| Karst is a common disadvantageous engineering geology,which is widely distributed in China,accounting for 1/3 of China’s land area,es pecially in the southern region,such as Yunnan,Guizhou,Guangxi,Hunan and Hubei.With the development of economy and the development of the western region,and the implementation of the "The Belt and Road" strategy,a large number of infrastructures are constructed in the karst areas.Karst is often encountered in the process of expressway construction.How to evaluate the stability of subgrade in karst area is one of the key technologies in engineering design and construction,which relates to the safety and economy of the whole project.Therefore,the method of the subgrade stability analysis in karst area has important theoretical significance and practice value.Following the previous studies,the effects of section shape,geometric size and distribution shape of the karst cave are considered in this paper.Combining theoretical and numerical analysis methods,the stability of circular and horseshoe-shaped caves under subgrade load is analyzed,and th e ultimate bearing capacity of subgrade is calculated,which has considered the effect of the rotation angle and the spatial distribution of caves.Considering engineering safety and reducing roadbed settlement,A Karst treatment method for pile-net composite foundation is proposed.Finally,a whole set of methods karst roadbed stability evaluation and treatment is formed.The main study contents are shown as follows:(1)To investigate the stability of circular karst cave under subgrade,based on the engineering practice,a theoretical analysis model is established,which considers the load of roadbed and overlying soil layer as a whole.Based on the theory of complex variable function,the analysis plane is mapped to the outer field of unit circle by mapping function.Using conformal transformation and Cauchy integral,the expressions of stress components in the stratum with circular cave under subgrade load are derived.On this basis,the maximum and minimum principal stresses at any point on the cave are obtained,and the Griffith failure criterion is introduced to judge the failure state of each point,so as to evaluate the overall stability of the cave.(2)Based on the stability analysis of circular karst cave underlying subgrade,the application of complex variable function method in stability analysis of roadbed in karst area is further deepened.Considering the stability of horseshoe-shaped karst caves under subgrade,three mapping functions of karst caves with arbitrary cross-section shape are introduced,including graphic combination method,polygonal approximation method and triangular interpolation method.Then,the formulas for calculating the stress at the edge of the cave in horseshoe-shaped cave stratum under subgrade load are obtained,and the maximum and minimum principal stresses at each point are calculated.Based on the Griffith failure criterion,the failure state of each point at the edge of karst cave can be judged,and the stability evaluation method of horseshoe-shaped karst cave under subgrade load is established.The method is verified by numerical analysis and engineering examples.(3)According to the theory of finite element limit analysis,the upper and lower limit theorems are transformed into corresponding mathematical programming models.Based on the platform of MATLAB,the finite element limit analysi s program is developed.This method combines the advantages of limit analysis method and finite element method,which does not need to construct maneuvering field and static field artificially,nor does it need load-displacement curve to judge the ultimate bearing capacity of subgrade.The upper and lower bounds can be obtained directly,which lays a foundation for stability analysis of karst subgrade under complex conditions.An adaptive meshing technique is developed to reduce the relative errors between the upper bound and the lower bound,which can automatically adjust the meshing density according to the different energy dissipation.(4)The stability of rectangular karst cave under subgrade load is analyzed by using finite limit analysis method.According to the engineering practice,the numerical analysis model of rectangular cave stability under subgrade load is established.Then,the ultimate bearing capacity of the subgrade is calculated,and the calculation results are dealt with dimensionless,which are summarized as design tables.The effects of physical and mechanical parameters of materials,the karst cave span,height and rotation angle on the ultimate bearing capacity of subgrade are discussed in detail.(5)The modified Hoek-Brown criterion is directly embedded in the finite limit analysis program to calculate the ultimate bearing capacity of the subgrade above multi-cave strata.Taking the bearing capacity of the subgrade above a single circular karst cave as a reference object,the dimension less parameters for measuring the bearing capacity of the subgrade under the condition of multiple karst caves are defined,and the design calculation tables for engineering practice are obtained.The influence of cave size,horizontal distance between cav es,vertical distance and rock mass parameters on the ultimate bearing capacity of subgrade has also beeb discussed.(6)Considering the engineering safety and reducing the settlement of the subgrade,a method of treating Karst by pile-net composite foundation is proposed for the condition that roadbed is easy to lose stability in Karst area.The pile-soil stress ratio without considering the influence of karst cave is calculated.Meanwhile,the relationship between deformation and stress of reinforcement after treatment of karst cave with pile-net composite foundation is analyzed to guide the setting of reinforcement in the process of treatment of karst cave with pile-net composite foundation. |
PDF Full Text Request