In Chicago most of the buildings having pile foundations have heavy timber grillage bolted to the tops of the piles, and on these timbers are laid the stone or concrete footings. For building foundations the grillage usually consists of 12x12 timbers of the strongest woods available, laid longitudinally on top of the piles, and fastened to them by means of drift bolts, which are plain bars of iron, either round or square, driven into a hole about 20 per cent, smaller than the iron. One-inch round or square bars are generally used, the hole being bored by a ¾-inch auger for the round bolts or a 7/8-inch auger for the square bolts. The bolts should enter the pile at least 1 foot.
If heavy stone or concrete footings are used, and the space between the piles and timbers is filled with concrete level with the top of the timbers, no more timbering is required; but if the footings are to be made of small stones, and no concrete is used, a solid floor of cross timbers, at least 6 inches thick, for heavy buildings should be laid on top of the longitudinal capping and drift-bolted to them.
Where timber grillage is used it should, of course, be kept entirely below the lowest recorded water line, otherwise it will rot and allow the building to settle. It has been proved conclusively, however, that any kind of sound timber will last practically forever if completely immersed in water.
The advantages of timber grillage are that the timbers are easily laid and effectually hold the tops of the piles in place. They also tend to distribute the pressure evenly over the piles, as the transverse strength of the timber will help to carry the load over a single pile, which for some reason may not have the same bearing capacity as the others.
Steel beams, imbedded in concrete, are sometimes used to distribute the weight over piles, but some other form of construction can generally be employed at less expense and with equally good results.
Objections to Pile Foundations. - It has been claimed that driving piles in a soil such as that under Chicago, within a few feet of buildings having spread foundations, has a tendency to cause the latter to settle so as to necessitate underpinning.
On driving the first piles for the Schiller Building it was found that an adjoining building had settled 6 inches, and it had to be raised on screws.
The driving of piles also causes a readjustment of the particles of clay and sand into a jelly, thus destroying the resisting qualities. These objections, however, are not of so much moment when the adjoining buildings are supported by piles.