In making a comparison of the cost of hand and machine excavation the figures given in Table 51 are from “Excavating Machinery” by McDaniel, who quotes the cost of machine excavation from the manufacturers of the Parsons machine issued as the result of several years’ experience with their excavator. In the comparison the hand crew is assumed to dig 315 linear feet of trench 28 inches wide by 12 feet deep in a day of 10 hours. This assumes that each man will excavate 7 cubic yards per day. The machine is assumed to excavate 250 feet of the same trench. The comparison indicates that an excavator will work at about 50 per cent of the cost of hand excavation, if the cost of moving the machine is not included.
Fig. 92.—Carson Excavating Machine on Trench Excavation in South Milwaukee.
Courtesy, Mr. C. F. Henning.
131. Cableway and Trestle Excavators.—Cableway and trestle excavators are most suitable for deep trenches and crowded conditions. They should not be used for trenches much less than 8 feet in depth. They differ from the continuous bucket excavators in that the actual dislodgment of the material is done by pick and shovel, the excavated material being thrown by hand into the buckets of the machine. A machine of the Carson type is shown in Fig. 92. The machine consists of a series of demountable frames held together by cross braces and struts to form a semirigid structure. An I beam or channel extending the length of the machine is hung closely below the top of the struts. The lower flange of this beam serves as a track for the carriages which carry the buckets. All the carriages are attached to each other and to an endless cable leading to a drum on the engine. This cable serves to move the buckets along the trench. The buckets are attached to another cable which is wound around another drum on the engine and serves to lower or raise all the buckets at the same time. In operation there are always at least two buckets for each carriage, one in the trench being filled and the other on the machine being dumped. There should be a surplus of buckets to replace those needing repairs.
The machines may be from 200 to 350 feet in length, and the number of buckets which can be lifted at one time varies from one to a dozen or more. On trenches over 5 to 6 feet in width a double line of buckets is sometimes used. The entire machine rests on rollers and straddles the trench. It is moved along the trench by its own power, either by gearing or chains attached to the wheels, or by a cable attached to a dead-man ahead.
The Potter trench machine differs from the Carson in that only 2 buckets are used at a time and these are carried on a car which travels on a track on top of the trestle. The movement of the buckets and the car are controlled by 2 dump men who ride on the car and who can raise or lower the buckets independently.
The organization needed to operate these machines is: a lockman who locks and unlocks the buckets on the cable, a dumper, as many shovelers as there are buckets on the machine, and an engineman who is usually his own fireman. From 50 to 400 cubic yards of material can be excavated in a day with one of these machines, dependent on the character of the material and the depth of the trench. H. P. Gillette in his Handbook of Cost Data reports that about 190 cubic yards were excavated per day with a Potter machine. The machine was 370 feet long. Six ¾-yard buckets were used, 4 in the trench and 2 on the carrier. The trench was 10½ feet wide and 18 feet deep in wet sand and soft blue clay. The organization consisted of an engineman, a fireman, 2 dumpmen on the carrier, and from 17 to 21 excavating laborers depending on the kind and the amount of the excavation. In general the capacity of such machines is limited by the amount of material which can be shoveled into them by hand.
132. Tower Cableways.—These are essentially of the same class as the trestle cableway machines. They differ in that the carriage supporting the buckets travels on a cable suspended between 2 towers instead of on a track supported on a trestle. As a rule only one bucket is handled in the machine at a time. They are used in sewer work only in exceptional cases as the towers must be taken down and re-erected each time that there is an advance in the trench greater than the distance between the towers.
133. Steam Shovels.—The use of steam shovels for the excavation of sewer trenches is becoming more prevalent because of their growing dependability and durability as compared with other machines, their adaptability for small trenches, and the relatively large number of widely different uses to which they can be put. In excavating a trench the shovel straddles the trench and runs on tractors, wheels, or rollers on either side of it. The shovel cuts the trench ahead of it. As a result it is difficult to set sheeting and bracing close to the end of the trench while the shovel is operating. Steam shovels are therefore not suitable for excavation in unstable material, unless the sheeting is driven ahead of the excavation. It is only in the softest ground that ordinary wood sheeting can be driven ahead of the excavation. Steel sheet piling is more suitable for such use. Fig. 93[[84]] shows a shovel at work on a trench in Evanston, Illinois.
Shovels are equipped with extra long dipper handles to adapt them to trench excavation. The dipper handle in the picture is longer than the standard for this type of machine. The method of supporting the shovel can be seen in the picture under the machine and the method of bracing and of finishing the trench by hand work are also shown. The excavated material is taken out in the shovel and dropped on the bank or into wagons.