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The second method seemsmaybe more efficacious; however, if the range or volumeof a slope failure is relatively small, the shapes of slidingblocks in different cross sections drawn from the failedzones are similar. The two continuous curves may be veryapproximative or overlapped; the back analysis proceduregoes back to the first procedure.Jiudingshan landslide is located in discontinuous mediaand the failure of rock mass is mainly controlled by thediscontinuity distribution. Inthisstudy,onthebasisofengineering geological investigation and survey forJiudingshan landslide, it can be found that the failure ofthe landslide initiates along the weak intercalated layer,which is filled with clay. To measure the shear strengthparameters on the sliding plane, the failure is back-ana-lyzed. The shear strength of the infilling (clay) is tested bylaboratory tests. To simulate the field conditions, thein situ direct shear tests are carried out. Due to the dif-ferences between the tests and back analysis, the discus-sion is made to determine where the sliding plane is (thesliding plane is through the filling or on the contactbetween the clay filling and the limestone boundaries).The ultimate strength parameters of the sliding plane aredetermined by contrast analysis. The accuracy of theconventional and earlier study results for the weak inter-calated layer is discussed.BackgroundJiudingshan slope is located in the east of Laiwu City, at adistance of about 20 km from the city center (Fig. 1). Thesite is located in the southeast fringe of Laiwu Basin, whichis characterized by hilly landscape, V-shape valleys, andthe ground evaluation is about 300–480 m. The length andheight of the slope are 100.0 m and 12.00–40.00 m,respectively. Because of construction of the new plants ofLaiwu Iron & Steel Co., Ltd., the slope was excavated;after excavation, the slope angle is above 70 and thedistance between slope toe and wall of new plants is3.5–6.0 m (Fig. 2).The Cambrian Fengshan group (”3f), which is widelyexposed in the study area, comprises mainly limestone(Fig. 2), characterizes moderately to intensely weathered,gray, with strike of 280 –310 , dip direction of NE anddipping angle of 25 –40 , in which RQD is about 40–80,and the rock quality is mid-poor. The thickness of the topintensely weathered is about 10 m; the lower is moderatelyweathered. Locally, quaternary loess covers part of theslope, comprises mainly rubble and soil which is theweathering residues of the limestone, with a thickness ofabout 0–0.5 m.Scanline surveys were carried out in the failed andadjacent zones; the range is limited by the site condition tobe 3 m 9 20 m along the strike of slope. The orientationand characteristics of 68 joints are collected from theclinohedron. The main discontinuities in the rock massinclude two joint sets and stratification planes of thelimestone (Fig. 3). Joint sets 1 (JS1) and 2 (JS2) are allsub-vertical, with strike of 300 –320 and 0 –20 , dipdirection of SW and NW, dip angle of about 70 –80 ,respectively. The two joint sets both have undulating andnearly smooth surfaces, with little soil packing. Joint set 1extends near 60 m, with fissure width of 0–3 cm; joint set 2extends about 3–10 m, with fissure width of 0–3 cm. Thestratification planes are closed. The bedding planes can beseen as joint set 3 (JS3).Due to excavation, the rocks move slowly. Duringconstruction stage of the plants, a tension crack developedalong the strike of joint sets 2, with width of 20 cm. Asmall-scale fast failure took place southeast of the slope.The instable block is about 10 m wide and 23 m high; thevisible sliding plane length along sliding direction is morethan 20 m. The failure consequently destroyed part of thenew plants. By measurement for the landslide body, con-necting the topographic contour map of the slope before failure, the section crossing the failed rock masses (SectionI–I indicated in Fig. 2b) could be precisely plotted. The siteinvestigation reveals that the sliding plane is through arelatively weak intercalated layer (Fig. 2b, c), in which themain component is clay. The weak layer gets distributedalong the bedding plane in the limestone.