| Large number of tunnel engineering practices have shown that the phenomenon of over-under excavation is widespread during tunnel excavation,especially in tunnels excavated by drilling and blasting methods.Over-under excavation will change the stress state of surrounding rock,cause stress concentration,and alter the distribution of plastic zone.Over-under excavation makes the contour of the surrounding rock become extremely irregular,which leads to the thickness of the shotcrete layer inhomogeneous,resulting in the change of the geometric shape of the supporting structure,even affecting the bearing capacity of the support structure.Based on a highway tunnel at the junction of Fujian province and Zhejiang province,this paper,in consideration of the unevenness of rock surface,conducts research from both theoretical analysis and numerical simulation,and analyzes the mechanical properties of support structures on the state of over-under excavation,and studies the influence of over-excavation on the stability of the surrounding rock.The main contents and conclusions are as follows:(1)Four types of methods for measuring tunnel boundary contour and over-under excavation data are summarized.The over-under excavation data,which are obtained by laser surface texture measurement,are used to study the fractal features of the boundary contour of over-under-excavated tunnel based on wavelet analysis.The result shows that the boundary contour has very strong fractal features and self-similarity.And as the level of the surrounding rock becomes larger,the fractal dimension of section boundary lines gradually increase.(2)Simplifying the irregular shape formed by over-under excavation,and extracting the basic over-under excavation shape.The surface contours of the surrounding rock after over-under-excavating are classified into three categories:incomplete filling,bottom filling,and complete filling.By using thick-wall cylinder theory,this paper analyzes the mechanical properties of support structures under the three types of surrounding rock surface contours,and deduces the supporting stiffness,maximum support force and other mechanical parameters of the anchor-spray support and the "steel grille + shotcrete" combined support,based on the stability principle of parallel system.(3)Based on the convergence-confinement method,this paper studies the interaction between surrounding rock and support structures under over-under-excavating condition.The results show that the stiffness and maximum support pressure of the bolt has a relatively minor effect on the stiffness and maximum support pressure of the combined support system.The stiffness and maximum support pressure of the initial support mainly depend on the stiffness and maximum support pressure of the shotcrete.The support pressure,when the support system reaches balance,and the maximum support pressure of over-under-excavating condition are smaller than of normal condition,which indicates that over-under excavation does reduce the bearing capacity of the support structure.(4)The software FLAC3D is utilized to analyze the influence of factors,such as the depth,range,and location of over-excavation,on the stability of surrounding rock.Under the condition of Grade ? surrounding rock,the impact of over-excavation on the stability of surrounding rock is gradually increased with the increasing of the depth and the expansion of the range.The influence coefficients of different characteristic values are all less than 1.1,which means that the influences of over-excavation of small-scale and shallow-depth on the stability of surrounding rock are limited.When the extent of over-excavation is same,the higher the surrounding rock grade,the greater impact on the stability of the surrounding rock.Under the conditions of Grade ? and? surrounding rock,the most rigorous occurs when over-excavation occurs in the vault zone.Under the condition of Grade ?,the most severe occurs when over-excavation occurs in the arching. |
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