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Introduction

Different methods (empirical, analytical, and numerical) can be used to evaluate the hydraulic conductivity of a rock mass, where, empirical methods, for example, may use data from in-situ field tests where a relationship between depth and permeability is derived by applying a curve-fitting method and establishing the relationship between hydraulic conductivity and a geological index. Analytical methods estimate the permeability of the rock mass with Darcy’s and cubic laws by considering the geometrical characteristics of the joint sets. Lastly, numerical modeling can evaluate the outcomes of both empirical and analytical methods as well as providing a sensitivity analysis of the parameters that affect the rock mass permeability.

The main purpose of this research is to develop a permeability index for a rock mass that considers the most significant fracture parameters and their relevant weight factors. By conducting sensitivity analyses using 3DEC software, the impact of each parameter is evaluated and then the permeability index will be developed considering the effective parameters and their relevant weight factors. The permeability index will be helpful to predict the permeability of the rock mass from data obtained from discontinuity surveying.

Validation of the Model

Initially, an analytical method for calculating the inflow rate into a tunnel was developed and validated with numerical modeling.

The following analytical models are being validated using 3DEC:

• Evaluation of the impact of block size and surface on the inflow rate.
• Calculation of the specific length of the tunnel that is representative to unlimited lengths.
• Calculation of the block volume is modified and improved.
• Calculation of the block surface and the volumetric fracture intensity (P₃₂).

3DEC as a Tool to Evaluate Hydraulic Conductivity

The main utilization of 3DEC in this research is to evaluate the fluid flow through fractures assuming that the rock body is impermeable. The numerical models consist of a rock mass with, or without, underground excavations and the dependency of the flow rate (i.e., hydraulic conductivity) to the joint set characteristics (e.g., aperture, spacing, orientation, etc.) for a set of boundary conditions applied.

As the general scope of the research was to specify an index for the hydraulic conductivity of the rock mass and consider its geological and geometrical characteristics, a series of numerical simulations were designed in order to derive the correlation between hydraulic conductivity of the rock mass (inflow rate to the tunnel) and the above-mentioned characteristics.