An analytical method for the determination of the stress state of sewer tunnel linings and evaluation of a decrease of their bearing capacity and durability due to concrete gas corrosion has been developed in the framework of the project supported by TNTAS grant 01–647. The method allows the actions of the soil own weight and weight of buildings or structures both existing on the surface before the tunnel driving and erected nearby the already constructed tunnel, to be taken into account and may be also applied for planning the location of new buildings with respect to the tunnel, thus reducing the negative consequences on tunnel durability. The paper presented describes some results of investigations on sewer tunnel lining durability depending on building sizes and their location with respect to the tunnel at different relations between deformation moduli of the soil Or rock and the lining material.
Construction of buildings or structures nearby to an already existing tunnel causes a redistribution of stresses in the tunnel lining and may result both in an increase (if the appearing compressive stresses lower that the tensile ones acting in the lining before the buildings erection) and in a decrease of its bearing capacity. The influence of a new building on the lining stress state depends on the tunnel depth, sizes, weight and location of buildings with respect to the tunnel axis and also on the relation between deformation moduli of the soil or rock and the lining material. For evaluation of this influence the methods for the design of shallow circular tunnel linings (Fotieva et al. 200 I) and shallow tunnel linings of an arbitrary cross-section shape (Fotieva et al. 2003) were proposed. These methods arc based on mathematical modelling of the interaction of underground structures and the surrounding soil or rock mass as elements of a united deformable system and on the analytical solutions of the corresponding plane problems of elasticity theory for a ring supporting the opening in a heavy semi-infinite linearly deformable medium subjected to the action of a uniformly distributed load on an arbitrary part of the semi-plane boundary. The methods may be applied to planning the locations of new buildings or structures on the surface with a minimum of risk to the existing tunnel.
The situation with sewer tunnels is substantially more complicated due to additional decrease of the linings bearing capacity caused by concrete corrosion in the aggressive gas atmosphere and resulting in a local lowering (up to 20mm per year) of the lining thickness in the upper part of the tunnel, which increases with time. This causes redistribution of stresses in the lining and may eventually result, especially under the influence of the new buildings weight, in the loss of its bearing capacity and failure, accompanied by environmental contamination through sewerage entering into the surrounding ground.
To prevention such situations an analytical method for the determination of the disturbed lining stress state and evaluation of its bearing capacity has been developed (Fotieva et al. 2003).