Page:The American Cyclopædia (1879) Volume II.djvu/721

 BLASTING 701 tached, either by blasting or other means, until the whole is removed and a new bench formed. The progress made in blasting at the Hoosao tunnel in Massachusetts during the month of March, 1872, in the east end, at a distance of 10,046 ft. from the entrance, was 120 ft. of heading 24 ft. wide and 9 ft. high. This head- ing was attacked by 12 Burleigh drills, mount- ed on two carriages manned by eight men and a foreman. On Dec. 12 of the same year the last portion of rock that divided the exca- vations was removed, and it was found that the axes of the two only differed by the re- markably small error of fiVe sixths of an inch laterally, and an inch and a half vertically. (See TUNNEL.) In submarine blasting on a large scale, by the modern method, a coffer dam is erected over the rock and a shaft sunk into it, from which tun- nels are excavated in radiating directions, and these connected by concentric galle- ries, while columns Fia. 6. Coffer Dam. o f roc ^ &Te J e ft as supports to the roof, and to maintain the water bed till the work is completed. A sufficient number of charges of an explosive compound are then introduced into the columns in chambers, and in the shell, and simultaneously fired by means of a galvanic battery. When the work is not too extensive and the superincumbent pressure of rock and water is not too great, the columns of rock supporting the roof may be replaced by wooden ones, thus allowing of the removal of a larger amount of material be- fore the final explosion takes place. This is an advantage, since its removal in this way is less expensive than by rakes and grappling irons after it is broken up and lying beneath the water. In such excavations many precau- tions are required which are unnecessary in boring a tunnel through a mountain. Mathe- matical calculations and estimates, requiring extensive engineering knowledge and sound judgment, must be made hi order to ascertain the amount of resistance required in the arches and in the columns of support, composed as they are of rock of varying composition, tex- ture, and degree and direction of stratification. If a breach should be made in the water bed, the works would be flooded, causing serious delay and expense in making repairs, which must be done by sinking rocks and cement into the breach and pumping the water from the caverns. Moreover, the breach might be so extensive as to be irreparable, in which case the remainder of the rock which had been tunnelled would have to be removed by sur- face blasting. It frequently happens that small fissures are opened, which under the great pressure of water from above cause serious annoyance, and all the ingenuity and knowl- edge that can be brought to bear are required to stop the leak. To avoid disturbing the water bed, it is also safer to fire the blasts of nitro- glycerine singly with a fuse, and not in num- bers simultaneously. It is thus perceived that blasting as now practised is an important branch of the science of civil engineering. With the materials and appliances at hand, in the form of gunpowder, nitro-glycerine, per- fect safety fuse, the ready and facile command of galvanic electricity, properly constructed drills, and compressed air engines to propel them, the problems presented to the civil en- gineer are exceedingly interesting, and offer no obstacles which careful and correct calculation cannot overcome. The removal of Blossom rock in the harbor of San Francisco is an ex- ample of the process of removing submarine rocks by conducting the excavation from with- in. It is the only operation of the kind which has been completed, although another and more extensive one, previously commenced, is now (1872) in progress at Hallett's point in the East river, opposite New York. The top of Blos- som rock was about 5 ft. below the surface of the water at mean low tide. A horizontal section at the depth of 24 ft. measured 195 x 105 ft. The quantity of rock contained with- in these boundaries was about 5,000 cubic yards, and consisted of a metamorphic sand- stone of irregular stratification. The great mass of it was so soft as not to require blasting. In October, 1868, brevet Brig. Gen. B. S. Alex- ander, lieutenant colonel of engineers U. 8. A., communicated a plan for the removal of this rock to Lieut. Col. R. S. Williamson, major of engineers, who had been placed in charge of its survey. Gen. Alexander's plan is briefly ex- plained in the following extract from his com- munication : " I propose to enclose a small surface of the rock by a water-tight coffer dam ; in this space to sink a rectangular shaft about 4 by 9 ft., which is the size I have seen in coal mines; from the bottom of this shaft to run tunnels and make powder chambers in such positions that when exploded the whole rock down to the level of 24 ft. below the level of the water will be lifted in the air and shiv- ered to pieces." In November following, Mr. A. W. von Schmidt, a civil engineer of San Francisco, sent in a plan for the removal of the rock, and offered to perform the work for $75,000, which plan and offer were in due time accepted. His plan was similar to Gen. Alexander's, except that instead of the ordinary coffer dam he proposed to sink an iron cylinder 6 ft. in diameter, carrying an in- dia-rubber flap at its lower end, pump out the water, bore into the rock, and slide another cylinder inside of the first down into the ex- cavation and secure it by cement. It was, however, found difficult to place the iron cyl- inder in position without first resorting to the ordinary cribwork coffer dam. The sinking of the shaft was commenced Dec. 7, 1869. Only one man could work at a time, but in the space of four weeks a depth of 30 ft. below low water was reached. Drifts were then run into