Influence of lixiviation by irrigation water on residual shear strength of weathered red mudstone in Northwest China: Implication for its role in landslides' reactivation
Highlights
► Lixiviation by irrigation water reduced residual strength weathered red mudstone. ► Reduction in residual strength of the soil was mainly led by decrease in φr′ ► The main effect of lixiviation was disaggregation of particles bonded by soluble salt. ► Results of this study clarified one of the roles of irrigation water in landsliding.
Introduction
Under thick loess deposits in northwest China, the predominant bedrock consists of a sequence of Cretaceous or Neogene red clastic strata (Wang et al., 1991; Derbyshire, 2001). This sequence comprises interbedded sandstone and mudstone horizons, and contains a significant amount of soluble salts as a result of being deposited in an arid lacustrine environment (Wang et al., 1991). Due to its weak resistance to weathering and impermeable nature, red mudstone horizon of the sequence underlying loess deposits is often highly to completely weathered into a wet to very wet and soft clayey soil (Figure 1). The presence of this weak horizon (weathered red mudstone, WRM) makes the overlying loess slopes prone to sliding along bedding plane of this horizon or its contact with loess (Derbyshire, 2001).
Over the loess covered area of northwest China, cultivation of crops and artificial afforestation for the improvement of ecological environment have largely relied on irrigation water pumped from the Yellow River due to insufficient rainwater. The cultivated and afforested areas have been irrigated using conventional flooding method, which facilitates much of water to infiltrate down to a depth of a few to tens of meters, particularly where cracks, pipes occur in loess (Derbyshire, 2001, Xu et al., 2011). As a consequence, irrigation has induced or reactivated numerous landslides, including those sliding along the contact between loess and the WRM or its bedding plane (Derbyshire, 2001, Xu et al., 2011). Salt sinters observed at the toes of many landslides where the WRM outcrops (Figure 1) suggest that lixiviation of soluble salts in the WRM by irrigation water may have played one of the key roles in reduction of shear strength of the landslides' slip surfaces. Clarifying this role of lixiviation is of great significance both for better understanding mechanisms of landslides related to the WRM and for stability evaluation of such landslides.
Shear strength reduction of clayey soils due to lixiviation by irrigation water is associated with physico-chemical interactions between the soils and water. For tens of years, considerable attentions have been given to such interactions and their effects on clayey soils' shear strength (e.g., Kenney, 1967, Rosenqvist, 1984, Di Maio and Fenelli, 1994, Anson and Hawkins, 1998; Gajo and Maines, 2007; Spagnoli et al., 2010). These studies unanimously confirmed that such physico-chemical interactions are complex and mainly controlled by both clay mineralogy and pore water chemistry, including pore water's salinity, pH, compositions and concentration of ions. Shear strength of bentonite was found to be very sensitive to pore water chemistry (Di Maio and Fenelli, 1994, Di Maio, 1996, Kenney, 1977), while that of kaolinite was observed to be insensitive to pore fluid salinity (Di Maio and Fenelli, 1994, Kenney, 1977, Anson and Hawkins, 1998, Wahid et al., 2011). Some studies showed that residual frictional angle of clays increased after salt concentration of pore water was incremented (Kenney, 1967, Moore, 1991, Di Maio and Fenelli, 1994, Di Maio, 1996, Anson and Hawkins, 1998). With respect to natural clays, Moore (1991) documented residual shear strength reduction of the weathered Weald clay and London clay when pore water was diluted. Tiwari et al. (2005) reported that residual frictional angle of the weathered Japanese coastal mudstone increased when treated with sea water and decreased when leached with distilled water. Effects of clayey soil–water interactions on the soils' shear strength due to variation in pore water's salinity were generally considered to occur through both fabric changes at particle level and variation in interparticle forces among clays (Kenney, 1967, Kenney, 1977, Mitchell, 1991, Moore, 1991, Di Maio and Fenelli, 1994, Anson and Hawkins, 1998, Tiwari et al., 2005). Tiwari et al. (2005) noticed that bentonite mixed with sea water contained less fines than that mixed with distilled water. Some field observations substantiated that a decrease in pore fluid salinity leads to a reduction of residual shear strength of landslides' slip surfaces and subsequently to landslides' reactivation (Moore and Brunsden, 1996, Tiwari et al., 2005, De Montety et al., 2007).
With the intention to clarify the effect of lixiviation by irrigation water on shear strength of the WRM in loess area of northwest China, particularly in the view of landslide occurrence, the shear strength of the WRM very close to two reactivated landslides in Lanzhou city was experimentally investigated in this study. The WRM's shear strength was tested after saturation with the Yellow River water, groundwater and deionized water after being leached several times. Furthermore, possible mechanisms of shear strength reduction of the WRM caused by lixiviation were discussed in terms of clayey soil–water physico-chemical interactions. Given that the investigated landslides were reactivated, this study focused on residual shear strength of the WRM.
Section snippets
Study area
Lanzhou city, the capital city of Gansu province, is one of the biggest cities in northwest China. In terms of geology, the city is well-known for its thick loess deposits and hundreds of landslides, about 30% of which involved the bedrock consisting of weathered red mudstone (WRM) and sandstone of Cretaceous or Neogene age (Ding and Li, 2009). In this study, the WRM samples were collected from outcrops of the WRM unaffected but very close to the two landslides (Wenchangge and Xujiashan) in
Shear testing and sample preparation
To explore the influence of irrigation water on residual shear strength of the WRM, the samples were saturated separately with groundwater, the Yellow River water and deionized water prior to shearing specimens of these samples. On the other hand, the effect of repeated irrigation on shear strength of the WRM was examined by leaching additional specimens using deionized water prior to shear testing. As the Yellow River water was nearly neutral with very low salinity, deionized water was used as
Results
The representative results of residual shear tests of samples WCG and XJS are given in Fig. 3. The results clearly demonstrated that WCG needed longer displacements to reach residual state than XJS, and that residual shear strength of WCG at each level of normal stress was lower than that of XJS. This could be mainly accounted for WCG's greater fine fraction, particularly clay, and lesser coarse fraction (Table 1). Although residual shear strength envelope of clayey soils may be nonlinear, many
Discussion
As negligible pore pressures were presumed to be induced during shearing at very slow rate of displacement (0.02 mm/min), variations in residual shear strength of the WRM with different pore fluids and leaching times were largely governed by water–soil physico-chemical interactions.
Examination of post-leaching fluid revealed a decreasing concentration of major ions with leaching cycles (Table 6). The ions leached out of the samples were primarily Na+, Cl− and SO42 −, with minor Ca2 + and Mg2 +.
Conclusions
This experimental investigation clearly demonstrated that lixiviation by irrigation water could significantly reduce residual shear strength of the WRM, which is the dominant bedrock in Lanzhou city, one of the biggest cities in northwest China. This gives an implication that such effect may play at least one of the important roles in reactivation of the landslides related to the WRM in the area. The following specific conclusions can be drawn from this study:
- (1)
Soluble salts in the WRM were
Acknowledgments
This research was funded by the National Natural Science Foundation of China (No. 40872182). The authors would like to express their heartfelt gratitude to Dr. A. Aydin of University of Mississippi for his extensive review on this manuscript both technically and editorially, which helped a lot to improve the quality of the manuscript. We also appreciate the constructive comments from two anonymous reviewers and editor that were valuable for improving this manuscript. The authors are thankful to
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