Abstract: The stability of air pressure in shield chamber during tunneling under "air pressure mode" and chamber opening under pressure should be guaranteed. Hence, the air pressure loose zones in 4 areas of soil in front of cutterhead, soil around the excavation gap, grouting area in shield tail and screw conveyor are analyzed, the Krabbe formula is improved, and based on which a semiempirical and semitheoretical calculation method of air loss Q is proposed by considering the permeability coefficient, earth chamber pressure and soil variation. The calculation method is applied to practical engineering, and the following conclusions are drawn by sensitivity analysis of the crosssection with the largest air loss: (1) There is a quadratic positive correlation between chamber pressure and air pressure loss, and the increase in air pressure within the scope of the project will result in a certain volumn of air loss. (2) The crosssection with the largest volumn of air loss locates in formation abrupt change area, and by carrying out sensitivity analysis on this crosssection, the power of air compressor equipped on shield can be reasonably judged.

Key words: shield, air pressure in shield chamber, chamber opening under pressure, air loss volumn, Krabbe formula, sensitivity analysis