Analysis Of Stability And Bolt Stabilization Of Flysch Layered Slope

Flysch is a kind of sedimentary structure composed of interlaced sandstone,shale and mudstone.Flysch slope is a typical layered slope,which is anisotropic and easilyweathered.If not treated properly,engineering disaster is easy to occur in flysch slope.Flysch is very common in Europe and there are some engineerings constructed by China in flysch area in Europe,so it is necessary to make a further understanding of the features of flysch slope.In this paper,the stability and bolt reinforcement effect of flysch layered slope were analyzed by finite element limit analysis method based on the field and literature investigations.First,the safety factors and failure modes of layered slopes were analyzed by explicit joint method and implicit joint method respectively and then the most unfavorable occurrence of bedding plane was discussed.The results of explicit joint method and implicit joint method both show that as dip angle of bedding plane increases from 0° to 90°,safety factor decreases first then increases and then decreases slightly.Failure modes show a progress of shear-sliding-out failure,sliding-tensile failure,overall bedding-slip failure,slip-cut-out failure,shear-slip failure and overall shear failure.When layer thickness is small,the result of implicit joint method is very close to that of explicit joint method.Implicit joint method is more applicable in subsequent analysis because of its higher efficiency.With the increasing of the slope angle,the most dangerous dip angle of bedding plane increases while the corresponding safety factor decreases.For gentlyinclined layered slope,the bedding plane through the slope toe plays a decisive role in slope stability,the stability of this kind of layered slope can be analyzed by simplified model with one single bedding plane.Second,the influence of anchorage angle and bolt length on the safety factors and failure modes of bolt stabilized layered slopes were analyzed and then the bolt length distribution was optimized based on equal-length bolts.The results show that as anchorage angle increases from 0° to 50°,the safety factor increases first and then decreases.There is an optimum anchorage angle,which is affected by dip angle of bedding plane and reduction coefficient of strength of contact surface between bolt and rock.When anchorage angle is less than optimum anchorage angle,deep failure occurs in layered slope.When anchorage angle is greater than optimum anchorage angle,shallow failure occurs in layered slope.In the range of 1m to 28 m,when bolt length is very small,the bolt is too short to stabilize the slope and the safety factor of bolt stabilized layered slope is same to that of slope without bolt.When bolt length is long enough to improve the slope stability,the safety factor increases first and then stays unchanged with the increasing of bolt length.There is an effective bolt length,which increases first and then decreases and then increases with the increasing of dip angle of bedding plane.When bolt length is shorter than effective bolt length,deep failure occurs in layered slope and the failure surface moves deeper as blot length increases.When bolt length is longer than effective bolt length,shallow failure occurs in layered slope.By the optimization of bolt length distribution,bolt materials could be saved with saving rates ranging up to 36%.The deeper the failure surface of the slope without bolt is,the longer optimized bolt needed to cross the failure surface is.Finally,the stability of the multi-step layered slope is analyzed and the design of slope of Bar-Boljare highway engineering in Montenegro is optimized according to engineering conditions and existed research results.The results show that the change of safety factor of multi-step slope along dip angle of bedding plane is same to that of singlestep slope.The bigger the step breadth is,the larger the safety factor and the smaller the most dangerous dip angle of bedding plane are.As the dip angle of bedding plane increases,the safety factor improvement rate of multi-step slope comparing to single-step slope decreases first and then increases and then decreases.The optimization of anchorage angle can improve the safety factor by about 5%.The optimization of bolt length distribution can save bolts more than 42%.Extending the bolt length first and then optimizing the bolt length distribution can get the optimum bolt length distribution which can maximize the reinforcement effect.