The function mode of influencing factors and recognition method for the loose zone of large span tunnels which are built in heavily jointed rock stratum are very important. Factors like the thickness of overlying weak rock stratum, tunnel depth and joints occurrences are influencing factors that need careful consideration. By using orthogonal array testing and distinct element method, a case of large-span metro station cavern on conditions of no support is taken, based on UDEC software, 64 different numerical simulation models of which the cross section share same form and size are two-dimensionally simulated. A new method based on joints occurrences is presented to determine the boundary of loose zone. The influencing rules of factors are summarized and new formulae for the area of loose zone and length of earth surface crack are formulated by using the liner least squares method. The errors of the empirical formulae are analyzed.
In recent years, more and more large-span rail way tunnels and metro station caverns are built in heavily jointed rock stratum. The existing specifications cannot meet the needs of design and construction of such tunnels in and abroad. The recognition method and function mode of influencing factors of the loose zone are the key to calculate surrounding rock pressure in ultimate limit state.
After excavation, the stress of surrounding rock mass will redistribute and the loose zone will be created, which is strongly associated with the stability of tunnel. So far, methods for recognition of loose zone can be divided into four groups: theoretical research, numerical simulation, field measurement and model test. These methods have the following deficiencies: firstly, most theoretical researches have at least one assumption, so the application of these methods is narrow; secondly, the numerical analysis software which based on continuum mechanics and cannot simulate the influence of joints on the area and the boundary of loose zone in rock stratum; thirdly, the accuracy and completeness of monitoring data cannot be guaranteed. In view of this situation, research on influencing factors and recognition method for loose zone in heavily jointed rock stratum is very necessary.
In this paper, by using orthogonal experiments and distinct element method, a case of large-span metro station cavern on conditions of no support is taken, based on UDEC (Universal Distinct Element Code) statistic simulation and liner least squares the function mode of influencing factors and recognition method for loose zone are analyzed, the study provides technical support for the calculation of surrounding rock pressure in ultimate limit state.