Xiaolangdi Key Water Control Project is located at the Yellow River;it has comprehensive effects of flood control

water supply

generating electricity

etc.Its underground powerhouse caverns belong to large-scale type and have a buried depth of 70-100 m.The process and method of numerical analysis was introduced

including: rock plastic yielding criteria adopted;three-dimension geometrical model;rock mass mechanical parameters;initial earth stress field;excavating process simulating method;sustaining structure simulating method.Calculation results of three cases under conditions of two earth horizontal stress coefficients 1.0 and 0.8 are given.The three cases are:（1） having no sustaining structures;（2） having rock bolts and concrete lining but no pre-tensioning cables;（3） having pre-tensioning cables and rock bolts and concrete lining.Several conclusions of this study are drawn as follows:（1） the sustaining structure has not significant effect in limiting rock displacement and plastic area

but has an indispensable role in fixing loose and broken rock pieces and preventing partial rocks around caverns from falling;（2） excavating process has little influence on pre-tensioning cable force;（3） excavating process causes great tension stress in concrete lining;and the steel-net on rock surface has important effect on confining lining cracking and preventing lining from falling.