This method is intended only to locate the surface of the ledge, as it is not adapted to penetrating rock as rock is generally classified. The most difficult material to go through is gravel or hardpan with many boulders. The smaller boulders may very often, by persistent drilling with the drill rod, be broken, or rolled aside sufficiently so that the casing may be driven through or by them. An abundant stream of water will assist materially in forming a lateral cavity into which the boulder may be pushed out of the way.
For boulders large enough to resist this treatment it is often necessary to employ dynamite. This explosive may be lowered through the casing until it rests on the top of the boulder; or it may be necessary even to drill a hole in the boulder. Before exploding the dynamite the casing should be drawn up 2 ft. or 3 ft. to save it from damage.
In some kinds of loose material it is necessary that the casing be kept driven several inches in advance of the drill rod in order that the material may be washed up; that is to say, if the drill rod were in advance so much water might be lost in the loose ground that none would come up. On the other hand, in fine sand containing sufficient clay binder it is often possible to run the drill rod far and rapidly in advance of the casing. In some borings under the observation of the writer the drill rod was run as much as 175 ft. as fast as the workmen could couple on the lengths. In fact, many borings after entering such material were run down to ledge at similar depths in a small fraction of the time required to drive the casing.
Fig. 8. Shoe for bottom of casing.
Fig. 9. Drive-head.
A gang for making wash drill borings should consist of a foreman who can take samples and record all measurements intelligently, also take any other notes that will assist in interpreting the results. There should be (depending on the length of casing being driven, and consequent weight of hammer) from two to four laborers on the hammer and one or two on the drill rod. This is based on the assumption that the water is supplied under pressure; it may be necessary to use a hand* force pump and take water from a nearby pool or stream, in which case about two more laborers are required.
If the borings are to continue for some time, and several gangs are employed, it may be desirable to install a power-driven pump. For several gangs working not too far apart one foreman will be sufficient, in which case the most intelligent laborer should handle the drill rod and be given some coaching so that he may do it intelligently.
It is obvious that the costs of such work may vary widely depending:
(a) Very largely on the character of the material encountered, the ease or difficulty of driving the casing; also whether or not the casing must be driven to the full depth of each hole.
(b) Somewhat on whether the holes are scattered, or bunched, this affecting the cost of moving not only the boring apparatus, but the also possible taking up and relaying of the water-supply line.
(c) Somewhat upon the cost of the water supply.
(d) Very little upon the cost of the apparatus, as that is not much.
(e) Somewhat upon the total amount of the work.
(f) Very little upon season, or temperature conditions. Roughly it may be said that the cost will range between 25 cents and $1 per lin. ft.; although lower costs have been recorded under particularly favorable conditions, and much higher costs might apply in certain localities, namely, to holes in the most obstinate material.