Study On Bedding Failure Mechanism Of Phyllite Slope In Qin-ba Mountain Area

Qin-Ba Mountain area was located in the central and western China with complex geological environments, namely, undulating terrain, multi-period tectonic movements, complex geological structure, strong weathering effect, rich rainfall and abundant human activities, making it prone to landslides etc. Metamorphic slopes were widely distributed throughout this area, and the main lithology was phyllite, slate and shale. Among them, phyllite slope was typical slope for its dense joints, low strength and strong weathered rocks. Under influence of natural environments and human activities, geological hazards such as sliding and rockfalls were frequent to occur in phyllite slopes and put great threat on local life and economy. Consequently, it was significant to study failure mechanisms of phyllite slopes for the prevention and control of phyllite landslides in the area.Taking Yaobai landslide as a representative phyllite slope in Qin-Ba Mountain area, engineering geology conditions including typical geological cross, rock mass characteristics, geological structures and landslide deformation characteristics were studied by landslide mapping and survey. And the results showed that all the slopes belong to sliding tension characterized by progressive slip. Landslide modeling tests were carried out to explore factors which had influence on failure of phyllite slopes, including gravity, layer strength, joint combinations and excavation at slope foots. A 3D numerical simulation was applied to acquire the effect of four factors mentioned above and deformation characteristics, stress evolution rules of different phyllite slope were concluded. Besides, calculation model along with calculation equation of the length of unstable rock mass during the failure of slope and relationship model between phyllite slope stability and depth of weak plane were set up. To monitor the movement of all rock masses of landslide model, a non-contact displacement monitoring system was developed and used in the landslide modeling test. Bedding failure mechanisms of phyllite slope with different layer strength, joint composites and excavation ways were summarized based on the results of landslide modeling test and computational simulation. At last, deformation rules, stress evolution progress and strain accumulation characteristics of slope during the initial failure stage and the entire failure stage were revealed through a three-dimensional discrete element software 3DECThrough the study mentioned above, several conclusions were derived as follows:(1) the bedding failure were results of natural factors including topography, lithology, geological structure etc., which played internal roles on sliding controlling, and human activities including excavation at slope foots etc., which played as induced factor on sliding, and the failure mode were revealed as sliding tension. Excavation at slope foots lead to increasing of maximum principle stress and decreasing of minimum principle stress in the middle-front part of slope and compression cracking were formed in superficial zone due to “tension-compression” stress combination. Shear stress was concentrated at slope foot resulting in the formation of slip band, and finally progressive sliding tension was occurred;(2) landslide modeling tests and three-dimensional numerical simulation showed that layer strength, joint composites and excavation at slope foot influenced failure laws of phyllite slopes: weak plane accelerated the sliding and slope stability decreased as the depth of weak plane increased; slope stability was higher in the angle of 20-30° between joint dip direction and slope aspect and the angle made the failure characterized by “counter-clockwise” sliding; multi-stage excavation at slope foots made the sliding characterized by “lagging” and the failure mode belonged to integral sliding tension, while integral excavation at slope foots made the sliding characterized by “rapidity” and the failure mode belonged to progressive sliding tension;(3) calculation model of length of unstable rock mass and relationship between slope stability and depth of weak plane were set up, respectively, and showed that the length was affected by layer strength, tensile strength of joint plane, rock density, dip angle and thickness of strata, while the stability decreased to minimum stability Fs,min as the depth increased and Fs,min was positively correlated with friction of weak plan and was negatively correlated with dip of weak plan;(4) two kinds of failure modes were revealed through the study, namely, progressive sliding tension and integral sliding tension, the sliding mechanism of the former was “creeping-tension-sliding-tension-sliding” model and the latter one was “creeping-tension-shearing-sliding” model.