ABSTRACT
Acid igneous rocks, like granites, basalts and gabbros, as well as metamorphic rocks, like gneisses, schists and slates, form more than 95% of the earth’s crust and make up the rigid basements of the continents, attaining more than 70 km depth at the root of high mountain chains. Basic igneous rocks, like petrified seafloor basalt extrusions, pile up to 7 km thickness under the oceans. However, igneous and metamorphic rocks emerge as outcrops only over 15% of the earth’s surface and are frequently covered by layers of sedimentary rocks, like sandstones, siltstones and limestone, up to hundreds metres thick, and also by less resistant thinner veneers, such as weathered rocks and residual soils or unconsolidated sediments like gravels, sand and clays. Yet, rockmasses near the earth’s surface, up to tens or hundredsmetres depth, are not
massive. Indeed, distinct groups of almost planar discontinuities split all igneous and metamorphic rock masses into contiguous and practically impervious blocks of quite resistant intact rock. From a strict mechanical point of view, even if genetically dissimilar, these rock masses, as well as some types of crystalline limestone and dolomites, can be collectively classified as “hard rocks’’ or, simply, as “fractured rocks’’. By custom, the generic term fracture stands for all kinds of restricted partitions or discontinuities within rock masses. Irrelevant and small amounts of groundwater, almost clogged, accumulate inside
the minute voids and micro cracks of intact rock blocks and hardly drop under the action of gravity. This kind of water remains practically unavailable for exploitation. On the other hand, significant quantities of groundwater may saturate the fractures of rock masses and other sorts of open discontinuities. This kind of water, called free water, may drain and percolate under its self-weight. Only this type of “gravity-driven’’ groundwater is the focus of the present book. In the last 50 years, many technical publications have dealt with the theoretical prin-
ciples and practical field and laboratory tests related to the appraisal of the groundwater flow through sedimentary rocks and unconsolidated sediments. In the same period, as “hard rocks’’ are not good aquifers by themselves, few publications have considered fractured rock hydraulics. In fact, groundwater storage volume is proportional to the porosity of the rock reservoir, including interconnected interstitial voids, fractures and dissolution discontinuities. For hard rocks, porosities seldom attain 2% of their total volume while may range from 5 to 15% for consolidated clastic rocks and up to 25%
for unconsolidated sands and gravels. Yet, during the last decades, the growing need of groundwater supply in “hard rock’’ land, mainly in poor countries, coupled with a better valuation of the seasonal groundwater recharge and corresponding transient storage at the top of deep weathered “hard rock’’ land in wet inter-tropical climate, has altered attitudes. Moreover, in the same period, new investigation methods and new technologies based on sounder theoretical concepts have been devised to improve “hard-rock’’ hydraulics models, commonly motivated by attempts to enhance the structural and environmental safety factors of increasingly larger engineering works founded and excavated in these rocks. Civil or mining facilities under the sea level or under the groundwater table, such as access drifts, stopes, panels, galleries, tunnels, shafts, underground hydropower plants, oil and gas storage caverns, and classed nuclear waste deep disposal, are examples of excavation works where their structural behaviour highly interacts with their hydraulics.
