The Bed of the Foundation

THE BED OF THE FOUNDATION.

The nature of the soil to be built upon is evidently the first subject for consideration, and if it has not already been revealed to a considerable depth, by excavations for buildings, wells, etc., it will be necessary to make an examination of the subsoil preparatory to deciding upon the details of the founda tion. Except for the heaviest structures, it will usually be sufficient, after having dug the foundation pits or trenches, to examine the soil by driving a steel rod or boring a hole with a post-auger from 3 to 5 feet further, the depth depending upon the nature of the soil and the weight and importance of the intended structure; but for the largest structures it is necessary to examine the soil to greater depths, in which case more elaborate devices must be employed. Some of the methods used for this purpose are: (1) driving an open pipe; (2) boring with an auger; (3) "washing a hole down" with a pipe and a water jet; (4) drilling with either a percussion or a rotary drill.

Driving a Pipe.

In soft soil, soundings 20 or 30 feet deep can be made by driving a rod or sections of gas-pipe with a ham 333 mer or maul from a temporary scaffold, the height of which will of course depend upon the length of the rod or of the sections of the pipe.

Good judgment is required in interpreting the results of such tests, particularly if the structure is to be a bridge abutment or pier in a stream liable to deep scour. A layer of compact sand or cemented gravel, which may be scoured away, may be mistaken for a ledge of rock; but the difference can usually be detected by striking the rod or pipe with a hammer, since rock will give a decided rebound while gravel or sand will not. A bowlder may be mistaken for bed rock; but the difference can usually be detected by making one or more additional tests, and accurately noting the depths at which rock is struck.

If samples of the soil are desired, use a 2-inch pipe open at the lower end. If much. of this kind of work is to be done, it is advisable to fit up a hand pile-driving machine (see § 751), using a block of wood for the dropping weight.

Boring with Anger.

Borings 50 to 100 feet deep can be made very expeditiously in common soil or clay with a common wood auger, turned by men with levers 3 or 4 feet long. Or the boring may be made with any one of several earth augers having a spoon like form for bringing up samples of the soil. An auger will bring up samples sufficient to determine the nature of the soil, but not its compactness, since it will probably be compressed somewhat in being cut off.

When the testing must be made through sand or loose soil, it may be necessary to drive down a steel tube to prevent the soil from falling into the hole. The sand may be removed from the inside of this tube with an auger, or with the "sand-pump" used in digging artesian wells.

Washing a Hole Down.

In soft soil or clay that can be washed with a stream of water, a hole can be sunk rapidly by dri ving a pipe, inserting a smaller pipe inside of it, and forcing water down the inside pipe, the debris and water flowing up between the two pipes.

Drilling.

When the subsoil is composed of various strata, particularly if there are strata of hard soil or rock, it is necessary to use a percussion or "chopping" drill in connection with some form of core drill; and in extreme cases the diamond drill is some times employed. Great care is needed in interpreting the results of such borings. In using the percussion drill, care must be taken that a stratum sufficiently hard to serve as a foundation is not passed by unnoticed. This can be prevented by taking dry cores at fre quent intervals. In using a core drill care must be taken to dis criminate between erratic bowlders and native ledge rock.

If the builder desires to avoid, on the one hand, the unnecessarily costly foundations which are fre quently constructed, or, on the other hand, those insufficient founda tions evidences of which are often seen, it may be necessary, after opening the trenches, to determine the supporting power of the soil by applying a test load.