Analysis of Water Migration and Spoil Slope Stability under the Coupled Effects of Rainfall and Root Reinforcement Based on the Unsaturated Soil Theory
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1. Introduction
Large spoil slopes are created during construction projects due to extensive engineering construction in China, resulting in increased risks of soil erosion, landslides, collapses, and other disasters. Landslide hazards in abandoned dreg sites pose a high risk. Rainfall and groundwater contribute to slope damage. Rainfall-induced landslides are caused by groundwater recharge and the advance of the wetting front due to rainfall infiltration. Changes in moisture disrupt the mechanical equilibrium [1]. Vegetation influences soil mechanics, soil suction, and hydrology by reinforcing the soil structure and decreasing the permeability coefficient [2]. Numerous studies have demonstrated that a combination of factors causes landslides [3,4]. Although engineering measures can prevent landslides, they are costly and often incompatible with sustainability. Root reinforcement presents a viable alternative for preventing shallow landslides and diminishing slope instability. Numerous researchers have integrated root reinforcement with effective vegetation management and engineering strategies to mitigate shallow landslides and enhance slope stability [5,6,7]. However, the hydrological response mechanism of the root–soil composite to rainfall and the effect of root reinforcement remains unclear, complicating the evaluation of slope stability in complex environments.
Previous research has focused on the influence of single factors on slope stability. Most studies have analyzed some rainfall patterns (uniform, antecedent, and delayed rainfall) and the effects of changes in rainfall parameters, including rainfall threshold, intensity, and duration, on the stability of unsaturated slopes [8,9,10]. However, long-term rainfall intensity can cause significant errors if the ponding depth is considered. Therefore, studies have investigated surface runoff and subsurface seepage. For example, numerous studies have coupled the surface flow equation and the groundwater flow equation to analyze the sensitivity of the influencing factors of infiltration [11]. A multilayer creep–tension mechanism was proposed to analyze slope stability. The combined effect of rainfall and groundwater on multiple weak layers on the slopes was assessed [12]. Furthermore, the slope angle is closely related to the soil’s internal friction angle under rainfall conditions. The larger the slope angle is, the more likely the slope is to collapse rapidly [13].
The mechanical reinforcement effect of plant roots plays an important role in slope stability. The root system can change the soil structure and improve the slope’s mechanical behavior by reinforcing the control of shallow soil movement [14,15]. Researchers usually quantify the root mechanical reinforcement effect through experimental research, theoretical calculation models, and numerical simulations [16]. The Wu model incorporates the root reinforcement effect [17]. The embedded beam element (EBE) model considers root pull-out and fracture damage modes in modeling the root–soil interaction [18]. The fiber bundle and root bundle models consider the asymptotic damage of roots and stress redistribution [2,19]. Researchers have developed a three-dimensional model to assess the effect of root morphology on soil stability [20]. An elastomer was utilized to substitute the root–soil composite. Slopes with plants with different root morphologies showed different mechanical properties. Furthermore, the plant configurations and their spatial variations affect slope stability. Researchers proposed a slope stability model for the regional distribution of shallow landslides, considering the influence of plant roots by analyzing local plant species and their spatial variations [21,22]. In addition to mechanical reinforcement, the root system changes the hydrological response of the soil. Matric suction increases due to the root uptake of water during transpiration and water retention after rainfall events, thereby contributing to slope stability [23]. Various factors affect root water uptake rates and hydraulic properties, including soil properties and hydraulic resistance. Researchers have analyzed rainfall seepage on unsaturated vegetated slopes using Green’s function and water–force coupling [24].
Although considerable work has been conducted to research the effects of a single factor, such as rainfall or vegetation, the coupled influence of rainfall and root reinforcement on slope damage remains unclear. Analyzing a single rainfall factor can improve our understanding of the rainfall-induced landslide mechanism. However, few studies have used the unsaturated seepage–stress theory for practical applications and synthesized hydrogeological, topographic, geomorphological features, and physical properties to study the destabilization of spoil slopes and the potential for landslides. The effect of root reinforcement and water absorption should be considered in slope stability models. The dynamics of the root–soil composite under the coupled effect of rainfall and root reinforcement remain to be addressed. Therefore, it is necessary to evaluate the landslide mechanism under complex multi-physical field coupling accurately.
This study analyzes an abandoned dreg site in Western Sichuan. The effect of root reinforcement on soil shear strength is quantitatively analyzed by direct shear tests. The seepage–stress theory was used to simulate the instability of unsaturated slopes in different restoration stages under the coupled effect of rainfall and root reinforcement. The water migration in the spoil, overburden, and vegetated slopes are comprehensively analyzed to elucidate the influence of root reinforcement on the slope safety factor. Field tests are performed to verify the reliability of the numerical model and method. The influences of the slope angle, rainfall amount, and vegetation cover thickness on slope stability are investigated. The results provide new insights to enable quantitative slope stability evaluations to prevent disasters and manage abandoned dreg sites.
5. Conclusions
This paper analyzed abandoned dreg sites in West Sichuan Province, China. The root reinforcement mechanism of the root–soil composite was quantified in terms of mechanical characteristics. According to the seepage–stress theory of unsaturated soil, the coupled effect of rainfall and root reinforcement on slope instability in different restoration stages was numerically investigated and verified by field measurements and tests. The influences of the slope angle, rainfall parameters, and vegetation parameters on the safety factor of the vegetated slopes were determined. The main conclusions are as follows:
(1) Root reinforcement enabled the soil to resist deformation and stress. The root system significantly increased the shear strength of the root–soil composite. Direct shear tests showed that the cohesion of the root–soil composite (crs = 33.25 kPa) was 177% higher than that of the engineering spoil (ces = 12 kPa) and 32.21% higher than that of the overburden soil (cos = 25.15 kPa).
(2) The plant roots improved slope stability by increasing the soil’s shear strength and decreasing the infiltration rate. The root system improved the slope’s mechanical strength. The safety factors of the spoil, overburden, and vegetated slopes were 1.741, 1.763, and 1.784 before the rainfall and 1.687, 1.720, and 1.763 after the rainfall, respectively. The reduction in the safety factor for vegetated slopes is 61.11% and 51.16% less than for spoil and overburden slopes. The slope safety factor met the code requirements, and the field test verified the model’s accuracy.
(3) The slope angle significantly affected slope stability. It decreased nonlinearly with an increase in the slope angle. The slope safety factor decreased significantly as the rainfall intensity and duration increased. After the restoration of the spoil slope, the slope should have an angle of less than 30°, and the cover thickness should be 0.5 for herbaceous vegetation and shrubs and 1.0 m for trees.
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