There are a lot of kinds of underground pipes and structures in the urban area; the safety and regularly running of which must be taken into account during newly constructions. There are no generalized criterions to evaluate the safety and regularly running of them at present. It should be investigated which sectors the pipes belong to before construction

then their design and construction specifications can be used in the safety evaluation. Drainage design specifications are used to evaluate the safety of 2 large diameter steel sewage pipes with diameter of 1.2 m and thickness of 12 mm

during the cofferdam construction in the Litchi River in Guangzhou city. In view of the calculation methods in the design specification can not consider the interactions of all the effects during the construction. A three-dimensional simulation model of the whole cofferdam construction field stratum with the 2 large diameter steel sewage pipes is established. Deformation and stress of the pipes under 4 different construction conditions are calculated with FLAC3D. The equivalent stresses are calculated by fourth strength theory. The maximum value is 136.61 MPa

which appears at the outer surface of pipe No.1 under the last construction condition. The maximum settlement of the pipe is 39.3 mm

which produces 0.275% declination of the pipe. The maximum stress value calculated by theoretical calculation formulas is 188.18 MPa

which is much higher than the result calculated by the current codes. The reason is that every effort is calculated independently and simply added in the theoretical methods. Finally

Deformation and strength of the pipes are checked according to relevant Chinese standards. It reveals that the sewage pipe lines are safe in the construction procedure and the results got by numerical simulation are more reasonable than those calculated by theoretical formulas.