Abstract

A concern during the design of an undrained tunnel lining is the possibility of a high groundwater pressure occurring behind the lining. If the lining is applied only to the tunnel walls and crown, flow to the invert may cause a pressure alleviation, possibly enhanced by an excavation damage zone (EDZ) with higher hydraulic conductivity than the undisturbed rock mass. This paper studies the groundwater pressure in the rock mass surrounding the Gevingåsen tunnel, a drill-and-blast railway tunnel in Trøndelag, Norway, lined in the walls and crown with an undrained lining structure consisting of sprayed concrete and a waterproof sprayed membrane. Numerical distinct element modelling in 3DEC is used to simulate the groundwater pressure. The results are compared to existing groundwater pressure measurements performed behind the lining. The construction damage zone (CDZ) is included in the model, and its hydraulic properties are back-calculated using the measurements. Assuming that there are no changes in the joint hydraulic apertures due to the tunnel excavation, the maximum groundwater pressure immediately behind the SCL is simulated to be approximately 470 kPa. This is lower than the hydrostatic pressure of 600 kPa, but higher than the measured pressure of 135 kPa. When hydraulic aperture changes caused by mechanisms not directly related to blasting are included in the simulations, the maximum pressure remaining behind the lining is approximately 400 kPa. A hydraulic aperture increase with a factor two to four in the CDZ, corresponding to a hydraulic conductivity increase of between one and two orders of magnitude, is necessary to obtain agreement with the measured pressures. Such an increase is considered reasonable, and results in a maximum pressure at the excavation boundary of approximately 160 kPa.

1 Introduction

According to the current technical specifications (Bane NOR, 2018), new drill-and-blast (D&B) railway tunnels in Norway should be lined with either a cast-in-place concrete lining with sheet membrane and drainage geotextile behind, or a sprayed concrete lining (SCL) waterproofed with a sprayed membrane (Fig. 1). The cast-in-place concrete lining results in a drained tunnel, while the SCL can be designed as either drained or undrained. This paper focuses on a D&B tunnel with an undrained waterproof lining in the walls and crown, and a drained invert. This situation will be referred to as partially drained. The theoretical maximum groundwater pressure acting on the lining is the hydrostatic pressure given by the elevation of the groundwater table relative to the tunnel. However, the drained invert will likely lead to a certain alleviation of the groundwater pressure behind the lining. This effect might be enhanced by a higher hydraulic conductivity in the excavation damage zone (EDZ). Several studies have shown that the hydraulic conductivity of the EDZ is significantly higher than that of the undisturbed rock mass (e.g. Chandler et al., 1996; Bossart et al., 2002; Bäckblom, 2008; Ericsson et al., 2015).

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