1518

THEORY AND PRACTICE OF ENGINEERING.

Book II.

Fig. 2780. QUAY-WALLS AND PLAN.

When the width of the quay and breadth ofthe bridge is small, and the passage over muchused, the quay-walls must be set out, as at thebridge of the Tuilleries at Paris , where by meansof a gathering over of the courses at A, B, C asplay is formed, uniting the bridge with the 'roadway, at the middle of the first arch ; atthe Pont au Change in the same city this splay -is extended still farther, but it is supported

Fig. 2781. QUAY-WALL AT THE TUILLERIES,

differently.

Foundations _The establishment of proper foundations for a bridge, where the points

of support carry all the weight, is of thegreatest importance : rock is almost the onlyunexceptionable foundation for hydraulic constructions, and even in that case it is necessaryto ascertain whether the thickness is sufficient to be entrusted with the weight intended tobe put upon it.

A soil composed of clay and gravel often forms a very compact bed, upon which themasonry may be placed without pilings; but sand, peat, and compressible earths requirepiles to render them sufficiently compact and solid. In deep rivers piles have been drivenover their entire beds, so that their heads stood level with the low water, and the spacesbetween filled with loose stones; but this method, though formerly much practised, hasbeen long discontinued.

Cofferdams are now generally made use of for laying in the foundations of the piersand abutments of a bridge, constructed to suit the particular position of their locality :they are commenced by driving two rows of vertical and plank piles, between which