| At present,the double tunnel is widely used in many fields such as highway,railway and hydraulic engineering.The design of double tunnel should not only consider the improvement of its capacity but also the influence caused by its geological,environmental and land expropriation.It is well known that the stability of the surrounding rock is the necessary condition for the normal construction of the tunnel,especially the net distance.The distance is an important factor that directly affects the stability of double tunnels.Therefore,elastoplastic theory and numerical simulation are used to study the net distance of double tunnels.The main conclusions are as follows:(1)the theory of tunnel surrounding rock stability related to the theoretical derivation of this paper is summarized and analyzed,that is,the elastoplastic formula of the stress of the surrounding rock of a single tunnel is summed up,and the failure and instability process of the tunnel is qualitatively given,and the factors affecting the stability of the surrounding rock are four aspects,such as the depth,the net distance,the surrounding rock grade and the support condition.The analysis is carried out.Finally,the principle of determining the net distance is given:the plastic zone of the surrounding rock is not intersected,the middle rock column does not destroy,and the stress field of the surrounding rock does not superimpose.(2)The middle rock column of the tunnel is divided into plastic zone and intermediate core area,and the force model of the middle rock column in the double tunnel is simplified.The elastoplastic analysis of the middle rock column in the shallow tunnel is carried out.The formula of the minimum net distance of the double tunnel under the condition of the side pressure coefficient is equal to 1,the side pressure coefficient is less than 1 and the support is supported.The minimum net distance formula of deep buried double tunnel is deduced by changing the pressure distribution pattern of double tunnel under the pressure arch theory.(3)The parameters involved in the minimum net distance formula under each condition are analyzed,and the relation between the parameters and the minimum net distance is obtained,that is,The minimum net distance of double tunnel nonlinearly increases with the depth of tunnel and volume weight.With the cohesion,the internal friction angle,the increase of the internal friction angle and the larger pressure coefficient increases,the minimum net distance nonlinearity decreases.When the lateral pressure coefficient is less than 1,the minimum net distance is larger than the lateral pressure coefficient equal to 1.and the minimum net distance under the support condition is smaller than the minimum net distance under no support.(4)The numerical simulation of a mountain tunnel project is carried out.According to the stress distribution in the plastic zone and the vertical stress on the middle of the tunnel in the middle of the two tunnel,it is safe to select 1.5 times the tunnel diameter.In the design specification of highway tunnel,the net distance of the separated tunnel under the surrounding rock is not considered when the interaction between the two tunnels is not considered,and the distance of the net distance of the tunnel under the surrounding rock is 2.5 times the diameter of the tunnel.In this paper,the tunnel diameter of the two hole is considered and the minimum net distance of the buried depth is 1 times that of the buried depth calculation theory.It is less than the actual condition and the net distance from the standard,but the vertical stress on the tunnel plastic zone distribution and the middle connection of the two tunnel shows that the net distance value obtained by the theoretical calculation is also safe and reasonable. |
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