Abstract: As the construction of underground project grows increasingly, more and more tunnels will be constructed in gassy soil (soil layer containing high-pressure gas), and the gas in soil may bring bad influence for the stability of tunnel by shield method. When the shield machine is driving in the soil layer, the gas may enters into the tunnel through segment seam because of bad air-tightness, it will cause longitudinal deformation of segment. Aiming at this phenomenon, the purpose of this paper is to establish a segment deformation model under the condition that the gas enters into the tunnel through single point at the segment seam. For the purpose, firstly the stratum deformation caused by gas releasing was studied; the curve fitting method was used to obtain the relationship between stratum deformation curve and the maximum settlement value. Then the numerical simulation software FLAC3D was used to verify results gotten by theoretical deduction; Comparative results show that the numerical results and the theoretical results are basically consistent, so as to verify the availability of the theoretical formula. By segment deformation formula and deformation compatibility equation, the influence of different gas layer thicknesses and different initial gas pressures on the segment deformation and the open value of segment seam are analyzed; and the condition of soil layer causing the damages of segment is discussed. Results show that, with the increase of the thickness of gassy stratum or the original gas pressure, the bending moment of segments will increase; and the opening values of segment joints will become larger. For the stratum with thinner gas layer (<1.5 m) or lower gas pressure (<150 kPa), bad air-tightness will cause the gas leak and enter tunnel and bring the damage of segment; but normally will not destroy the waterproof facility of segment; however, for the stratum with large gas layer depth or higher gas pressure, that the gas leak will cause larger segment deformation and destroy the waterproof ability, and make underground water entering into the tunnel. When the deformation of segment becomes larger, the sand will flow into the tunnel with groundwater, so as to bring serious damage of segment.

Key words: shallow natural gas, segment deformation, gas releasing.