Research PaperStress-dependent water retention of granite residual soil and its implications for ground settlement
Introduction
Subgrade soil settlement beneath pavements is a major concern for engineers. Previous studies have shown that any nonuniform deformation of the subgrade soil may cause crack propagation and affect the performance of the pavement (Brown, 1996). Subgrade soil is normally unsaturated due to seasonal variations in the soil moisture content (Yang et al., 2008, Ng et al., 2013, An et al., 2018). Similar engineering problems also occur in other earthen infrastructures, such as slopes, river dikes, and excavations (Kuriqi et al., 2016, Muceku et al., 2016, Ng et al., 2020b; Qin et al., 2020, Qin and Chian, 2020). The strength and deformation of unsaturated soil is governed by two stress state variables comprising the net normal stress (difference between the total stress and pore air pressure) and matric suction (difference between the pore air pressure and pore water pressure) (Fredlund and Xing, 1994). In early studies, the mechanical component that describes the stress–strain relationship was not coupled with the saturation-suction relationship (hydraulic component). However, many recent theoretical and experimental studies have demonstrated that the mechanical and hydraulic components should be fully coupled in order to scientifically describe the deformation behavior of unsaturated soils (Wheeler et al., 2003, Sheng et al., 2008, Sheng et al., 2004).
The matric suction from contractile skin provides a resistance to maintain the stability of the particle system in contrast to the sliding action of external stress (Chai and Khaimook, 2020). As the water content decreases in the soil, the matric suction increases because shrinkage of the pore water decreases the curvature radius of the meniscus. The relationship between the soil water content and matric suction, which is known as the soil water retention curve (SWRC), is widely considered an inherent characteristic of a soil. Fig. 1 shows a typical SWRC (Fredlund and Rahardjo, 1993). The water content can be expressed by the degree of saturation and the suction is transformed into a logarithmic coordinate. The initial saturated soil gradually loses water and becomes desaturated with the increase in suction. According to the shape of the SWRC, this drying process can be divided to three stages: (i) close to saturation point in the boundary effect stage; (ii) a significant decrease in the degree of saturation in the transition stage; and (iii) an almost constant degree of saturation in the residual stage. In particular, although the water has drained from the soil, it is still saturated during the boundary effect stage. Air then passes through the largest pores between the soil particles as the suction increases to the air entry value (AEV). After exceeding the AEV, the degree of saturation continues to decrease until the residual stage is reached. According to the mechanics of unsaturated soils, it is generally considered that the SWRC is a constitutive relationship for the water phase and it plays key roles in predicting the shear strength and permeability characteristic of soils (Sun et al., 2007, Gao, 2017, Nguyen and Likitlersuang, 2019). SWRC is also an important soil hydraulic property for transient seepage analysis of unsaturated geotechnical infrastructures (Ng and Shi, 1998; Ng et al., 2020c). Therefore, many attempts have been made to describe the characteristics of soil and water using mathematical models, where most employed empirical formulae are obtained by fitting experimental data. Typical prediction models were developed by Brooks and Corey (1966), Van Genuchten (1980), and Fredlund and Xing (1994).
Previous studies have demonstrated that the water retention of an unsaturated soil is affected by many factors, such as the initial compaction water content, degree of compaction, void ratio and mineralogy. When the initial compaction water content increases, the compressibility of the aggregates increases and the soil fabric and pore size tend to become more uniform (Birle et al., 2008). The micro- and macro-pore systems cannot be easily differentiated (Delage et al., 1996). The soil has a higher desorption rate. However, the influence of the initial compaction water content on the SWRC is negligible when the water content is below a threshold value (Vanapalli et al., 1999b). Soil compaction directly affects the soil pore structure (i.e., soil size distribution, pore shape and orientation) and thus water movement in the soil (Pasha et al., 2020). Soil density increases and void ratio decreases under the higher degree of compaction. With the increase of soil density, the water retention ability of soil also increases due to the decrease in average void ratio (Gallipoli, 2012, Pasha et al., 2017). Therefore, SWRC obtained at a specific compaction state cannot be used at the other compaction states. However, soil density or soil void ratio-dependency of SWRC is not equivalent to the stress effects on SWRC and can only partially explain the stress effects on SWRC (Ng and Pang, 2000). This is because the stress not only affects the average soil void ratio, but also average pore size distribution, pore shape and orientation (Zhou and Ng, 2014).
Granite residual soil is widely distributed in tropical and subtropical areas, such as Southern China, Hong Kong, and Singapore, and it is frequently used as the subgrade soil in pavement engineering in these areas. Granite residual soil is generally classified as silty sand, sandy silt, or clay depending on the particle size distribution according to the British Standards Institution (Rahardjo et al., 2004). Recent studies found that soil minerology influences the SWRC significantly. The decrease in the content of clay and other minerals (kaolinite and illite) causes a lower water retention (Mile and Mitkova, 2002). With the high content of sesquioxides, the soil has many large-size inter-aggregate pores and hence its SWRC cannot be predicted by the “apparent” pore size distributions (Ng et al., 2020a). The main minerals of the granite residual soil are quartz, kaolinite and gibbsite (Kong et al., 2018). The formation of soil aggregates especially during the drying and heating causes higher soil shear strength. This soil is also characterized by structural collapse under wetting and significant strength reduction after disturbance (Rahardjo et al., 2004, Lan et al., 2003). Granite residual soil is treated as a special soil in China and its engineering properties are quite different from those sedimentary clay or sand (Luan et al., 2018). Even though some SWRC results have been reported for this type of soil, they are all under zero stress conditions (Heidemann et al., 2016, Ye et al., 2019). The influence of stress on water retention ability of this type of granite residual soil at different compaction states has not been investigated yet. The soil water redistribution and soil deformation during rainfall have been studied (Orense et al., 2004, Wang et al., 2010, Wu et al., 2016), but the influence of stress and the compaction state on soil water retention curves were generally ignored. In pavement engineering, it is necessary to consider the influence of the stress- and compaction-dependent SWRC on subgrade soil settlement.
The present study aimed to investigate the influence of soil compaction and vertical stress on water retention of a granite residual soil and its significance for ground settlement during both rainfall and drying events. Laboratory tests were conducted using soil specimens prepared at four degrees of compaction (DOCs) over a wide suction range (0–940 kPa) by using a stress-controllable pressure plate apparatus. The measured stress-dependent SWRCs were then used as input hydraulic parameters in order to explore their effects on the soil suction distribution and subgrade soil settlement in a flat ground by using SEEP/W and SIGMA/W. Two scenarios comprising rainfall and drying were considered in the numerical simulation. A flowchart illustrating the overall research methodology employed in this study is shown in Fig. 2.
Section snippets
Soil type and sample preparation method
The soil tested in this study was sampled from a construction site at the Guangzhou–Foshan–Zhaoqing Highway Project in China. The soil type was granite residual soil, which is commonly distributed in areas of Southern China, such as Guangdong and Hunan Provinces. A representative soil sample weighing 500 g was collected to measure the particle size distribution, according to ASTM D7928-17 (2017). Fig. 3 shows the particle size distribution for the granite residual soil investigated in this
Interpretations of experimental results
Fig. 5 shows the relationships between the VWC and suction for the granite residual soil under different vertical stresses and DOCs. Table 2 presents the fitting parameters (a, n, and m) based on the Fredlund and Xing (1994) model. As the suction increased, the specimens at zero vertical stress retained the highest VWC when the suction was less than 200 kPa, regardless of the DOC. The retained VWC was lower when the vertical stress increased. Moreover, the rate of desorption decreased as the
Numerical simulations
To investigate the influence of the stress-dependent SWRC on the suction response and ground settlement in unsaturated subgrade soils, finite-element transient seepage and coupled stress-deformation analyses were conducted using SEEP/W and SIGMA/W, respectively. For simplicity, a one-dimensional vertical profile of flat ground was selected for the numerical simulation. The suction results obtained from transient seepage analysis in SEEP/W were imported into SIGMA/W for the subsequent coupled
Influence of stress-dependent water retention on suction distribution
Fig. 9(a) shows the suction distributions with depth before and after the heavy rainfall (Case A). Before the application of heavy rainfall (381 mm/d for 1 day), the initial suction distribution was at a steady state. The initial soil suction distribution differed greatly between the conventional (without considering stress effects on SWRC) and current transient analyses (considering stress effects on SWRC). The initial steady suction in the current analysis was up to 18% higher than that in
Conclusions
The present study investigated the influence of stress and soil compaction on the SWRCs for a granite residual soil and its implications for subgrade ground settlement. A stress-controllable pressure plate apparatus was used to measure the stress-dependent SWRCs in the laboratory. Subsequently, the measured stress-dependent SWRCs were used to derive the permeability functions for transient seepage and coupled stress-deformation analyses of a typical subgrade ground settlement problem. Both
CRediT authorship contribution statement
Yongsheng Yao: Conceptualization, Methodology, Project administration. Junjun Ni: Data curation, Writing - original draft, Writing - review & editing, Software. Jue Li: Investigation.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the National Natural Science Foundation of China, grant number 51908562; Centra South University of Forestry and Technology, grant number QJ2018008B; the Research Foundation of Education Bureau of Hunan Province, China, grant number 19B581; Open Fund of Engineering Research Center of Catastrophic Prophylaxis and Treatment of Road & Trac Safety of Ministry of Education (Changsha University of Science & Technology), grant number KFJ170405 and the Hunan Provincial
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