ABSTRACT
There are still some problems in present grade division both at home and abroad. For example, uncertainty of various factors which can influence the result, small applicability scope of calculation theories and so on. These will make construction cost be increased. Currently, the grade of excavation is divided according to deformation values (Specification for Excavation in Shanghai Metro Construction, SZ-08-2000). Every grade has its control values. When the grade division is not precise, if higher which means the safety warning value is smaller, it will forecast the safe condition as dangerous condition, then the excavation requests high rigidity retaining structures, also more bracings and bottom strengthening may be needed to resist deformation.Actually in this condition, investment has been increased. If lower which means the safety warning value is larger, the condition of excavation has already been dangerous before forecasting, it may be too later to remedy, so it can cause great loss and the work may be delayed.
