+------------------------------+------------+-----------+------------+ | CHARACTER OF MATERIAL. | KEYSTONE. | SPRINGERS.| INVERT. | +------------------------------+------------+-----------+------------+ | | Ft. | Ft. | Ft. | |Laminated clay, first variety |2.15 to 3 |2.75 to 3.5| 1.6 to 2.5| |Laminated clay, second variety|3 to 4.5 |3.5 to 5.5| 2.5 to 4 | |Laminated clay, third variety |4.5 to 6.5 |5.5 to 8.1| 4 to 4.5| |Quicksand |2 to 3.28|2 to 4.1| 1.33 to 2.5| +------------------------------+------------+-----------+------------+ TABLE III. Showing Thickness of Masonry Lining for Side Tunnels through Soft Ground. +------------------------------+----------+----------+-----------+ | CHARACTER OF MATERIAL. | KEYSTONE.|SPRINGERS.| INVERT. | +------------------------------+----------+----------+-----------+ | | Ft. | Ft. | Ft. | |Laminated clay, first variety |1.6 to 2.3|1.8 to 3 |1.5 to 2 | |Laminated clay, second variety|2.3 to 3 |3 to 4.1|2 to 2.6 | |Laminated clay, third variety |3 to 4 |4.1 to 5 |2.6 to 3.29| |Quicksand |1.6 to 2.5|1.3 to 2 |1.3 to 2 | +------------------------------+----------+----------+-----------+ CHAPTER IX. TUNNELS THROUGH HARD ROCK; GENERAL DISCUSSION; REPRESENTATIVE MECHANICAL INSTALLATIONS FOR TUNNEL WORK. The present high development of labor-saving machinery for excavating rock makes this material one of the safest and easiest to tunnel of any with which the engineer ordinarily has to deal. To operate this machinery requires, however, the development of a large amount of power, its transmission to considerable distances, and, finally, its economical application to the excavating tools. The standard rock excavating machine is the power drill, which requires either air or hydraulic pressure for its operation according to the special type employed. Under present conditions, therefore, the engineer is limited either to air or water under compression for the transmission of his power. Steam-power may be employed directly to operate percussion rock drills; but owing to the heat and humidity which it generates in the confined space where the drills work, and because of other reasons, it is seldom employed directly. Electric transmission, which offers so many advantages to the tunnel builder, in most respects is largely excluded from use by the failure which has so far followed all attempts to apply it to the operation of rock drills. As matters stand, therefore, the tunnel engineer is practically limited to steam and falling water for the generation of power, and to compressed air and hydraulic pressure for its transmission.