Fracture apertures hydraulic conductivity

This model assumes laminar flow between two perfectly smooth and parallel plates. However, Experiments have shown that the real mechanical aperture (E) and the hydraulic conducting aperture (e) are not equal. The cubic law (e = E) is only valid for very open fractures and/or for fractures with smooth fracture surfaces (low JRC). The mechanical aperture E can be converted into the hydraulic conducting aperture e, by using Eq. (2) (Barton et al., 1985) ... 

The results for flow on a single fracture surface are incorporated in the derivation of hydraulic properties of unsaturated fractured rock mass. Liquid retention and hydraulic conductivity in partially saturated fractured porous media are modeled in angular pores and slit-shaped spaces representing rock matrix and fractures, respectively. A bimodal distribution of pore sizes and apertures accounts for the two disparate pore scales and porosity. These considerations provide a framework for derivation of retention and hydraulic conductivity functions for fractured porous media (Or Tuller, 2001). 

For the hydraulic base case, constant hydraulic apertures and fracture densities were considered. Homogeneous hydraulic conductivity tensors and porosities were applied to each formation. The case with a constant hydraulic aperture of 10 im and medium fracture density for all formations is illustrated in Figure 5, where the streamlines and the particle travel times are shown. The time of travel between each marker is 1000 days. Owing to the low effective porosity values and the relatively high fracture density, particle travel times through the host rock are fast. The mean particle travel time from the repository to the seabed is only 123 years, when a constant hydraulic aperture of 10 pm is used. 

The results for the particle travel times for various constant hydraulic apertures and fracture densities from the repository to the seabed are presented graphically in Figure 4. For the low fracture density case, the hydraulic conductivity estimated for a block size of 25 m x 25 m has been used even though this size does not correspond to the REV, which is estimated to be greater than 100 m x 100 m. This block size does allow, however, a calculation of the mechanical closure of the fracture apertures for the HM case and therefore a comparison of the results between the two cases for low-density conditions. The results of the continuum model based on constant hydraulic apertures display very rapid mean particle travel times from the repository to the seabed. For example, for the low and high fracture density networks adopting a constant hydraulic aperture of 10 pm, particle travel times from the repository to the seabed are 580 years and 106 years respectively. A doubling of the aperture increases the conductivity by a factor of... 

For the HM base case the mean mechanical properties have been used to calculate the hydraulic aperture distributions over the depth of the model. The continuum model and the applied methodology for the HM coupling in fractured rock does not allow the modelling of a fully HM-coupled system, hence the HM-modified hydraulic conductivity tensors were calculated at the mid point values of several depth ranges (Table 1). The results were assigned uniformly to the formation within each depth range (25m=>0m-50 m, 75 m => 50 m - 100 m, 175 m => 100 m -250 m, 375 m => 250 m - 500 m and 750 m 500 m - 1000 m). The variation in the calculated aperture values decreases as depth increases, which allows for the larger depth bands at the base of the model. 

Neretnieks 1993) formulated a simple channel network model that does not use detailed information on aperture variations and fracture orientations. It uses information obtained from hydraulic measurements in boreholes on the flowrate distribution and the frequency of conductive fractures found in the boreholes. The model assumes that every measured flowing fracture in a borehole represents a channel with constant properties... 

The conductivity and specific storativity of the fracture were obtained by calibration in a purely-hydraulic computation. In each fracture, two perpendicular sets of channels 2 meters apart were generated. At first, the same initial aperture of lO was assigned to all fractures, but since a joint in 3FLO represents several in situ discontinuities (see Section 4.1), the conductivity Cond was calculated as follows ... 

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