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16 RAPID EXCAVATION OIL RIG I S O U T E D / B U L K S T O R E , SELF SUPPORT TUBE K M OIL OiifV.tir TO TANKER -LOCKTUBE FOR ACCESSORIES BULK STORE r W I D SSPAKA7ION WELL FIGURE 4 Artist's sketch, concept for subterranean and sub sea oil recovery, storage, and delivery system. Published by pennission of the Naval Weapons Center, China Lake. U.S. Department o f the Navy. CURRENT CONSTRAINTS ON INCREASED UNDERGROUND-EXCAVATION PRODUCTIVITY From what has been described above, it can easily be surmised that the cur- rent capability of underground-excavation technology is insufficient to lessen the growing concern about environmental quality, and that this is in large measure a reflection of the current cost and time differentials between surface- and subsurface-excavation activities. These differentials call for comment here. Underground excavation is the process of digging out and removing mate- rial either to form a usefid cavity beneath the earth's surface or to derive profit from the removed material. The process comprises a survey of geological con- ditions and rock properties, disintegration of in situ material, transportation of
SIGNIFICANCE 17 disintegrated material from the excavation, support and lining of the excava- tion, and provision of a safe working environment. Although both surface and underground excavation involve the same steps, the process is appreciably more difficult in a subsurface environinent than in a surface environment because of two distinctive features of underground ex- cavation. First, in underground excavation, geological conditions are the prin- cipal design consideration, being both the source oftoadsto be resisted and the basis of resisting these loads. Moreover, once the location and dimension of an excavation are determined, geological conditions are the major factor in choosing excavation methods. Yet, the predominant characteristic of the earth's crust is heterogeneity; major differences in rock types, arrangements, and con- ditions occur within relatively small vertical and horizontal distances. Seldom is the true extent of this heterogeneity known before the excavation process begins. Consequently, the cunent process closely resembles a trial-and-enor effort, calling for great ingenuity on the part of excavators to devise suitable techniques as work progresses. Second, in underground excavation, space is confined, ahnost tailored, to the final dimensions of the excavation. These space limitations require high concentration of rock-disint^ration energy at the working face, complete coordination of disint^ation, removal, and support systems, and extraordinary provision for the comfort, health, and safety of the working force. But integral underground-excavating systems, e.g., "one- piece" continuous driving, removal, and support systems, do not now exist for general excavation operations. Rather, even the best current process is at most a start-stop process, requiring excavators to assemble and coordinate available components into a system having limited synchronization. Inevitably, the re- sult of these two features is that the current process is expensive and has a rather slow rate of sustained linear advance. But other interrelated technical and commercial features of underground excavation also tend to retard the rate of sustained advance as well as the cur- rent rate of technological change in the process. Chief among these features are (1) iiudequate technical knowledge to promulgate rational design criteria for both the excavation and the equipment and (2) inadequate industrial in- centive to develop better equijnnent. The inadequacy of techiiical knowledge, principally geological and rock- mechanics knowledge, now penalizes the undergroimd-excavation process in several ways. For example, the base of knowledge is insufBcient to develop geological methods needed for properly assessing, in advance of actual excava- tion operations, the geological characteristics of underground rock masses (e.g., the degree of jointing, faulting, or other weaknesses of geological origin) and of the possibility of unfavorable groundwater conditions. Thus, at the present time, it is not possible to ensure choosing the best site for an
18 RAPID E X C A V A T I O N underground excavation or for the route of a tunnel. Further, the base of knowledge is inadequate to develop rock-mechanics methods needed for de- termining the in situ properties of rock masses (e.g., strength, creep rate, thermal and electrical conductivity, and the state of stress). Thus, it is not possible to predetermine rationally the type and extent of linings or ground support that will be required. The base of knowledge is also insufficient to develop geological methods needed for determining the quality of rock or groundwater conditions immediately in advance of the working face. Too often, even with comparatively slow conventional excavating procedures, areas of incompetent rock or excessive groundwater are exposed without warning, and inordinate delays result. Moreover, the base of knowledge is insufficient to permit correlation of geological and rock-mechanics information with excavating process and machinery design and performance information. Thus, the development of im- proved blasting and mining and tunneling-machine techniques - for reducing damage to excavation surfaces, for improving hydraulic conditions in unlined tunnels, for reducing the need for support, for minimizing damage to the rock behind tunnel walls - is retarded. In sum, these inadequacies create a high degree of conservatism among designers and excavators, produce inordinate delays in excavating operations, and add substantially to the cost of subsurface facilities. The inadequacy of industrial incentive to produce better equipment likewise penalizes the underground-excavation process in a variety of ways. Most ob- viously, it denies the excavator theflexibilityin equipment and operating procedures cunently needed to perform efficiently in the face of geological uncertainty. More importantly in the long run, it denies the excavator the co- ordinated, assembled excavating systems that are needed to increase drastically the sustained rate of advance of excavation. To a large degree, the inadequacy of motivation is a reflection of the highly fragmentary nature of the construc- tion industry and its work. The construction industry is a loosely organized industry of designers, contractors, and equipment manufacturers. That branch of the industry most seriously interested in excavation is usually classified as the "heavy-construc- tion industry," and it is generally concerned with the creation of dams, tuimds, canals, and the larger hi^way and railroad projects. Seldom does any member of this branch have a detailed interest in the whole excavation process - planning, design, construction, and the manufacture and supply of excavating equipment. Thus, there is no internal inducement to develop an integrated attack to improve the entire process. Ihe great variability of underground-excavation projects is also an impedi- ment to industrial incentive. Heavy-constructionfirmsseldom have two or
