THE MEMPHIS BRIDGE.

11

and building their lower portions hollow, which, in the mild climate ofMemphis, was considered entirely unobjectionable.

The method adopted to limit the scour was to carpet the bottom ofthe river with a woven mat similar to those which are used by the Mis-sissippi River Commission for the protection of eroding banks. Thisdevice, which is believed to have been entirely original, proved perfectlysuccessful.

The general rule followed in determining the size of the foundationswas based on making the caissons of such construction that the weight ofmaterial in the foundations below the bottom of the river should not begreater than that of the sand which they displaced; and that after de-ducting four hundred pounds per square foot for friction on the sides ofthe caissons the weight placed on top of these caissons should not pro-duce a pressure on the bottom of the foundation exceeding two tons tothe square foot.

The size required for the base of such foundations was 47 feet wideby 92 feet long, and it was thought best to build the caissons with verti-cal sides below the bottom of the river.

The caisson for Pier II is shown on Plate 16. It is 59.4 feet high,47 feet wide and 92 feet long.

The caissons were built of Southern pine timber most of whichcame from the State of Mississippi. The cutting edge is of iron, of aform used by the Chief Engineer on other bridges. This shape is pre-ferred, because it at once gives convenient access to the actual edge whenobstacles are encountered, and provides a shoulder above the edge onwhich the caisson can come to a bearing when sinking through sand, theedge projecting below this shoulder preventing an influx of sand fromwithout. The V shaped walls surrounding the working chamber, and theentire space between the timbers for a height of 17.3 feet above the bot-tom were filled solid with concrete which was put in after placing thecaisson in position. Above this solid concrete filling, for a height of 26.7feet, the interior portion only of the structure was filled with concrete,the outer parts being left empty. The upper 15.4 feet of the 59.4 feetwere built of solid timber. i

The vertical side walls are bound by 54 two inch rods the lowerlengths of which pass through the timbers, the nuts being screwed upagainst the under side of the shoulder of the cutting edge and againstwashers on top: in the upper part of the work these rods are placedimmediately inside of the timbers. There are also 24 two inch rodssimilarly placed in the cross-walls, these being connected with l£"

rods extending through the concrete. Besides these, 1 112 1-J" rods

are placed near the timber intersections, extending from the roof ofthe caisson to the top of the concrete. The inclined walls of thecaisson are tied to the outside vertical walls with 96 H" rods passingacross the V shaped space. The two sides of each cross wall are tiedtogether by 36 one inch bolts, and the timbers of the roof are tiedtogether by 390 one inch bolts. The successive courses of timber inthe outside walls are fastened together with one inch round drift bolts34 inches long, spaced five feet apart, and as these bolts alternate insuccessive courses the real distance between the bolts is only 30inches. The timbers of the inclined walls of the working chamber arefastened with drift bolts 30 inches long three feet apart, and the crosswalls are fastened with drift bolts in the same manner as the outsidevertical walls. The timbers of the solid filling above the concrete arefastened with 34 inch drift bolts driven eight feet apart in every stick,thus making the actual distance between drift bolts four feet. The out-side planking was secured by two 7" X i" boat spikes to each square footof plank, and all other planking was fastened with two 7" X f" boatspikes to each square foot of plank. The corners are rounded and platedwith f" iron.

Pier II contains 1560 M. B. M. of timber and 450 000 pounds of

iron.

The great depth to which the foundation was to be sunk, as well asthe fact that a considerable amount of clay was to be penetrated, made itimportant to provide special machineiy both for passing the men up anddown and for removing the clay. The caisson was provided with four24-inch shafts for the removal of material, which same shafts were used tosend in the concrete with which the working chamber was finall y filled.Besides this there was one 36 inch shaft with a double air lock at thebottom, of a form used on the piers of other works built by the ChiefEngineer, and also one six foot shaft with a special air lock at the bottom,and fitted with an elevator cage for the use of the men. Besides this theusual provisions of pipes for air and water supply and the removal ofsand were made.

The device selected for the removal of the clay was what is knownas a clay hoist. It was originally designed by the Chief Engineer for useon the Rulo Bridge and had been used at Sioux City and Riparia. Thisarrangement is shown in detail on Plate 25. The clay hoist consists ofan air lock at the top of the shaft, back of which is placed a cylinder inwhich runs a piston; the speed of this piston is multiplied by two sets of

sheaves so that the short stroke of the piston will lift a bucket from thebottom of the caisson to the air lock on top; the air lock is provided withtwo doors, one of which opening into the shaft below, is closed by a leverwith a balance weight on the outside, and the other opening into theopen air is worked by the attendant outside; the air is equalized througha large valve worked from the outside; the only power used is the airpressure of the caissons; the full working of the apparatus will appearfrom the plan. The bucket carried 6£ cubic feet, and when the work hasbeen pushed 12 buckets have been passed out by a single hoist in anhour. Four hoists were provided, but no more than two were ever usedat a time.

Plate 25 also shows the form of sand-pump used to remove sand bya water column; this sand-pump has the same principle of action as theEads’ Pump used on the St. Louis Bridge, but is very much simpler inconstruction.

The details of the special air lock and passenger hoist are given onPlate 26. It is simply a hoisting-engine placed on top of the workingshaft, and so arranged that it could be taken off with a derrick andquickly replaced when it was necessary to add a section to the shaft.The engine was made with very large cylinders, so that it could beworked at low pressure and instead of being driven by steam was drivenby air from the caisson. The upper shaft through which the elevator-cage runs is a cylinder six feet in diameter, the air lock itself is a cylin-der six feet in diameter, and the shaft leading to the caisson acylinder four feet in diameter; the three cylinders are tangent to eachother, and the shells are connected by cast iron door frames carryingdoors, while a fourth door opening outwards was placed at the bottom ofthe lower shaft; in working, the door between the two shafts wasalways kept closed, and the door at the bottom of the bottom shaft wasalways left open; it was possible, however, if an emergency had arisen,to use the lower section of the shaft as an air lock by itself; when thefilling of the working chamber was completed the bottom door was per-manently closed. The only power used to raise either men or material wasthe air of the caisson. While this arrangement may not be as economicalof power as the direct use of steam, it has the great advantage of con-venience, and the further advantage of improving the ventilation of thecaisson.

The plans for this work were determined on in January, 1889. Billsof material were prepared and arrangements made for the immediatebeginning of work with the hope that the two river foundations could be