| A large number of tunnel construction practice shows that the non-filled cave has little impact on tunnel engineering safety.The construction of the Feifengshan tunnel(3,400 m in length)from the boundary to the Leshan line also encountered unfilled caves,and the tunnel was excavated to DK35.In the +267 ~ DK35+261 section,the vault reveals a non-filled cave with a width of about 3 m.After the conventional treatment,the tunnel continues to excavate to the DK35+249 position,and the surrounding rock of the DK35+259 ~ DK35+249 section appears in a wide range.The deformation causes the initial support of the section to break with the front part of the support,and the extrusion deformation occurs.The emergence of this problem has attracted the attention of survey,design and construction units.In view of the above problems,on the basis of on-site investigation of local overall Karst Characteristics and geological conditions such as surrounding rocks,combined with the data and maps obtained by geological exploration and advanced detection,the most representative section of tunnel excavation uncovering the instability of arch roof without filling karst cave is selected to establish a generalized model,and then the excavation support scheme is designed according to actual working conditions,and the three-dimensional finite element Midas/GTS is adopted.By simulating different excavation methods,the mechanism of surrounding rock deformation induced by this kind of karst cave and the causes of its mechanical effect on supporting structure are analyzed,the construction scheme is optimized,and the supporting forms are adjusted to solve the key technical problems of tunnel safety construction.The main research results are as follows:(1)According to the forward results of FDTD,the GPR response characteristics of the unfilled cave are obtained:1 Waveform characteristics: a.The curvature of the in-phase axis is affected by the ratio of the shape of the cavity to the horizontal and vertical axes of the model.The smaller the ratio of the horizontal and vertical axes,the greater the curvature of the same phase axis,the larger the ratio of the horizontal and vertical axes,and the bending of the same axis.The degree is small;b.When the electromagnetic wave propagates through the karst region,the interface reflection at the top interface of the karst is more intense than the interface generated at the bottom interface,and the effect is more obvious;c.As the detection distance of the shale formation increases The electromagnetic wave has obvious energy attenuation,the bottom interface of the rock layer is not obvious,and the bottom interface resolution of the rock layer is reduced.2 Spectral characteristics: The cave is filled with air,and electromagnetic waves propagate in the air with less energy loss.Perform spectrum analysis on the forward data,from which the relationship between energy and frequency can be seen.Since the karst cave is filled by a single gas phase air medium,the main frequency value obtained after spectrum analysis reflects the characteristics of the single gas phase air medium filling karst cave.The main frequency value can be used as a quantitative index for presuming empty karst caves;since the karst cave is only filled by the gas phase single medium air The filled air medium is uniform,and the unimodal characteristic near the dominant frequency in the spectrum analysis diagram can also be used as a criterion.(2)Based on the three-dimensional finite element Midas/GTS simulation of the excavation construction process of the Feifengshan tunnel research section,after the excavation and support of the tunnel top unfilled karst tunnel section,on the tunnel arch and the left and right arch shoulders Due to the existence of the unfilled cave in the vault,the difference between the DK35+265 ~ DK35+262 section and the deformation of the section before and after the section caused the fracture of the rear lining of the cave.In addition,the maximum deformation position of the left and right arch waists and the minimum deformation position in the middle of the bottom plate occur in the range of the unfilled caves in the vault.The difference in the deformation between these positions and the front and rear regions is also the factor that causes the lining of the cavern to break.(3)Simulated the construction process of four different excavation methods in the tunnel research section.On the control of the difference between the DK35+265 ~ DK35+262 segments and the front and rear positional deformation of the four excavation methods,the CD method is used to excavate the arch.The deformation control effect of the top,the arch shoulder and the bottom plate is the best.The three-step method has the best effect on the deformation control of the arch waist.Compared with the deformation of the full-section excavation at the position of the DK35+265,two steps and three steps,The phenomenon of deformation of the CD method at this mileage position is not obvious.(4)Through the monitoring of the displacement of the arch,the arch shoulder and the arch waist of the lining of the DK35+259 lining,the results show that the CD method has the best displacement control effect on the dome,and the limitation of the three steps on the horizontal displacement of the arch waist is There are slight advantages in the four excavation methods.The maximum principal stress at the vault,arched shoulder and arch waist of the DK35+259 lining break is monitored.Among the four excavation methods,the CD is excavated in the vault,the left arch and the left arch.The stress value is the smallest,and the maximum principal stress value of the three-step excavation at the right arch shoulder and the right arch waist is the smallest.The DK35+259 lining dur ing the CD excavation process has the least effect on the combined stress at the five monitoring points.(5)The three types of advanced support methods have reduced the deformation difference between the unfilled karst section of the DK35+265 ~ DK35+262 dome and its front and rear positions.The advance small conduit support scheme is better than the advanced bolt support scheme in the control of the difference between the DK35+265 ~ DK35+262 and its front and rear arch deformation.The advanced grouting backfill support scheme is used in the cave area DK35+265 ~ The difference between the DK35+262 segment and its front and rear position dome deformation is minimal. |
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