Study On The Optimization Design Of Initial Support In Loess Road Tunnel

With the construction of the mountain highway and the rapid development of theeconomic in western area, the road tunnels which through the loess area are increasing.Because of the complexity of tunnel engineering and the special engineering properties of theloess, many diseases were appeared during construction and operation of the loess tunnelwhich were constructed by the method of engineering analogy, the optimization design ofsupporting structure need to be further studied.Combining with a large number of loess tunnel has been built, through informationresearch, theoretical analysis and numerical simulation method to propose the optimizationdesign philosophy of initial support in the loess road tunnels,which including using theconcrete-filled steel tube to replace steel arch,increasing the thickness of the initialsupport,using the molded concrete to replace the shotcrete support. Plane finite elementmodels were established by finite element software MIDAS/GTS. Orthogonal experiment ofthe type and size of steel tube were built and the effect characters of the different size ofsurrounding rock and the tunnel were analyzed and engineering economy were combined toput forward the type and size of the concrete-filled steel tube in optimization design. Thesafety factor which was calculated by assumed resistance method and the supporting effectbetween the original design and the optimization design were contrasted, the structure wassafety and the displacement and plastic zone of the surrounding rock has been effectivelycontrolled.Soaking model after tunnel excavation and supporting was established by finite elementsoftware MIDAS/GTS. To evaluate the safety of the new support structure. under thecondition of soaking, the results showed that the soaking reduce the bearing capacity ofsurrounding rock, and make surrounding rock producing the vertical displacement, thesurrounding rock stress were distributed again, stress concentration appeared around tunnel,the soaking lead to the plastic zone of surrounding rock and the maximum plastic strainvalues are increased. To the support structure, axial force of inverted arch is larger, axialforce and bending moment of other parts were less affected by the soaking. The effect ofsoaking on optimization design is greater than that of original design, but the supporting effectis better.