**1. Introduction**

Large-scale mountain excavation and city construction project in the Loess Plateau of China not only brings grea<sup>t</sup> opportunities for development, but also risks of slope disasters and safety hazards [1,2]. Loess is loose in texture and easily softened by water [3]. Rainfall is a common source of surface water, which may infiltrate along the interface between loess fill slope and original slope, affecting the stability of loess fill slope in mountain excavation and city construction project [4,5], triggering slip and instability (Figure 1). Filling body is the main part of city building project, and is the most important part of the whole project; its strength directly affects the success of the whole project. Therefore, the stability of filling slope seriously affects the safety construction and operation of the project, and it is more important to study the interface effect and instability mechanism between fill slope and original slope under rainfall [6].

For the filling slope, predecessors have done a lot of research. Day [7] studied the filled slope and concluded that the slope was unstable due to the loading of the top of the slope and the shallow slope was not cleared in time. The damaged slope can be repaired by removing the sliding soil and building a retaining wall. Cheuk et al. [8] discussed the characteristics of soil nailing in fill slope by numerical simulation. The results show that the soil nailing structure can reduce the deformation of the loose fill slope caused by rainfall infiltration, so as to maintain the stability of the slope. Zhang [9] carried out the centrifugal model test of loess high fill embankment to study the development process and distribution

**Citation:** Tan, W.; Huang, Q.; Chen, X. Physical Model Test on the Interface of Loess Fill Slope. *Land* **2022**, *11*, 1372. https://doi.org/10.3390/ land11081372

Academic Editors: Matej Vojtek, Andrea Petroselli and Raffaele Pelorosso

Received: 5 July 2022 Accepted: 16 August 2022 Published: 22 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of embankment settlement. The results show that the settlement in the middle of the embankment is greater than that of the shoulder, and the stability of the embankment slope will be significantly reduced when the slope rate is too large or the construction speed is too fast. Duan et al. [10] took the high fill slope as an example, the deformation and stability were analyzed by finite element method and limit equilibrium method, and compared with the monitoring data in construction. It was found that the stress and displacement results calculated by finite element method were in good agreemen<sup>t</sup> with the field monitoring data, indicating that the combination of finite element calculation and monitoring analysis can guide the construction and reinforcement of high fill slope. Zhao et al. [11] divided the rainfall infiltration process of the newly filled slope into three stages: free infiltration, scouring infiltration and stable infiltration. During rainfall, the slope top is mainly vertical expansion and contraction deformation, while the slope surface is mainly lateral free surface displacement. Wang et al. [12] combined the relative displacement sensing technology and GSM technology to monitor the fill slope near an airport, and the monitoring results can be fed back to the monitoring station in real time, which successfully warned the collapse of the monitoring point. The interface between the original slope and the fill slope will inevitably be generated in the process of mountain excavation and city construction project, and sliding failure may occur due to the different strength properties of the soil on both sides. Through indoor physical model tests, Chang et al. [13] monitored and analyzed the hydromechanical parameters of the loess fill slope, simulated the failure mode of the loess fill slope, and proposed engineering measures to prevent and control the instability of the loess fill slope.

**Figure 1.** Loess filling slope disaster. (**a**) Local cracking of filling subgrade, (**b**) filling slope top cracks, (**c**) local slip of fill slope and (**d**) collapse of channel fill slope.

Rainfall can induce slope failure [14–16], and more attention should be paid to the effect of rainfall on loess filled slope. Through a series of laboratory slope failure tests, Tohari et al. [17] recorded the hydrological response of the model slope to the saturation process by using the volumetric soil moisture sensor, and proposed the concept of slope failure prediction method induced by rainfall. Saadatkhah et al. [18] used the instantaneous rainfall infiltration and grid-based regional slope stability analysis model, combined with spatial rainfall distribution model, found that local daily rainfall is not the only factor affecting slope stability, and long-term early rainfall may play a certain role in the formation of slope failure mechanism. Hakro [19] believed that the failure of a slope is caused by the increase of water content and pore pressure through indoor rainfall model test, and

the pore pressure will increase sharply in the failure process. Gallage et al. [20] studied the influence of slope inclination on slope stability by artificial rainfall test. The results show that the slope is more prone to sudden collapse with the increase of slope angle in the process of rainfall. Model test is an important means to study the deformation and failure mechanism of slope affected by rainfall [21]. Therefore, a physical model test can be carried out on loess filled slope.

In summary, previous scholars have studied the deformation and failure process, instability mechanism, seepage field and stability of fill slope by using physical model test, field test, field monitoring, numerical simulation or a combination of multiple methods, and achieved a series of important results. However, there is a little research on the interface effect between the loess fill slope and the original slope, and the research on the variation of water content and pore pressure at the loess filling interface and the stability of the slope under rainfall conditions is not deep enough. In this paper, the variation of hydrological parameters of original slope and fill slope caused by rainfall infiltration and some understandings of the interface effect of loess fill slope are obtained by physical model test. The deformation characteristics and failure process of loess fill slope caused by rainfall are also summarized. The research results can provide reference for the deformation and stability of fill slope in mountain excavation and city construction project.

#### **2. Experimental Design**

#### *2.1. Experimental Flume*

A rigid model box with transparent organic glass on both sides is selected for physical model test, and the front end is a water tank (Figure 2a,b). The size of the model box is 3.2 m × 1.4 m × 1.5 m and the size of the water tank is 1.4 m × 0.5 m × 0.3 m.

**Figure 2.** Schematic diagrams of the experiment design. (**a**) Original slope cutting, (**b**) model stereogram, (**c**) model side view, (**d**) slip zone model-Plastic mesh, (**e**) sensor embedding and (**f**) layout of slope tracing points-Plastic mesh.
