Li et al. Geoenvironmental Disasters (2015) 2:24 DOI 10.1186/s40677-015-0032-7 RESEARCH Open Access Investigation for the initiation of a loess landslide based on the unsaturated permeability and strength theory Ping Li*, Xingting Zhang and Hao Shi Abstract Background: The Yanlian landslide, occurring on 21-22 October 2010, destroyed many facilities of a big oil refinery in Shaan’xi Province, China. It led to a suspending of the refinery work for a week and caused near 700 million RMB economic losses. Results: Site exploration shows that the sliding mass is unsaturated-saturated loess. The groundwater is rich in the landslide and shortage in the surrounding slopes. Further investigation finds that the water drop released from the vapor heating furnaces on the top of the slope is the only source of the groundwater. Laboratory tests were performed to get the unsaturated strength parameters and hydraulic conductivity of the loess layers which were applied to a pre-failure slope model to simulate the water infiltration process and the stress field based on which the factor of safety is figured out to analyze the slope stability. Analysis shows that during the first ten years, the factor of safety has no prominent decrease. In the following 5 years, the slope stability decreases significantly till failure. Conclusions: A little water infiltration has minor effects on slope stability for some time. As a result it is easy to be ignored. However, when the period of water infiltration is long enough to raise the groundwater level, it will have detrimental influence on the stability. In conclusion, any minor water produced by engineering or other activities for a long period may have harmful effect on slope stability. Therefore it is essential to take account of this kind of water and adopt measures to curb the surface water infiltration and to drain the groundwater. Keywords: Unsaturated soil; Loess; Landslide; Strength parameters; Soil-water characteristic curve; Hydraulic conductivity function Background moves down intermittently for two days and the people Loess landslides occurring in China lead to a huge eco- working on it has enough time to escape. So there is nomic losses every year (Liao et al. 2008; Bai et al. 2012; no fatality in the accident. The cause of the landslide Li et al. 2012). It has been proved that rainfall, irrigation, is unclear and deeply concerned by the managers and water canal leakage, submerging of reservoir, melted researchers. The aims of the paper are to find the trigger- snow and earthquake are the main factors triggering ing factors by site exploration and to study the failure loess landslides (Lei, 1994; Dai and Lee 2001; Tu et al. mechanism by laboratory tests and numerical analysis. 2009; Li et al. 2013; Zhang & Peng 2014). However, there are no water resources and earthquake mentioned above Methods, results and discussions before the Yanlian landslide occurred, which is located at Characters of the landslides Luochuan county, Shaanxi province, China as shown in The landslide occurred on the erosive bank of the Luohe Fig. 1, and the slopes nearby are stable even though they river which is the second tributary of the Yellow river. are higher and steeper than the failed. The landslide The first and the third terraces of the river develop on that bank and the second terrace is eroded off. The first terrace, about 5 m above the river bed, is flat and wide * Correspondence: [email protected] Department of Geological Engineering, Chang’an University, Xi’an, Shaan’xi, on which there is an oil refinery named Yanlian (Fig. 2). China Against the rear of the first terrace is a 20 m high rock © 2015 Li et al. Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Li et al. Geoenvironmental Disasters (2015) 2:24 Page 2 of 11 Fig. 3 The spring at the toe of the slope Fig. 1 Location of the Yanlian landslide (Fig. 5). The refinery had to stop working until 7 days cliff composed of hard horizontal sandstone of the upper later when the facilities were recovered. There were no Triassic formation. The rock cliff is the base of the third fatalities but it caused 700 million RMB losses. terrace which is overlain by about 2 m thick alluvial peb- The surface gradient of the slope before failure is about ble and nearly 50 m thick Quaternary loess successively. 30° and the vertical height is 47 m. The failure mass has a The pebble is filled with silt-sand and cemented by leached width of 240 m, a length of 150 m, a thickness of carbonate calcium, so it has a very low permeability. A 13.8 m and a total volume of 400,000 m3.Theslopeis spring flows out from the top of the pebble with an average composed of the unsaturated loess of Late (Q3 loess), flow quantity of about 0.5 L/s (Fig. 3). The sliding mass, Medium (Q2 loess) and Early (Q1 loess) Pleistocene shearing out on the pebble bed, is totally loess. Around formationsasshowninFig.6.Thegroundwaterlevel the failed slope, there are 34 large oil tanks lying on is only 7 m deep at the center of the landslide, but no the top and 24 oil pipelines on the toe. A road wanders groundwater is found in the surrounding slopes. The on the slope from the toe to the top. A boiler house region is generally shortage of groundwater because of and a coal storehouse stand on the first terrace against the small precipitation (annually 596 mm/a) and the the cliff (Fig. 2). deep cut landform. The rainfall during the rainy season Several days before October 21, 2010, a small part of may quickly run off on the surface. Investigation on loess at the toe of slope started to collapse from where a the top platform where oil tanks stand finds that there spring flowed out and buried part of the coal storage is lots of water vapor released from the pressure house. When the workers were trying to clear the col- adjusters on the heating pipes which are used to keep lapsed material, the whole slope moved downwards for the oil in the transfer pipes from frozen in winter two days. (Fig. 7). The hot vapor condenses in winter and partly The landslide buried part of the coal storehouse (Fig. 2) penetrates into the ground. The condensed water of the and cut off all the 24 oil pipelines (Fig. 4) and the road. vapor is possibly the water source in the landslide The oil tanks were only 5 m away from the head scarp Fig. 2 The Yanlian landslide Fig. 4 The oil transfer pipes cut off by the landslide Li et al. Geoenvironmental Disasters (2015) 2:24 Page 3 of 11 Fig. 5 The oil tanks hanging over the top of the head scarp Fig. 7 The release water vapor from the pressure adjust valves on the heating pipes which has been ignored at the beginning. Therefore it is assumed that the groundwater in the landslide was (>0.05 mm) less than 10 %. From Q3 to Q2 and Q1 generated by unsaturated infiltration of the condensed loess, the mean particle size and the void ratio decrease water drops and the landslide was trigged by the rising as well as dry density increases. It demonstrates that of the groundwater level. the lower the loess layer lies, the longer the time of pe- dogenesis is and the heavier the load of consolidation Determination of the loess physical properties experiences, the more compacted the loess is. The sliding mass and surface are composed of loess. Basic physical properties of loess include permeability, Determination of SWCC and HCF water content, density, plasticity, grain components. Soil-water characteristic curves (SWCC) were mea- The coefficient of saturated permeability was measured sured in laboratory with undisturbed block specimens through the falling head method. The water content of 300 mm × 300 mm × 300 mm in dimensions. The and density were measured by oven drying and metal specimens were collected from Q1,Q2 and Q3 loess ring respectively. The plastic limit and liquid limit were respectively. The matric suctions were measured in measured by hand rolling and Cassgrande apparatus wetting processby the TEN−15 tensiometer and the respectively. The results are shown in Table 1. Laser corresponding moisture contents by the oven-drying Particle Analysis were utilized to measure the grain size method. It was accomplished through the following distributions of Q1,Q2,Q3 loess as shown in Fig. 8. It steps: First, a block of specimen was air dried and a is shown that major component is silt (0.005−0.05 mm) hole of 80 mm in depth and 300 mm in diameter taking up 60 %. The second is clay (<0.005 mm) taking was punched in the center of one face of the speci- up 20−40 %, and the minor component is fine sand men. Second, insert a tensiometer into the hole with Fig. 6 The main longitude section of the Yanlian landslide Li et al. Geoenvironmental Disasters (2015) 2:24 Page 4 of 11 Table 1 The basic physical properties of the loess samples Loess Density Moisture Specific Dry Void Volumetric moisture Liquid Plastic Plastic Coefficient of content gravity density ratio content limit limit index permeability 3 3 ρ/(g/cm ) w/(%) Gs ρd/(g/cm )e0 θ/(%) wL/(%) wP/(%) IP/(%) k/(m/day) Q3 1.67 16.8 2.71 1.43 0.895 24.0 30.9 18.9 12.0 0.110 Q2 1.86 20.5 2.71 1.54 0.760 31.6 37.0 22.8 14.2 0.044 Q1 1.89 19.9 2.71 1.58 0.715 31.4 33.7 20.8 12.9 0.027 the ceramic head saturated in advance and fill the gap Where Cr is a constant related to the matric suction cor- with the same soil powder.