scheme C was 36.1% higher than that for scheme A and 40.1% higher than that for scheme O. As discussed previously, the deformation area was located at the front edge of the deformation body of the deposit slope. e anchors and concrete frame beams provided stress compensation for the artificial scarp and restricted the upper soil from continuing to be pulled. us, the analysis shows that the anchors and concrete frame beams significantly improved the stability of the deposit slope. e front prestressed anchor cables and stability piles, the rear stability piles, and the anchors and concrete frame beams displayed different degrees of reinforcement effectiveness in the slope reinforcement system. Figure 6 and Table 4 show the numerical analysis results for the four different reinforcement schemes (Table 3). For scheme D, critical SRF values of 1.428, 1.440, and 1.515 were obtained for the front, middle, and rear of the deposit slope, respectively. ese factors of safety are consistent with the GB 50330-2013 standard, which specifies a value of more than 1.30 for a grade III slope. Scheme D was therefore adopted for this engineering application.

Slope Reinforcement System.
e deposit slope reinforcement systems were constructed before excavation to ensure the safe construction and operation of the Taihedong tunnel, including stabilization of piles, anchors, and concrete frame beams and establishment of groundwater drainage using collector wells. Figure 1 shows a plan view of the deposit slope reinforcement systems. Figure 2 shows a typical slope cross section used in this study.
Two rows of stabilizing piles were constructed as a primary slope reinforcement. ese were buried piles with a cross section of 2.0 × 3.0 m 2 . e front stability piles, ranging in length from 15 to 25 m, were located approximately 4.9-12.3 m from the top of the scarp and were tied back by two prestressed anchor cables. e prestressed anchor cables were 45 m and 50 m in length and consisted of stranded wire cable. e prestressed anchor cables were installed at orientations of 20°to 25°downward, with a bond length of 10 m. A total of 13 stability piles were installed to the right of the right tunnel at a spacing of 6 m. A total of five stability piles were installed between the right tunnel and the left tunnel at a spacing of 5.5 m. A total of four stability piles were installed to the left of the left tunnel at a spacing of 5.5 m. e spacing between stability piles was 5 m near the tunnel. e rear stability piles, which were 30 m long and spaced 6 m apart, were located at elevations of 82 to 84 m and were 50 m away from the front stability piles. Anchors and concrete frame beams were the other primary slope reinforcement measures. e rock bolts consisted of 40 mm diameter deformed steel bars, 12 and 15 m long, spaced 3 m apart.
e prestressed anchor cables were stranded wire cables with lengths of 30 and 35 m. e prestressed anchor cables were installed at an orientation of 20°downward with a bond length of 10 m. Figure 7 shows the sequence of the completed reinforcement works.

Field Measurement.
To observe the behavior of the deposit slope during the installation of the stability piles, anchors, and cable frame beams, three displacement monitoring holes were established, designed to monitor the deflections of the deformation body. Figures 1 and 2 show plane and cross-sectional plan views, respectively, of the displacement monitoring holes. e purposes of monitoring hole JC01, which was located ahead of the front stability pile, were to determine the deformation characteristics of the  Advances in Civil Engineering front slope and to forecast and warn geological disasters that may be caused by the construction of the slope reinforcement and tunnel excavation. e purpose of monitoring hole JC02, which was located between two rows of stability piles and between the left and right tunnel, was to monitor the potential sliding surface and the deformation of the deep sliding surface. e purpose of monitoring hole JC03, which was located outside the boundary of the deformation body, was to monitor the deformation of the back edge of the deposit slope. Sliding borehole inclinometers were installed to measure the deflections of the deformation body. Figure 8 shows   Advances in Civil Engineering 7 situ monitoring. An inspection of the deposit surface was also carried out. e deformation of the deposit slope was analyzed based on the monitoring hole arrangement and the monitoring data collected. Monitoring hole JC01, which was located at the front slope and above the artificial scarp, fully reflected the deformation behavior of the front slope before, during, and after the reinforcement construction. Figure 9 shows the horizontal displacements at the three monitoring holes. e largest horizontal displacement, 56 mm, occurred at the front of the deposit slope. e maximum deformation occurred at the top of the inclinometer tube. Monitoring hole JC02, which was located in the potential deformation area, reflected the deformation of the middle slope. Figure 9(b) shows the larger deformation observed (within 28 m below the inclinometer tube). e largest horizontal displacement of the middle deposit slope was 35 mm. e maximum deformation occurred near the top of the inclinometer tube. Monitoring hole JC03, which was located outside the potential deformation slope, reflected the deformation of the back edge of the slope and provided an early warning as to whether the deformation area of the slope would be enlarged. Figure 7 shows that the maximum horizontal displacement was 28 mm. e maximum deformation occurred near the top of the inclinometer tube.
e measurement results show that the horizontal displacement decreased gradually from the front to the rear of the deposit slope. e artificial scarp significantly reduced the stability of the slope and increased the deformation of the front slope. e horizontal displacement of the middle slope was caused by traction of the front slope. In addition, due to the thrust of the rear slope, the horizontal displacement was distributed to a certain depth. e horizontal displacement of the rear slope was small. e rear slope was pulled by the front and middle slopes. e rear edge of the slope had no obvious thrust effect on the rear slope. e results were confirmed by inspection of the deposit slope surface.

Deposit Slope Stability Analysis.
Given the spatial relationship between the location of the monitoring holes and the section selected for numerical analysis, the monitoring results for hole JC01 and the numerical analysis results were judged to be comparable. Scheme D of Figure 5 shows the horizontal displacements for numerical analysis. Figure 10 shows a comparison of the measured and simulated horizontal displacements at JC01. e measured and simulated horizontal displacements exhibited the same deformation trend. e maximum horizontal displacement occurred at the borehole top, and the minimum displacement occurred at the borehole bottom. e maximum measured and simulated horizontal displacements were 56 mm and 66 mm, respectively. e measured value was approximately 84.8% of the simulated value.
As shown in Figure 10, the measured horizontal displacements of the deformation body differed from the numerical results. e reasons are as follows: (1) although monitoring hole JC01 is adjacent to the section considered in the numerical simulation, it is not in the same position, as shown in the plane layout in Figure 1. Both the measured and simulated horizontal displacements exhibit the same deformation trend, which indicates the accuracy of the simulated results to a certain extent. (2) A series of temporary measures were taken to reduce the groundwater level during the construction of the reinforcement system, which effectively improved the stability of the deposit slope. (3) Grouting reinforcement was used in a localized area above the tunnel roof during the construction of the reinforcement system. is further improved the deposit stability above the tunnel roof. e drainage holes and grouting reinforcement mentioned above were not simulated in the numerical model, but they did strengthen the reinforcement system during construction, which explains why the numerically simulated horizontal displacements were larger than the measured ones, as shown in Figure 10. Factor of safety for grade II slope = 1.30 Figure 6: Changes in the FOS of the deposit slope for different reinforcement systems.

Collector wells
Stabilizing pile Anchor frame beam Advances in Civil Engineering simulation results were judged to be consistent with the measured results. e proposed comprehensive reinforcement scheme was therefore judged to be a suitable guide for tunnel slope reinforcement. Figure 11 shows the displacement increment data for monitoring hole JC01 during the construction of the slope reinforcement and the initial excavation of the tunnel. e deposit slope reinforcement began in the middle of May and lasted for about two months. e tunnel excavation was carried out after the completion of the reinforcement construction. e monitoring data indicated that the horizontal displacement of the deformation body was found to increase quickly at the beginning of the reinforcement construction and tunnel excavation data. e subsequent rate of horizontal displacement was notably reduced. Before the slope was disturbed, the  Figure 11: Field-monitored displacement increment data for the monitoring holes during slope reinforcement and tunnel excavation. displacement increment was relatively small, less than 5 mm, and the slope was in the ultimate stability state. During the construction of the stabilizing piles, the excavation of the numerous pile holes disturbed the slope and caused a release of stress in the slope. e displacement of the slope tended to increase. However, no obvious deformation of the slope as a whole was observed. During the construction of the prestressed anchor cable frame beam, the displacement of the slope was significantly reduced because of the reinforcement effect of the stabilizing pile and the cessation of disturbance. After the completion of the comprehensive reinforcement construction, the whole reinforced slope was affected by the tunnel excavation. e displacement tended to increase, but the displacement values were all less than 18 mm. e displacement increase tended to be gentle as the tunnel excavation continued to advance. e whole slope was stable. In summary, the analysis results show that the comprehensive reinforcement treatment significantly improved the stability of the deposit slope and ensured the safety of the tunnel construction. e measured and simulated results together indicate that the failure mode of the deposit was a typical retrogressive landslide type [36,37]. e failure process is as follows: the scarp formed by a long-term excavation reduced the slip resistance of the toe of the deposit slope. Significant displacement occurred in front of the deposit. e strength of the rock and soil mass was further reduced by the surface cracks and rainwater infiltration. e front of the deposit slope was in a state of limit equilibrium. If no reinforcement measures are taken, the middle and rear of the deposit slope will be driven to slide. Finally, the whole deposit slope will slide. e slope failure will cause significant property damage and harm to those in the area. As for the retrogressive landslide, the optimal reinforcement site is the lower region of the deposit slope. In this engineering application, the anchors, concrete frame beams, prestressed anchor cables, and front stability piles that were installed at the front of the deposit slope effectively enhanced the safety factor and stability of the deposit slope. e rear stability piles are able to prevent the front deposit from being pushed and the middle and rear of the deposit from being pulled.

Conclusions
A deposit slope at an exit of the Taihedong tunnel, which is located in the city of Qingyuan in Guangdong Province, China, was analyzed in this study. e deposit slope, with surface cracks, had a scarp formed by a long-term excavation at the toe of the slope and was in a state of limit equilibrium.
To determine how to best reinforce the deposit slope, four reinforcement schemes were analyzed based on field measurements and 2D numerical analysis results. e major findings of the study can be summarized as follows: (1) e maximum horizontal displacement and the minimum safety factor of the deposit slope with the artificial scarp formed by excavation were located at the front of the slope. e horizontal displacements of the middle and rear of the slope decreased gradually, and the safety factor increased steadily.
(2) At the beginning of the reinforcement construction, a large number of excavation piles disturbed the slope and released stress within the slope, resulting in a significant increase in horizontal displacement. e construction of the prestressed anchor cable frame beam reinforcement resulted in less disturbance to the slope. e slope was strengthened by the stability pile, and the horizontal displacement of the slope was notably reduced as a result. e tunnel excavation disturbed the reinforced slope. e horizontal displacement increased notably at first and then became more stable. At all construction stages, the slope as a whole was in a stable state.
(3) e slope stability of this retrogressive-type landslide was improved significantly by the use of prestressed anchor cable frame beam and front stability pile reinforcements, which provided stress compensation for the artificial scarp at the foot of the slope. e antisliding force of the slope was improved. e rear stability piles were found to be effective in preventing the front slope from being pushed and the middle and rear slopes from being pulled and in improving the overall stability of the deposit slope.
Data Availability e data used in this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.