A chemo-mechanical insight into the failure mechanism of frequently occurred landslides in the Loess Plateau, Gansu Province, China

Highlights

• Frequent landsliding in the Heitai loess terrace, China, triggered by the extensive irrigation

• Role of long-term saturation and pore fluid chemistry changes in controlling the loess strength.

• Chemical analyses, oedometer and ring shear tests

• Pore ion concentration drop causes shear strength loss in the clay component and volume decrease.

Abstract

Thick loess deposits are considered problematic soils due to their susceptibility to collapse suddenly and in large volumes upon increase of water content. In the past decades, a large number of landslides occurred in the Loess Plateau in Gansu Province, north-western China, triggered by the extensive irrigation. The increase of water content has been identified as a key factor controlling the loess strength. However, the role of long-term saturation and pore fluid chemistry changes has not been investigated systematically. This paper reports on an experimental study on loess samples from Heitai loess terrace (Yongjing, Gansu) carried out through chemical analyses, oedometer and ring shear tests. The natural pore fluid was found to be a concentrated saline solution in unsaturated soil condition. Upon saturation, the pore ion concentration drop can contribute to shear strength loss in the clay component and volume decrease. A prolonged exposure to the irrigation water might facilitate cement dissolution, which could result in further weakening.

Introduction

Loess is a wind-blown, loosely consolidated deposit, mainly consisting of lightly cemented silt-sized, angular, poorly polished grains of quartz, feldspar and mica. Clay minerals are generally not abundant, but they can sometimes exceed 30% in dry weight (Derbyshire, 2001). As a result of its genesis, loess features high void ratios – commonly higher than 1.05 – and well-developed vertical joints which facilitate the subsurface water transport (e.g. Billard et al., 1993; Derbyshire et al., 1994).

Thick loess deposits can be considered “problematic soils” due to their susceptibility to collapse suddenly and in large volumes. At high matric suctions, they show significant cohesion and can stand in nearly vertical slopes, some tens of meters high; however, they may lose most of their strength upon increase of water content, even while still being unsaturated, and be rapidly mobilized as slides, slumps or flows (Derbyshire et al., 1994; Derbyshire, 2001; Lin, 2008; Xu et al., 2012).

Although loess is widespread throughout the world, the most extensive and thickest deposits are in China, where they cover approximately 6.6% (631,000 km²) of its total area. Among them, the Loess Plateau of north-western China is the largest one, covering several provinces: Gansu, Ningxia, Shaanxi, Shanxi, Henan, and Xinjiang (Liu, 1985; Derbyshire et al., 2000). In these areas, due to insufficient rainwater, cultivation of crops and afforestation has largely relied on irrigation water pumped from the Yellow River (also called Huang River). The flooding method has been commonly used, due to which large volumes of water can infiltrate to depths of tens of meters, particularly through the vertical discontinuities (Xu et al., 2011b; Derbyshire, 2001). Water infiltration is a possible cause of landslides in unsaturated soils, both through wetting of a shallow top layer due to short-term rainfall or irrigation (Brand, 1981; Anderson and Sitar, 1995; Sorbino and Nicotera, 2013; Wu et al., 2017) and through the rising of the groundwater table in the deep soil after prolonged irrigation (e.g. Dai et al., 1999; Jin and Dai, 2008). This latter process has been shown to be the cause of several landslides in the Plateau (Derbyshire et al., 2000; Wu and Wang, 2006; Tu et al., 2009; Zhang et al., 2009; Xu et al., 2011a).

The clay component plays a key role in the development of shear strength in the unsaturated zone (Derbyshire et al., 1994), and changes in water content have been identified, indeed, as a key factor affecting the loess strength (e.g. Hu et al., 2014). It has also been suggested that the chemical composition of the pore fluid may play a relevant role (e.g. Dijkstra et al., 1994). High salt concentrations in groundwater have been found in loess areas (Xu et al., 2011a; Zhang et al., 2013) and salt sinters are commonly observed at the base of the slopes (Wen and He, 2012).

Soil deposits can undergo variations of pore fluid composition due to various processes, which can result – especially if a relevant clay component is present – in significant changes of mechanical properties. Typically, a decrease of pore ion concentration can cause loss of strength and volume changes in illite and smectite bearing soils (e.g. Bjerrum, 1954; Bjerrum and Rosenqvist, 1956; Mesri and Olson, 1970; Di Maio and Fenelli, 1994; Di Maio, 1996, Di Maio, 2004, Anson and Hawkins, 1998). The pH of the pore fluid can also play a major role, as it can control surface charge and soil fabric, particularly in kaolinitic soils (e.g. Santamarina et al., 2002; Wahid et al., 2011).

The initiation and the state of activity of landslides in clay formations can be affected by changes in the composition of the pore fluid. For instance, low pore solution concentrations have been correlated to high displacement rates by Moore and Brunsden (1996) in a coastal landslide in marls in England. Tiwari and Ajmera (2015) reported shear strength decrease due to NaCl leaching in mudstone landslides in Japan. In the Chinese Loess Plateau, Wen and He (2012) and Zhang et al., 2009, Zhang et al., 2013 hypothesized that salt leaching due to irrigation can be a possible cause of landslides. In a deep earthflow in marine clays in Italy, Di Maio et al. (2015) observed a gradual ion concentration decrease in the pore solution of a clayey slope, from the depth to the ground surface, and discussed the possibility that such a decrease might be responsible of the landslide's viscous displacements. Di Maio and Scaringi (2016) and Pontolillo et al. (2016), by means of shear-controlled direct and ring shear tests, showed that a reduction of ion concentration in the pore fluid can indeed cause shear displacements with a tertiary creep pattern under constant effective stresses.

In order to get a deeper insight into the failure mechanisms of loess landslides in the Chinese Plateau, a number of samples have been taken from the Heitai loess terrace in Gansu Province, where several instabilities have occurred. A series of ring shear tests have been carried out to investigate the effect of the degree of saturation and the effect of a prolonged exposure to water on the soil strength. Oedometer tests have been carried out to study the volume change behaviour upon saturation. The chemical composition of the pore fluid has been evaluated after different durations of exposure to water, in order to highlight possible processes of ion diffusion and mineral degradation. Microstructural changes have been observed by means of Scanning Electron Microscopy (SEM).