Research On The Reinforcement Technology Of Tunnel Portal Section In Weak Surrounding Rock

In recent years, China has seen a rapid development in highway industry. With more andmore highways built in mountainous area, numerous highway tunnels will emerge as a resultof terrain and road alignment constraint. As an important part of a whole tunnel, with its weaksurrounding rock, the tunnel entrance requires pre-reinforcement measures to ensureconstruction safety.Based on the geological condition of Huashan tunnel, NingBo, combine with existingresearch results, then build a numerical model to explore the action mechanism and itsapplication conditions of big pipe shed and pre-grouting small pipe supporting. Main researchcontents are as following:1. On calculating the internal force situation of the pipe in big pipe shed supportingsystem, analysis the varying rules of the pipe with the tunnel excavating. Also, thereinforcement principle of the pre-grouting small pipe supporting and grouting was explored.After these works, we point out that only when the small pipe elements best match thegrouting elements, can we get the best reinforcement result.2. By using the MIDAS/GTS software, A tunnel model was built under threecircumstances: deep-buried, shallow-buried and super shallow-buried, its surrounding rockranges from five class to four class rock. During the construction period, the surrounding rockstress, plastic zone and initial supporting structure stress are analysed, then draw a conclusionthat pipe shed and small pipe can all reinforce the surrounding rock at a certain extent, but thepipe shed is better.3. Analysis the tunnel crown settlement values and ground settlement values whenusing the pipe shed and small pipe to reinforce the surrounding rock, we come to a conclusionthat it’s better to use the pipe shed to reinforce the surrounding rock if the rock class is verybad.4. Comparing the simulation result and the field measurement result of pipe shedreinforcement measure used in relying tunnel entrance, the conclusion of this article havesome reference value, and it can offer some reinforcement choices for the tunnel entrancewith weak surrounding rock.