| When selecting lines,the uplink and downlink tunnels were often restricted by topography and geomorphology,so small net distance tunnel form was often used.in mountain highway.This paper aims at the research status like: the excavation method of small net distance tunnel was uncertain,the surrounding rock deformation was relatively special,the reasonable net distance and construction spacing were not clear and the stability of the soil column was not specific and so on.Through the methods of theoretical analysis combines numerical calculation,comparison verification model test and engineering case and so on,further studied the mechanical property of net distance and construction spacing change surrounding rock space effect,analyzed the strengthening method of the soil column,and proposed the safe net distance value.Through numerical simulation optimized a excavation method that more fit for the small net distance tunnel,putting forward the phenomenon that there was a certain bias in the deep or shallow buried surrounding rocks based on the formation subsidence characteristics under different buried depth conditions,and revealed two types of surface subsidence curve changeed with buried depth and the unique pressure arch subsidence phenomenon in middle clip region.Analyzed the influence drgree of different net distance and construction distance value on the double hole effect and the response characteristics of the soil surrounding rock affected by the double-hole effect,and proposed the reasonable construction spacing value.The excavation key points put forward by highway tunnel design details were optimized,and the effect of this point in the soil surrounding rock was clarified.Strengthened the middle clamping column,optimized longitudinal prestressed bolt placement spacing,putted forward reasonable layout density range,worked out the value range of after reinforcement minimum net distance,and provided reference for similar engineering design and construction.1.Respectively adopted the CRD method,the arc guide pit keeps core soil method and the two-side wall guide pit method to simulate excavation the small net distance tunnel.Considered from controlling ground settlement,medium clamping soil displacement and the development of plastic zone and the growth of initial structural stress,used contrast analysis,choosed the two-side wall guide pit method as the most suitable excavation method in soil surrounding rock small net distance tunnel.2.Researched the deformation characteristics of surrounding rock under different buried depth conditions,found that the vault settlement value was much larger than that of the middle column,and the upper displacement of the arch and middle column increased with buried depth,while the lower displacement have liitle influence with the buried depth.There was a certain bias phenomenon in both shallow and deep buried cave surrounding rocks.On the premise that the net distance was constant,the pressure value on both sides of the tunnel increases with the depth of buried depth,the deeper the buried depth,the greater the inside and outside pressure difference value and the greater the bias pressure,the buried depth change was one of the causes of medium clamping soil stress concentration phenomenon aggravation.As the buried depth increased,the surface subsidence curve gradually changed from "single hump" to "subsidence groove",the specific stratum in the middle region began to appear the pressure arch subsidence phenomenon,the location of the stratum gradually closed to the vault with the increase of the buried depth,and gradually leaved to the vault with the increase of the net distance and the depth increase of longitudinal excavation.3.Through numerical simulation of different net distance conditions,surrounding rock levels and tunnel design parameters,studied the mechanical properties and deformation rules of the surrounding rock.As the net distance decreases,the displacement of the medial arch and the soil column appeared a sharp growth,the lateral arch changed very little,showed the net distance change was the main reason in influencing the double-hole effect strength,at the same time,it was another reason that making the clip soil column stress phenomenon intensified,the smaller the net distance,the greater the bias phenomenon of the surrounding rock.When the net distance was 24m~ 9m,the both sides distribution of the middle column two-hole strong influence area was 60% of the total area,the relatively weak central region was about 40% of the total area,at the same time,the plastic zone develops slowly.When the net distance was 9m~ 7m,the distribution rate of strongly affected areas began to increase slowly,the plastic zone develops quickly.When the net distance was 6m~ 3m,the increase of strongly affected areas was significant,the plastic zone grew rapidly and was well connected.Both the weakeness of the surrounding rock and the increasement of span would increase the influence of excavation disturbance,and made the stress environment of the surrounding rock more complicated,and the sensitivity of tunneling structure span increases was larger than the surrounding rock weakening.4.The response characteristics of the hole surrounding rock under the construction of different vertical palm surfaces were studied.The vertical displacement of each layer was the maximum at the entrance of the cave,and gradually decrease as the depth increases,gradually stable at 10 m distance in front of the palm.The differential settlement of the left and right holes and the main stress growth rate of the middle soil column change was not obvious when spacing?0.5D and?2.0D,the change was significant between 0.5D~1.5D;The increase of spacing will also expand the distribution of the antecedent plastic zone,A new secondary disturbance region was formed in the arch shoulder and the inverted arch,the change of spacing was the secondary reason to change the double-hole strength effect,and noted that the reasonable palm surface construction distance should be?0.5,or?2.0D.5.Optimized the excavation technique key points of the double-side wall in the detailed rules,and supplemented the effect of this point in soil surrounding rock.With the increase of the construction distance of the left and right pits,when excavated to one-side guide pit,both the first-forming pit and the post-forming pit were affected,the first-forming pit showed that the displacement and stress value increased slowly,but the post-forming pit showed that the transverse displacement and stress value decreased gradually,and the post-forming pit change rate was relatively large,therefore,the technical points of the excavation were not significant to the stability of the first-forming pit,but it made a great contribution to the stability of the post-forming pit.The change of the construction distance of the left and right pits had less influence on the vertical displacement of the surrounding rock,but considered from controlling the growth of horizontal displacement in the palm,the appropriate distance range should be?1.0D.6.The medium clamping column should be divided into upper,middle and lower parts to reinforce,it was concluded that the overall reinforcement + prestressed bolt + grouting was the best to control the stability of the medium clamping soil column.Researched on the implant density of longitudinal prestressed bolt,suggested that the implant spacing was not proportional to the reinforcement effect: When the set spacing range was 0.6m~1.0 m,the development of vertical displacement and the increase of stress value of the medium clamping soil column were better controlled;When the set spacing range was 0.3m~0.6m,the control effect was secondary;When the set spacing range was 1.0m~1.5m,the control effect was the worst;The development of the lateral displacement of the surrounding rock was very small regardless of the set spacing range.The after-reinforcement value range of the minimum net distance was proposed,? level surrounding rock of two-lane minimum interval value was 4 ~ 7 m,the third lane was 5~ 8m;? level surrounding rock two-lane was 5 ~ 8 m,the third lane was 6~ 9m.To sum up,through the study of soil mechanics characteristics of small clear distance tunnel,select the suitable excavation method,the stress of the surrounding rock deformation law reveals the hole,put forward the clip in the soil column reasonable reinforcement measures,for the soil surrounding rock out in the form of small clear distance tunnel design and construction provides a certain reference. |
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