Key Technology Study On Forked Tunnel Design And Construction

Forked tunnel is selected to construct in Hurongxi highway Baziling tunnel because of landform condition limit and in order to reduce project cost. Forked tunnel is composed of large-arch tunnel, multi-arch tunnel, neighborhood tunnel and separated runnel, whose design and construction is complex also. In this paper, the key technology in design and construction of forked tunnel is researched by theory analysis, geo-mechanical model test and numerical simulation methods. In the end the research results are contrasted and validate by in-situ monitor results.Tunnel middle wall bearing weight model is set up, the middle wall limit intensity is proposed according to concrete middle wall of multi-arch tunnel and rock middle wall of neighborhood tunnel, and middle wall stability criterion is presented. The support measure of neighborhood tunnel middle wall is researched, when tunnel depth is constant, the neighborhood tunnel middle wall can be reinforced by enhancing its c,φvalue and increasing side force by pre-stress through bolts.Forked tunnel is researched by geo-mechanical model test using new type test-bed and new type analogy material IBSCM. Two models are researched, model 1 is from large-arch section tunnel to multi-arch section tunnel, model 2 is from multi-arch section tunnel to neighborhood section tunnel. Tunnel surrounding rock stress, displacement, failure are analyzed in construction process and overloading safety of model 2 is also studied. The model test results show that vault displacement of large-arch section and multi-arch section tunnel is 4mm, vault displacement of multi-arch section and neighborhood section tunnel is 4.5-6.0mm, horizontal displacement is small. Stress in middle wall is 6.5MPa. The transition of large-arch section and multi-arch section tunnel is not safe, model 2 shows that surrounding rock in vault, hance and middle wall is easy to damage, neighborhood section tunnel near multi-arch section tunnel is damaged worse. The tunnel overloading safety degree is 3.5.Using damage and elasto-plastic coupling method, according to irreversible thermodynamics principle, the damage softening failure theory is presented. The large 3-D damage elasto-plastic finite element code D-FEM is programmed using FORTRAN langue. D-FEM can simulate geotechnical construction process including excavation, backfill and it has quickly solving, group function and many material models. Comparing with FLAC^3D, D-FEM can simulate rock mass failure really considering damage strain softening. The reasonable width of multi-arch tunnel straight and bend middle wall and neighborhood tunnel middle wall are researched respectively by D-FEM. The influence of different surrounding rock type and different tunnel depth to middle wall is considered, construction sequence of forked tunnel is also optimized. The conclusion is drew as follows: (1) straight middle wall is more stable then bend middle wall, straight middle wall should be used in multi-arch tunnel. (2) The construction sequence of lift tunnel 32m ahead and lift and right tunnels excavation in-phase should be adopted in Baziling forked tunnel.Forked tunnel whole line type design optimization model is presented. We can get most economical line type according to this model. Support measure of forked tunnel is also presented, according to original design of Baziling forked tunnel, the rational advice is presented.In different sections including large-arch tunnel, multi-arch tunnel and neighborhood tunnel, displacement, bolts stress, steel structure stress, lining stress, middle wall pressure and released zone are measured. The model test and numerical result is correct by contrast with monitor result.