| Taking HuRongxi expressway as the research background, considering the characteristics of rock mass structure of the soft-rock slope in Badong formation, a series of laboratory and in situ tests are carried out in order to study the engineering characteristics of rock mass in Badong formation. Based on these systematic researches, the failure mechanism of two typical failure soft-rock slopes containing gently inclined consequent slope and counter-tilt slope in Badong formation is studied here, the law about engineering properties of soft-rock slope in Badong is gained. At the same time, the failure mechanism and failure mode are put forward. The main contents of this paper are stated as follows:(1) Based on statistical analysis on the failure of many soft-rock slopes in Badong formation during construction, it can be found that there are mainly two major failure patterns that are consequent slide and counter-tilt failure. The dip angle of rock stratum for consequent slope is mainly between 10～20°, which belongs to gently inclined consequent slide failure. While the dip angle of counter-tilt slope is mainly between 50～70°. According to the two typical failures, investigations and analyses are carried out on those typical failure slopes. It can be found that the failures of the slopes are close related with the special rock mass structure and mechanical behavior of soft-rock slope in Badong formation, especially the weak intercalation which is the controlling factor for slope stability.(2) Based on classification scheme of rock mass structure by Sun GZ, the rock mass structure of soft-rock slope in Badong formation is considered as layered slab-rent structure according to its self-characteristics, and three subclasses containing cataclastic texture, intermittent texture and scattering texture are classified. According to these classifications, analyses on basic characteristics by laboratory test are carried out, and conclusions on the disintegration and expansibility of the rock are obtained.(3) The laboratory triaxial test results show that water influence the characteristics of rock intensity and deformation obviously. Based on constitutive model gave by Sun GZ, considering the close of structural surface, elastic constitutive model for natural and saturated state is established here. By doing load-bearing plate test, under different argillaceous content, the deformation characteristics of two kinds of soft rock in Badong formation are analyzed here. The rock characteristics containing stress and displacement distribution and the influence range of depth under cycling load are also studied here. On the basis of researches above, we totally understand the deformation behavior of soft rock in Badong formation. Advices to the design of soft-rock slopes in Badong formation are given. It can also offer references to the research on reinforcement effect of pre-stressed cable. According to the rheological test, rheological constitutive equation considering the closure of structural surface is established. It can offer theoretical support to research on long-term stability of slope.(4) By doing direct shear laboratory test, we analyzed the shear characteristics of intercalation existing widely in the soft rock in Badong formation. The results show that the shear intensity reduced significantly as the increase of water content and thickness of intercalation, especially the shear friction angle.(5) Based on the method of safety factor for independent blocks and user-defined constitutive model considering the closure of structural surface, further analysis on the failure mechanism of gently inclined consequent soft-rock slopes in Badong formation are carried out. Conclusions on the failure of mentioned slope type above are:the failure start from excavation of toe and come with intercalation failure from constant rate creep to acceleration creep. From the view of failure mode, the failure belongs to the mode of disintegration, expansion of cracks, infiltration of surface water and so on. The failure model is motivation and traction combination model, the frontal slope is the motivation area and the toe can countercheck the slide of this area.(6) Based on the cantilever beam limiting equilibrium model by D.P.Adhikary and A.V. Dyshin, the article made some improvements mainly on the form of fracture plane and influences of cohesion and severe by different layers, at the same time, an improved cantilever beam limiting equilibrium model is proposed. A new method taking residual unbalance force as standard is used to analyze the stability of counter-tilt rock slopes, and the influence factors of counter-tilt rock slopes are further studied. Conclusions and laws useful for design and construction of these slope types are suggested.(7) Based on improved cantilever beam limiting equilibrium model and numerical simulation by UDEC, combining with the influence of excavation and intercalation, analysis on a typical counter-tilt slope are carried out. The main failure pattern of counter-tilt soft-rock slopes in Badong formation is toppling-sliding-bending. Rocks in the middle of slopes and intercalations are the beginning point of the slope failure, and the toe of slopes is very important in stability analyses which need to be mainly considered during the construction. These three points are the most important part for the reinforcement and protection of counter-tilt soft-rock slopes in Badong formation.
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