Improved technology
Worldwide efforts are under way to accelerate improvements in the technology of underground construction and are likely to be stimulated as a result of the 1970 OECD International Conference recommending improvement as government policy. The endeavour involves specialists such as geologists, soil- and rock-mechanics engineers, public-works designers, mining engineers, contractors, equipment and materials manufacturers, planners, and also lawyers, who aid in the search for more equitable contractual methods to share the risks of unknown geology and resulting extra costs. Many improvements and their early applications have been previously discussed; others are briefly mentioned here, including several that have not yet moved from the research stage to the pilot, or trial, stage. Projects in rock are emphasized, since the field of rock engineering is less developed than its older counterpart, soils engineering.
Geologic prediction and evaluation are universally recognized as deserving a high priority for improvement. Since ground and water conditions are controlling factors in choosing both the design and construction method underground and seem destined to be even more so with greater use of moles, efforts are directed toward improving boring information (as with borehole cameras), faster borings (the Japanese are trying to bore one to three miles ahead of a tunneling mole), geophysical methods to estimate rock-mass properties, and techniques to observe pattern of water flows. For evaluation, the new field of rock mechanics is concentrating on measuring geostress and rock-mass properties, failure mechanics of jointed rock, and analytical methods for applying results to design of underground openings.
For rock excavation, improved cutters are generally considered the key for expanding economic ability of moles to include harder rock. Much effort is being devoted to improving current mechanical cutters, including technical advances based upon space metallurgy, geometry of cutter shape and arrangement, mechanics of cutting action, and research in presoftening rock. Concurrently, there is an intensive search for entirely new rock-cutting methods (some nearing a pilot application), including high-pressure water jets, Russian water cannon (operated at high pressures), electron beam, and flame jet (often combined with abrasive powder). Other methods under research involve lasers and ultrasonics. Most of these have high power requirements and might increase ventilating needs from an already overtaxed system. Though some of these novel methods will eventually reach the stage of economic practicality, it is not possible to predict at present which ones will eventually succeed. Also needed is a means for testing rock in terms of mole drillability plus correlation with mole performance in different rocks, where promising work is under way at several locations.
A decided change in current materials-handling systems seems inevitable to keep up with fast-moving moles by matching the mole’s rate of excavation and fragmentation sizing of the muck produced. Schemes now under study include long belt conveyors, high-speed rail with completely new types of equipment, and both hydraulic and pneumatic pipelines. Useful experience is being accumulated with pipeline transport of ore slurries, of coal, and even of such bulky material as canned goods.
For ground support, rock-mechanics engineers are working toward replacing past empirical methods with a more rational basis of design. One key factor is likely to be the tolerable deformation for mobilizing but not destroying the strength of the rock mass. There is wide agreement that progress will best be aided by field-test sections at prototype scale in selected ongoing projects. While several newer types of support have been discussed (rock bolts, shotcrete, and precast-concrete elements), developments are under way toward entirely new types, including lighter material plus yield-controllable types as a corollary to above tolerable deformation concept. For projects using concrete lining, major changes seem inevitable to keep pace with fast-moving moles, probably including some entirely new types of concretes. Current efforts include work with precast elements, plus research into stronger and faster set materials which use resins and other polymers in lieu of portland cement.
Preservation of ground strength is beginning to win acceptance as vital for the safety of large rock chambers and also often a means of cost saving in tunnels. For preserving strength of the rock mass around tunnels, a mole-cut surface provides a solution. For large chambers, consideration is being given to cutting a peripheral slot with a wire saw of the type used to quarry monument stone. Where chambers are blasted, engineered sound-wall blasting has provided a solution in Sweden.
Ground strengthening by precementation with chemical grouts is a technique notably developed in France and Britain through extensive research by specialized grouting firms. The world’s outstanding application at the Auber Station of the Métro Express beneath the Place de L’Opéra traffic centre of Paris has a large chamber 130 feet wide by 60 feet high by 750 feet long in chalky marl below an existing subway, at a depth of 120 feet, about 60 feet below water table. This was completed in 1970 without interrupting surface traffic and without underpinning the many old masonry buildings above (including the historic National Opera Building), a truly courageous undertaking made possible by surrounding the chamber with a pregrouted zone to seal out water and to precement the overlying sand and gravel. Different types of chemical grout were successively injected (totaling about two billion cubic feet), working from crown and side drifts; then the chamber was mined and supported both top and bottom by prestressed arches of concrete elements. Similar procedure was also successful at the Étoile Station adjacent to the Arc de Triomphe. While this technique of ground strengthening by grout solidification requires highly skilled specialists, it is an instructive example of how a new technology is likely to make economically possible future projects previously considered beyond engineering ability.