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Abstract Analysis and research results show that thesliding plane of Jiudingshan landslide is along the weakintercalated layer (clay-filled) in the limestone and thesliding block is separated by the tension crack on the slopecrest. The earlier study results show that for the rough, upsand downs structural plane, when i is greater than 2.0, theshear strength of the structural surface is intensely close tothe strength of the filling. By the earlier theory, this failuremust be through the clay filling. In this study, the failure isback-analyzed and the shear strength of the infilling istested by the laboratory direct shear tests for which sampleswere retrieved in the field. 50954
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The failure cannot be explainedby the laboratory results of shear strength parameters. Tosimulate the field conditions, the real strength parametersof sliding surface are measured by the in situ shear tests forthe weak intercalated layer. By the in situ tests, it is shownthat the failure initiates along the contacts between the clayinfilling and the limestone boundaries, but not through theclay itself. Though the contact surface is the interface ofthe clay–limestone, the cohesion is not 0 and it is notnegligible too.Keywords Bedding landslide The weak intercalatedlayer Shear strength Back analysisIntroductionMany studies and practices show that the weak layers havesignificant impact on the stability of the stratified rock mass from the analysis by the large number of projects. Thestability of a rock slope, when containing an inclined planeof weakness, has been discussed by many authors (Ter-zaghi 1962; Jaeger 1971; Goodman 1976; Goodman andGen 1985; Okubo and Nishiatsu 1993; Hatzor 1997). Inaddition, case studies have been carried out on cut slopes inJordan, concluding that slide movements always coincidedwith weak layers (Alhomoud and Tubeileh 1998).A separated block is the primary condition for rockinstability, which means there is the separation surface asthe side border of the instable block in the rock mass, andwhich has little sliding resistance; the inclination of slidingplane must be consistent with inclination of the slope.Secondly, the dipping angle of the sliding plane must beless than the slope angle of inclination and greater than theinternal friction angle on the sliding plane. When thesliding force of the sliding plane is greater than resistancesliding force, the rock block will fail. According to theMohr–Coulomb theory, the resistance sliding forcedepends on the strength parameters c and / of the slidingplane.The earlier study results (Barton 1971, 1973; Barton andBandis 1982; Patton 1966; Ladanyi and Archambault 1977;Saeb and Amadei 1992; Maksimovic 1996; Grasselli andEgger 2003; Haberfield and Johnston 1994;
Indraratna andHaque 2000; Kodikara and Johnston 1994; Ghazvinian andTaghichian 2010) show that for the rough, ups and downsstructural plane, when the value of i (the filling thickness/the evaluation of the ups and downs of the structural sur-face) is greater than 1.0, the shear strength of filled surfaceis governed by the strength of the filling material. When i isgreater than 2.0, the shear strength of the structural surfaceis intensely close to the strength of the filling. Hatzor(1997) pointed out that the strength of the filling clay maybe greater than the contact of the clay–limestone, but the strength on the contact is mainly governed by the effectiveinternal friction angle; the cohesion is negligible.Each slope failure process along the discontinuities inrock mass can be reasonably accounted as a large-scalein situ shear tests. On the other hand, failure zones of rockmasses induced by excavation can be measured easily andreliably. Therefore, the back analysis techniques havealways been a hot research topic since the 1970s andextensive studies have been conducted to develop differentmodels of back analysis (Sakurai and Takeuchi 1983;Gioda and Locatelli 1999; Zhang et al. 2006). Back anal-ysis has been used also based on field measurements ofstrains or stresses (Kaiser et al. 1990). The shear strengthparameters obtained by back analysis of slope failureshould be more reliable than the results obtained by labo-ratory, because of the influences of scale effect (Mostafaet al. 2010).The shear strength parameters of a failed slope havebeen back calculated in the following procedure: assumingthe value of the angle of internal friction / or of thecohesion c to calculate another (Fookes et al. 1977).Assuming that the material of the sliding plane obeysMohr–Coulomb failure criterion; therefore, the results ofback analysis provide a range of combination of apparentfriction angle and cohesion, which brings difficulties to theparameter selection. The above difficulties usually can besolved by utilizing two cross sections in a failed slope toestablish two equations and evaluate the values of / and c;the continuous curves corresponding to the two equationsmay have an intersection point, which represents the values/ and c of the sliding plane.