Research On Stability Evaluation And Construction Technology Of Surrounding Rock Of Sand And Pebble Stratum Tunnel Based On The Physical Properties Index

With the rapid development of cities in our country,urban traffic problems have become one of the issues that people are paying more and more attention to and urgently need to be solved.Because the subway has the advantages of"safe,fast,and land saving",it has become a sharp sword to solve the traffic problems of urban development.The difficulties and problems brought about by large-scale subway construction are also huge,among which the construction of sand and pebble stratum mining method tunnels is the most representative.The self-stabilization ability of sand and pebble stratum is poor,and the equilibrium state is easily broken under construction disturbance,which can cause problems such as surrounding rock collapse and ground subsidence.Construction is extremely difficult and risky,and the urban river system is usually relatively developed.Tunnel construction underneath the river is a common phenomenon.Therefore,how to make the sand and pebble stratum mining method tunnel realize rapid construction while satisfying the safety,economy and construction period is a problem worthy of study.Based on the mining method tunnel project of Chengdu Metro Line 9,this paper studies the stability and evaluation methods of the tunnel surrounding rock,excavation and advance support technology,and protection measures for the tunnel which down through the river,and puts forward the rapid construction method of the sand and pebble stratum mining method tunnel,the main research content and results are as follows:（1）The sand and pebble stratum is divided into two types:sand and pebble soil.Through numerical analysis,the critical stability span D_s of the tunnel face of different sub-level surrounding rocks of sand and pebble soil is obtained.This paper introduces the basic quality index of sandy soil surrounding rock SBQ and the basic quality index of gravel soil surrounding rock GBQ.According to the relationship between sandy soil SBQ,E₀ and physical property indexes,as well as the sandy soil SBQ,E₀ and D_s obtained in this paper.The fitting formula between pebble soil GBQ,E₀,physical property index and D_s,combined with the method of judging sandy soil/pebble soil surrounding rock grade based on SBQ/GBQ,this paper established a method which can assess the sand and pebble stratum surrounding rock grade and predict the tunnel surface critical stability span D_s by using physical property indexes（the method for evaluating the stability of the tunnel face）,this method is suitable for mining method tunnels in the sand and pebble stratum with a buried depth of not more than 20m after precipitation and a span-to-height ratio of about 1.（2）According to the on-site anchor rods test and the concept of the ADECO-RS,it is believed that the construction of sand and gravel stratum tunnels should reduce the surrounding rock disturbance.When the tunnel span is small and the construction space is limited,it is difficult to use traditional radial anchor rods and lock-foot anchor rods.Radial reinforcement measures should focus on maintaining the stability of the longitudinally advanced core soil.Through data investigation and numerical analysis methods,the tunnel excavation and advance support measures and parameters of different sub-level sand and pebble stratum are proposed.（3）Numerical analysis shows that when the river channel is not intercepted but only precipitation+advanced pipe shed auxiliary measures are adopted,instantaneous instability and failure of the upper step face directly below the river channel will occur,which verifies the necessity of river channel closure;and numerical calculations are carried out.The reasonable river closure length L with different distances between the vault and the river bottom h₁ is obtained.Based on the survey and research of the aforementioned data,the excavation and advance support measures are determined,and the excavation and advance support measures and parameters of each sub-level surrounding rock tunnels which down through the small rive have been obtained.（4）According to the aforementioned research results,the application of construction technology to the supporting project can ensure the construction quality and improve the construction progress.The excavation progress reaches 50-60m/month,realized rapid construction.On this basis,the key construction steps were further clarified,and a set of rapid construction methods for mining method tunnels with sand and pebble stratum was formed.