**1. Introduction**

With the development of social economy and the requirements of modern transportation, expressways have become a focus during city construction [1]. Different from buildings, strict requirements are considered for the foundation of expressways such as the evaluation index of the expressway (i.e., fast, security, economy, and comfort). However, many problems still exist after an expressway's construction. Firstly, the safety of the expressway is affected directly by the instability of the subgrade. Secondly, if different settlement occurs on the expressway pavement, the driving speed is reduced. Moreover, the impact force caused by slowing down the driving speed is further exacerbated by the extent of the unevenness of the expressway pavement. Therefore, a vicious circle is formed, which further affects driving safety and causes huge economic losses [2]. These problems have seriously affected the use of expressways, especially in areas in southeast coastal provinces where the economy is developing rapidly. They are located in the region of a continental monsoon climate zone with relatively high rainfall in China. Therefore, in these areas, the subgrades have been greatly affected after completion by atmospheric conditions that include rainfall and evaporation. The instability and different settlement become more prominent under the influence of wet conditions [3].

For expressways, the foundation soil is located above the groundwater table isoline and is in an unsaturated state. In the unsaturated subgrade, the volumetric moisture content increases with rainfall. Therefore, the strength of the foundation soil is minimized, and it leads to instability and different settlement of the subgrade. In order to prevent

**Citation:** Wu, Y.; Zhou, X.; Liu, J. Long-Term Performance of the Water Infiltration and Stability of Fill Side Slope against Wetting in Expressways. *Appl. Sci.* **2022**, *12*, 5809. https://doi.org/10.3390/ app12125809

Academic Editor: Bing Bai

Received: 2 April 2022 Accepted: 4 June 2022 Published: 7 June 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/).

instability and reduce different settlements after completion, a long–short pile composite foundation [4,5] is adopted. This enables piles of different lengths and stiffnesses to carry more loads with a decrease in the proportion of the load on the soil in the subgrade. Moreover, it makes up for the decrease in the soil strength caused by wetting deformation. However, the pile foundation greatly increases the cost of an expressway's construction, and the long-term performance of the pile is still unclear. The composite foundation using foamed cement banking (FCB) as a replacement is another effective treatment measure [6]. FCB is a light-weight material with low density, high strength, and low permeability. By minimizing its weight and permeability, the instability and settlement problems are effectively delayed due to the wetting deformation of the expressway pavement. However, FCB is still in the research stage. The influencing factors of its strength and permeability are still unclear. Wang [7] used a soil stabilizer to reinforce the silty clay for the subgrade. He studied the pavement's performance with a mixture composed of the best proportion, which provided the basis and reference for similar engineering applications. Apart from using a soil stabilizer, Zhao [8] proposed that the recycled fine soil of construction waste with a diameter under 5 mm could be used in expressway subgrade. The soil was validated as a good subgrade material through basic experiments. However, the existing improvements were all treatment methods based on materials without considering the wetting distress on the long-term performance. The construction cost has increased because of these methods, and long-term performance under wetting and drying cycles is unclear. Hence, we propose effective measures to block the infiltrating rainwater in the subgrade through a capillary barrier.

A capillary barrier has been widely used in landfill cover systems, because of its advantages of long service life, ease of construction, and environment conservation [9,10]. It consists of fine layers over coarse layers, which are composed of different properties. Soil properties are very important in engineering. Different soil particles play an important role in the subgrade soil, which affect soil erodibility and the infiltration rate of water. Arunrat et al. [11] pointed out the variations in soil properties and explained how soil erodibility is affected. Wang et al. [12] showed that the different properties and the different poultry compost amendments to the soils resulted in distinct runoff, sediment yield, and soil erodibility values. With the different permeability coefficients of these two soil layers under unsaturated conditions, the fine-grained layer, used as a buffer, can store and transfer infiltrating water effectively [13]. Ross [14] and Stormont [15] derived the evaluation index of capillary barriers through the theory of the permeability coefficients of two soils. The evaluation index is the distance downslope, called the diversion length. It has been verified that a capillary barrier can effectively block the infiltrating rainwater within the diversion length [16,17]. Rahardjo [18] proposed that a capillary barrier could be used to protect the slope. The waterproof effect of a capillary barrier was verified through field tests and through the same way the application of a capillary barrier to the slope was studied and found to be feasible. Wu et al. [19] proposed that a capillary barrier could be applied as a cover barrier to protect the expressway subgrade from wetting distress. Previous studies focused on the effect and mechanism of a capillary barrier applied to landfills in dry climates. However, the performance of a capillary barrier in humid climates is still unclear. In other words, the performance of a capillary barrier applied to a slope shoulder is still unclear in humid climates in China's southeast coastal areas. Meanwhile, the environment of expressways in the southeast coastal areas of China is generally humid compared with the relatively dry landfill cover system. Due to the presence of these peculiarities, it is necessary to verify the rationality of the application of the capillary barrier on the expressway.

This paper firstly explains the mechanism of moisture migration in the unsaturated subgrade and proposes effective measures to block the infiltrating rainwater in the subgrade through capillary barrier. Then, the anti-seepage effect of the capillary barrier is introduced. Comparing the numerical simulation results of the subgrade with or without a capillary barrier, the feasibility of applying a capillary barrier to the subgrade was verified. Finally, the results were evaluated and focused on the role of the capillary barrier in the antiseepage performance by simulating their changes in moisture content, pore-water pressure, settlement, and safety factor of the subgrade under rainfall conditions in the southeast areas of China.
