12

THE MEMPHIS BRIDGE.

completed during the low-water season of the following fall, a hopewhich was destined to disappointment.

The first timber for the caissons was received on the 1st of Apriland framing was begun on the 18th of that month. The timber wasreceived in a material yard on the east side of the river about 2000 feetbelow the bridge line. A Daniels planer was set up here and used tosize the timber, the 12 inch timber being thus reduced to Ilf inches;this reduction explains the slight irregularity in vertical dimensions. Allother work except this sizing was done by hand with ordinary carpenters’tools. a

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The caisson was put together on launching-ways, the ways beingbuilt directly in front of the framing yard and at the edge of the water.These ways, which were very substantial, are shown on Plate 24. 'Thefoundation was of cypress piles which were generally driven about 25feet into the ground. The piles were cut off and capped with 12” X 12”timbers running parallel with the river, and on these were placed theways proper, which were drift-bolted to the caps, and given an inclinationof.one in four for a width of 48 feet, and one in 3.43 for the remainingdistance. The launching-shoes, which were simply transverse timbers,were placed on these ways, the cutting edge was set up on these shoes,and the caisson built up to a height of 17.8 feet from the lower edge ofthe iron in this position.

The first pile for the ways was driven on the 24th of April, and theways were sufficiently completed to place the first iron cutting edge uponthem and begin building the first caisson on the 18th of May. Thebuilding of the caisson and the construction of the lower portion of theways, including the submerged portion, went on simultaneously. Theways were entirely completed on the 27th of July, and two days later thecaisson for Pier II was successfully launched. The caisson was fittedwith a false bottom, which bottom, however, was simply intended toprevent the lower edge of the caisson settling too rapidly in the waterand was not made tight; when the caisson was launched it drew about12 feet. The caisson was then kept near the bank until the river shouldbe in proper condition to put it in position.

Before the caisson could be placed in position the mats which wereto protect the pier sites from scour had to be placed. Two barges, thegeneral plan of which is given on Plate 22, were fitted with ways forweaving the mats, and two mooring barges were anchored above the piersite transversely with the stream. The weaving barges were then placedbelow the mooring barges and the material barges were brought up to the

lower side of the weaving barges; the mat was then woven on the waysand the upper end of it fastened to the mooring barges and also to theanchors which held the mooring barges, each anchor carrying two lines, oneleading to one of the mooring barges and the other under the mooring-bargeto the mat. As the weaving proceeded the weaving barges were droppeddown stream so that when the mat was entirely completed the weavingbarges were at the lower end and the whole mat was floating on thewater. The mat woven in this way was 240 feet wide and 400 feet long.It was then loaded with stone until it barely floated, and enough stonewas thrown on the upper end to sink this end. The upper end of themat was then submerged first and held near the surface by the line lead-ing from the mooring barges. Two barges loaded with stone were thenfloated over the upper end of the mat and stone was thrown from themon the floating mat below; as the mat was sunk the barges were droppeddown stream until the entire mat was settled on the bottom. When thesinking barges had passed over about half the length of the mat, the linesconnecting the upper end of the mat with the mooring barges were castoff and the mat allowed to sink to the bottom. The whole time requiredto sink the mat was not over ten minutes. In the case of the first matsunk, soon after the upper end had been released from the mooring barges,the anchors dragged, and the mat took a position about 120 feet fartherdown stream than was intended. Two mats were placed at the site ofPier II, the first mat being sunk on the 27th of August and the secondon the 10th of September. The exact position of these two mats isshown on Plate 15. Each mat contained 1000 cords of brush and poles,900 tons of riprap, and 10 000 pounds of wire.

The weaving and sinking of the first mat are shown on the illus-tration on the opposite page.

To facilitate handling the caisson in position two special bargeswere built which were anchored above the pier site; the lines by whichthe caissons were held in position were fastened to these barges, and adouble-drum winding engine placed on one of them gave power to adjustthe lines. The anchors used were of the pattern known as box anchorsconsisting of timber cribs, each a ten-feet cube inside the timbers, filledwith stone. They were built on barges, thrown overboard at the placewhere they were wanted and left there, the anchors never being recov-ered. Each barge was held by six steel wire ropes If inches in diameter,each leading to a separate anchor, so that 12 anchors and 12 wire cableswere used for one caisson.

On the 23d of September, these anchorage arrangements having been

completed, the caisson for Pier II was brought up into position and heldat first by two wire ropes leading to the anchor barges. It was alsoanchored laterally by fastening two anchors on each side about 500 feetdistant and leading a 4f-inch Manilla rope from each of these anchors tothe caisson and thence through a snatch-block to one of the anchor-barges above.

Fortunately the current in the river was very light at this time andno special difficulties were experienced in placing the caisson. After itwas once securely anchored the concrete filling of the spaces above theworking chamber was begun and the building of the upper works wascarried on at the same time. As this proceeded the draught of thecaisson increased, and the number of anchor lines leading from the caissonto the barges was increased until the strength of the lines leading to thebarges was about the same as that of the lines from the barges to theanchors. On the 7th of October, the caisson grounded on the bottom atelevation 151.6 in 36 feet of water.

The whole method of anchoring and handling the caisson in theriver was devised and carried out by Mr. Noble, the Resident Engineer.

Air pressure was applied on the 10th, and 14 days, from the 11th tothe 24th, were spent in cutting away and removing the two thicknesses ofmat. Meanwhile the concrete filling of the caisson was finished on the11th of October; and the entire caisson, including all upper works, wascompleted on the 24th of that month. The sinking proceeded regularlyfrom this time, the sand being excavated by columns of water driventhrough sand pumps. On the 23d of October the laying of masonry wasbegun ; about the middle of November the caisson reached the clay, andthe removal of material by the clay hoists was started on the 17th. Onthe 30th of November all work on this pier was suspended for ninemonths, the water having risen to such a height that it was consideredinexpedient to do anything more at that time. The shoulders under thecutting edge were blocked and the foundation abandoned till low-waterseason of the following year.

On the 2d of September, 1890, work was resumed. No work hadbeen done on this foundation for nine months; for more than threemonths the entire work had been submerged, though a barge had beenkept anchored to the pier and a light maintained on it. Early in themorning on the 10th of February the steamer PORT EADS, going downthe river with a tow of barges, struck the submerged pier and sunk; theaccident, which was evidently due to a mistake of position by the pilot,unfortunately resulted in the loss of five lives. The steamer LINCOLN