Seepage‐Induced Effective Stresses and Water Pressures around Pressure Tunnels

The change in stresses in the rock mass generated by the internal loading of a pressurized conduit, has traditionally been estimated assuming that the rock mass is impermeable and that the internal pressure in the tunnel can be treated as a mechanical load applied to the walls of the opening. The deleterious effects of pressurized seepage, percolating through the natural discontinuities in the rock mass, have generally been ignored, resulting in tunnel alignments with inadequate lateral and/or vertical cover as well as in erroneous interpretations of the in‐situ hydrojacking test results. This paper presents an approach that takes into account the effect of seepage forces by treating the fractured rock mass as a continuous porous elastic medium. The distribution of effective stresses along a radius intersecting the springline of the tunnel was estimated from closed‐form solutions derived for the simplified case of negligible circumferential seepage forces in the medium. The validity of the simplifications was assessed using the finite‐element method. The analysis presented in this report resulted in specific recommendations to establish minimum cover criteria in pressure tunnels, and also provided the basis for reevaluating the methodology currently used to determine the minimum in‐situ stress from hydrojacking test results.