Editors of other technical journals are invited to reprint articles NOTE. The Club, as a body, is not responsible for the statements and opinions advanced in its publications. THE failure of the dam at Austin, Pennsylvania, on September 30, 1911, affords a sad but immensely valuable lesson to hydraulic engineers. It teaches that in building such structures the first requirement is the absolute safety to human life; the second, that it shall serve its purpose; third, that it shall cost a minimum. Apparently, in this case the ends attained were in reverse order with the last term negative. It is a principle of humanity that the most valuable lessons are taught and firmly impressed by discovery of disastrous error. Some one has said that no engineer is safe until he has made some serious blunder. The consequence of the Austin dam failure will probably result in making unduly expensive for the next few decades all similar structures. The Johnstown flood taught the importance of the spillway for earth dams, while subsequent experiences in the Allegheny Mountains gave specific data which indicated that in small drainage areas of 1 to 10 square miles it is not unreasonable to provide for a flood flow of 400 cubic feet per second per square mile. The design of a masonry dam, as far as its section is concerned, can be successfully executed by any technically trained boy just out of college. It is only necessary to be familiar with the simple principles of statics, the resolution of forces, and a few empirical rules. To resist overturning he makes the section such that the resultant of the water pressure and the weight of the assumed section of unit thickness will fall within that section at every point. If this condition is fulfilled, under the most severe conditions of exterior load that will ever take place, the dam will not overturn unless the masonry or rock crushes. To avoid this, as well as to provide for a factor of safety, the shape is made such that the resultant falls at all depths within the middle third of the section. In making this calculation it is considered conservative to allow for possible upward pressure. Under the worst conditions this might amount to half the hydrostatic pressure, but no one allows for more than a third, which is even then considered ultraconservative. It is sometimes considered proper to calculate the safety of the structure against sliding, and for that purpose he assumes the entire horizontal pressure of the water resisted by the weight of the dam, less the assumed upward pressure, which net weight is multiplied by the coefficient of friction between masonry and rock, using about 0.7. In both cases a factor of safety of two is considered good practice. The effect of sliding, however, can be eliminated by carrying the structure down into the rock ledge a sufficient distance to effectually prevent sliding. Theoretically, the top thickness may be zero, but when the effect of unequal strains, frost, and the possible blow from a log, or tree, or ice thrust is considered, 4 feet is none too much, and for a large dam 6 feet is better. Now, when all these requirements are fulfilled, the pure mathematics portion of the work is done, but the real difficulties are yet to come. The material upon which the dam is to be founded must be considered. If the foundation is not excavated deep enough the structure. will fail, and if it is taken down too deep the cost will be prohibitive, and within these two extremes the engineer has abundant opportunity to tax his judgment to the utmost. The author remembers an excavation for a masonry dam in the South. The bottom looked good and solid, gneissic rock at a depth of about 15 feet, but a series of drill holes revealed a large cavern at a depth of only a few feet below the excavated surface. Again, if the rock is fairly solid but pervious to water, shall the entire structure be carried down to full depth or only a cut-off wall under the up-stream toe? If the rock is found compact and impervious to water, but too soft to resist the erosive effects of falling water below the down-stream toe, how shall this be treated? But these are only one or two of the innumerable questions that tax the judgment of the most experienced engineers during the construction of an important dam, when it is necessary to save every dollar possible without taking chances. In constructing an important dam in the Allegheny Mountains, where the cracked and distorted rock structure presented the greatest difficulties, the following methods were pursued: The excavation was carried down the full width of the assumed bottom section of the dam to a depth which indicated that the rock base was amply solid and hard to afford sufficient bearing power. A narrow trench some 6 feet wide, having its upper face in a plane with the up-stream face of the dam, was then excavated. In two of the most important cases this trench was excavated by means of a channelling machine. The full section was carried down some 30 feet, and the cut-off trench a maximum of 30 feet farther, until the bottom appeared to be impervious to water. In order to test this and insure its water-tightness, hand drill holes were sunk in the bottom 15 feet deeper and spaced about 8 feet apart. A special stuffing-box device was inserted into each one of the holes, commencing, say, near the center, and water was pumped into these holes, under as high pressure as possible, using a hand pump for the purpose. Pressures usually ran from 20 to 50 pounds per square inch. If the rock was at all porous water would appear in the adjacent holes. While pumping, Portland cement was stirred into the barrel from which the pump obtained its suction, until the cement water appeared in the adjacent holes. More and more cement was stirred in until the proportion of cement to water by weight was about one to two. Finally, pumping became more difficult until it was impossible to pump the slightest amount of grout into the hole, and the pressure increased to the limits of the pump, say 100 pounds per square inch, or more. The same process was continued for the alternate hole, and after all the holes were treated in this way the intermediate holes were grouted in the same manner. This is a very effective method, as was demonstrated in a large number of cases. The construction of the dam was then begun, and after the work was completed, it was assumed that the down-stream rock was too soft to continuously resist the erosion of the falling water. Therefore, a large space was excavated and filled with heavy stone, each containing a yard or more, and to a depth of 10 or more feet. The interstices between these stones were filled with grout containing one of Portland cement to three of sand. On October 3d, in company with Mr. Geo. S. Cheyney, member of the Engineers' Club, who took the photographs (Figs. 1 to 6) herewith shown, the author made a visit to the Austin dam and spent the afternoon in making as close an inspection of the conditions as possible. No detailed description will be given, as the Engineering News, Engineering Record, and other papers, in their issues of the week commencing October 2d, published very full accounts with abundant illustrations and photographs. Briefly, the dam was approximately 544 feet long, had a width of about 30 feet on the base, and a maximum height of 51 feet. The top width is 211⁄2 feet. The up-stream face was vertical and the down-stream face pitched at an angle of about 36 degrees with the vertical. This slope commenced about 12 feet from the top, where |