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" ~. (' J.~j.J section, either on the sides as wall-plate drifts or in the top center, and opened out to full top heading, followed by bench excavation. However, following the early adaptation of the drill carriage or jumbo to these tunnels the method of excavation known as full face has been universally adopted, except in very bad ground, where some variation of heading and bench is generally used. The estimated rate of tunnel advance was 4 to 5 feet per heading per eight-hour shift. The actual progress was at a much higher rate, as shown in the following tabulation. However, it will be noted that the average for all tunnels in the first period and for San Jacinto in 1937 and 1938 approximates the estimated rate: MAIN AQUEDUCT TUNNELS 151 Linear feet of tunnel advance Linear feet Linear feel per shift, excl. per shift San Jacinlo and Vah'erJe 1933 .. ......... 46.094 . ......m.4.62..............mmm. 1934 ....._.._...m.....2S0.066 .........6.34....m_m.. 1935 ............._.......140.854 m..... .............6.05.. ........... 1936 .m......._____n. 22,t\'0i\ ......___.....5.'2D........_ 1937 ....._......_m.... 1O,li91 ....__00...4.77....____ 1938 (10 June 30).. 9.9031 ...m..mm. ......m....4.S6.m...... ... .m.m......m... TOTAL ...........479,500 m ..........mm. .......5.89......... . 4.71 m ....m 7.17 mm 7.84 ......10.50 m......m.. 7.04 , , When conditions remained constant, with the section either con- tinuously supported or entirely unsupported, remarkable progress was made. In a number of tunnels an average daily advance of 30 feet was maintained for long periods. As shown in the above table, the average advance per eight-hour shift up to June 30, 1938 for all tunnel excavation, including the placing of support, waS 5.89 feet and, excluding San Jacinto and Valverde, where difficulty was experienced from water and bad ground, was 7.04 feet. The Whipple tunnel, driven from an adit with two headings or faces, and 19 per cent supported, averaged 44.1 feet per day of elapsed time and 46.9 feet for the actual days worked, or 23.45 feet per actual heading day of three shifts. The Coxcomb tun- nel driven from a single heading and 50 per cent supported had corresponding advances of 20.1 and 23.7 feet; Cottonwood tunnel, driven from both portals and 100 per cent supported, made 38.7 and 45.7 feet; Iron Mountain East tunnel, driven with two head- ings from a vertical shaft and 59 per cent supported, made 28.6 and 36.2 feet. In the Coachella division the two headings of the East Coachella 1 San Jacinto only; an other main a~ueducl tunnel exc3\'ation wus c(Jmplet~d prior to January 1, 1937. /'