In cases where the ground is soft, and when it is not considered prudent to excavate the foundations deeper for fear of disturbing the stability of the coffer-dam piles, rows of large, square bearing-piles may be driven in the floor of the foundation, as shown in Fig. 111. The tops of these bearing-piles must all be sawn off to the same level, and a platform of strong double planking securely fixed to the piles to receive the foundation course of concrete, masonry, or brickwork. The spaces around the tops of the piles and the under side of the timber platform should be filled in with good cement concrete. The interior of the coffer-dam is kept dry by constant pumping, either by hand pumps or steam pumps, according to the volume of water finding its way into the foundations. When the finished pier or abutment has been carried up above the river water-level, the coffer-dam is no longer required, and may be removed. Sometimes, to save the timber, the piles are drawn by means of strong tackle fitted up for the purpose; but in doing this there is considerable risk of disturbance to the foundations, and it is better to leave the piles in the ground and employ divers to cut off the tops a little above the bed of the 129 river. In preparing the design for a large foundation it is absolutely necessary to first ascertain by careful borings the description of material upon which that foundation must be placed, so as to proportion the area of bearing surface to the weight to be sustained. Some materials will naturally carry more weight than others, and although the engineer cannot always select the material he would prefer, he can, however, control the superficial area of the foundations. Much valuable information has been obtained both from experiments and from comparisons of actual practice, and the following memoranda may be useful for reference, as indicating the pressures per superficial foot which may be safely put on various materials:-- Moderately stiff clay 2½ tons. Chalk 4 ” Solid blue clay 5 ” Compact gravel and close sand 6 ” Solid rock 12 ” Doubtless the above weights have been exceeded in many cases, but it is better to be on the safe side, and leave a good margin for stability. Large subaqueous foundations for heavy piers and abutments are costly and tedious, and especially so when the pneumatic process has to be adopted. Special appliances and well-trained, experienced workmen are requisite, and if all the men and materials have to pass through the air-locks, the progress of the work must necessarily be slow. When the foundations have been completed up to the level of the water, the construction can be pushed on more rapidly, as the work of scaffolding, hoisting, and building, can all be carried on in the open air.