In order to determine whether rock exists and the depth to the rock (questions Nos. 1 and 3a) the procedure is to cover the site with wash drill borings. Briefly the method is to drive down to rock an iron pipe of suitable size, say 2$ in. inside diameter. Inside of this casing, so called, and accompanying it as it is being driven, is a smaller pipe called the drill rod furnished with a chisel point and supplying a stream of water. The function of the inside pipe is to loosen the material which is being driven through and wash it to the surface, thus facilitating the driving and furnishing samples of the material.
Fig. 2. Clamp to pull casing used with pries or jacks.
Fig. 3. Upper end of drill rod, showing handle and the hose supplying wash water.
For doing this work there are several different rigs which, however, vary only in minor particulars. Figs. 1 to 9 illustrate the details of the outfit very satisfactorily employed on the Boston Metropolitan Water Works.
End Elev. Of Pulley.
Fig. 4. Yoke.
The casing and drill rod are introduced, as required, in lengths of about 6 ft.; the casing is inserted just below the drive head (see Fig. 9), and the drill rod just below the hose connection.
The hammer slides up and down on a length of casing screwed into the top of the drive head; ropes from the hammer go over the pulleys in the yoke. The yoke is mounted on a coupling 2 in. or 3 in. below the top of the same length of casing.
Fig. 5. Light hammer.
Fig. 6. Heavy hammer.
Fig. 7. Various forms of drill rod points.
Two or four men are employed on the ropes according to the weight of the hammer. One or two men, according to length and weight of drill rod in use, stand upon planks supported at any desired height by two wooden horses and operate the drill rod.
Though the casing as used on the Metropolitan Water Works had outside couplings it has since been amply demonstrated that inside couplings are much to be preferred, on account of the lesser resistance from the material penetrated.
These borings should cover the entire site and some consideration should be bestowed upon the order in which they are taken, so that each boring as made may settle the largest possible amount of remaining uncertainty; that is to say, the first few holes should properly be broadly scattered in order to supply general information, leaving the details to be filled in later. The results from each hole assist in determining the number and location of further adjacent holes which may be necessary.