The Numerical Analysis On Constructional Mechanical Responses Of The Weak Surrounding Rock In Tunnel By Excavation Methods

With the enlargement of scale of highway tunnels and the development ofconstruction standard, there are many complicated kinds of soft surrounding rock area.However, tunnels’ landslide with soft adjacent and workers’ casualties happenedfrequently. This article will make further development in excavation methods of softsurrounding rock of tunnel and structural mechanics of tunnel support.In this article engineering geological features of tunnels with soft surroundingrock, factors affecting tunnels surrounding rock’s stability and deformationcharacteristic are analyzed. This article gives a brief summary of excavation methodsof soft surrounding rock of a road tunnel. Combining the construction of Qishupotunnel’s soft surrounding rock area and monitoring data, this article analyzes softadjacent rock’s deformation characteristics and makes numerical simulation andsupporting structural mechanics about different excavation models of the softsurrounding rock tunnel by using elastic-plastic finite element method(FEM).Furthermore, author demonstrated and sum up reinforcement methods and measuresof the weak surrounding rock. The primary fruit is listed as follows.①Selecting Qishupo tunnel’s soft weak area to make monitoring andmeasurement, this article makes research on characteristic feature of the typicalprofile’s surrounding rock displacement-time curve and surrounding rockdisplacement-space curve. Results show that the vertical convergence deformation oftunnel’s arch crown is bigger than the horizontal of tunnel’s side wall. Release rate ofdisplacement is slower in V-grade surrounding rock’s displacement of Qishupotunnel’s soft weak area and IV-grade surrounding rock’s displacement release rate isfaster. Especially, the release rate of displacement in down-step’s excavation is fasterthan rate in up-step’s excavation.②Using full section method step excavation method CD method and CRDmethod in shallow cover load part of Qishupo tunnel’s soft weak area, this articlemakes numerical simulation on process of excavation and construction. With the fullsection method，for the excavation large cross-section, the surrounding rock constraintlift fast, although the final value of the displacement is not too big, but the changespeed is faster, it is not conducive to control the deformation of the surrounding rock;Excavation on the upper of the step excavation method, because the span is large andthe rise-span ratio is small, in the two lower ends of the steps can make a stressconcentration.It is not conducive to bear external load, so it need to support quicklyand timely.The supporting strength has to enhance and the coupling placereinforcement of the up and down of the steps has to draw attention; With the CDmethod,the surrounding rock of both ends of the wall and primary support generate abig stress concentration and tensile stress and the local damage emergence, using thismethod, the wall need to use the flexible supporting design;With the CRD method, therapid formation of bearing structure of the surrounding rock is disadvantageous forthe disturbanceand and irresponsible supporting of wall.③According the analyzed results of the above four construction methods: all of the methods can ensure construction safety and the displacement of each method canmeet the requirement of criterion; however, we decide to use step method to makeexcavation and construction measurement considering the time, economic factor andthe results of numerical simulation. By the comprehensive analysis of measurementdata, construction schedule and economic conditions, etc, choose the most suitablemethod for this tunnel.