| Landslides are frequent in the Loess Plateau, and they are mainly induced by rainfall. The sedimentologic characteristics of the loess formation, influenced by endogenetic and exogenetic processes, determine its permeability. Therefore, the rate and extend of water infiltration are controlled by loess porosity. Rainwater converges at the preferential seepage channel, permeates the slope soil rapidly, and controls the formation, expansion, and connection of potential sliding planes. However, there is limited rainfall in the loess region, and it is concentrated on specific areas with complex porosity. The sliding control mechanism for preferential flow is relatively under-researched, and traditional research methods are difficult to apply to landslide stability with preferential flow. Therefore, it is important to study the seepage characteristics of loess preferential flow and identify the preferential sliding planes for revealing the landslide mechanism under preferential flow and manage landslide risk.In the present study, we investigate the seepage characteristics of preferential flow based on the loess microscopic structure, the meso-loess mass, and the macro-loess slope. Furthermore, we propose a rapid search method for the largest shear-and-strain zone within the loess plastic region, when the loess dilatancy angle is zero, in order to determine the preferential slide plane of the loess slope. The main results of this research are outlined below:(1) We apply the percolation theory of seepage through fissured rock mass for analyzing the loess microscopic pore connectivity. We redefine the pore connectivity (p), introduce the fissure length coefficient (ω), build the double percolation model of loess, and give the mathematical expression for the threshold combination fc (n, D, p, λ, ω)= 0. The loess percolation phenomenon is connected with the formation and the spatial position, and it is closely related to loess porosity (n), fractal dimension (D), pore connectivity (p), porosity area coefficient (λ)fissure length coefficient (ω), and other factors. We attempt to apply the ARCGIS network analysis method to determine the percolation thresholds. We define the microscopic fracture connectivity (pc) and the combination of percolation thresholds. When pc≥ 0.8, percolation occurs in the fissured loess, and combination of percolation thresholds of double permeability model containing crack are as follows:(2) The tracer test on the loess mass is conducted in Nanyuan of Jingyang to reveal the relationship between the infiltration rate, infiltration depth, moisture content, porosity, and fractal parameters and their inherent laws. It is based on the calculation formula for the improved nonuniformity coefficient (Cμ) of the preferential flow infiltration depth and the nonuniformity coefficient (PFIA) of infiltration. The test results indicate the following:a) Infiltration has a significant impact on the infiltration depth distribution; the nonuniformity of the preferential flow infiltration depth becomes stronger with increasing infiltration, b) However, the nonuniformity of the preferential flow infiltration depth is less obvious when the initial moisture content is high; the development of the preferential flow is lower, its velocity is faster, the average infiltration is greater, and the fractal dimension of the wet peak trace is smaller, c) The loess porosity size also significantly influences the correlation coefficient of the preferential flow infiltration depth. The nonuniformity of the infiltration coefficient and the nonuniformity of the preferential flow infiltration depth become more obvious and the fluctuation range of depth thresholds is greater when porosity increases, d) The test scale of loess of the same type does not show an appreciable effect on the fractal dimension of the wet peak trace, e) Within the loess 0-0.5 Zmax depth range in Nanyuan, Jingyang, when the value of the fractal characteristic (y) is high, the proportion of the active field in the entire seepage field is larger, the nonuniformity of the preferential flow is higher, and the preferential flow is more developed.(3) According to the results of the case study of Nanyuan, Jingyang, a simulation study is conducted for preferential flow seepage of the loess slope based on the SEEP/W seepage software. Three slope models and four sub-models are built, particularly for the paleosol layer, to discuss its effect on the preferential flow seepage of the loess slope. The simulation results show the following:a)Irrigation strength is an important factor affecting the preferential flow of the loess slope, and the funnel flow of loess slope is generated under a premise that irrigation strength is greater than the infiltration coefficient of saturated soil. b) The irrigation duration is proportional to the infiltration velocity and the infiltration depth of the preferential flow. c) The initial moisture content of the loess slope is a sensitive factor for the preferential flow seepage of the loess slope. When the initial moisture content of the loess slope is high, the infiltration velocity of preferential flow is faster, and the preferential flow infiltration depth is greater.d) The location of the fractures also has a significant impact on the distribution of seepage fields of preferential flow inside the slope. e) The presence of a paleosol layer significant affects the seepage characteristics of the preferential flow. A lower infiltration coefficient of the paleosol layer results in a reduction of the vertical infiltration rate of the preferential flow and in an increase of the lateral infiltration rate, and it results in changes in the distribution pattern of the seepage field. In addition, the paleosol layer has stagnant water, the extent of which is related to the thickness and inclination angle of the paleosol layer. The distribution of the stagnant water increases with the thickness of the paleosol layer, whereas the amount of the stagnant water accumulated increases with the counter-inclination angle.(4) In the slopes of Nanyuan Jingyang and Huangling Yintaishan, a study is conducted on the preferential sliding plane under conditions of preferential flow seepage. It is proposed that the preferential sliding plane of the loess slope may be determined by locating the largest shear-and-strain zone in the loess plastic region for a zero dilatancy angle. Three potential sliding modes are proposed under conditions of preferential flow seepage:a) rapid sliding of the slope surface layer, which is caused when the pressure of the pore water inside the soil is increased as a result of preferential flow seepage under strong irrigation (or rain) conditions; b) overall sudden sliding of locked sections of the slope, when the irrigation (or rain) strength equals to the soil infiltration and the irrigation duration is longer; and c) sliding of the entire slope, which is caused when the creep slippage occurs to the leading edge of the slope along the preferential seepage structural plane due to increased pore water pressure and dead weight, generating continuous tension on the fractures of the trailing edge. Overall sudden sliding is the most dangerous circumstance in the case of a sudden connection of the fixed sections, which would result in high-speed loess landslides. |
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