280
HISTORY OF ENGINEERING.
Roo: I.
Fig. 289
SECTION OF BRIDGE OF JENA.
CTTLJ J J j J J LI J J'J'J'J J J m~J J J J"lTj~J J j J J J J J'lf '3 jU
Fl,?. ‘210. BRIDGE OF JENA.
Bridge of Bordeaux , on the Garonne , consists of seventeen stone arches resting on sixteenpiers and two abutments. The length between the abutments is 1590 feet, and the width48 feet 6 inches. The arches are segments of circles, whose versed sine is one-third of thechord. The seven middle arches, which are equal, are 86 feet 11 inches span, and the fivefirst on each side are successively 68 feet 10 inches, 72 feet 6 inches, 76 feet 1 inch, 79 feet8 inches, and 83 feet 3 inches. The thickness of the piers at the springing of the arches is13 feet 9 inches.
The Garonne has a general depth of 22 feet, and in some places of S3 feet at low water.The tide rises twice a day to 13, 16, and even 20 feet above this level. The currents inboth directions have occasionally a velocity of more than 10 feet in a second. The riverflows over a bed of sand and mud easily displaced. The borings gave a resisting soil at39, 49, and 52 feet below low water. Two hundred and fifty fir piles were driven undereach pier, and cut off 12 feet 3 inches below low water with a circular saw.
Before the piles were driven, a large frame was sunk 1 foot 6 inches below the plane ofcutting off, to regulate their position and their distances apart, formed of strong pieces oftimber placed longitudinally and transversely ; the stones which fill the spaces between thepiles, from 3 feet 3 inches to 8 feet 2 inches above the bottom, kept the heads of the pilessteady, and are levelled even with the foundation. All the bases of the piers, and thewater-way under the arches, are covered with a pavement of rubble work, the stones whichcompose it being enveloped by the mud which is deposited in their interstices, presents,as the experience of fifteen years proves, a mass impervious to the erosive action ofwater.
The masonry of the piers was raised in a caissoon of a pyramidal form at the base, andthe upper part of the sides rising vertically to a height of 25 feet 10 inches abave the planeof the heads of the piles with a length of 78 feet 8 inches, and a breadth of 27 feet 2 inchesat the level of the same plane.