| Karst landform is widely distributed in our country with many different types.Along with the continuous advancement of transportation engineering projects into Karst mountainous area,Karst has become one of the key engineering technical problems faced with the tunnel construction engineering.The shape and filling material of Karst have direct i MPacts on the normal tunnel construction and safe operation.To avoid the adverse i MPacts of Karst on engineering construction,the present transportation engineering tends to adapt high-grade route selection,and the routes are always within the vertical infiltration zone of Karst,among which the vertical no-filling Karst cavities are the most commonly seen.Different shapes of this type of Karst produces conspicuous effects on unloading characteristics of surrounding rocks,and vertical no-filling Karst cavities with different shapes and sizes pose relatively huge difference of i MPact on the stability of surrounding rocks around tunnel.How to correctly evaluate the i MPact on surrounding rock stability produced by the Karst cavities of different shapes becomes a hard nut to crack in design.This paper takes a segment of Laobishan tunnel of Cheng Kun double railway track,where Karst exists,as research object.It begins with characteristics of geometrical shapes of vertical no-filling Karst cavities,and studies the detection methods and response characteristics of this type of Karst.Through survey and three-dimensional scanning technique,it summarizes characteristics of tunnel surrounding rocks and Karst,and builds threedimensional geological fine model and generalized model on tunnel and Karst cave.It employs three-dimensional finite element of MIDAS/GTS to simulate the i MPact of forward vertical Karst caves on tunnel,and co MPares deformation of surrounding rocks and mechanical effects of support structures under different excavation methods.Then,it modifies the construction project of this segment,co MPares deformation of surrounding rocks and mechanical effects of support structure under different advanced support methods,and discusses on the calculation results from Karst models with different precision.The main content and achieved results are as follows:(1)Based on a large number of field survey resources like geological structure,stratigraphic lithology,rock mass quality,Karst development state of Laobishan tunnel of Chengkun double railway track,it employs early forecast methods like geological radar detection,TSP and early horizontal drilling to detect and find out the adverse geological conditions of tunnel in DK207+507～480 segment.The bad conditions include relatively low strength of surrounding rock,relatively high joint and fissure development,relatively broken surrounding rock,high intensity Karst development,existing erosion phenomenon like Karst cave and cavity,and low stability surrounding rocks.It comprehensively utilizes acquired geological logging and physical detection to construct development density of small joints as geological symbols of vertical Karst cavity development,and finds out the bad geological condition of Karst development in location of DK207+508 with this symbol.(2)It conducts fine scanning on Karst cavities of DK207+508 of the tunnel with three-dimensional laser scanning technique.Based on traditional generalized Karst cavity calculation model,it builds a finer three dimensional Karst cave geological model.It respectively analyzes the i MPacts on displacement and strength response characteristics of tunnel of DK207+608～508 segment by traditional generalized Karst cave model and three dimensional scanning Karst cave model,using three dimensional finite element software MIDAS/GTS.The simulation results show that: the change patterns of overall displacements of feature points around the cave all demonstrate that firstly gradually increase to the maximum and then gradually decrease after the completion of tunnel excavation.Calculation results of the two Karst cave models all illustrate that displacement of feature points around the cave reaches maximum value in the 20 th cross-section,and the biggest difference of maximum displacement between traditional generalized model and scanning Karst cavity model is about 5mm,which is relatively small.Response strength distribution patterns of the two models are overall similar,while the difference value is rather big.The difference of maximum principal stress in soffit reaches 0.21 MPa,while the minimum reaches 1.39 MPa.Thus,fine model can be adopted to simulate and analyze to improve the reliability of support structure when using strength theory to conduct support structure design.(3)It respectively analyzes construction procedures without early support on the tunnel segment of DK207+608～508 using two-bench method,under the condition of having or having not Karst cave in the front with numerical simulation software.The simulation results show that: distribution patterns of displacement and principal stress with or without the Karst cave resemble,and the displacement and principal stress with Karst cave are bigger than that without Karst cave.The maximum displacement of arch crown reaches 55.39 mm with Karst cave after the completion of excavation,which is 27.53% higher than that without Karst cave.The maximum principal stress on arch crown increases 31.66% than that without Karst cave,and the minimum principal stress increases 28.67%.The 20 th cross-section of the tunnel is the most unfavorable location for the construction due to the overlap of construction plastic zone and Karst cave plastic zone.Combined with the increasing extent of displacement and principal stress on arch crown co MPared with no Karst cave,it can be inferred that deformation risk exists on the arch crown of the 20 th cross-section of the tunnel.(4)Numerical simulation software is employed to respectively simulate and analyze the construction procedures of tunnel in segment DK207+608～508 under three different excavation methods.The simulation results demonstrate that: the characteristics of displacement,principal stress and preliminary support stress distribution of three excavation methods resemble with that of two-bench method.Three-bench seven-step method has the best controlling effect on the displacement,and the displacement decreases 12.59 mm at maximum co MPared with that of two-bench method.Three-bench seven-step method has the best controlling effect on the principal stress of arch crown and inverted arch,and the stress decreases 0.75 MPa at maximum co MPared with two-bench method.Three-bench temporary cross-bracing method has the best controlling effect on principal stress of arch waist,and the stress decreases 0.89 MPa at maximum co MPared with two-bench method.Three-bench seven-step method has the best controlling effect on stress of preliminary support arch crown and inverted arch,and the stress decreases 0.62 MPa at maximum co MPared with two-bench method.Three-bench temporary cross-bracing method has the best controlling effect on stress of preliminary support arch waist,and the stress decreases 1.13 MPa at maximum co MPared with two-bench method.Stress on the arch waist can be well controlled using three-bench temporary cross-bracing method,whereas the number of steps of construction is large,the requirement on construction conditions is high,and construction is highly difficult.Thus,three-bench seven-step method is recommended to construct.(5)Construction procedure of tunnel segment of DK207+608～508 is simulated and analyzed under three different early support methods with numerical simulation software.The simulation results show that: The characteristics of displacement,principal stress and preliminary support stress distribution of the three support methods resemble with that of two-bench no pre-support model.Two-layer small-pipe supporting has the best controlling effect on displacement of arch crown,and the displacement decreases 12.58 mm co MPared with that of no pre-supporting.Middle shed-pipe supporting has the best controlling effect on displacement of other feature points,and the displacement decreases 11.60 mm at maximum co MPared with that of no pre-supporting.Middle shed-pipe supporting has the best controlling effect on principal stress of arch crown and inverted arch,and the stress decreases 1.12 MPa at maximum co MPared with that of no pre-supporting.Double-layer small-pipe supporting has the best controlling effect on principal stress of arch waist,and the stress decreases 1.30 MPa at maximum co MPared with that of no pre-supporting.Middle shed-pipe supporting has the best controlling effect on preliminary supporting stress of arch crown and inverted arch,and the stress decreases 0.95 MPa at maximum co MPared with that of no pre-supporting.Double-layer small-pipe supporting has the best controlling effect on stress of preliminary supporting arch waist,and the stress decreases 0.47 MPa at maximum co MPared with that of no pre-supporting.The maximum difference of stress between middle shed-pipe supporting arch waist and double-layer small-pipe supporting reaches 0.09 MPa.The results are very close,and thus,middle shed-pipe pre-supporting method is recommended after comprehensive consideration. |
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