This paper presents three closely related ideas and technologies: (1) The secure, repairable, long time confinement of nuclear radioactive waste underground by a large surrounding region of compressive overstress; (2) The inherent tectonic weakness and vulnerability of the normal underground environment and its modification by overstress; (3) The process of creating overstress by the sequential periodic high pressure injection of a finite gel strength rapid setting grout.

Nuclear Waste: The secure, long-term confinement of radioactive nucleotides has traditionally required the assurance of confinement integrity over many thousands of years. In view of the tectonic activity of this earth, this seems to be unrealistic and not obtainable. Every location on the continents is visited by the damage of a major earthquake roughly every 10,000 years. Because of the usual, initial, large anisotropy of the tectonically relaxed underground stress field, an earthquake easily alters this local stress field in a major fashion so that it is unlikely that any underground structure can be certified against damage during such an event. However, it is possible to both periodically augment the tectonic security of the underground structures and furthermore to offer the possibility of in situ but remotely opperated repair. This assures the integrity of underground confinement of nuclear wastes by a technically competent society indefinitely in the future. To accompish this it must be shown to be both feasible and relatively inexpensive to engineer a large region of compressive stress surrounding any underground cavity. This curtain of overstress (over and above the local in situ stress due to overburden pressure) can be reestablished at a future time by a future society whenever necessary. The process requires the technology of drilling and pumping high pressure fluids. A society that has given up these technologies would be hampered from such post-repair work, but it is unlikely that the population density at such a time would ever be threatened by exposure to greatly attenuated and decayed nuclear waste.

Results of Overstress: A region of compressive overstress surrounding any underground cavity not only multiplies the integrity against failure by collapse of such a cavity by orders of magnitude but also insures that the exchange of fluids either into or out of a cavity is greatly inhibited by the compressive overstress of the medium itself. Therefore the integrity of underground confinement can not only be greatly improved at the time of its initial inception, but more important can be assured in the indefinite future. Underground stress engineering is not only feasible but is a relatively inexpensive repair process.

Creating Overstress: The process of creating overstress results in altering the underground stress distribution. It can be achieved by the periodic injection of a settable fluid that has the rheological properties of finite gel strength and rapid setting to a rock-like material. Each cycle of injection fractures the medium followed by the setting of the injected material to a rock-like, hard solid. This setting processes locks-in the increment of pressure used to fill and open the fracture in the first place. By successive increments of pressure and successive fractures, the local stress locked into the medium with each cycle can be progressively increased to an arbitrarily high value, limited only by the strength of the materials used to inject the special fracture fluid and the compressive strength of rock. The process of underground stress engineering has been partially tested in the field but needs a much larger effort to demonstrate its feasibility for the important role that it can play in the safety of our underground confinement structures.