Modeling unsaturated flow in fractured rocks with scaling relationships between hydraulic parameters

Modeling unsaturated flow in fractured rocks is essential in various subsurface engineering applications,but it remains a great challenge due to the difficulties in determining the unsaturated hydraulic properties of rocks that contain various scales of fractures. It is generally accepted that the van Genuchten(VG) model can be applied to fractured rocks, provided that the hydraulic parameters could be representatively determined. In this study, scaling relationships between the VG parameters(a and n) and hydraulic conductivity(K) across 8 orders of magnitude, from 10-10m/s to 10-2m/s, were proposed by statistical analysis of data obtained from 1416 soil samples. The correlations were then generalized to predict the upper bounds of VG parameters for fractured rocks from the K data that could be obtained more easily under field conditions, and were validated against a limited set of data from cores, fractures and fractured rocks available in the literature. The upper bound estimates significantly narrow the ranges of VG parameters, and the representative values of a and n for fractured rocks at the field scale can then be determined with confidence by inverse modeling using groundwater observations in saturated zones.The proposed methodology was applied to saturated-unsaturated flow modeling in the right-bank slope at the Baihetan dam site with a continuum approach, showing that most of the flow behaviors in fractured rocks in this complex hydrogeological condition could be properly reproduced. The proposed method overcomes difficulties in suction measurement in fractured rocks with strong heterogeneity, and provides a feasible way for modeling of saturated-unsaturated flow in fractured rocks with acceptable engineering accuracy.