Underground Caverns

Displacement data provide a direct means of evaluating cavern stability. Displacements that have exceeded the predicted elastic displacements by a factor of 5-10 generally have resulted in decisions to modify support and excavation methods. In a creep-sensitive material, such as may occur in a major shear zone or zone of soft altered rock, the natural stresses concentrated around an opening cause time-dependent displacements that, if restrained by support, result in a build-up of stress on the support. Conversely, if a rock mass is not sensitive to creep, stresses around an opening normally are relieved as blocks displace towards 

The walls of a cavern may be influenced by the prevailing state of stress, especially if the tangential stresses concentrated around the cavern approach the intact compressive strength of the rock (Gercek and Genis, 1999). In such cases, extension fractures develop near the surface of the cavern as it is excavated and cracks produced by blast damage become more pronounced. The problem is accentuated if any lineation structures or discontinuities run paraiiei with the walls of the cavern. Indeed, popping of slabs of rock may take place from cavern walls. 

Rock bursts have occurred in underground caverns at rather shallow depths, particularly where they were excavated in the sides of valleys and on the inside of faults when the individual fault passed through a cavern and dipped towards an adjacent valley. Bursting can take place at depths of 200-300 m, when the tensile strength of the rocks varies between 3 and 4 MPa. 

The support pressures required to maintain the stability of a cavern increase as its span increases so that for larger caverns, standard-sized bolts arranged in normal patterns may not be sufficient to hold the rock in place. Most caverns have arched crowns with span to rise ratios, B/R, of 2.5-5.0. In general, higher support pressures are required for flatter roofs. Frequently, the upper parts of the walls are more heavily bolted in order to help support the haunches and the roof arch, whereas the lower walls may be either siightly bolted or even unbolted (Hoek et al., 1995). 

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Deep-Well Injection Deep-well injection for the disposal of liquid wastes involves injecting the wastes deep in the ground into permeable rock formation (typically limestone or dolomite) or underground caverns. 

The problem faced after separation of the C02 is what to do with it. The uses of C02 are so small relative to the volumes produced by combustion that the only realistic solution sequestration. This involves finding a stable location for the C02 (e.g. by injection into oil wells or underground caverns). Thus, whilst it is technically feasible to recover C02, it is both expensive and difficult to dispose of the C02. 

An in-depth discussion on natural gas storage in underground caverns may be found in Gas Engineering and Operating Practices, Supply, Book S-1, Part 1, Underground Storage of Natural Gas, and Part 2, Chapter 2, Leached Caverns, American Gas Association, Arlington, Va. (1990). 

An underground cavern of the city drinking-water supply system consists of a water reservoir (area A0 = 2000 m2, depth hK = 4 m) and an air space for maintenance above it (mean height hz = 2.5 m). The flow rate of the water is gw = 1600 m3 Ir1. The air space is exchanged in 2 hours. 

As the calcium bicarbonate solution drips through cracks in the rock and into an underground cavern, some carbon dioxide bubbles out of the solution. 

Stalactites and stalagmites are found in underground caverns in limestone areas. They are formed from the slow decomposition of calcium or magnesium hydrogencarbonates in water (Figure 11.34). 

The intoxicating action of certain chemical vapors is not a new discovery. Some researchers have claimed that Pythoness, a mystic in ancient Delphi, delivered her famous oracles while under the influence of heady fumes escaping from an underground cavern. 

For stationary hydrogen storage on a large scale, underground caverns or cavities are an appealing option, often offering storage solutions at a very low cost. Three possibilities of interest are salt dome intrusions, cavities in solid rock formations and aquifer bends. 

Although erosion and deposition are most easily observed where solid sediment is being moved about, invisible chemical reactions also produce landforms. As water moves through the soil it becomes acidic, in part because of the addition of carbon dioxide produced by the decay of organic matter. This weak acid is able to dissolve some kinds of rock, particularly limestone, giving us spectacular underground caverns, such as the Carlsbad Caverns of New Mexico. If such caves develop near the surface they often collapse, and a landform called a sinkhole develops above the cave-ins. 

More recently, salt beds have been considered for storing nuclear and solid wastes. One possibility is to inject water to remove some of the salt and thereby form an underground cavern. The cavern could then be filled with the waste, plus (possibly) a solidifying agent, and then cement sealed. 

As the Arsenal continued to pour 165 million gallons of waste into the underground cavern over the next five years, the area suffered no less than 1,500 earth tremors. When, in 1966, the dumping was called to a halt, the army announced it would investigate whether the stuff could be pumped out again. Their conclusion, that the liquid wastes could only be removed at a rate of 300 gallons a day, indicated that it would take over a thousand years to empty the well. Although the earth tremors stopped after only part of the waste had been removed, the incident did little for the popularity of chemical weapons. 

Underground caverns then form. In the process of dissolving the limestone and creating the cave, the water containing the dissolved CaCOj drips from the ceiling of the cave. As the water forms drops, it tends to lose some of the dissolved CO2, which lowers the amount of H" present (by the reversal... 

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