| With the rapid development of transportation construction in the west,experessways will inevitably pass through a large gro-up of karst areas,such as the highways of Yanru,Ruchen,Chenning,Ningdao,Guiwu and Louxin in Hunan Province and mostof the expresswaysof Guangxi and Guizhou provinces have a large number of karst roadbeds.At this time,how totreat the karst roadbed properly and effectivelyand evaluate its stability has become a key issue to be solved in engineering construction.Therefore,it is necessary to propose a more effective and economical treatment method based on comprehensive analysis of existing karst road treatment techniquesand evaluate its stability.To this end,this paper relies on the Guiyang to Linwu(Guiwu)Expressway in Hunan Province as the project,and uses theoretical analysis,numerical simulation and field test comprehensivelyto carry out research on the dynamic compaction treatment of highway subgrade in karst area,and proposes dynamic compactio treatment design principles and design parameters and stability evaluation methodsof highway subgrade in karst area.in order to provide reference for similar projects in the future.The main research contents of this paper are as follows:(1)Through the karst formation and development conditions,karst morphology and its characteristics,karst roadbed disease and karst roadbed stability issues,comprehensive analysis of karst roadbed diseases,and then the use of six conventional methods and normative methods for highway karst roadbed collapse,the possibility analysis is carried out.Based on this,the karst roadbed strong tamping treatment technology is proposed,and recommend values for design parmeters of the effective reinforcement depth and impact depth,sniper energy,slamming times,karma,and impact depth of karst roadbed are proposed based on the definition of karst roadbed dynamic tamping treatment.(2)According to the on-site experimental research on the dynamic compaction of roadbed treatment of the highway subgrade in the karst area,the test purpose and content of the highway subgrade in the karst area are determined according to the existing method of dynamic compaction of foundation treatment.That is to say,the detailed geological exploration of the dynamic compaction point is carried out.On the basis of static penetration test,the surface vibration acceleration and horizontal dynamic earth pressure at different horizontal distances,the vertical dynamic earth pressure at different depths,and the surface settlement of dynamic compaction point should be systematically tested to determine the selectioncriteria,instrument embedding methods and precautionsof dynamic compaction experiment.The comparison of ground vibration acceleration,dynamic stress,slamming number and horizontal distance was obtained through the comparison and analysis of field intensive test results,andverify the rationality of the design parameters of dynamic compaction for karst roadbed proposed in the paper.(3)Considering the effect of roadbed load and pavement load on karst roof,a stress analysis model for stability analysis of karst roadbed is proposed.Based on the calculation method of road vehicle load and karst roof load,the structural mechanics analysis method is used to establish the stability analysis method of karst roof considering the spatial shape of cave.That is to say,the karst roof fixed beam,parabolic arch,dome,two-way plate or shell analysis model is established,and the calculation method of minimum safety thickness of karst roof is established,which is determined by tensile strength.Through the typical engineering case,the interaction law between karst roof failure mode and karst shape,geometric plane size,vector height and roof surrounding rock strength is discussed.It is determined that the stability evaluation of karst roof should focus on the spatial shape and vector height of the cave.(4)In order to analyze the stability of the circular soil cave with double holes under the foundation,the method to obtain the stress of any point in the soil layer with double holes is proposed with the Cauchy integral method,Schwarz alternaing method and iteration method.Meanwhile,the stability evaluation method of the soil cave is presented based on the stress coordinate transformation and the Mohr-Coulomb strength criterion.The computational accuracy of the proposed method in this paper is verified through the comparison of the results with the exact analytical solution and the ABAQUS numerical simulation results.The influence laws of the lateral pressure coefficient,the soil cave radius ratio and the soil cave relative position to the stability of the double soil caves are analyzed,and the change rules of the soil cave stability coefficient with various influence factors are obtained.(5)The strength reduction method and numerical method are used to analyze the roof stability of the underlying caves under the construction load and the strong load.By discussing the variation law of karst roof deformation,principal stress and safety factor under differentspans,heights,buried depths and roof thickness.The safety and stability criteria of karst roof under different working conditions are obtained.If the depth is more than 20 m or the thickness of the roof rock layer exceeds 3m,it may not be treated.By comparing and analyzing the numerical simulation results of dynamic compaction treatment of karst roadbed under different working conditions,various possible working conditions corresponding to karst roof collapse under(200 × 20)kN·m single energy dynamic compaction are obtained.The rationality of the design parameters of dynamic compaction for karst roadbed of expressway is verified.(6)In order to reflect the influence of various uncertain factors in the stability analysis of karst roadbed,a risk analysis method for the stability of karst roadbed of expressway was proposed.Meanwhile,the fuzzy energy reliability analysis method was used to calculate the probability of the instability of karst roadbed,and a risk loss determination method for karst roadbed was established.The fuzzy energy reliability analysis method is used to determine the instability probability of karst roadbed.Based on the risk analysis theory,the risk loss determination method of karst roadbed and the stability risk analysis method are established.(7)Dynamic compaction treatment technology is applied to the treatment of karst roadbed of Ningdao expressway.Based on the comprehensive analysis of the engineering geological conditions of the Ningdao Expressway,the basic principles and specific treatment plans for the roadbed treatment of the karst area of the Ningdao Expressway are proposed.The original design scheme of karst cave grouting is analyzed through comprehensive optimization,the comprehensive treatment plan of karst roadbed based on dynamic compaction + excavation backfill + cover plate spanning,etc.In combination with the engineering geological situation of karst roadbed in six markers,the design scheme of dynamic compaction and crossing design scheme of cover plate are put forward. |
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