Controlling Factors of Loess Landslides in Western China

Environ Earth Sci (2010) 59:1671–1680 DOI 10.1007/s12665-009-0149-7

ORIGINAL ARTICLE

Controlling factors of loess landslides in western China

M. Zhang Æ Jie Liu

Received: 26 April 2008 / Accepted: 9 March 2009 / Published online: 28 March 2009 Ó Springer-Verlag 2009

Abstract Based on detailed field data from recent geo- latitudes. China has the most extensive, most complete hazards surveys in the Loess Plateau region of western and thickest loess deposits in the world. Loess is mainly China, this study analyzes the key factors that control loess distributed in Shandong, Hebei, Henan, Shanxi, Shaanxi, landslides. The evolution phase of river valleys, the geo- Gansu, Qinghai, Ningxia and Xinjiang Provinces or logical structure of slopes and the geometry of slopes are autonomous regions, and covers a total area of over all found to play a role in determining the occurrence, 0.6 million km2 (Duan et al. 1993). The thickness of loess distribution and other characteristics of loess landslides. in China generally ranges from 30 to 200 m, but reaches a Groundwater and vegetation also contribute to their for- maximum of 500 m in Jingyuan, Gansu Province. The mation. A combination of human engineering activities and well-known Loess Plateau (also known as the Huangtu precipitation events are the principal triggering factors for Plateau, Fig. 1), an area in the upper and middle reaches of the instability of loess slopes. the Yellow River, has been undergoing serious soil erosion, and in consequence is one of the regions in China most Keywords Loess Landslide Western China affected by serious landslide hazards (Liu 1985). The study area—Baota District, Yan’an City—is located in the middle reaches of the Yellow River, in the central Introduction part of the Loess Plateau (36°110N–37°020N, 109°140E– 110°070E) (Fig. 1). The north–south length of this area is Loess is a windblown under-consolidated deposit formed 96 km, the width along the east–west direction is 76 km, during the past 2.5 million years in arid and semi-arid and covers an area of 3,556 km2. The topography is climatic conditions. It is fine-grained, silty, pale yellow or characterized by a terrain of interwoven hills and gullies, at brownish yellow in color. It consists of angular grains of elevations between 800 and 1,400 m. The major rivers in quartz, feldspar, mica and other minerals between 0.005 this area are the Yan and the Fenchuan Rivers. This area is and 0.05 mm in size that are little polished. Loess is located in a warm and temperate climatic zone, with four macroporous, with well-developed vertical jointing and distinct seasons. The annual mean temperature is 7°C, the susceptibility to collapse on wetting. Loess deposits are annual average precipitation around 550 mm and the located mainly in dry continental zones at the middle annual average frost-free period is 150 days. Baota District is an important area that serves as the political, economic and cultural center for the Northern Shaanxi region. Due to its coal and petroleum resources, this area is a nationally M. Zhang J. Liu (&) Center for Water Research, College of Engineering, important center for energy production and industrial Peking University, 100871 Beijing, China chemicals. However, the thick loess deposits and the loose e-mail: [email protected] loess structure make Baota District one of China’s most geo-environmentally vulnerable areas. Geo-hazards such as M. Zhang Xi’an Regional Center, landslides and mudflows frequently take place in this area, China Geological Survey, Xi’an, China resulting in significant damages to the infrastructure and 123 1672 Environ Earth Sci (2010) 59:1671–1680

Fig. 1 Location of the Loess Plateau and the study area: Baota District, Yan’an City. (The area circled by the red line is Baota District; the triangle and the star represent the locations depicted in Figs. 2 and 3, respectively

Fig. 2 An example of 3D remote sensing interpretation map showing landslide locations (red squares), and the location of this area is depicted in Fig. 1 using red triangle

economic losses (Yin 1996). These factors justify investi- interpretation of SPOT-5 satellite images for the entire gations of the controlling factors of loess landslides in this area, together with remote sensing interpretation based on area to assist the understanding and prediction of the haz- Quickbird satellite images covering 225 km2 for the urban ard and safeguard the continued economic development of area. Figure 2 shows an example of 3D remote sensing the area. The study ﬁndings are expected also to provide interpretation map with landslide locations marked. Land- useful information, examples and experiences for other slides, unstable slopes and other geomorphologic features similarly afﬂicted areas of the world. were mapped and analyzed at 1,081 locations. Based on In 2005 and 2006, Xi’an Regional Center of China remote sensing interpretation, geological surveying at a Geological Survey (CGS) conducted detailed landslide scale of 1:50,000 was carried out and 428 locations were surveys in Baota District. This included remote sensing investigated. Detailed geological mapping at a scale of

123 Environ Earth Sci (2010) 59:1671–1680 1673

1:200–1:2,000 was done for 30 locations subject to serious severity of landslides. The three stages of the river val- landslides. These studies confirmed that loess landslides leys, their distribution in Baota District and the main are the dominant geo-hazard in this area. The results of an characteristics of river valleys at each stage are listed in analysis of the controlling factors for these landslides are Table 1. presented in terms of the evolution phase of river valleys, the geological structure and the geometry of slopes. The Advanced river valley (I) impact of groundwater, vegetation and the combination of human engineering activities and precipitation events are An advanced river valley is usually U shaped and wide with also considered. well-developed floodplains and river terraces up to 600– 800 m wide. Bedrock outcrop between 4 and 45 m may be present in the lower river terraces, on valley sides or at the Effect of the river valley evolution phase bottom of the valley, and both downward and lateral erosion is gentle. Generally, valley slopes are gentle (5°–30°) This study divides the complete evolution of river valleys and the interfluvial hills are more than 200 m high. There into three phases: (1) advanced river valleys, including are few new natural landslides along the two sides of the the Yan and the Fenchuan Rivers, which belong to the valley and most of the slides are ancient and advanced ones first level tributaries and are perennial rivers; (2) mature (Wang and Wang 2004) which are medium and small in river valleys, including the first and second level tribu- size (Table 1). Here, the size of the landslides is defined taries of the Yan and the Fenchuan Rivers, which are according to the volume of sliding mass: small size indi- either perennial, seasonal or rainstorm rivers; and (3) cates the sliding mass volume less than 10 9 104 m3; incipient river valleys, including the third and lower level medium size indicates the volume between 10 9 104 and tributaries of the Yan and the Fenchuan Rivers, which 100 9 104 m3; large size 100–1,000 9 104 m3 and super have flowing water only during storms. Rills, shallow large size indicates the sliding mass volume greater than gullies, suspended gullies, gullies and dry vales all belong 1,000 9 104 m3. However, because Yan’an City and urban to this third phase (Hydrogeological Team No. 2, 2000; population centers are located in the major river valleys, Hydrogeological Team 908, 2000). Figure 3 shows the human engineering activities, which are intensive, are often three phases of river valleys and their distribution in the cause of many landslides and related serious damages. Baota District. The survey results show that the phase of Survey data in the study area show that around 60% of the river valley evolution and development has obvious 293 investigated landslide locations are in advanced river controlling effects on the deformation characteristics valleys. For example, the Level I river valleys of Yan and and failure mode of slopes, as well as on the scale and Fenchuan River indicated in Fig. 3 belong to this category.

Fig. 3 Three evolution phases of river valleys and their distribution in Baota District

123 1674 Environ Earth Sci (2010) 59:1671–1680 Human activity Intensive Moderate Low landsides, small-scale falls landslides and falls and falls Geo-hazard type and scale shapes V Small-scale landslides

Fig. 4 Percentage of the number of landslides in different slope gradient intervals

Mature river valley (II)

gully, dissected gully, shallow gully A mature river valley has a well-developed drainage sys- Valley types Profile River valley U Large and medium scale River valley, gully U, V Medium and small-scale Dry gully, flushing tem. The slope gradient range is 20°–60°, the valley width 100–600 m, the slope height 100–200 m, and the valley profile U- or V-shaped. Vertical erosion is relatively slow, while lateral erosion is prominent. The valley sides gully gully gully are steep and many convex slopes on valley sides are formation Paleo-erosion Recent erosion unstable. Again, river valleys of this type are in the area that is densely populated and intensive engineering activities are ongoing such that frequent human-induced landslides occur. Geo-hazards are also associated with lateral river erosion and poorly designed engineering stepped linear

Slope form Time of works. Such destabilization mechanisms frequently lead to small to medium size (deﬁned above) loess landslides, which are indicated as Level II river valleys of Yan and 200 Concave shaped, 100 Protruding,

Slope height (m) Fenchuan River in Fig. 3. [ \

Young river valley (III) 100 Valley width (m) 600–800 100–600 100–200 Linear Paleo-erosion \

° A young river valley is still in the incipient stage of its ° °

–60 evolution. It will go through evolution in the sequence of ° –30 ) 60 ° ° Slope gradient ( 5 20 rill, shallow gully, suspended gully or vale, flushing gully, [ dry vale and in the end, river valley. The profile of a young river valley is generally V-shaped with varying depth. Vertical erosion is intense, while lateral erosion is not very obvious. The valley slope is steep, usually more than 60°, or almost vertical. The main form of slope instability consists of frequent small-scale loess falls where failure is controlled by the presence of structural joints and weath- the Fenchuan Rivers tributaries of the Yan and the Fenchuan Rivers (mounds) sloping land, steep valley slope ering and unloading fractures. Because a young river valley Area of distribution Main stream of the Yan and is usually narrow, few people live there and few engineering facilities are present; this instability is usually not Classification of evolution stages of the river valleys in Baota District and their main characteristics hazardous. The Level III river valleys indicated in Fig. 3 represent this category. The three development stages of valley Table 1 Level of evolution Advanced Mature valley First and second level Young valley Liang (ridges) and Mao the river valleys, their distribution in Baota District and the 123 Environ Earth Sci (2010) 59:1671–1680 1675 main characteristics of river valleys at each stage are listed Slope morphology in Table 1. Of the 293 investigated landslides, 261 took place on forward-tending slopes (107 convex slopes and 154 linear Effect of strata and structure slopes). 37 of 52 falls took place on forward slopes (21 linear slopes and 16 convex slopes). Of the 51 unstable Slide-prone strata slopes, 31 are forward-tending slopes (24 linear slopes and 7 convex slopes). Due to the stress state and distribution in The stratigraphic units in the area investigated include slopes, backward-tending concave and stepped slopes are Triassic, Jurassic, Neogene and Quaternary strata, among liable to be more stable as this reduces stresses acting in the which the Quaternary loess and the Neogene red clay are slope direction (Zheng 2002). On the contrary, in forward- slide prone. The Triassic and Jurassic strata are deeply tending slopes there is a concentration of stresses which buried under the Quaternary and outcrop only at low ele- leads to lowered stability. Forward-tending linear and vations in large river valleys. As a result no bedrock sliding convex slopes become unstable more liable than backward- was found during the investigation. However, along the tending concave slopes and stepped slopes (Wang 1992). two sides of the Yan and the Fenchuan Rivers, especially the side slopes along roads and railways, bedrock falls Slope gradient often take place. The Neogene red clay is not continuous in the investigated area and outcrops only in isolated valley Slope gradient data were extracted from the DEM map bottom locations, so even though this material is slide- (1:50,000) of the Baota District, in which there were prone few landslides occurrences are present. Quaternary 5,672,922 grid cells in all. Taking 10° as the step length, loess almost covers the entire area. Because of its relatively the number of grid cells in each interval and correspond- loose texture, metastable strength characteristics, collaps- ingly the percentage of each gradient interval were ibility and unique jointing structure, this material is very computed (Fig. 5). According to the number of landslide susceptible to instability in this area. In fact, all of the locations in each slope gradient interval and the percentage landslides located in this investigation are either related to of each gradient interval among all the slope gradient loess or took place in loess (Sun 2005). intervals, the probability of landslides occurred in each slope gradient interval was calculated (Fig. 6). It is easy to Rock–soil mass structure see that the probability is low when the slope gradient is less than 30°; the value is basically less than 0.0002 in the The rock–soil mass structure in the area falls into four gradient interval of 30°–50°, and this number increases categories: (1) loess ? nearly horizontal paleo-soil layers, with the increase of the slope gradient. When the slope (2) loess ? inclined paleo-soil layers, (3) loess ? paleo- gradient is greater than 60°, the probability of landsliding is soil layers ? bedrock and (4) loess ? paleo-soil lay- as high as 0.0016. Figure 4 shows the percentage of the ers ? the Neogene clay. The rock–soil mass structure number of landslides in different slope gradient intervals. predetermines the mode of instability and location of structural weaknesses in the slope and obviously has a controlling effect on the sliding plane location. Vertical loess slopes (with angles between 30° and 60°) or cliffs (with angles greater than 60°) usually result in falls, while loess slopes (with angles between 10° and 30°) are usually subject to sliding.

Effect of topography

Unfavorable slope geometry is a prerequisite for the initi- ation and evolution of natural landslides and loess falls. Evolution of river valleys and gullies and eroded terrains provide the conditions for landslides and falls in the entire area. Slope geometry determines the state and distribution of stresses in the slope mass and controls the stability and mode of instability of the slope. Fig. 5 Percentage of each slope gradient interval 123 1676 Environ Earth Sci (2010) 59:1671–1680

levels of stress. With increase in height, stresses increase linearly and for the same slope gradient, instability becomes more likely. Therefore, slope height also controls the occurrence of landslides. Loess landslides occur most commonly in slopes with the height of 50–120 m. Loess falls mostly occur in slopes with the height of 10–20 m, accounting for 69.23% of the total number; then they occur in slopes with the height of 20–30 m, accounting for 17.31% of the total. The higher the slope, the lower the ratio of loess falls. The reason is that through long-time weathering, high slopes have reached a stable condition and the slope angle is low. On the contrary, in lower slope steep slopes are readily eroded by river action and erosion, Fig. 6 Probability of landslide occurrence in different slope gradient and are also susceptible to human engineering activities. intervals The slope is still in equilibrium and in an adjusting phase, therefore the ratio of loess falls is high.

Slope orientation

The 293 landslides investigated are plotted in Fig. 8 in terms of slope orientation and gradient. Gullies crisscross the study area and slopes are oriented in various directions. Landslides also appear to have an even distribution of orientations. However, the probability of landslides appearing in each interval of slope direction (Fig. 9) show that most of the slopes fall into the orientation range of 45°–135° and 225°– 315° which is related to the direction of rivers in this area. As Fig. 3 shows, the general direction of the Yan and the Fenchuan Rivers is east–west, and their second level tribu- Fig. 7 Relationship between the safety factor and slope gradient at taries are mostly in the north–south direction. The slope different slope heights

Among the 293 landslides investigated, 279 took place on steep slopes (slope gradient greater than 60°), accounting for 95.22% of the total number. Only four took place on cliffs and ten on gentle slopes. Falls only occurred on cliffs. The slope gradient significantly affects stress distribution along slopes. With increase of the slope gradient, increases occur in both the stress in the slope and stress concentration at the bottom of slopes. According to the results of finite difference numerical analysis shown in Fig. 7, the safety factor decreases logarithmically with slope gradient increase thus showing that slope gradient has a significant effect on the stability. Cliffs with gradient larger than 60° are fall-liable. With decrease of the slope gradient, landslides will take the place of falls.

Slope height

Figure 7 shows the controlling effect of slope height and gradient on landslides. Even though slope height does not Fig. 8 Scatter point map of loess landslides at all slope orientations change the distribution of stresses in slopes, it controls the (0°–360°) and all slope gradients (0°–90°) 123 Environ Earth Sci (2010) 59:1671–1680 1677

in the alluvial deposits of river valleys at low altitudes and although elsewhere it occurs at the contact between loess and bedrock, it generally does not form a regional water table. However, precipitation will flow into and along the well-developed systems of joints, fractures, sub-surface flow channels and sinkholes in the loess during rainstorms. Local perched water or even phreatic water can be formed above fossil soils or bedrock. Groundwater reduces the strength of loess and changes the stress state within the slope mass, and triggers instability of slopes. There are three impacts of groundwater: (1) impact of perched water in the upper layers of slopes reduces the strength of the soil mass, increases the soil weight and therefore causes instability of slopes. (2) Downward infiltration causes increase of soil water content. Although the soil remains unsaturated, the strength of the Fig. 9 Distribution of probability of slopes in different slope soil mass is reduced which triggers instability of slopes. (3) orientation intervals Impact of surface water infiltration from surface water bodies, including reservoirs which raise the groundwater level and have a significant effect on the stability of slopes. For example, the reservoir impounded by the ZhaoJia Bank in Hezhuangping District, locally raised groundwater levels and resulted in the formation of springs in front of the slope mass, which causes distortion of houses and cracking of walls, and thus seriously threatens the security of local residents.

Vegetation

Vegetation protects slopes through preventing soil and water erosion and thus influences slope evolution and stability. It can also remove moisture from the soil causing desiccation and increasing pore suction. In the northern Fig. 10 Probability of landslide occurrence in different slope orien- part of the area investigated, vegetation is poor and land- tation intervals slides are extensive. On the contrary, in the southern area, vegetation coverage is high and landslides are scarce aspects along the two sides of rivers happened to fall into the (Fig. 11). Thus, there is significant relationship between range of 45°–135° and 225°–315°. Figure 10 shows that vegetation coverage and the distribution of landslides, but the slopes with aspects of 0°–45° and 315°–360° (especially vegetation is not the essential factor determining the dis- the north-east aspect) are more prone to landslides compared tribution of landslides. The impacts of vegetation on slope to slopes in other direction intervals. That is, more instability movements, evolution and landslides are reflected mainly occurred on the shaded slopes, that is, the west facing slopes. in three aspects: (1) The hydrogeological effect: vegetation The difference in the water and temperature regimen hinders runoff to a certain degree, thus increasing precip- between shady and sunny slopes results in differences in the itation infiltration and the recharge into the slope mass. (2) soil moisture content, weathering and slope gradient, and The mechanical effect: the root systems of plants can sta- therefore has a certain effect on landslide occurrence. bilize soil mass and enhance its ability to resist shear stresses. Some root systems embedded in the bedrock or paleo-loess act as anchors to the overlying materials. Effect of groundwater and vegetation Besides, the weight of the trees on slopes adds to the load on the slope mass and transfers the dynamic wind load to Groundwater occurrence the slope, which has a destabilizing effect. (3) The slope protection effect: in the area of abundant vegetation soil There is no integrated and continuous groundwater flow erosion is reduced. On the contrary, serious soil and water system in the study area. Groundwater is present here mainly loss, extensive erosional down-cutting and accompanying 123 1678 Environ Earth Sci (2010) 59:1671–1680

Fig. 11 Comparison between vegetation coverage (left) and landslide of slide occurrence (numbers and scales of the slides that took place in risk (right). The risk of landslide was derived based on the basic the past), as well as the possible triggering factors (such as information of slope (steepness, height, morphology, etc.), the history engineering activities and precipitation) deformation of slopes are always prevalent in areas of low Human engineering activities vegetation cover (Wang and Dai 1997). Vegetation in the Fenchuan River basin is well devel- With fast economic development, human engineering oped where the main tree species include broadleaved trees activities have become more and more intensive and like Liaodong oak, aspen and birch, and conifers like oil inﬂuential. Poorly sited excavation in natural slopes can pine and oriental arborvitae. The vegetation coverage is destroy slope equilibrium reached by long-term slope generally more than 60% and in some local areas more than processes and results in deformation of the ground and 80%. The slope body in this area is relative stable and instability of slopes (Peng 1997). This has become one of landslides are rare. While in the northern Yan River basin the most important triggering factors for landslides. the vegetation coverage is poor, and the dominating plants Loess strata have been subjected to long-term erosion in are forest, shrubs and grass, including shrubs like Vitex recent times. This has led to the formation of loess Liang negundo, wild jujube and Sophora viciifolia, and herbs like (ridges) and Mao (hills or mounds) with gentle gradients Bothriochloa ischaemum and Themeda japonica. The (see Fig. 3). Since Holocene times (about 10,000 years vegetation coverage is generally less than 30%, mostly less ago), most slopes are in a stable or nearly stable condition than 60%. Serious river erosion is always present in this (Zhang 1994). Under natural conditions and without active area and slopes are unstable with well-developed land- erosion by rivers, there would not be signiﬁcant landslides. slides. Based on the investigation statistics, almost 293 However, human engineering and economic activities, landslides occurred in the northern Yan River basin, while such as slope cutting, excavation and loading, and changes only one took place in Guanzhuang, located in the southern to the groundwater conditions all have the effect of Fenchuan River basin. disturbing the original equilibrium. Usually most engineering work is carried out in dry seasons when steep slopes can be created. These remain stable until the next Effect of human engineering activities and precipitation rainy season or longer, and may become unstable after an exceptionally wet period or because of other trigger- The geo-hazard surveys in the study area indicate that ing factors. The landslides investigated this time all relate human engineering activities and precipitation have a to poorly executed human engineering activities (Lei triggering effect on landslides. 2001).

123 Environ Earth Sci (2010) 59:1671–1680 1679

Precipitation changeable. The percentage of landsides which took place in the study area in recent years shows good correlation with Loess is composed of silty material or silty clay with rel- the average monthly precipitation and human engineering atively low permeability. Normally, it is not easy for activities. 10 out of 13 landslides were related to slope precipitation to infiltrate and form perched water or phre- cutting for construction of cave dwellings and houses; 2 atic water and the time lag is significant. Therefore, were related to highway and railway construction; and 1 precipitation events alone will not normally trigger land- was caused by hydraulic engineering. The coupled influ- slides. However, where structural joints, sinkholes and ence of precipitation and human activities is the most unloading and weathering fractures are present, precipita- important factor in landslide triggering. tion can infiltrate downward and form perched water or saturated zones on top of impermeable layers, so as to reduce soil strength. Precipitation also adds to the weight Summary and conclusions of rock and soil mass, creates pore water pressure and reduces suction. The effective strength of the loess is This study has utilized field investigated data to analyze how reduced, and thereby landslides are triggered. river valley evolution phase, slope structure and geometry, According to the meteorological data of Baota District, groundwater occurrence, vegetation, human engineering the average annual precipitation is 496.2 mm. The pre- activities and precipitation control and trigger landslides in a cipitation is unevenly distributed within a year, with 71.4% loess plateau landscape. The following conclusions may be of the total amount concentrated from June to October. The drawn from this study. statistic data of landslides in this area since 1985 showed The occurrence of loess landslides is controlled by all that 84.6% of the landslides take place from June to these factors. Attention should be given to river valley October and there is a close correspondence between evolution stage, slope structure and slope patterns in land- landslide occurrence and the pattern of precipitation slide prevention, risk assessment and prediction. Loess (Fig. 12). For example, rainfall was higher in June, 1986 landslides are also affected by groundwater regime and when the monthly precipitation was the highest of the vegetation. Because of the large thickness and low perme- whole year with a daily precipitation of 35.9 mm. Two ability of loess, precipitation generally does not give rise to landslides located, respectively, at Baota Mountain and in landslides by causing rise in the water table. Therefore, Fenghuang village were triggered by precipitation at this groundwater is not a primary controlling factor of landslides time. in the study area. However, in strata with fractures, hidden Among the controlling factors of landslides, the geo- holes and sinkholes, precipitation can easily infiltrate into environmental conditions are relatively stable, while pre- the interior of loess formations and cause the water table to cipitation and human engineering activities are more rise, thus trigger landslides. Vegetation can prevent soil and

Fig. 12 Relationship between precipitation and landslide occurrence (the values above the columns represent the number of loess landslides and loess falls in the study area)

123 1680 Environ Earth Sci (2010) 59:1671–1680 water erosion and thus influence slope stability, but it is not Lin, Sun Qiaoyin, Zhang Rui, Li Qing, Wang Xiaoyong, Zeng Lei, the essential factor determining the distribution of Xue Qiang and Yi Hao. Professor Lin Zaiguan provided valuable suggestions and great help in editing this paper. landslides. The practice of modifying slopes and excavating caves for living and storage space has long since been established References in the Loess Plateau and the practice of modifying slopes for housing construction and other engineering activities is Duan Y et al (1993) Geo-hazards in China (1st version). China unavoidable because of terrain limitations. Living and Construction Industry Press, Beijing engineering facilities generally are built in wide river Hydrogeological Team No. 2 (2000) Shaanxi Geological Bureau. Regional Environmental Geological Survey Report of Shaanxi valleys. As loess slopes have undergone stress release and Province adjustment through various stages of erosion in geo-his- Hydrogeological Team 908 (2000) Shaanxi Geological Bureau. tory, Holocene in particular, the strata are generally stable Landslides Survey Report of Baota Mountain, Yan’an City, and insusceptible to landslides under natural conditions. Shaanxi Province Lei X (2001) Human activities and geo-hazards on the Loess Plateau. Artificial slope modifications, however, disrupt the original Geological Press, Beijing equilibrium, leading to the formation of unloading, tension Liu D (1985) Loess and environment. Scientific Press, Beijing and weathering joints. As a result, landslides are more Peng J (1997) Study of the stability of engineering field. Xi’an likely to occur in these locations in rainy seasons. Cartographic Press, Xi’an Sun J (2005) Loess sciences (1st version). Hong Kong Archaeology Enhanced by human activities, precipitation infiltrates Association, Hong Kong into the interior of the loess formations along structural Wang J (1992) Mechanism of high speed loess landslides—saturated joints and weathering openings. This increases the weight loess creeping liquefaction. Geol Rev 38(6):532–539 of rock and soil mass, changes the pore water pressure, Wang S, Dai F (1997) Evaluation and prediction of engineering geoenvironment and countermeasures to coordinate human reduces the strength of rock and soil, and thus initiates engineering activities. J Geol Hazards Environ Preserv 8(1): landslides. Landslides occur most frequently in the period 31–35 from June to October in the study area, and their occur- Wang Z, Wang N (2004) A summary of present study on loess rence has a close correlation with the precipitation regime, landslides. Soil Water Conserv China (SWCC) 11:16–18 Yin Y (1996) Prediction and mapping of geo-hazards trend in China. which can be useful for landslide prediction and warning. Quat Study (2): 123–130 Zhang Z (1994) Typical human engineering activities and the impact Acknowledgments The authors would like to thank the funding on geological environment. Southwest Transportation University support from the National Land Resources Survey Project Press, Chengdu (1212010541106-3, 1212010740907) and the National Scientific Zheng Y (2002) Finite element strength reduction method and its Support Project (2006BAC04B05). The authors also greatly appre- application on soil and rock slope. Collected paper of Rock ciate the help in field survey and data collection from Xiao Peixi, Wei Mechanics and Engineering Society Xingli, Huang Yuhua, Nie Haogang, Wang Jiayun, Wu Wenying, Li

123 本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP 图书馆。图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页文献云下载图书馆入口外文数据库大全疑难文献辅助工具