The preceding specifications are applicable to open-trench excavation. Rigid restrictions are placed about tunneling because of the greater difficulty of doing good work, the greater danger to life and property and the possibility of later surface subsidence if the backfilling is done improperly. A common clause in specifications is:
All excavations for sewers and their appurtenances shall be made in open trenches unless written permission to excavate in tunnel shall be given by the Engineer.
127. Hand Excavation.—Earth excavation by pick and shovel is the simplest and most primitive mode of excavation. Only small jobs are handled in this manner in order to save the investment necessary in machines or the expense of hiring and moving one to the work. The tools used in the hand excavation of trenches are: picks, pickaxes, long-handled and short-handled pointed shovels, square-edged long- and short-handled shovels, scoop shovels, axes, crowbars, rock drills, mauls, sledges, etc. The excavating gangs are divided up into units of 20 to 50 men under one foreman or straw boss, and among the men may be a few higher priced laborers who set the pace for the others. Each laborer on excavation should be provided with a shovel, the style being dependent on the character of the material being excavated and the depth of the trench. In stiff material and deep trenches requiring the lifting of the material in the shovel, long-handled pointed shovels should be used. In loose sandy material loaded directly into buckets short-handled, square pointed shovels are satisfactory. Picks are used in cemented gravels or where hard obstructions prevent cutting down with the edge of the shovel. Very stiff but not hard material can be cut out in chunks with a pickaxe and thrown from the trench or into a bucket with a scoop shovel. Scoop shovels are also useful in wet running quicksand. The number of picks, axes, crowbars, and other tools must be proportioned according to the material being excavated. Under the worst conditions of excavation in a hard cemented gravel it may be necessary to provide each man with a pick as well as a shovel, whereas in sand only a shovel is necessary. Two or three crowbars, axes, a length of chain, two or three screw jacks, etc., are provided per gang in case of an unexpected encounter with an obstruction in the trench, such as a boulder, a tree stump, a length of pipe, etc.
In laying out the work the foreman marks the outlines of the trench on the ground by means of a scratch made with a pick, chalk marks, tape, or other devices. These marks are measured from offset or center stakes set by the engineer. Center stakes are less conducive to error but are more likely to be disturbed before use than are offset stakes, but careless foremen make more errors with offset than with center stakes. The inspector should assist or be present at the laying out of the trench. After the trench has been laid out each laborer should be given a certain specific portion of it to dig and this portion is marked out on the ground. In this way a check can be kept upon the performance of each laborer and the knowledge of this fact tends to a uniformly better performance. The amount of work that can be performed by one man with a pick and shovel is as shown in Table 49. Some men may exceed these rates, many will not attain them. The allotted task must be gaged on the character of the ground in order that the tasks may be equal and a spirit of competition fostered. The hard worker will set the pace for the lazy man. Some contractors have adopted the expedient of dismissing laborers for the day as soon as the allotted task is done.
| TABLE 49 | |
|---|---|
| Amount of Material Moved by One Man with a Pick and Shovel | |
| (From H. P. Gillette) | |
| Material | Cubic Yard per hour |
| Hardpan | 0.33 |
| Common earth | 0.8 to 1.2 |
| Stiff clay | 0.85 |
| Clay | 1.00 |
| Sand | 1.25 |
| Sandy soil | 0.8 to 1.2 |
| Clayey earth | 1.3 |
| Sandy soil (frozen) | 0.75 |
The opening of the trench may be facilitated by breaking ground with a plow. In hard ground or on paved roads it may be necessary to cut through the surface crust with a hammer and drill, although in some cases a plow can be used successfully. Frozen ground can be thawed by building fires along the line of the trench, or greater economy may be achieved by placing steam pipes along the surface with perforations about every 18 inches and either boxing them on the top and sides or burying them in the frozen earth with a covering of sand. Another arrangement is to blow steam into a line of bottomless boxes in which each box is about 8 feet long. Holes are left in the top of the boxes into which the pipe is shoved, and after its withdrawal the holes are covered. Blasting of frozen earth is sometimes successful but cannot be resorted to in built up districts where it is unsafe unless properly controlled. Once the frost crust is broken through it can be attacked from below and frequently broken down by undermining.
A laborer cannot dig and raise the earth much more than to the height of his head, and preferably not quite so high, without tiring quickly. After the trench has passed a depth of 4 feet he cannot throw the earth clear of the trench. An additional laborer is needed then at the surface to throw the earth back. He should shovel the earth from a board platform placed at the edge of the trench as a protection to the bank. When the trench passes the 6–foot depth a staging is put in about 4 feet from the top on which the lowest laborer piles his materials. It is then passed up to the surface by a second laborer on the staging, and a third laborer on the surface throws the material back clear of the trench. Stagings are put in about every 5 or 6 feet for the full depth of the trench.
When the trench has come within half the diameter of the pipe of the final grade, if the material is sufficiently firm, the remainder of the trench should be cut to conform to the shape of the lower half of the outside of the pipe, with proper enlargements for each bell.
128. Machine Excavation.—On work of moderately large magnitude excavation by machine is cheaper than by pick and shovel alone. In comparing the cost of excavation by the two methods all items such as sheeting, pipe laying, backfilling, etc., should be included, since these items will be affected by the method of excavation. The cost of setting up and reshipping the machine must be included as this is frequently the item on which the use of the machine depends. Because of the cost of setting up and shipping, which must be distributed over the total number of yards excavated, the cost per cubic yard of excavating by machine varies with the number of cubic yards excavated. The point of economy in the use of a machine is reached when the cost by hand and by machine are equal. For all work of greater magnitude, excavation by machine will prove cheaper.[[81]] Items favoring the use of machinery which may cause its adoption for small jobs are: its greater speed, reliability, ease in handling, economy in sheeting, economy in labor, and small amount of space needed making it useful in crowded streets. Continuous bucket machines, drag lines, and occasionally steam shovels are not adapted to conditions where rocks, pipes and other underground obstacles are frequently met.
The following problem is an example of the work necessary in making a comparison of the relative economy of machine and hand excavation: