| In the current China's large-scale tunnel construction, one of the main problems which puzzled the majority of tunnel builders is the design and construction of tunnels in the soft and weak rock. It usually presents a great deformation, or even collapse and other accident; and a slow construction progress which is a severe restriction of the construction time. Such problems are mainly due to shortage of knowledge on the deformation mechanism and law of development and evolvement of tunnel surrounding rock, especially the soft and weak surrounding rock. Another reason for the problems is that the technical measures and methods which have been taken are lack of pertinence. This article studied in a system and deep-going way on soft and weak surrounding rock, including the geological features, evaluation of engineering influence, deformation mechanisms and time-space effect, interaction system of supporting structure and surrounding, and deformation control technique. Following key achievements have been achieved in the article.(1) The meaning of the soft and weak surrounding rock tunnel has been expatiated systemically, and itsgeological features, deformation characteristics and strength characteristics have been analyzed. The soft and weak surrounding rock has been classified three levels, grade I soft rock, grade II soft rock and grade III soft rock according to it mechanical characteristics.(2) The deformation mechanism and the deformation control criteria of the soft surrounding rock are influenced by the size of the tunnel-section. The recommended classification of tunnel sections and span of railway tunnel are proposed for the convenience of the study on deformation control of soft and weak surrounding rock. The tunnel sections are classified as small, medium, large and extra-large size, and the span of tunnel are also distributed into small, medium, large and extra-large span.(3) The deformation distribution of surrounding rock is divided into three parts in space, which are deformation ahead of the tunnel face, at the tunnel face and back from the tunnel face. And all of them occurred at the same time. The large-value, long-lasting deformation, the large change range ahead of the tunnel face and high change rate back from the tunnel face in initial excavation period are considered as the main characteristics of the soft surrounding rock tunnel.(4) Based on the study of the stress release and the dynamic relationship of stress control in the process of tunnel construction, the structure system of the soft rock tunnel, which is composed of surrounding rock, support in advance, primary support and lining, is established. The mechanical role of each component is explained. The support structure is divided into advancing support, primary support and lining. The advancing support and primary support forms the load bearing structure together with the surrounding.rock and the lining serves as safety margin, mainly supporting the additional load caused by the residual deformation.(5) Establish the tunnel structure theory model considering the rock stress release and verify the exactness with the numerical method, then point out that there are two sides should be careful in order to avoid or reduce the rock plastic deformation, for one is timely supporting, reducing the rock stress release before the construction of the support structure, and another is enough stiffness of support structure, especially the early stiffness to provide enough support force to resist the occurrence of rock plastic deformation.(6) Excavation method and support measure are two sides to control the rock deformation. Excavation causes the rock stress release and the support is the method to control the stress. Stability of the support structure and rock is the destination of deformation control. The control principle of rock deformation is to keep the rock deformation, including deformation ahead of the tunnel face, at the tunnel face and back from the tunnel face, under the deformation control criteria and make the surrounding rock and support structure stable for a long time by using appropriate excavation method and support meausure according to the time-space distribution regularities of rock deformation.(7) The deformation predictions of weak surrounding rock are distributed in two stagesas prediction before the construction and after the construction. Numerical method and the statistic method based on its original measure data are the deformation prediction methods approaching the reality. The management reference suggestions of clear displacement, crown settlement, precession displacement of excavation face, extrusion displacement of excavation face, arch subsidence and displacement velocity behind the excavation face have been presented in the article for the railway tunnel in our country.(8) The control method of weak surrounding rock and its design principle of support structure have been presented. That is using the advanced support method to control the displacement before the excavation face and the excavation face, using the primary support to control the displacement behind the excavation face, using the auxiliary support to control the stability of excavation face and the arch subsidence of primary support, and using the secondary lining support as reasonable emergency capacity to control the residual deformation. The structure design principle, design process and design parameters have been presented for weak surrounding rock.(9) Taoshuping Tunnel of the Lanzhou-Chongqing railway and Tianpingshan Tunnel of the Guilin-Guangzhou railway have been taken examples in the article to state the control practice of deformation before and behind the excavation face in the weak surrounding rock. Perfect result of deformation control has been got in above two tunnels by using the research achievement of this article. |
PDF Full Text Request