Failure Mchanism And Countermeasure Of Silicastone Cuesta Cutting Slope Of Ziyuan-xing'an Expressway

Cuesta slope is a long low ridge with a steep scarp slope and a gentle back slope which composed of monocline.And the silicastone ranks the fourth place of the sedimentary rock with silicon dioxide as the main chemical composition.Untile today,the railway and highway projects have not yet been covered by such large scale,long distance and wide area distribution of cuesta slope,so there is no successful experience to be borrowed.Aiming at silicastone cuesta cutting slope of Ziyuan-Xing'an Expressway,the field survey statistics,laboratory tests,in-situ direct shear tests,numerical simulations and field monitoring have been used to research the silicastone mechanical properties,the discontinuity mechanical properties,stability factor analysis,the rock discontinuity network modeling,cuesta cutting slope failure mechanism and engineering control effect evaluation.The following conclusions are observed:(1)The cuesta slope of research area influenced by the Ziyuan-Xingning tension-torsion fault,and the strike of the slope perpendicular to the fault.The dip angle of the slope is 25° approximately and two discontinuities were developed,because of the dicontinuites are unstable structural plane,and the cutting may caused rock collapses or bedding sliding(2)Summarized the relationship between the structure and the failure type of the silicastone cuesta cutting slope,and the silicastone rock mass structure are divided into five types which are bonded structure,block structure,layer structure,cataclastic structure and bulk structure.The silicastone cuesta cutting slope failure type are main sliding-tensile failure or controlled by the discontinuity.And the stability factors rank slope angle> internal friction angle> excavation height> cutting slope angle> cohesion=layer thickness.(3)In uniaxial compression tests,when the compressive stress at the 50~65% of the peak stress,rock enters the crack sharply extension stage,and the stress-strain curves appears a “platform”.But after the “platform”,the stress still increase until splitting failure.And the triaxial tests show that the shear failure not cut-through under low confining pressure,with the confining pressure increasing,the rupture angle gradually decreases.(4)In order to understand the discontinuity plane strength ofsilicastone,in-situ shearing tests are done in Tianmenvillage,Guangxi Zhuang Autonomous Region.Then estimate the discontinuity plane strength by Barton model based on field survey.The tests results indicate that shear stress-displacement curve have the characteristic of plastic deformation with climbing,cavitation and dilatancy,and the shear curve can be divided into dilatancy,cutting salient and complete contact three progresses.At the beginning of shearing,the shear stress increased rapidly and the shear displacement is increasing linearly with shear stress in the elastic state.With the increasing of shear stress,climbing and cutting salient are occurred in the discontinuity plane.After the cutting salient,the shear stress increase with gradient decreasing till peak strength.The results of two methods are consistent but the shear strength parameters of Barton model are lower than in-situ shearing tests.As the parameters determining of Barton model is subjective,it is very necessary to do the in-situ shearing tests in a region without empirical value and reference engineering.The in-situ shearing tests.(5)The three-dimention model was established by Midas GTS NX software,and the slope failure is sliding-tensile.The excavation cuases stress concentration at the slope foot then the microfrature gradually accumulates untile the fracture surface formed.The rock discontinuity network modeling based on the K-means clustering method show that as the slope cutting reveal the discontinuity surface,the slope slides along the discontinuity surface.And the simulated results indicate that the slope is stable in the original and secondary cutting slope,however,the slope failue probability during the first cutting slope cutting is 76%.(6)The bedding slide rapping range are affected by slope angle,discontinuity strength parameter,excavation height.The reinforcement of the slope adopts the prestressed cable-bolting and lattice beams,part ndependent cable boltings are also used.The surface deformation monitoring shows the displacement tendency is increasing then stabilizing.And the cable-bolting stress passes through the stress attenuation stage,fluctuation phase and stabilization phase three stages.And the monitoring results show that the slope presents a stable state after the current governance design scheme,which means the slope reinforcement effect is obviously effective.