Occasionally a seam is encountered of such width that the drill runner can perceive the drop of the cutting tool. The amount of information that an experienced drill runner can get from feeling the drill rod is little short of marvelous, and on important work he should be encouraged to keep very extended notes. After completing the boring it is highly desirable to test the hole at a number of depths by pumping in air, or preferably water, under pressure. This test is to show whether the seams encountered are tight or open, and if carefully conducted it may furnish as much and as valuable information as the core itself.
The process and required apparatus are very simple. Two pipes are used of such diameter that the smaller will go inside the larger, and the larger inside the hole to be tested, and in lengths sufficient to reach nearly to the bottom of the hole. (See Fig. 11).
On the lower end of the smaller pipe is screwed a plate upon which is placed a thick rubber ring or gasket some 2 in. or 3 in. in height, both plate and rubber being just enough smaller than the hole to be readily introduced. On top of the rubber ring rest a washer and the larger pipe. At the upper end a screw thread on the smaller pipe with nut and washer are used to draw up on the smaller pipe, bringing the rubber ring in compression and expanding it against the sides of the hole. Water can then be pumped through the smaller pipe into the section of hole below the gasket, and the pressure recorded by gage. The amount pumped to maintain the pressure is of course a measure of the leakage into the rock. Care should, of course, be taken that the gasket is properly expanded against the sides of the hole so as to make a tight joint. The results may not always be conclusive, for while very slight or no leakage necessarily indicates tight rock, much leakage may or may not indicate open seams, as it is not always possible to get a tight joint or even to know whether it is tight or not. If the surface of the rock at the hole is under water there is no way to detect leakage past the gasket. If it is above water and accessible, however, leakage past the gasket may be detected and measured, although of course such quantity would be subject to loss in seams above the gasket. If the hole is in still water the test might be made with air, and any leakage would show up as bubbles.
Fig. 11. Apparatus for testing tightness of seams encountered by a boring in rock.
In the case of a hole in very seamy or distorted rock, or rock tilted up to approach the perpendicular or where it may be known or suspected that the sides of the hole are full of cavities from the dropping of fragments into the hole while boring, the only way to obtain a tight closure may be to cement the pipe (one pipe in this case) into the hole and make the test after the cement has hardened sufficiently.
The test may be made at as many points in a hole as desirable, each one of course showing the total leakage below the gasket. Let each test be directed toward solving the maximum remaining uncertainty. Thus assume a hole 30 ft. deep test first at 15 ft. Then if large leakage is found move to 22.5 ft. Then if no leakage is observed move to 18.75. Whatever is the result the seam has been located within 3.75 ft. by three tests.
In boring across seams fragments are liable to break off and drop into the hole; this is more liable to occur if the stratification makes but a small angle with the perpendicular hole. In the holes under discussion this was very much in evidence, and many of the tests were inconclusive. Hole No. 1 was found to be absolutely tight as 75 lb. pressure below elevation 72. A leak of 1.64 cu. ft. per minute at practically no pressure was located between elevations 72 and 75, but as at that elevation it was only 7 ft. horizontally to the river, the leakage indicated nothing objectionable. That the soft streak was sound and entirely suitable for a foundation was indicated by a test in Hole No. 5 at elevation 70 where the hole was found absolutely tight at 75 lb. pressure.
The above-described procedure for testing holes is entirely suitable and sufficient for the purpose of a preliminary examination, i.e., to determine feasibility, design and estimate of cost.
When, however, the foundation has been unwatered and the bed rock has been stripped of all loose overlying material, both opportunity and time are afforded for a more exhaustive and detailed examination. Such further examination, possibly involving additional holes, might be desirable in connection with the question of necessity for or extent of grouting the foundation.
An apparatus similar to the above, but employing two gaskets, may be used. One application of the pressure tests only that portion of the hole between the two gaskets, which may be set at any desired distance apart. The following description and illustration of such an apparatus is taken from an article appearing in Engineering Record, July 4, 1908, which described methods of testing the foundation of the Olive Bridge dam.