This paper studies the potential impact of glaciations on the deposition boreholes of the future deep geological repository for nuclear waste at Forsmark, Sweden. A multiple-borehole numerical model is considered, in which fracture growth leading to spalling is simulated in three-dimensions, using the Imperial College Geomechanics Toolkit, a finite element-based discrete fracture growth simulator. Fractures initiate in tension due to in situ stress redistribution after drilling, and grow based on stress intensity factors computed at fracture tips. Glaciation is modelled through its effect on the horizontal stresses determined by ice-crust-mantle simulations, and the vertical stress is determined by the weight of the overlying ice sheet. Numerical results indicate that changes in the in situ stresses influence nucleation and growth patterns, increasing the extent of the damaged rock around the deposition boreholes during the glacial cycle.
Spalling is a common failure mechanism in brittle rock under high compressive in situ stresses, and it is expected to occur around the deposition boreholes of deep-drilled geological disposal facilities for nuclear waste (Martin 2005). Spalling-inducing fractures initiate in tension and propagate as a response to the mechanical and, if present, thermal loading of the borehole. The fractures interact and coalesce in the vicinity of the borehole, forming excavation damage zones (EDZs), which can be classified based on the intensity of the damage (Tsang 2005). The damaged zone closest to the borehole wall contains interconnected fractures and is generally referred to as highly damaged zone (HDZ). The fractures within the HDZ may cause slabs of rock to spall from the borehole wall, resulting in a distinctive V-shaped notch, i.e. the "spalled" zone, in the direction of the minimum compressive stress (Figure 1a). Beyond the spalled zone, the host rock becomes progressively less damaged, but the fractures in the outer excavation damage zones still alter host rock properties such as permeability (Siren et al. 2015).
Observations from field studies indicate that mechanical spalling is a common occurrence during the excavation stage of deep-drilled boreholes and tunnels in hard, brittle rock (Read 2004, Martin 1999), and that it may be aggravated during the thermal stage of a deep geological repository (DGR) for nuclear waste disposal (Anderson 2007). DGRs in high latitude regions also have the additional long-term risk of being influenced by glaciations (SKB TR-10-23), which may cause new fractures to nucleate in the EDZs, or cause pre-existing fractures to reactivate.
