Evaluation of rock-mass permeability tensor and prediction of tunnel inflows by means of geostructural surveys and finite element seepage analysis

In this paper, a new practical technical approach for the evaluation of hydraulic conductivity and tunnel water inflow in complex fractured rock masses is presented. This study was performed in order to evaluate water flow into tunnels planned along the new highway project from Firenze Nord gate to Barberino di Mugello (Tuscany). The results are based on detailed and comprehensive geostructural characterization of rock masses, by means of field surveys, geological and hydrogeological studies.Starting from discontinuities properties surveyed in the field, the permeability K tensor was calculated using the Kiraly equation, integrated with the introduction of the effective hydraulic opening of fissures (e). Principal directions of tensor K were calculated for each geostructural survey station using an automated software script especially developed for this purpose.Available K data from Lugeon tests were also collected and analyzed with results that do not fully represent the rock mass due to its structural (hydraulic) variability.In order to evaluate water flow into tunnels planned for excavation under the water table (for a total number of eight), a finite elements seepage analysis was performed on 38 representative geological sections transverse to tunnel paths (α-planes). Each section referred to the nearest and geologically most compatible geostructural station.Principal directions of K tensors were projected on the α-planes by means of trigonometric transformations.Unitary water inflows were then evaluated for long-term steady-state, as well as for initial state immediately after tunnel excavation. Inflow values calculated for each unitary section were extended to geologically homogeneous lengths of the tunnel, according to the variability of the water head above excavation, and then summed up for the whole length of each tunnel. Inflow values obtained with FE seepage analysis were also compared to other inflow evaluation methods.