At the Äspö underground rock laboratory spalling was induced in part of walls of a 1.75 m diameter, 6.5 m deep hole in granite. A damaged wedge about 50 cm wide and 10 cm deep extended along the walls of the hole. The rock fragments were collected, in all about 176 kg, and their shapes and sizes, which ranged between a fraction of mm and tens of cm were measured. Frequencies of mass of different particle sizes as well as shapes of particles were determined The particles were typically flat slabs with a thickness 0.1 of the length. They were much thinner at the edges than in the middle. The width was about half the length or less. The data were used to estimate the specific surface of the fragments. This information was used to estimate the anisotropic hydraulic conductivity of the damaged zone. Particle shapes were used to estimate the tortuosity in the zone, which influences the effective diffusivity of solutes that migrate in the zone.
There are concerns that spalling may occur in deposition holes for spent nuclear fuel canisters. A zone damaged by spalling may have a multitude of small fractures and the rock has dilated increasing the porosity of the damaged zone. The hydraulic conductivity is expected to be larger than that of the intact rock. Water bearing fractures intersecting the deposition hole can lead in water in the pore system in the damaged zone and the water will have a longer residence time and larger contact area with the buffer between the rock and the canister than if it just passed in the fracture. Larger mass transfer of corrosive agents and of potentially leaking radionuclides will then occur. Information of the number of induced fractures and on the tortuosity of the flow paths in the damaged zone is needed to assess this effect. The particles sizes, forms and particle size distributions of the rock in the damaged zone are needed to make estimates of flow and transport properties.
The Äspö Pillar Stability Experiment (APSE) was carried out to examine the failure process in a heterogeneous and slightly fractured rock mass when subjected to coupled excavation-induced and thermal-induced stresses. (Andersson 2007) The pillar was created by the excavation of two large boreholes (ø 1.75 m, 6.5 m deep) so that a rock web of ~ 1 m was left in between them. The first of the two large holes was confined with a water pressure before the excavation of the second hole commenced. This was done to enable the effect of a confinement pressure on the response of the rock mass to increased loading to be studied. During heating, the rock damage formed a V-shaped notch that propagated down the un-confined hole wall. It was closely followed by visual observation and an acoustic emission system.
After the experiment all the rock fragments were collected. They amount to totally 176 kg.