Rock-mass permeability

Abstract Modeling of the drift-scale heater test at the Exploratory Studies Facility at Yucca Mountain, Nevada, U.S.A. was performed. The objectives of the analysis were to investigate the (i) temperature effects on mechanical deformation surrounding the heated drift and (ii) thermal-mechanical effects on rock-mass permeability. The continuum representation of a deformation-permeability relationship based on fracture normal stress was developed to assess rock-mass permeability variations because of temperature changes. The estimated rock-mass displacements and permeability variations as a function of heating time were compared with field measurements. The estimated trend of permeability responses using a normal stress-based deformation-permeability relationship compared reasonably to that measured. 

Thermal-Mechanical effects on rock-mass permeability... 

To assess the thermal-mechanical effects on rock-mass permeability, a deformation-permeability relationship based on fracture normal stress was developed. 

A normal stress-based deformation-permeability relationship was proposed to investigate the thermal-mechanical effects on rock-mass permeability. The estimated trend of permeability responses to heating compared reasonably to that measured. The modeling results, however, were not able to predict the permeability recovery observed at certain locations. 

Parameter improvements (reduced rock mass permeability and rock mass thermal expansion by the KTH/SKI team, and increased thermal expansion coefficient and reduced swelling pressure constant of the buffer by JNC team) -Inclusion of the sealing of rock fractures by penetrating bentonite by the KTH/SKI team, which can explain the uniform (axisymmetric) wetting of the bentonite. 

With respect to the safety assessment, the importance of the couplings will be appraised from the following indicators the evolution of the THM state of the buffer (in particular the maximum temperature, the time to re-saturation and the maximum stresses), and the rock mass permeability changes and possible failure. 

All teams assumed linear elastic behaviour for the different media in addition, SKI used a notension model for the buffer and the backfill. Because of the crucial role of the rock mass permeability, three values of its initial intrinsic permeability have been considered lO lO (base case) and lO m. Moreover, a variation of tbe permeability with respect either to the porosity tp or to the effective mean stress Om (SKI) has been assumed ... 

As might be expected, the value of the initial intrinsic rock mass permeability has a low influence on these results (Figure 3). 

Figure 3. Evolution of temperature at B4 for different rock mass permeability ( CEA)...

The thermal gradient induced by the heat generated from the wastes causes first a desaturation close to the heater, followed by a resaturation controlled by the rock mass permeability. For the base case, the predicted time necessary for a full re-saturation of the buffer, varies between 7 years (CEA) and 55 years (SKI) for the three dimensional models. The evolution of the degree of saturation at point B4 is plotted on Figure 4. 

As expected, it strongly depends upon the rock-mass permeability, as can be seen on Figure 5 for the low permeability case. 

Figure 5. Evolution of degree of saturation at point B4 for low rock mass permeability (10nr)...

In this paragraph, the importance of the couplings on the safety indicators is evaluated by comparing the THM calculations to the partially decoupled ones TH, TM and HM. However, all calculations start from the same initial conditions. The influence of the initial rock mass permeability on the results has been investigated as well. 

The situation is completely different for the saturation. In fact, for medium or high rock mass permeability, the TH and THM analysis lead to similar evolution of the liquid saturation in the buffer, close to the canister. In these cases, it is controlled mainly by the vapour diffusion associated with the thermal gradient, while the rock supplies water sufficiently rapidly. This is illustrated on Figure 8. 

In addition to this table, the importance of the initial rock mass permeability has been highlighted. For confidence building and demonstration purposes, a fully coupled approach is necessary to interpret monitoring data that would be collected during the first several decades after repository closure. 

Initial rock mass permeability values of lO, 10 and 10 m will be considered by the research teams, with the median value of lO defined as the base case. Additionally, the rock mass permeability is assumed to vary with the effective porosity according to the following function ... 

In Phase B, the research teams used the T-H-M models to perform scoping calculations for a repository in a homogeneous rock mass. For that case the fully coupled THM analysis predicted localized rock mass failure and also important features related to the buffer/backfill resaturation, the rock mass permeability evolution and the swelling stress development in the buffer which might be overlooked by more simple analyses. The effects of coupling on the above features seem to be amplified when the rock mass permeability is lower. Temperature is the only output parameter which is not significantly influenced by coupling (Millard et al. 2003). 

Stress-induced changes in fracture aperture can give rise to significant changes in the rock mass permeability, through a sensitive cubic relationship between fracture aperture and fracture flow. This section discusses how to determine this relationship for large-scale heterogeneous fractured media (characterized by fLm) from small-scale laboratory measurements. The determination is based on the assumptions that an equivalent... 

Coli, N., Pranzini, G., Alfi, A., et al. 2008. Evaluation of rock-mass permeability tensor and prediction of tunnel inflows by means of geostructural surveys and finite element seepage analysis. Engineering Geology 101(2008) 174-184. 

From the overview from the top of the highwall, students have observed the relationship between the wetland and the deep void in fractured rock, and appreciated the consequences of rock mass permeability... 

Students have considered the consequences of rock mass permeability and the proximity ofthe adjacent wetlands in the context of the site s new function as a landfill, and considered the measures being employed to manage the potential for groundwater contamination. 

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