This Symposium General Report attempts to highlight important developments in Rock Mechanics for applications to engineering problems at great depth. Much of the report was presented orally in session # 9, but in an abbreviated form. The many excellent written and oral contributions to the Symposium represent a technical richness that will take time to assimilate. We cannot mention all the technical content nor do justice to the effort that the Proceedings represent. We recommend that the reader scan the various General Reports and Invited papers to appreciate the breadth of technical content and the scope of the topic "Rock at Great Depth". Then, with this as background, the detailed papers will provide insight into a number of relevant Rock Mechanics problems in the areas of civil, mining and petroleum engineering, with links to geochemistry, solid body mechanics, geo-hydrology, applied mathematics, tectonics and geophysics. The interdisciplinary scope of the papers is astounding; when a mine designer and a geochemist sit down and discover common interests, when a drilling engineer and an applied mathematician outline similar views of a phenomenon, it speaks for the excitement of Rock Mechanics applications at great depth. The Session General Reporters and Workshop Presidents provided us with useful oral and handwritten comments. In particular, we thank A. Nur, D. Ortlepp, J. Geertsma, M. Chenevert, M. King, R. Widman, W. Bradley, C. Spiers, and O. Coussy for their opinions and comments. Conclusions from the Workshops are summarised, based on oral summaries provided by A. Guenot, W. Kessels, F. Cornet, and written comments by J. Sulem, I. Vardoulakis, P. Berest and Ph. Weber. The last-minute communication by B. Come has also been used. Before a general Symposium conclusion is attempted, we will address a number of current and central issues in Rock Mechanics
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Unique aspects of the mechanics of geo-materials at great depth
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Rapidly developing areas related to Rock Mechanics and
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Specific issues that have arisen during workshops.
"Deep" is defined as 3 000 to 7 000 m, occasionally somewhat deeper, but still involving situations where the rock is physically encountered, as opposed to remotely probed. Below 10,000 m, one must speak of "geological" or "geophysical" depths. This may be the domain of the next generation of petroleum and rock engineers and applied earth scientists; currently, only the deep Kola Peninsula scientific borehole exceeds this limit. A number of special and important cases were addressed during the technical presentations.
Exceptionally high porosities at depth are found in chalk, silt, and sandstone reservoirs as well as undercompacted shales. Examples include the Chalk reservoir of the Valhall Field in the North Sea, with beds of 50% porosity at 3 000 m depth; the "geo-pressured" reservoirs of the Gulf Coast, California, and many other basins; and high porosity shale cap rocks in many areas, usually with pore pressures well above hydrostatic.
