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Modeling How Fractures Develop

Source:  Daisuke Asahina and Dan Hawkes

Geomechanical processes are known to play an important role in hydrogeological behavior. Linkage between mechanics and hydrogeology occurs in two fundamental ways: (1) through interactions between rock strain, the geometry of pores and fractures, and their permeability and porosity; and (2) through interactions between fluid pressure and rock mechanical stress. Modeling of hydromechanical coupling, with mechanistic representation of damage and fracture initiation/propagation, remains a major difficulty. However, such processes are of particular importance for mechanically weak geomaterials such as clays and shales.

ESD hydrogeologist Daisuke Asahina recently led a group of scientists (including ESD’s Jim Houseworth, Jens Birkholzer, and Jonny Rutqvist) in developing a modeling approach for studying hydromechanical coupled processes, including fracture development, within geological formations. They did this through the novel linking of two codes: TOUGH2, a widely used simulator of subsurface multiphase flow based on the finite volume method; and an implementation of the Rigid-Body-Spring Network (RBSN) method, which provides a discrete (lattice) representation of material elasticity and fracture development. The TOUGH–RBSN simulator predicts fracture evolution, as well as mass transport through permeable media, under dynamically changing hydrologic and mechanical conditions. The group compared their numerical results from the simulator with those of two independent studies involving hydro-mechanical coupling: (1) numerical modeling of swelling stress development in bentonite; and (2) experimental study of desiccation cracking in a mining waste. The comparisons show good agreement with respect to moisture content, stress development with changes in pore pressure, and time to crack initiation. The observed relationship between material thickness and crack patterns is captured by the proposed modeling approach.

Fig. 1. Typical lattice element ij with a zero-size spring set located at centroid C of area Aij, which is the area of the Voronoi facet common to neighboring Voronoi cells associated with matrix nodes i and j.
Fig. 2. Uniaxial tension test of concrete: (a) load-displacement result (experimental results of van Vliet and van Mier, 1999), and (b) crack propagation simulated by RBSN. (Adapted from Sukumar and Bolander, in press).

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Citation: Asahina, D., J.E. Houseworth, J.T. Birkholzer, J. Rutqvist, J.E. Bolander (2013), Hydro-mechanical model for wetting/drying and fracture development in geomaterials. Computers & Geosciences, DOI: 10.1016/j.cageo.2013.12.009.

Funding: UFDC