Study on the Mechanism of Loess Landslide Induced by Chlorine Salt in Heifangtai Terran

Japanese Geotechnical Society Special Publication 7th Asia-Pacific Conference on Unsaturated Soils (AP-UNSAT2019) Study on the mechanism of loess landslide induced by chlorine salt in Heifangtai terran Juan Wang i ,ii), Wei Liu i ,ii), Wenwu Chen i ,ii), Peng Liu i ,ii) , Bobo Jia i ,ii), He Xu i ,ii) , Li Wen iii) i) Department of Geological Engineering, Lanzhou University, Tianshui Road, 222, Lanzhou 730000, China ii) Key Laboratory of Mechanics on Disaster and Environment in Western China, Ministry of Education, China iii) School of Information Science and Technology, Lanzhou University, Lanzhou 730000,China ABSTRACT It is well known that water content is the key factor affecting the loess strength. In arid and semi-arid regions, the salt content in pore solution of loess is relatively high. Rainfall or irrigation can change the water content of loess and induce geological disasters such as loess landslide and loess mud flow. In this paper, Heifangtai, Gansu Province, is chosen as the study site, where is the most typical place of loess landslides. The occurrence of loess landslides is closely related to water, salt. In order to investigate the interaction rules of these three factors, a series of pressure plate apparatus tests and undrained shear tests were conducted on saturated loess, to investigate the degradation mechanism of loess strength caused by the interaction between water and salt.The results showed that: NaCl concentration had a significant effect on the matrix suction, i.e. the water retention capacity of loess sample increased as the increase of NaCl concentration,especially in the boundary effect zone (low suction zone, soil almost saturated), concerning the remolded loess samples having a same dry density.The increase of NaCl concentration in pore water can also lead to the decrease of shear strength of saturated loess, especially for the cohesion. The results of Scanning Electron Microscopy (SEM) tests gave a microscopic explanation for the above results. In conclusion, the enrichment of salt leads to the increase of water holding capacity of approximately saturated loess, which is closely related to the decrease of undrained shear strength in saturated conditions. This will help us to understand the mechanism of loess landslide in this area. Keywords: NaCl, Loess landslide, chemomechanical behaviors, SWCC, sensitivity 1 INTRODUCTION the most typical areas of irrigated loess landslides, it is known as "the microcosm of the loess plateau landslide The chemomechanical behaviors of porous media geological disaster", and has been attracted by wide is a major topic in soil mechanics (Nitao and Bear,1996; attention of scholars (Meng et al., 1998;Xu and Dai, Lu and Likos, 2006;Wei, 2014;). Pore water in soil is a 2014; Xu and Peng, 2016; Fan and Xu,2017; Xu and complex chemical solution. As an active and sensitive Coop,2017). Another characteristic of this area is the factor in soil, the chemical composition of pore water is high salt content in the loess pore solution (Chen et al., crucial to the mechanical strength of soils. The effect of 1999), as located in arid or semi-arid area. The decrease pore water chemistry on the mechanical strength of soil of saturated loess strength induced by irrigation is has been investigated widely by previous studies. (Di closely related to the migration and enrichment of Maio and Fenelli,1994; Tiwari et al.,2005;Naeini and soluble salts in loess. The chemomechanical properties Jahanfar, 2011; Thyagaraj and Rao,2013; Ismeik and of loess in this area have been studied in previous Ashteyat, 2013; Zhang and Sun,2016; Ajmera and studies. For example, Li and Xu (2017) analyzed time Tiwari,2018). In the above studies, it has been found effect characteristics of loess strength weakening by that as the salt concentration of pore water solution water, and discussed the influence of soluble salt alters, the surface potential of soil particles will alter content on the shear strength, collapsibility and other accordingly, which led to the changes in the thickness strengths of loess. Zhang et al.(2013) investigated the of the diffusion double layer and in the strength of effects of NaCl concentration on pore water and saline soil. desalinization on the shear behavior under undrained Heifangtai in Gansu Province (Figure 1), as the conditions by a series ring-shear tests on saturated resettlement area of Liujiaxia Reservoir, has suffered loess. from frequent loess landslides caused by large-scale The shear strength is the core index of loess agricultural irrigations, which has caused serious losses landslide stability study (Stark 2013), which is affected to the lives and property of local residents. As one of https://doi.org/10.3208/jgssp.v07.024 159 by loess density, water content, mineral composition, and Wei,2017). Within this context, to shed new soluble salt content and gradation. Among all factors, insights into the coupling effects of water-salt-strength, water content, salt type and concentration are we investigated the influence of NaCl concentration on important. the loess shear strength, based on the results of NaCl Due to the long-term drought and lack of water in concentration on loess water retention, and analyzed the heifangtai area, agricultural irrigation mainly depends possible mechanisms for the occurrence of loess on pumping river water from the Yellow River. After landslides. that, irrigation water infiltrates through dominant channels such as joints, fissures and falling water 2 STUDY SITE tunnels in the loess, resulting in the rise of groundwater The study area is located in Yongjing County, level in the deep soil layer (Xu and Dai et al.,2011a, Gansu Province (Figure 1). In terms of geological 2011b; Xu and Peng et al., 2016). Water content has structural unit, it belongs to the Qilian Mountain been identified as a key factor in influencing the shear geosyncline of the Qin-Qi-Kunlun geosyncline system. strength of loess, for the inherent water sensitivity of Strong tectonic activities destroyed the integrity of loess (Zhang et al,2016).Water infiltrates through the strata, surface water is easy to infiltrate and make the pores of the loess to a relatively impervious bed and self-weight of slopes increase. At the same time, the causes the accumulation of groundwater level to rise, river gradient of each river system in the basin leading to the increase of loess saturation around the increases, the erosion datum level decreases and the bed and the decrease of matrix suction. As the river cuts down, creating a favorable free surface for saturation of loess increases, a significant shrinkage the formation of landslides. phenomenon occurs because of the aggravation of plastic deformation. As a result, the soil pore water can’t be drained smoothly and pore pressure increases sharply to form static liquefaction. It causes a softening zone at the bottom of the loess. Meanwhile, undrained shear failure occurred in a short time, resulting in a sharp increase in deformation. A whole sliding forms as the sliding surface penetrates. This is the general mode of loess landslide failure in this area. Salinity moves dynamically with water(rainfall or irrigation), and accumulates in the vicinity of the slip zone. So the loess are filled with highly saturated saline.Thus, the relationship between the Fig. 1 (a) Location of study site (b) Salt deposition in this study chemomechanical behaviors of loess and the formation site of landslides is worth further study. However, previous studies have focused on the unsaturated soils with low The base of terrace IV where Heifangtai located, is saturation, for which the electrochemical potential of the Lower Cretaceous Hekou Group,with the loose pore fluids is affected shiftly by osmosis capillarity and strata of the Upper Pleistocene overlying it (Wang and adsorption. And only a few research studies have been Wu etal,2004;Xu and Peng etal,2016) (Figure 2). performed to focus on the chemoplasticity of fully saturated soils (Huechel, 2002; Loret et al., 2004; Witteveen et al., 2013). As a matter of fact, landslide failure occurs principally under the condition of high saturation, thus, there is a clear need to develop the work in this field. On the other hand, soil-water characteristic curve (SWCC) is an crucial tool to describe the flow and mechanical behavior of unsaturated soil, and becomes the link between the microscopic chemomechanical interaction and the macroscopic properties of soil(Fredlund and Xing ,1994; Thyagaraj and Rao, 2010), which can help us to understand the influence of Fig.2 Salt deposition during irrigation and lithological profile chemical composition in soil pores on water retention of Heifangtai area performance.It has been found that the increase of salt content will lead to the increase of saturation in the The Lower Cretaceous Hekou Group is composed boundary effect region from SWCCs of saline soils(Ma of brownish red, fuchsia thick or thick silty mudstone and Lin,2013; Ma and Wei,2015;He and Ye,2016; Ma with purple lime, purple-red sandstone and argillaceous 160 siltstone. The Upper Pleistocene strata are composed of relative to the slip body, the loess in the slip zone has alluvial sandy gravel, silty clay and aeolian loess. little residual soluble salt after repeated leaching, which Aeolian loess covers the fourth terrace. The Aeolian basically meets the test requirements. Loess in Heifangtai is typical gray yellow named Malan We used remolded loess samples matched with with uniform structure, porous porosity and vertical different concentrations of NaCl solution , the concrete joints, which softens rapidly in the presence of water. preparation scheme is as follows: Concentration of The general situation is that loess in east is thicker than NaCl solution: As is known that the saturated that in the west, the loess in the south is thicker than concentration of NaCl solution is 5.43 mol/L, our target that in the north.

Load more