ABSTRACT

A practical technical approach for the preliminary evaluation of hydraulic conductivity and tunnel water inflow in fractured rock mass is presented. The procedure is based on the geostructural characterization of the investigated area by means of detailed geostructural field surveys. The principal directions of the anisotropic hydraulic conductivity tensor K are calculated by using the Kiraly equation, integrated with the introduction of the effective hydraulic opening. The water flow into the tunnel planned for excavation under the water table is then estimated by a 2D finite element seepage analysis performed on representative geological cross sections of the Tunnel path. In order to verify the reliability of this approach, it has been applied on a 4.5 km length of the Vaglia High Speed Railway Tunnel, excavated a few years ago in order to connect the city of Bologna to the city of Firenze (Italy). The analysis showed that the estimated inflows resulted of the same order of magnitude of the inflows measured in the field.

INTRODUCTION

The prediction of water flow into tunnels is an important aspect which must be taken into account for tunnel designs. This prediction could result in a very difficult and time-consuming operation, due to the anisotropic hydrogeological properties of fractured rock mass, which are deeply influenced by the physical properties (orientation, aperture, roughness, persistence, spacing) and interlocking of discontinuities. As a consequence, a practical technical approach for the preliminary evaluation of hydraulic conductivity and tunnel inflows is presented. The procedure is based on the geostructural characterization of a fractured rock mass by means of field surveys and hydrogeological considerations. It allows for the investigation from simple to more complex geostructuaral scenarios and requires only field data. On the basis of a detailed geological and geostructural characterization, the procedure works by subdividing the investigated area in several homogeneous geostructural domains. Each domain is characterized by a homogeneous distribution of the ubiquitous discontinuities (bedding and joints) where their properties basically remain the same (Representative Elementary Volume, REV).

Within each domain, the main properties of discontinuities are collected by means of field surveys and the principal directions of tensor K are calculated. The tunnel water inflows are then evaluated by means of a 2D finite element seepage analysis, performed on representative geological sections transverse to tunnel path. Unitary water inflows are then extended to geologically homogeneous sectors and then summed up for the whole length of the tunnel. In order to verify the reliability of this procedure, it has been applied on 4.5 km length of the Vaglia High Speed Raylway Tunnel, which was excavated a few years ago (from 1998 to 2005) in order to connect the city of Firenze to Bologna (Italy).

Geological characterization

In order to define the structural homogeneous domains that characterize the investigated area, a field survey is carried out, locating geostructural survey stations homogeneously distributed according to the tunnel path and the geological framework.

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