A foundation of rock may be considered in the abstract as the most solid base to be obtained, but it must be treated judiciously, and a proper surface secured. The outer portion of many descriptions of rock consists of blocks or layers of stone partially or entirely separated from the main bed, and these, lying in a loose condition, are deceptive and treacherous as a foundation base. The exposed rock should be carefully examined, and all detached or outlying pieces or layers removed before placing any foundation course. Special care must be paid to all
shelving rock, and a level seating cut into it for the entire width of the foundation, as shown in [Fig. 110].
A thick bed of concrete, as in [Fig. 109], makes an excellent foundation course. When firmly set it becomes one solid massive base from end to end, and prevents the yielding or dropping of masonry at any intermediate points.
There are many places in soft, wet ground where instead of attempting to excavate all the soft material down to a harder stratum, it is better to adopt timber pile foundations, as shown in [Fig. 111]. The size of the piles and their distance from centre to centre must be regulated by the description of material into which they have to be driven and the weight they have to sustain. Double waling pieces should be properly checked and bolted on to the heads of the piles, and trimmed or levelled off to receive a double floor of thick planks. The spaces round the heads of piles and walings should be filled in and levelled up to under side of flooring, with cement concrete.
For bridges of moderate span, over soft ground or over shallow fresh water, strong cast-iron screw piles can be adopted with great advantage. [Fig. 112] shows a very usual form of screw pile, made with an external screw at the lower end and with a sharp cutting edge to facilitate penetration into the ground. The upper portions are made in suitable lengths, and all to one pattern and template, for convenience in carrying out the work. The screwing into the ground is generally effected by means of a capstan or cross-head fixed to the top of the first working length of pile, and which is pulled or turned round by ropes worked from stationary windlasses. In some cases long bars or levers are attached in radiating positions to the capstan-head, and a number of men are employed to walk round and round, pushing the levers, and in this way screwing the pile into the ground. As the pile goes down the capstan-head has to be removed, and additional lengths bolted on, until the pile enters a solid stratum, or is considered deep enough for the duty it has to perform. The last or top length has generally to be cast to a special length to bring the work up to the exact height to receive the girders. The core of excavated material passes up into the interior of the pile, and in some cases becomes so compressed or tight as to require the use of an internal augur to remove a portion of it to enable the screwing to proceed. The pile shown in [Fig. 112] is one of a number which were successfully screwed
into the ground to depths varying from 42 to 48 feet. A toothed or serrated edge, as in [Fig. 113], is sometimes given to the lower edge for screw piles which have to cut their way through a hard stratum.
All bolting flanges should be accurately turned and fitted to ensure close, parallel surfaces when bolted together.
The joint shown at A, [Fig. 112], is one the writer has used to a large extent for the bolting flanges of cast-iron screw piles and cylinders. It is very simple in form, readily coated with white lead to ensure a water-tight joint, and as the upper length is practically recessed, or let into the lower length, the exact continuity of the different castings is secured.