| During the construction of municipal and mountain tunnels,they often pass through complex accumulation areas.Due to the poor self-stabilizing ability of the fly ash filling body,the force is broken into scattered particles,and the physical and mechanical properties are discrete.The stability of surrounding rock in the coal ash field is poor which makes the tunnel body pass through the powder,and large-scale instability easily occurs when the supporting structure is not supported in time or the bearing capacity of the supporting structure is insufficient.At present,there are few cases of building tunnels in the stratum of fly ash accumulation,and there are no engineering cases of underground construction of double-arch tunnels for reference in underground engineering construction in China,which brings huge challenges to the design and construction of tunnels..Therefore,in view of this situation,it is necessary to study the mechanical behavior of underground tunnel construction in fly ash accumulation stratum.Relying on the Yanpingba multi-arch tunnel project,this paper studies the construction mechanical behavior of the double-arch tunnel under the fly ash accumulation stratum by means of data investigation,numerical simulation and model test.The main research contents and conclusions are as follows:(1)The stratum of the fly ash accumulation body has poor self-stability,low shear resistance,and high permeability.The engineering properties are poor after encountering water,and the phenomenon of vault collapse is very easy to occur during construction.Its properties are similar to those of silt.According to the results of the triaxial test,the relationship between the principal stress difference and the principal strain is similar to the curve of loose sand and normal consolidated clay in the triaxial test,so the mechanical properties are somewhat similar.(2)The development of the instability shape of the double-arch tunnel was simulated by the model test.During the test,the instability of the multiple-arch tunnel was analyzed by releasing the support pressure on the face and weakening the initial support and without the initial support.The results show that when the support pressure on the face of the left and right holes and the middle pilot hole is released at the same time,the slump arch appears "bull’s horn".(3)The different excavation methods of the double-arch tunnel are studied by means of model tests.The results show that the stress of the double-arch tunnel is mainly concentrated in the upper half of the structure,and the different excavation methods have different effects on the bearing arch height of the structure.The stress and deformation of the single side wall pilot pit method and the CD method are similar.The stress and settlement above the vault of the three-step method are significantly larger than those of other construction methods,which have a greater impact on the safety of the structure.(4)The plastic hardening constitutive in FLAC3 D can be used to describe the relationship between the principal stress difference and the principal strain in the CU test.The results show that when the excavation distance between the main tunnel and the middle pilot tunnel is kept below 10 m,the stress and deformation of the structure are reasonable.(5)The numerical simulation and on-site monitoring results show that the combination of three-pipe high-pressure rotary jet grouting and advanced small conduit can effectively reduce the displacement during tunnel excavation,and the self-stability of the surrounding rock itself is good. |
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