Study On Surrounding Rock Mechanical Behavior During Construction Of Double Tunnel Obliquely Going Through High And Steep Slope

When shallow tunnel is inevitably built through the high and steep slope landslides, the rock mechanical behavior will be very complex due to the bias stress condition, which can seriously impact on the construction and structure safety. Thus this issue must be analyzed thoroughly in order to take specific measures to ensure safety and eliminate risk in the process of double hole tunnel construction in the high and steep slope.So in this paper, based on the significant and difficult issues occuring in the construction process of Caopingzi tunnel which is under through high and steep slopes located between Kala and Yangfanggou hydropower with a history of landslide and mud-rock flow, the in-depth analysis for the mechanical behavior and failure modes of the surrounding rock are conducted by a comprehensive apply of literature research, theoretical analysis, numerical simulation, site surveys and other methods. The major research work and achievements are listed as follows.1. After Systematical analysis of Caopingzi tunnel condition, such as geology, tunnel structure, two-hole spatial distribution, construction methods, as well as their impact on the construction safety, conclusion can be drawn that suitable construction methods and control measures must be taken for double tunnel construction in high and steep slope with such poor geological conditions as landslides, shallow depth, bias stress distribution and so on. Otherwise landslides and tunnel collapse will certainly happen. Incidentally landslides generated would cause severe damage to the tunnel lining structural. Therefore close attention should be paid to the double tunnel surrounding rock stress, displacement distribution, their changing laws as well as failure modes in such mountain geological conditions.2. The existing theories and methods of tunnel engineering are summarized systematically. There are the second, third stress and the displacement solution of the deep tunnel surrounding rock, the second stress and the displacement solution of the deep bias tunnel surrounding rock, and the solving method of the second stress and displacement of the deep two-hole tunnel surrounding rock. Then the distribution characteristic of rock stresses and displacements in deep tunnel were analyzed by using deep tunnel rock analytical solution. Although this solution is completely accurate, but could not consider the impact of body force. So the scope of its use is restricted indeed.3. According to the transportation engineering practice in Kayang, the concept and the mechanical model of sub-deep tunnel are put forward firstly. This model take body force impact into account so that it can better meet the engineering practice. Meanwhile it overcomes the shortcoming of deep tunnel analytic solution, which can not consider body force. Based on this mechanical model, applying the complex variables functions method of the elastic theory, the analytical solutions of the surrounding rock stress and displacement distribution regularities in the sub-deep tunnel have been deduced. Then using the formula deduced, the sub-deep tunnel problems above are calculated. Compared with numerical solutions under the same conditions, the analytical solutions show good symmetry characteristics, which can well reveal the secondary stress and displacement distribution of sub-deep tunnel surrounding rock. The analytical solutions and numerical solutions prove well to each other. Moreover, the rules revealed are in line with people’s understanding. Thus the analytical results are reasonable. It has a high reference value in the future design and construction of sub-deep tunnel, even deep tunnel.4. Reference to plane issues, space issues and numerical calculation theory, excavate working conditions were analyzed respectively for deep buried section and shallow buried bias sections. The former mainly consider three sets of variables such as excavation methods (full section method and bench cut method), support patterns (the supporting structure followed in time or not), and tunnel through forms (single hole through and double holes through). The latter is basically the same with the former, only the third variables change to the sequence of excavation (the outside hole firstly or the inside hole firstly). In view of the above variables to be analyzed, four conditions are set for both section at last. Finally, each working condition of are calculated and the effects of different variables on the mechanical behavior of surrounding rock are analyzed. The results showed that:change the method of excavation and support programs can greatly impact on the mechanical behavior of rock, while tunnel through forms and excavation sequence have a smaller effect.Specifically bench cut method can effectively control the vault settlement and sidewall vertical stress concentration, while full section method can effectively control the arch bottom bulge, side walls horizontal displacement, vaults and arch bottom stress concentration.Supporting structure followed in time can largely reduce rock displacement, for instance vault settlement and the arch bottom bulge displacement can be reduced by more than40%. So supporting effect of the structure designed is quite obvious.There is no big difference of the surrounding rock displacement and stress change rule for single hole through and double holes through. The surrounding rock stress and displacement distribution of double hole can be gained by symmetric replication from single hole distribution.Dig the left hole (deep buried lateral hole) firstly is advantageous to lower right hole vault subsidence and horizontal displacement convergence of the right side wall. On the contrary, dig the right hole (shallow buried side hole) firstly can help control right hole arch bottom bulge and horizontal displacement convergence of the left side wall. But the effect is very limited, and the surrounding rock displacement of left hole is not sensitive to the change of the excavation sequence.The above related conclusions about tunnel through forms and excavation sequence show that the double hole interval of Caopingzi double tunnel can satisfy safety net distance requirements for the deep buried section and shallow bias section. The mutual influence between two holes in the construction process is quite weak and negligible. So the tunnel face distance between the double holes is unconstrained, and the excavation sequence also don’t have special requirements. As long as the single hole construction is reasonable, the safety of the whole project construction can be ensured.5. According to theoretical and numerical analysis, combined with the tunnel construction practice, the surrounding rock stress and displacement distribution rules and failure modes is obtained during construction of double tunnel under through high and steep slope or landslide.The analysis results show that the surrounding rock in deep buried section display a x-shaped shear failure. Whereas the mechanical behavior and failure mode of surrounding rock are very different. They vary greatly in shallow buried bias sections. The existence of bias condition of shallow buried tunnels, especially at the tunnel entrance makes initial stress field deflect. The failure modes of surrounding rock for the shallow buried side show overall collapse damage (U-shaped or V-shaped) or extrusion damage (L-shaped), while deep side show y-shape shear failure.Therefore, in the construction process, enough attention should be paid to both sides of vault and left side of arch bottom for left hole (deep buried lateral hole), and also the upper right part of right hole, where shear failure much more likely occur. Specifically for the left hole, strong support includes forepoling should be applied and must be closed as soon as possible. For the right hole, weak blasting and tight lining must be strictly implemented. At the same time slope reinforcement should go ahead of tunnel excavation in order to ensure the slope stability and tunnel safety. The surrounding rock failure mode put forward above has important guiding significance for the construction of this project, also it can provide some reference for other similar projects.