Permeability Characteristics and Mechanism of Silicone-Hydrophobic-Powder-Modified Compacted Loess

1. Introduction

In loess filling projects, post-construction settlement and potential wetting deformation risk are typically alleviated through the remodelling and compaction of the filler. At present, large-thickness loess filling projects, such as the first phase of Yan’an New Area and Lanzhou New Area in China, are entering the operation stage. Relevant monitoring and experimental research has shown that the loess filling body with large filling thickness, complex original terrain, and local hydrogeological conditions is expected to affect the filling site in the long-term operation stage [ 1 3 ]. Owing to surface water infiltration and groundwater field rebalancing, uneven settlement of the fill site, local cracking, and collapse of excavation and fill joint frequently occur [ 4 6 ]. Considering the creep characteristics of filling loess [ 7 ] and the humidification deformation mechanism [ 8 ], the improvement of loess strength and permeability has emerged as a promising method for controlling settlement development and alleviating the risks in filling engineering. Many researchers have used microbial reinforcement technologies [ 9 ] and introduced lime, fly ash, and other additives to improve the compressive strength [ 10 ], shear strength, impermeability of loess [ 11 ], and the loess surface electrochemical properties [ 12 ]. Notably, as a porous medium, the permeability of loess is closely related to its pore structure and distribution [ 13 ]. Therefore, the optimization of the internal structure of loess by introducing additives to reduce the moisture migration in the loess and, thus, the humidification deformation of loess has attracted significant research attention. Hu et al. [ 14 ] obtained the loess water characteristic curve of lime-modified loess and predicted the unsaturated seepage coefficient. The unsaturated seepage coefficient of the loess with lime was smaller than that of compacted loess without lime. Zheng [ 15 ] performed an infiltration test to determine the optimal content of lime and fly ash for improving the impermeability of loess. Gao et al. [ 16 ] and Zhang et al. [ 17 ] reported that the impermeability of fly ash- and lime-improved loess is attributable to the hydration, filling, and adsorption of free water of fly ash and lime.

In large-thickness loess filling projects, the exhaust conditions in the consolidation of unsaturated compacted loess directly affect its settlement development [ 18 ]. The improvement of remolded loess, such as by the addition of lime, cement, and fly ash, decreases the water circulation ability by blocking the connected pores of loess. This phenomenon changes the basic physical and mechanical properties of loess, with the effect being similar to that of curing, and the corresponding environmental impacts are difficult to estimate. Furthermore, the loess permeability is lowered and the air and water flow velocity in the large-thickness fill body is reduced, which increases the complexity of the post-construction deformation problem of the large-thickness loess fill body in the unsaturated state. To address these problems, some scholars have adopted new materials, such as nano-clay [ 19 21 ], silica [ 22 23 ], and hydrophobic curing agents [ 24 ], to reduce the permeability of loess while maintaining the permeability of its pore structure.

As a green hydrophobic material that can enhance the impermeability of materials, silicone hydrophobic agents have been successfully applied to improve cement mortar, concrete, and pavement materials. These agents can effectively reduce the capillary water absorption capacity of concrete and improve the waterproof and anti-permeability performance [ 25 26 ]. When silicone hydrophobic materials are coated on the surface of concrete specimens as a protective agent, the nature of the specimen surface changes from hydrophilic to hydrophobic [ 27 ] and the penetration depth is significantly reduced [ 28 ]. Notably, the existing research on the application of silicone hydrophobic materials for loess improvement is limited. Xu [ 29 ] applied silicone hydrophobic agents to the surface loess of the central green belt of an expressway and reported the satisfactory ground seepage control effect. Zhao et al. [ 30 ] applied silicone materials for loess slope protection. Choi et al. [ 31 ] and Haquie et al. [ 32 ] used hydrophobic materials, such as organic silane, to improve the permeability of kaolin. The authors reported that organic silane changed the particle size distribution, enhanced the adhesion between particles, and reduced the water absorption.

In general, silicone materials have been widely applied for concrete protection. However, only a few researchers have focused on the permeability characteristics of compacted loess modified using silicone hydrophobic agents. To examine the feasibility of using silicone hydrophobic materials for improving loess properties, the corresponding effect and mechanism must be understood.

Therefore, in this study, the permeability coefficient of loess with different dry densities and organic silicone contents was measured through flexible wall permeability tests. These tests quantitatively analyze the improvement effect of hydrophobic materials on loess impermeability, determine the optimal ratio of hydrophobic materials, and reveal the improvement mechanism from the perspective of macroscopic surface contact characteristics and microstructure changes. The findings can provide guidance for the anti-seepage design and risk control of various loess filling projects. These aspects can help alleviate the engineering risks associated with water erosion or internal water field changes in the loess fill site.