The limiting depth to which trenches can be excavated by steam shovels is about 20 to 25 feet, where the trench is too narrow for the shovel to enter. Wider trenches are cut in steps of about 15 feet, the shovel working in the trench for additional depths. Shovels are now made to cut trenches as narrow as a man can enter to lay pipe. The greatest width that can be cut from one position of the shovel is from 15 to 40 feet, dependent on the size of the shovel. Occasionally a combination of a drag line and a steam shovel can be used, as on the construction of the Calumet sewer in Chicago. On this work the first step was cut by a steam shovel. It was followed by a drag line resting on the step thus prepared, and excavating the remaining distance to grade. The depth of the trench in this work averaged about 25 to 30 feet. [Illustration: FIG. 93.—Steam Shovel at Work on Sewer Trench for North Shore Intercepting Sewer, Evanston, Illinois. ] Steam shovels are rated according to their tonnage and the capacity of the dipper in cubic yards. Both are necessary as the size of the dipper is varied for the same weight of machine, dependent on the character of the material being excavated. For rock the dipper is made smaller than for sand. Gillette in his Hand Book of Cost Data gives the coal and water consumption of steam shovels as shown in Table 52. The performance of steam shovels is recorded in Table 53. The conditions of the work have a marked effect on the output of the shovel. A shovel in a thorough cut, i.e., in a trench just wide enough for the shovel to turn 180 degrees but too narrow to run cars or wagons along side of it, will perform less than one-half of the work that it can perform in a side cut, i.e., where the cars can be run along side the shovel which turns less than 90 degrees. TABLE 52 COAL AND WATER CONSUMPTION BY STEAM SHOVELS (From Handbook of Cost Data, by H. P. Gillette) ───────────────────────────────────┬─────┬─────┬─────┬─────┬─────┬───── Weight in tons │ 35 │ 45 │ 55 │ 65 │ 75 │ 90 Dipper, cubic yards │ 1¼ │ 1½ │ 1¾ │ 2 │ 2½ │ 3 Coal, tons per 10 hour day │ ¾ │ 1 │ 1¼ │ 1½ │ 2 │ 2¼ Water, gallons per 10 hour day │1500 │2000 │2500 │3000 │4000 │4500 ───────────────────────────────────┴─────┴─────┴─────┴─────┴─────┴───── TABLE 53 PERFORMANCE BY STEAM SHOVELS