10
THE MEMPHIS BRIDGE.
The first foundation put in was that of Pier IV, which was made ina considerable degree experimental, the observations during the placingof this foundation being used in working up the plans of the two deepestfoundations. The principal information obtained in this way related tothe character of the clay; and this, in connection with the borings madeby Mr. Duryea and other borings made immediately before beginningwork (the position of these borings is shown on Plate 5), were the au-thorities for the character of the bottom. This information proved cor-rect in nearly all respects, but the slight details in which it was incom-plete led to features in the plans which would not otherwise have beenadopted.
The general plan of caisson used was similar to that of the variousbridges which have been built under my direction across the Missouriand some other rivers, the general principle followed being timber wallsfilled with concrete. In the case of the two principal foundations (PiersII and III) these pians were necessarily materially modified on accountof the great size and other conditions of the foundations.
It was thought that a good foundation for Pier II could not befound above elevation 83, but that a safe foundation for Pier III couldbe found at elevation 103, and the plans of the caissons were made onthe basis of this difference of twenty feet. This was an error in judg-ment. The foundation of Pier III is actually a little deeper than that ofPier II. The caissons, which were designed to be 60 and 40 feet highfor these two respective foundations, should have been made preciselyalike, and each 50 feet high. The difference, however, does not affectthe stability of the structure.
It has seemed to me best to describe the several piers in the order oftheir position rather than that of time. The time at which the founda-tions were put in is sho wn graphically and briefly on Plate 28.
PIER I.
The location of Pier I was fixed by the Secretary of War at a pointwhere the west face of this pier should come in water nine feet deep atthe highest stage, and while the irregularities of a shore which is con-stantly subject to slight changes prevented this being done accurately,it was practically accomplished.
The ground at the site of the pier was levelled off and the caissonwas built in position on blocking. The plans of this caisson are givenon Plate 12. It was 70 feet long, 30 feet wide, and the total height ofthe timber work was 52.84 feet. The framing of the timber of this cais-son was begun December 11th, 1889. Piles were driven above the siteof the pier as a protection from drift during high water. The cuttingedge was placed December 21st and the erection of the timber workbegun on the following day.
The first concrete was placed in the walls of the caisson on the 2d ofJanuary, and on the following day the blocking was removed from underthe edge. Air pressure was applied January 9th and sinking prosecuteduntil the 19th. Rainy weather delayed and a rise in the river impededthe construction of the upper works, and it became necessary to suspendsinking from January 19th to February 14th. Sinking was then resumedand prosecuted continuously until completion.
The construction of the timber and concrete portion of this pier wasfinished March 9th, 1890, and masonry begun on the following day.
On April 18th, 1890, the caisson reached its full depth and wasstopped in hard clay with the cutting edge at elevation 137.85. On the22d of April the filling of the working chamber was completed.
This pier is located at the foot of the bluff and was sunk for practi-cally the whole depth through the class of material of which the bluff iscomposed. This material, however, was by no means uniform. Its char-acter is shown on Plate 5. As a general rule, none of the sandy stratawere free from clay and few of the clay strata entirely free from sand.It was generally removed either in sacks or by the clay hoists (Plate 25).For twenty-four feet, from elevation 183 to elevation 159, the materialwas so largely of sand that it could be removed with a sand-pump, andprogress was more rapid. The last twenty feet were almost entirelythrough clay. The clayey nature of the material for the greater part ofthe distance explains the time consumed in sinking this foundation.
Detailed statements showing the cost and progress of sinking of thisfoundation are given in Appendices I and J.
The total volume of the foundation is 30 by 70 by 52.84 feet, equalto 110 964 cubic feet, so that the cost of the foundation, iiot includingsinking and protecting foundation, was $0,369 per cubic foot, and thecost, including everything, $0,637 per cubic foot.
The masonry of Pier I was finished June 23d, 1890, it being thefirst pier entirely completed.
The total cost of the entire pier was as follows:
Material.
Labor.
Total.
Protection pile work.
$204.6413 170.64
9 893.86
2 115.79
$545.01
8 119.49
5 523.76
1 143.67
$749.6521 290.1315 417.62
3 259.46
Caisson...
Concrete above chamber.
Concrete in chamber.
Cost, not including sinking, protection, etc.
$25 384.93
$15 331.93
$40 716.86
$40 716.86
Sinking.
$6 566.05
1 738.00246.24
$21 300.9181.22
$27 866.96
1 819.22246.24
Riprap protection.
Insurance.
Sinking, protection to foundation, etc.
$8 550.29
$21 382.13
$29 932.42
29 932.42
Total foundation. $70 649.28
Masonry (2695 cu. yds.). 70 386.66
Total cost of pier . $141 035.94
Total foundation. $70 649.28
Masonry (2695 cu. yds.). 70 386.66
Total cost of pier . $141 035.94
PIER IL*
The plans of the caissons for Piers II and III were worked up to-gether in January, 1889. Soundings had shown the bottom of the riverto be at elevation 145 at the site of Pier II, and borings had found cleanriver sand to elevation 98 where clay was encountered.
The two prominent facts which had to be faced in the preparation ofthese plans were that the caissons must be placed in water from forty tofifty feet deep on a sandy bottom, which in its natural condition wouldbe washed away from under the edge of the caisson as this settled on thebottom, and the foundations when finally placed would rest on clay, thecompressibility of which was but imperfectly known.
The three requirements which were borne in mind in the preparationof these plans were:
1. To limit the weight of the pier as much as possible.
2. To limit the scour at least during the construction of the work.
3. To make the base of the foundation large enough to keep thepressure within safe limits on a compressible clay.
To keep down weights it was determined to build a very highquality of masonry, reducing the dimensions of the piers to a minimum
* The account of Piers II and III is slightly modified from a paper entitled “ The River Piers ofthe Memphis Bridge, ” published in Volume CXIV of the Minutes of the Proceedings of the Institutionof Civil Engineers, London. The apparent discrepancy in the cost of the piers in the two accounts isdue to some changes in the distribution of cost of plant.