Each course of stone around the outside should be laid header and stretcher alternately ; the stretchers should be from 18 inches to two feet in breadth ; and the headers, which should be about one third of the whole face, should each be from three to four feet in length : their upright, or end joints, should be correctly squared, at least one foot in from the face, and in no part be more than one inch in width. The interior, or filling in stones, should be of equal height to the outside stones, nnd have their upright joints not more than one inch in wid.h : they should break joint at least one foot. The first, and ant the succeeding courses, should- be laid flushed, both their bed and upright joints, in proper mortar. flie Frem.i engineers allow from two to four and six lines for the thickness of the outside mortar joints : in England about an eighth of an inch, when compressed, is usual. All the joints should be run full of grout, where there is any vacancy after the first operation. The French cramp all the outside stones with iron cramps, from 15 to 18 inches in length, run and covered with lead ; but if the masonry be composed of large stones, well worked, and laid to break joints properly, iron cramps appear an unnecessary expence. Great care should be taken to select the hardest and most perfect stones for the projecting points of the piers, especially loose on the upper side of the bridge. The points should be carried up at least to above high water mark, and at that height they are usually finished by sloping them back to the face of the spandrels. In some cases, a projection of a circular or polygonal form is carried up to the level of the roadway. The courses of stone may vary in thickness, 18 inches being a good average.
The abutments are managed in the same manner as the piers, only their backing is in general made of good rubble stone, laid in lime mortar. This rubble work must be levelled and grouted at the height of each course of square masonry ; great care being taken to have the whole properly bonded and connected together. If the bridge is wide, a buttress, or counterfort, should be placed behind the middle part of the abutment. This
should he made of rubble work, well bonded into the body of the abutment ; and having, besides, thin hoop iron, laths, or half-inch boards, laid in as they are carried up. This is a necessary precaution in all large buttresses constructed with rubble stone.
After all the precautions which can be taken to secure the foundations of piers, accidents sometimes happen to the best constructed works. In the bridge of Orleans, though conducted by the best engineers in France, one of the piers sunk 18 inches, French measure, although the foundation had shewn no symptoms of being worse than the others. The points did not sink with the body of the pier, but both the masonry and platform broke off at these points. The pier was loaded with 1,200,0001bs. for five months. The points were then taken down under low water and rebuilt. This was in 1759 ; we have not heard of its sinking more. But, in 1761, the water having swept away about two feet from the piers, induced them to drive two rows of piles quite across the river, six feet distant from the lower points of the piers, and fill in rubble stone.
In 1747, one of the piers of Westminster bridge sunk 18 inches at one end, which caused the taking down of two arches. The pier was loaded with 700 tons, or 1,568,0001bs. It was cased round the foundation with strong piles, to prevent any more gravel running out. The pier was taken clown for some distance under low water, and rebuilt level ; the two arches were also rebuilt ; but, to lighten the pier, arches were construct ed in the spandrels, and the same was afterwards done at Orleans.
At Orleans, the cause of the sinking of the pier was not discovered. At Westminster, it took place from there being no piles under the foundation, and from the ballast men lifting gravel for covering the bridge too near to the foundation of the pier. This circumstance ought to prove a caution to engineers, never to leave unprotected, a foundation composed of gravel, sand, or mud.