10
THE CAIRO BRIDGE
the two rivers is more than a mile below the junction at the time thecity was laid out, and Cairo is now on the Ohio River, about a mile anda half away from the Mississippi.
The bridge crosses the Ohio River about two miles above the oldpoint of junction and about three and a half miles above the presentpoint of junction. This location is shown on the two maps, Plates1 and 2. It will be noted that the distance between the two rivers atthe bridge line is less than the width of the Ohio River; if the bridgeproper had been made two and a half times as long as it is, it wouldhave crossed both rivers and the intervening space.
The neck of land between the two rivers is in Illinois ; the southshore of the Ohio is Kentucky, and west shore of the Mississippi, Mis-souri. The entire alluvial land, extending several miles back into eachState, is from ten to twenty feet below high water. The town site ofCairo is protected by a levee completely surrounding it. All otherparts of this alluvial country are liable to be overflowed in any highwater season.
Although the two rivers are in the same alluvial delta, their phys-ical characteristics differ very materially. The bed of the Ohio River isstrengthened by the heavy sands which are characteristic of that river,and which overlie the lower deposits, leaving the regimen of the riverabout as stable here as it is within the limits of its own proper valley. Onthe other hand, the upper deposits of the Mississippi are the light alluvialsilts and sands which are characteristic of that river and the Missouri,and the same instability which characterizes the Mississippi everywherebelow the mouth of the Missouri is to be found here. The two riversare of about equal size, but the problem of bridging the Ohio River wasa much more simple one than the bridging of the Mississippi wouldhave been. Foundations which are perfectly safe in the Ohio wouldhave been of doubtful character in the Mississippi.
On the other hand, the Ohio River is liable to more violent floodsthan the Mississippi, and the floods which generally cause the mosttrouble in the Lower Mississippi come from the Ohio and its tribu-taries.
Borings taken at the bridge site had been sunk to 86 feet on theIllinois side of the river, and to 193 feet below low water on the Ken-tucky side ; nothing was found but alluvial deposit, which varied fromfine sand to gravel with occasional masses of clay, there being no regu-larity in the variations. It was evident that the foundations of the
bridge must be sustained in the sand, and this was recommended in thereports already mentioned.
The bridge and approaches may be divided into five classes :
1. The Bridge Proper, being the main structure embraced in theoriginal contract.
2. The Iron Viaduct on the Kentucky Approach.
3. The Iron Viaduct on the Illinois Approach.
4. The Timber Trestle on the Kentucky Approach.
5. The Timber Trestle on the Illinois Approach.
The bridge proper consists of nine through spans and three deckspans. Two of the through spans are each 518.5 feet long and the otherseven 400 feet long between centers of end pins. The deck spansare 249 feet between centers of end pins. The through spans reston ten masonry piers, all of which have pneumatic foundations.The deck spans are supported on two of these piers and on threemasonry piers with pile foundations.
The total length of the bridge proper from center to center ofend piers is 4044 feet.
The Viaduct on the Kentucky Approach consists of 21 spans of150 feet each and one span of 106.25 feet, which rest on iron cylinderpiers filled with concrete and supported by piles driven within thelimits of the cylinders.
The Illinois Viaduct consists of 17 spans of 150 feet each and onespan of 106.25 feet, and is precisely like the Kentucky Approach inconstruction.
The total length of the metal work from end to end is 10560 feet.It is the longest metallic structure across a river in the world, being33 feet longer than the Tay Bridge. The total length of the bridge,including the timber trestles, is 20461 feet, or 3.875 miles.
The bridge proper crosses as nearly as possible at right angles to theriver and is straight.
On each approach there is a five degree curve (1146 feet radius)there being 90 degrees of curvature on the Kentucky Approach and96 degrees and 45^ minutes on the Illinois Approach.
The general law fixes the elevation of the bridge at 53 feet abovehigh water. The high water selected by the Chief of Engineers as thegoverning high water Avas that of 1867, Avhicli is 51.20 feet above thestandard Ioav Avater.
The bridge is built 104.42 feet in the clear above Ioav Avater.
The deepest foundations are 75 feet beloAV Ioav Avater and the totalheight of the structure, from bottom of deepest foundation to top ofhighest part of iron Avork (top of ornamental casting), is 248.94 feet.
The grade is level across the Avliole length of the main structure,but descends from each end of this structure to the ends of theapproaches. The grades on the approaches are in both directions 0.75per cent. (40 feet per mile) on the straight line and 0.56 per cent. (30feet per mile) on the five degree curves. The five degree curves areeased by compounding them into tAVO degree thirty minute curves,117.4 feet long at each end. The general features of the bridge gradesand alignment are given on Plate 3.
III.
SUBSTRUCTURE.
The substructure comprises a small masonry pier on pile founda-tion on the Kentucky shore, ten large masonry piers on pneumaticfoundations in the river, the three Avestern piers being someAvhat largerthan the others, and tAvo small masonry piers on pile foundations,similar to the one on the Kentucky shore, on the Illinois shore. Thethree larger piers in the river next to the Illinois shore have beencalled the channel piers, and the other seven the river piers, as dis-tinguished from the three shore piers.
The dimensions of the pneumatic caissons are given in the tablebeloAV:
Caisson
II,
60 feet long,
26 feet
Avide and 16
44
III,
60
44
u
26
44
44
44
16
u
IV,
60
u
44
26
44
44
44
16
a
V,
60
u
u
26
4 4
44
44
16
a
VI,
60
u
44
26
44
44
44
16
u
VII,
60
u
u
26
44
W
44
16
u
VIII,
60
u
u
26
44
44
44
16
IX,
70
u
u
30
44
44
44
16
44
X,
70
u
a
30
44
44
44
16
u
XI,
70
44
u
30
44
44
44
16