THE MEMPHIS BRIDGE.
15
tons of riprap were used in it. In March, 1893, 2504 tons of heavy rip-rap, brought from the Ohio River, was placed upon this mat.
PIER IV.
The foundation for Pier IV was the first one put in. It was madein a measure an experimental foundation, and the greater part of thework upon it was done before the construction of the entire bridge wasauthorized.
The caisson, including the upper works of concrete and timber, is 26feet wide, 60 feet long, and 50.43 feet high. The details of this caissonare given on Plate 20. The caisson was built on launching ways on thewest bank of the river, about 100 feet below the bridge line.
The framing of the timber for this caisson, which was realty thebeginning of the construction of the Memphis Bridge, began November7th, 1888. The cutting edge was set up on the launching ways Novem-ber 23d, and the caisson launched on the 15th of December and placed atthe site of the pier two days later; it was launched with a false bottomwhich was then removed. The concrete filling was begun on the 22d ofDecember and air pressure applied and sinking actually begun on the27th. The timber and concrete work?were completed January 5th, 1889.
As this foundation was put in before any arrangements had beenmade for building the masonry of the bridge, it was surmounted by atight curb made of 12" by 12" timbers planked on the outside with onecourse of three inch plank. This curb was made 20 feet high, and whenthe foundation was completed the top of it was about 12 feet above lowwater. During the progress of the work the river rose 23 feet above lowwater, and a rough timber crib was built above the curb to protect theshafts and pipes from floating drift.
When air pressure was first put on the cutting edge of the caissonwas at elevation 172.09. The material excavated was clean sand whichwas handled easily by the sand pumps, and progress was rapid untilJanuary 27th, when clay was reached at elevation 126.56. A clay hoistwas then brought into use, and the sinking continued until February 5th,when the caisson was brought to its final position at elevation 122.94.
The clay was tested in the same manner as was afterwards done atPiers II and III, the average crushing strength of two-inch cubes being440 pounds, or 110 pounds per square inch. To determine better thenature of the material, especially with reference to the river piers, a wellwas sunk thirteen feet below the bottom of the caisson, or to elevation
109. This well passed through six feet of hard sandy clay similar tothat on which the caisson rests, and penetrated seven feet of hard toughclay nearly free from sand, which when tested showed a crushing strengthin two-inch cubes of 408 pounds.
The sealing of the working chamber with concrete was begun on the6th of February and finished four days later.
The clay on which this foundation rests, a strong hard clay, notentirety free from sand, is not as well fitted to resist scour as that foundat the bottom of the well. This pier, however, is a shore pier and standsbehind the line of shore protection hereafter described. It is, therefore,entirety secure.
Nothing further was done for six months; but in August 1889, con-struction of the bridge being fairly in progress, the curb was pumpedout, the mud which had collected in it during high water was removed,and laying of masonry was begun. The first stone was laid on the 27thof August 1889, but the work was not prosecuted continuously and thepier was not finished until September 3d, 1890.
The rate of progress of sinking the foundation is shown graphicallyon Plate 28. Detailed tables showing the cost and progress of sinking ofthe foundation are given in Appendices I and J.
The total cost of the entire pier is given in the following table:
Material.
Labor.
Total.
Total.
Launching ways....
$927.42
$716.87
$1 644.29
21 066.96
Caisson.....
14 856.94
6210.02
Concrete above chamber.
6 744.71
2 070.94
8 815.65
845.21
445.85
1 291.06
Cost, not including sinking, protection, etc..
$23 374.28
$9 443.68
$32 817.96
$32 817.96
Sinking...
$3 053.72120.78
$8 489.63
1 062.99
$11 543.35
1 183.77
Pumping and removing coffer-dam.
Riprap.
1 366.98
415.75
1 782.73
Insurance....
210.99
210.99
$4 752.47
$9 968.37
$14 720.84
14 720.84
Total Foundation. $47 538.80
Masonry (3761 cu. yds.).. 71 828.21
Total cost of Pier . $119 367.01
The total volume of the foundation is 26 feet by 60 feet by 50.43feet, equal to 78 671 cubic feet; so that the cost of the foundation, notincluding sinking, etc., was $0,417 per cubic foot and the cost includingeverything $0,604 per cubic foot.
PIER V.
Pier V is located in the bottom land west of the shore line. Theconstruction of this pier was not originally contemplated, it having beenintended to extend the iron viaduct to Pier IV, but as the west shore wascut away during the high water of 1887 and 1888, it was thought best toreplace the eastern portion of the viaduct by a deck span, which calledfor Pier V.
This pier differs from the other piers in not being a masonry struc-ture. The lower portion is of timber and concrete, and the upper portionconsists of two cylinders of soft steel f inch thick, which extend wellinto the concrete mass below and are covered by copings of Bedfordstone. This pier is illustrated on Plate 21.
The caisson is 22 feet wide by 40 feet long and the total timber-work is 80 feet high. The first timber was framed February 21st, 1890;the cutting edge was set up on blocking at the site of the pier April25th, 1890; and the lower section of the caisson was completed so thatthe blocking was removed on the 7th of May and air pressure was appliedthe following day.
The timber work with its concrete filling was completed May 30thand the clay was reached at elevation 126 on the 7th of June. The cais-son reached its final position with the cutting edge at elevation 124.93,about one foot below the top of the clay, on June 8th, 1890. The work-ing chamber was at once sealed with concrete, the sealing being completedon the 12 th of June.
The clay on which the caisson rests was tested in the same manneras that under the other piers, four two inch cubes showing an averagestrength of 180 pounds, or 45 pounds per square inch. The clay was of asomewhat sandy character but entirety satisfactory for a foundation atthis location.
A well was sunk eight feet below the cutting edge and a hole wasbored with an auger 8£ feet further. At elevation 109 a hard tough clay,like that found under the bed of the river, was reached, but it was notconsidered expedient to go to the additional expense of sinking this foun-dation to that elevation.
The steel cylinders were put in position soon after the completion ofthe foundation, but the concrete filling and the coping were not hurried,and the pier was not finally finished till May 15th, 1891.
The detailed tables showing the cost and progress of sinking of thisfoundation are given in Appendices I and J.
The actual cost of the pier is as follows: