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It has long been recognised that the size of a rock sample can have a significant impact on the strength recorded of the sample. In this paper the scale dependence caused by the presence of larger weaknesses when the sample volume increases is investigated. An approach based on linear elastic fracture mechanics is proposed to define the behaviour and results are presented.

High-level waste canisters in bored holes are generally proposed to be isolated from the rock by dense smectitic clay, termed buffer. Other rooms can be backfilled with less smectite-rich soil. Smectites expand on hydration and confine the canisters effectively. These clays are very tight and practically the transport of water and ions takes place by diffusion. Maturation of the buffer and backfill is slow and associated with precipitation of salt. Long-term changes include slow settlement of canisters and conversion from smectite to non-expanding minerals associated with precipitation of cementing agents like silica and iron compounds.

Two large-scale field tests in granite for identifying possible excavation-induced rock disturbance have been performed in Sweden as part of the on-going research for developing techniques for disposal of highly-radioactive waste from Swedish nuclear reactors. The tests involved determination of the effect of blasting and stress changes on the near-field rock and have led to a preliminary model of excavation damage. Blasting produces a disturbed zone with enhanced axial hydraulic conductivity from a few decimeters to more than 1 m from the tunnel periphery, depending on the change. The axial hydraulic conductivity is increased by 2–3 orders of magnitude in this zone, a net value being about 10 −8 m/sec in rock with a virgin conductivity of 10 −11 −10 −10 m/sec. The increase in conductivity is partly due to stress changes, which raise the axial conductivity also further out in the rock. Such influence may take place to a distance from the tunnel periphery of up to 50% of the tunnel radius and the net axial conductivity of the zone is estimated to be about 10 times higher than the average virgin conductivity if the tunnel is suitably oriented with respect to the fracture sets. This change is associated with a reduction in radial conductivity of around 5 times.

Publisher Summary This chapter describes mechanical and basic physico-chemical properties of clays and clay minerals that are important to the understanding of their behavior. In civil engineering, the relationship between the visual mechanical behavior of clays and the properties of clay minerals at the nano- and meso-scales (usually referred to as ‘‘microscale’’) is now well recognized. Investigating the properties at these scales would allow predictions to be made of macroscopic behavior. The most important properties of clays in the fields of soil mechanics are hydraulic conductivity, gas conductivity, ion diffusion capacity, swelling potential, compressibility, and rheological properties. All these properties depend on the type and content of clay minerals in, and the bulk density of, the soils.

Publisher Summary Eliminating the risk of transportation of radionuclides from radioactive waste stored underground to the biosphere is the goal set by all countries that make use of nuclear power. Different countries plan geologic disposal in rocks, salt and clay sediments. The design principle is that highly radioactive waste, such as burnt-out fuel and products from processed waste, is enclosed in containers—canisters—isolated by smectitic (smectite-rich) clay. These canisters are placed in vertical boreholes or long horizontal bored or blasted tunnels at a depth of 300–800 meters. Low- and medium-level waste has shorter lifetimes and can be stored at smaller depth with less effective isolation. Selection of smectitic clays with suitable bulk density to embed the canisters is based on the valuable properties of clay buffer: (1) very low hydraulic conductivity, (2) very low anion diffusion capacity and fairly low transport capacity of positively charged radionuclides, (3) a high swelling potential for self-sealing of gaps and openings in the buffer and its contacts with the rock and canisters, (4) favorable rheological properties, such as sufficient bearing capacity to minimize settlement of the heavy canisters and sufficient softness to avoid transfer of high tectonically induced shear stresses to the canisters, (5) sufficient thermal conductivity to transfer heat caused by the radioactive decay to the rock without being too hot, (6) colloid filtering capacity, and (7) capacity to filter microbes.

Smectite clay is used for sealing purposes in repositories for radioactive waste because of its excellent isolating properties. Typical examples are grouts injected into rock fractures for sealing the host rock, and blocks of highly compacted clay powder for plugging shafts and tunnels and for embedding metal containers with waste. The most important clay materials are bentonites, which usually have the smectite species montmorillonite as the major clay mineral. Such clays have a low hydraulic conductivity and a swelling potential that makes them fill up and seal off the space in which they are placed. They also have a significant cation exchange capacity and, at least for some ion species, a low diffusion transport capacity. The migration of radionuclides is of course of great importance for the performance of repositories for high-level radioactive waste, a matter that will be dealt with here. This chapter starts by describing the nature of smectitic clays and by giving examples of bentonite seals, and proceeds with explaining microstructural processes that control physical properties like hydraulic and gas conductivities and swelling ability. The chapter ends by outlining the basics of the longevity of smectite clay materials and by describing the performance of smectitic seals under repository conditions.

Theory as well as small- and large-scale experiments indicate that tunnel excavations in rock result in an increased axial permeability close to the tunnels. This is due to stress relief as well as to blasting, of which the former is generally assumed to be most important, at least when smooth blasting is applied. The Buffer Mass Test in Stripa offered a possibility of quantifying the increase in axial permeability and it was concluded from this experiment that the increase may have been as high as 1000–10,000 times in a 0.5–1.0 m wide zone adjacent to the excavation, assuming that the value 10−10 m/sec derived from the preceding, large-scale “Macropermeability Test” is representative of the hydraulic conductivity of the undisturbed rock. If this figure is correct, disturbance by blasting would be the major cause of the increased perviousness.