Dripping in multi-segmented rough fractures: model and assumptions The enhanced model A modification of the Wang and Narasimhan model (Wang and Narasimhan 1985, 1993) has been used to establish a probabilistic, semi-analytical formulation to model dripping into a tunnel from a single rough multi- segmented fracture set. Ground surface local saturation conditions are assumed to exist intermittently and peri- odically. The same is assumed for the fracture at the tunnel level, where dripping occurs or is initiated according to the concept of water pooling at fracture termini presented by Su et al. (2003). A fracture is gen- erated from the top of the tunnel to the ground level. The fracture set, considered here, is formed by a finite number of connected fracture segments of given strike and dip. These parameters can be sampled from either data-driven or hypothetical Probability Density Func- tions (PDFs). Unsaturated flow occurs in these fracture segments with each segment being partially filled with water volumes that have characteristic shapes in the lower and upper boundary, called menisci. The upper (r u ) and lower (r d ) menisci can differ in size both within each fracture segment and from segment to segment. Only the meniscus at the tunnel level (R) is considered to be different from the other menisci in the fracture seg- ments. A schematic of a fracture segment and the varying menisci is shown in Fig. 1 for the case of a triangular groove. The fracture segments are considered as rough frac- tures. The roughness is characterized by an aperture distribution function that can be derived from field data. The roughness along each fracture segment is considered to be constant, that is a constant aperture, and it is Environ Geol (2007) 51: 1295–1306 DOI 10.1007/s00254-006-0423-x ORIGINAL ARTICLE Amvrossios C. Bagtzoglou Daniele Cesano Dripping into unsaturated rock underground excavations: model testing at the Yucca Mountain exploratory studies facility Received: 11 November 2005 Accepted: 25 May 2006 Published online: 2 August 2006 Ó Springer-Verlag 2006 Abstract This paper presents a probabilistic formulation of unsatu- rated flow through rough multi-seg- mented fractures, with emphasis on Dripping Initiation Time (DIT), and attempts to explain the fast flow paths detected at the Yucca Moun- tain (YM) Exploratory Studies Facility (ESF). Three scenarios of parameters were used to test the validity of the model with these fast flow paths. The scenarios differed in the DITs and the corresponding parameter configurations were also tested for three different strati- graphic horizon depths. Each depth represented a different location where fast flow has been detected at YM. The first depth corresponds to the Bow Ridge Fault; the second represents a network of steep frac- tures and cooling joints with large variability in dip; and the third location, probably connected to the Diabolous Ridge Fault, has a flow path that is low dipping. Monte Carlo simulations for each configu- ration and depth produced proba- bilistic results from nine scenarios for DITs, which are quite consistent with field observations. Keywords Dripping Æ Fractured rock Æ Numerical modeling Æ Tracers Æ Nevada Æ USA A. C. Bagtzoglou (&) Department of Civil and Environmental Engineering, University of Connecticut, 261 Glenbrook Road, U-2037, Storrs, CT 06269-2037, USA E-mail: acb@engr.uconn.edu Tel.: +1-860-4864017 Fax: +1-860-4862298 D. Cesano Ambiente Italia, Istituto di Ricerche, Via Carlo Poerio 39, 20129 Milano, Italy E-mail: daniele.cesano@ambienteitalia.it Tel.: +39-02-27744239