TRENCHING MACHINE WORK
In 1909-1910, the City of New York installed between Valley Stream and Amityville, Long Island, (a distance of 83,800 feet) a 72-inch lock bar 7-16-inch steel pipe as a portion of its Brooklyn conduit system The contract was awarded to the T. A. Gillespie Company, and the total estimated cost for all the work which included culverts, valve chambers, valves, and other appurtenances, was $1,879,390. That portion of Long Island traversed by the pipe is almost level. The material to be excavated was sand with some gravel and a light sandy top soil. A right of way, in general 200 feet in width, with few cross roads gave opportunity for the use of any excavating system. The contractors used practically every known nethod in excavating the trench, including hand, horse and scraper, clam shell buckets steam shovels, and the Austen trenching machine. The trenching machine was used for the greater part of the work. It could be and was operated from the shallowest trench section up to a maximum depth of about ten feet, the limiting depth being determined by the resultant width of trench, it being necessary to have a secure track foundation on each side of the trench on which the machine traveled, and by which it was supported. Where the depth of the trench was greater than to feet, the contractor removed a portion of the material by other methods and then used the trenching machine.
Two machines were regularly employed, working in tandem, one machine removing approximately half the material and the other machine completing the trench. This method was considered to give maximum rate of progress. The two machines used were not owned by the T. A. Gillespie Co., hut were rented from the T. C. Austen Drainage Excavator Co., at a yearly rental of $8,500 and $9,300, respectively. This rental was based on the total yardage excavated by either machine being 100.000 cubic yards or less, all over and above 100,000 cubic yards being paid for at the rate of 55 cents per cubic yard. To determine the cost per cubic yard for excavating the trench by use of the trenching machine, a length of trench was taken, extending from Sta. 969+32 to Sta. 1090+92 or a total of 12,140 feet. The machines were worked in this section three shifts per 24 hours for one month from May 15th, to June 16, 1909. An accurate force account was kept by the department for this period. The total cost of excavating per cubic vard was subdivided under the following: (1) Rental of machines. (2)
Repairs and coal for machines. (3) Labor force The total amount of excavation made by the two machines was approximately 400,000 cubic yards or 200,000 cubic yards for each machine. The cost for rental per cubic yard would therefore be for Machine No. 1 the first 100,000 cubic yards $.085 per cubic yard; the second 100,000 cubic yards $.055 per cubic yard.
Machine No. 2, the first 100,000 cubic yards $.093 per cubic yard, the second 100,000 cubic yards $.055.
The average for both machines would be $.072 per cubic yard.
The trenching machine excavated the trench with side slopes of 1 on 1 and the bottom of the trench rounded to conform with the curve of the pipe. The average depth of the trench for the 12,140 feet excavated was approximately 8.0 feet. This gave a total of 48,560 cubic yards or 4 cubic yards per linear foot of trench. The repairs on the two machines for the first six months, including the cost of setting up, amounted to $6,000, and the cost of coal for the same period was $2,000. The cost for repairs, coal, etc., for one month would be $1,334 or $.0275 per cubic yard.
The force included : (a) The men who operated the machines, (b) The gang laying and shifting the track and moving machines, (c) The gang who trimmed the trench to grade after the machine had passed. This force was as follows: General foreman, 2 months at $125, $250; foreman, 143.5 davs at $2.50, $358.75 ; laborers, 2,368 davs at $1.40, $3,315.20; teams, 266 days at $5.00, $1,330; toys, 45 days at $.75, $33.75; engineman, 176 days at $4.00, $704; foreman, 171 days at $3.00, $513, and skilled laborers, 15 days at $2.00, $30. Total, $6,534.70. This cost of labor shows a cost per linear foot of trench of $.5382 and a cost per cubic yard of excavation of $.1345. The total cost for excavation would be: Rental of machine $.072 per cubic yard; repairs and coal for machine, $.0275; labor. $.1345; total cost, $.2340 per cubic yard. This cost per cubic yard is equivalent to $.936 per linear foot of trench.
The following summary of cost of laying the pipe will enable a comparison to be made between the excavation cost and that of other factors in the work: Clearing and grubbing, $.037 per linear foot; excavating, $.928 per linear foot ; unloading and distributing, $.1911 per linear foot; laying, $.2166 per linear foot; digging hell holes, $.084 per linear foot; riveting, $.345 per linear foot; caulking, $.288 per linear foot; testing, $.24 per linear foot; backfilling, $1,061 per linear foot; cleaing up, etc., $.0993 per linear foot; total $3.49 per linear foot. It is interesting to note that on this job the backfilling cost 15 per cent, more .than the excavation. The usual experience warrants an estimate to backfill at a materially lower cost than the excavation.