Agricultural irrigation has increased the groundwater level in the Heitai terrace (part of the Heifangtai terrace) by 20 m over nearly five decades, which causes 3–۵ landslides each year at the edge of the terrace. The Heitai terrace is of great interest in the study of loess-related landslides; but there is no unanimous agreement on the types of either the landslides in this study site or the loess-related slope failures in general. On the basis of aerial images (res. 5 cm), Digital Elevation Model (res. 10 cm), and field investigations, we analyzed the distribution and failure mode of the landslides in Heitai. The geological structure and characteristics of 69 landslides (vol. 5 × ۱۰۳ –۶ × ۱۰۶ m3 ) are studied. The preliminary results of groundwater recharge in the terrace and formation of the apparent spring lines on the slope surface are analyzed to better understand the failure modes. We divided the landslides in Heitai into two groups based on the location of the failure surface, i.e. loess landslide and loessbedrock landslide, of which the development is governed by the angle between the principle direction of slope deformation and the dip of bedrock bedding. We further analyzed the failure mode of each type observed in Heitai, defined as follows: loess-bedrock planar slide, loess-bedrock irregular slide, loess flowslide, loess slide, and loess flow. The proposed types of loess-related landslide are to be incorporated in the Varnes classification (Varnes, 1978) in consideration of the engineering properties of loess, and to provide backward compatibility for Heitai and potentially other regions in the Loess Plateau of China.

Introduction

The Heitai terrace is formed by the fourth river terrace of the Yellow River in Gansu Province, China (Fig. 1a). More than 50 gullies are formed at the edge of the terrace. The terrace has a surface area of 9 km2 with the maximum length of 5.2 km (W–E) and a width of 2.5 km (NeS). It is part of the Heifangtai terrace with high concentration of loess-related landslides due to surface irrigation started in 1968 (Derbyshire, 2001; Peng et al., 2016; Zeng et al., 2016; Zhou et al., 2014). Loess slope failure is common upon wetting due to its high porosity and collapsibility (Dijkstra et al., 1994; Jiang et al., 2014; Liu et al., 2016; Smalley et al., 2001; Wu et al., 2017; Yuan and Wang, 2009). Irrigation provides 4–۶ cycles of groundwater recharge annually, which has increased the groundwater level by 20 m at an average rate of 0.18 m yr−۱ (Xu et al., 2014) and caused about 200 landslides along the edge of the terrace (Fig. 1b). Study on the loessrelated landslides in Heitai sustained its popularity due to frequent failures and significant societal impacts. Landslide classification system forms a long-lasting research topic (Alimohammadlou et al., 2013; Casagrande, 1940; Cruden and Varnes, 1996; Görüm et al., 2011; Hungr et al., 2011; Hungr et al., 2014; Hutchinson, 1988; Qi et al., 2010; Varnes, 1978; Wang and Zhou, 1999; Xu et al., 2016). In many cases, defining the landslide type serves as a ‘baseline’ of the problem because it reflects the landslide material and behavior (Hungr et al., 2014). Landslides in loess provide a unique collection of landforms and failure mechanisms that are often undervalued in the global classification systems. Loess is recognized as a unique material, created by aeolian deposition in the Loess Plateau of China, comprising predominantly silt-sized particles that form cemented meta-stable structures with characteristically high void ratios. Landslides in loess are often generalized as a single type of landslide based on the material composition, such as loess flow by Varnes (1978), and loess flowslide by Hungr et al. (2014), under the category of earth flow. In the context of landslide behavior, loess-related slope failure was divided into different types based on either the movement and failure mechanism or the location of the failure surface.