This may be necessary when a pile breaks, or for other reasons.
A pile may also be drawn by means of a large screw, one end of which is fastened to the head of the pile while the other passes through a cross head temporarily but firmly supported above it.
For instance it is evident that in the building shown in Fig, 405 there would, in the absence of the inverted arches, be a great pressure upon the foundation immediately under the abutments and piers, but none at all on the portions under the voids; the arches, however, cause the weight to be uniformly distributed over the whole.
Such arches are of course only necessary when the foundation is of a compressible nature.
Arches of 9 inches in thickness are sufficient for ordinary purposes, but for large and heavy buildings they may be increased in thickness; the angle subtended by the arch should not be less than 45°.
It is most important that inverted arches should have an efficient abutment on both sides ; if not, they may do more harm than good by thrusting out the corner of the wall as shown in dotted lines in Fig. 405.
When a chasm or bad soft place occurs in a foundation immediately under a pier and cannot be filled up, it may be bridged over by an ordinary arch whose extremities spring from or rest upon those of the inverted arches which lie under the openings on either side ; or, where there are no inverted arches, the ordinary arch may spring from the sides of the chasm.