Embedded Discrete Fracture Networks to Analyze Groundwater Inflows during Tunnel Drilling

Adriana Piña,Diego Cortes,Leonardo David Donado,Daniela Blessent

doi:10.15446/ing.investig.v42n1.89889

Abstract

Tunnels commonly go through fracture zones that used to be analyzed as an equivalent porous medium with homogeneous permeability. However, it is a rough simplification that overlooks the connection triggered by underground works in fractured massifs. This study introduces the use of synthetic discrete fracture networks (DFN) to analyze groundwater inflows through tunnel excavation in a fractured zone considering the daily advance of the drilling front. First, a hypothetical case with six different settings varying the fracture density, the fracture length, and the aperture distribution is analyzed. Each setting has about 100 iterations. DFN hydraulic properties were estimated and compared with previous DFN studies, displaying the same behavior even though the magnitude of the estimated parameters differs. As an application example, structural measurements of the Alaska fault zone in the La Linea massif (Colombia) are used to obtain the statistical parameters of the fracture length and aperture distributions to generate the DFN. Five settings varying the fracture density are built, obtaining measured and simulated groundwater inflows of the same order of magnitude. These results highlight the potential of the synthetic DFN to analyze tunnels’ effects on groundwater flow.

Highlights

Fractured media refers to lowpermeability matrix rocks that may acquire moderate to good permeability thanks to fractures (Singhal and Gupta, 2010)
Some analytical formulations based on the equivalent porous media (EPM) approach have been developed for steady state conditions and homogeneous materials (Goodman et al, 1965; Heuer, 1995; Karlsrud, 2001; El Tani, 2003), and some have been extended to the transient-state
This work highlights the possibility of implementing synthetic discrete fracture networks (DFN) to analyze the effects of the advance of tunnels excavation in fractured zones

Summary

Introduction

Fractured media refers to lowpermeability matrix rocks that may acquire moderate to good permeability thanks to fractures (Singhal and Gupta, 2010). Some of the reported effects of tunnels are groundwater inflows during and after their construction (Celico et al, 2005; Perrochet and Dematteis, 2007), groundwater and surface water level drawdown, (Molinero et al, 2002; Maréchal and Etcheverry, 2003; Vincenzi et al, 2009; Font-Capóet al., 2011), the triggering of preferential flow paths (Evans et al, 2001), and rock deformations and instabilities (Preisig et al, 2014; Shen et al, 2014; Loew et al, 2015; Valenzuela et al, 2015). Previous approaches do not consider the effect of the excavation-induced drawdown leading to over/underestimated groundwater inflows For this reason, some authors have proposed analytical or semianalytical approaches to predict the height of lowered water level under the steady (Su et al, 2017) and the transient states (Liu et al, 2017), providing a better prediction of tunnel inflows

Methods

Results

Conclusion