THE MEMPHIS BRIDGE
13
was brought to this pier while the BERTRAM remained at Pier III.The cutting edge reached its final elevation at 88.09 on the 16th ofOctober. An experimental well was sunk to elevation 86.0 and a boringcarried from the bottom of this well to elevation 76.3. The sealing ofthe working chamber was completed on the 24th.
The rate of progress of sinking this foundation will be readily seenfrom Plate 28. An examination of this plate will show that for 28 daysin September and October the work was carried on in over 100 feet ofwater. Full detailed records of the cost and progress of this work aregiven in Appendices I and J.
The total cost of the entire pier was as follows:
Material.
Labor.
Total.
Total.
Launching-ways.
$1 799.81
44 165.87
10 177.26
3 799.21
$1 824.51
24 233.12
3 847.05
1 310.46
$3 624.3268 398.9914 024.31
5 109.67
$91 157.29
f , . , •
$109 791.00
Concrete above chamber.
Concrete in chamber.
Cost, not including sinking, protection, etc.
$59 942.15
$31 215.14
$91 157.29
t. '■ ■ a
$17' 003.62
5 949.89
14 631.71260.70
5 514.05552.51
$9 520.87
2 087.55
51 031.46
2 398.71839.93
$26 524.49’ 8 037.4465 663.17
2 659.41
6 353.98552.51
Anchoring caisson.
Sinking caisson.
Lighting pier.
Sinking, protection to foundation, etc.
$43 912.48
$65 878.52
$109 791.00
Total Foundation. $200 948.29
Masonry (4 197 cu. yds.)..... 108 669.14
Total Foundation. $200 948.29
Masonry (4 197 cu. yds.)..... 108 669.14
Total cost of pier ... $309 617.43
The total volume of the foundation is 47 by 92 by 59.4 feet, equalto 256 846 cubic feet; so that the cost of the foundation, not includingsinking, protection to foundation, etc., is $0,360 per cubic foot; and thecost, including everything, $0,783 per cubic foot.
The whole pier, including masonry, was finished on the 25th ofApril, 1891. This foundation had been sunk six feet less than had beenexpected, and the differences required to correct dimensions of masonrywere obtained by a slight offset. This is shown on the plan of themasonry on Plate 13.
The foundation of this pier is sunk 13 feet into the clay, which isentirely solid and free from sand.
Special tests were made to determine the bearing strength of thisclay. The method adopted was to cut out a large block of the clay andbring it out of the caisson; then, without giving time for the clay to dryout or slack, it was trimmed so as to make a two inch cube on top at-tached to a large mass of clay below; pressure was then applied to thetop of the two inch cube.
Four samples were tested which were taken from the chamber ofPier II about one foot above the final position of the cutting edge.These gave the following results:
First sample. Broke under 423 lbs. pressure. As weight was ap-plied it compressed gradually. When it was compressed inch it gaveway suddenly at bottom, two thirds of top being uninjured. Bottommashed as though much softer than top.
Second sample. Broke under 343 lbs. This sample compressed ^inch as above and then broke suddenly into several pieces.
Third sample. Broke under 303 lbs. Did not compress but brokesuddenly along dry cracks.
Fourth sample. Broke under 343 lbs. No compression. Topslipped off very suddenly along joint in clay. Broke in two nearlyequal pieces.
In behavior the clay resembled rock rather than an ordinary clayand was evidently safe for very much greater pressures than the founda-tions put upon it. It is also a material well adapted to resist scour. If
It is important to- compare the 'strength of the clay as thus testedwith the actual weights put upon it. Each foundation being 47 feet by92 feet, has an area of 4324 square feet. The four tests made of clayfrom Pier II show an average strength of 93 pounds per square inch, or13 400 pounds per square foot, when entirely unsustained at the sides.
The greatest actual weight which the foundation of Pier II will everbe called upon to bear is as follows:
130 041 cu. ft. timber at 50 lbs... 6 502 050 lbs.
Iron work. 450 180 “
3 379 cu. yds. concrete at 3 780 lbs. 12 772 620 “
4 197 cu. yds. masonry at 4 200 lbs. 17 627 400 “
Total weight of pier. 37 352 250 “
Superstructure.
Live load 791 ft. at 4000 lbs.
4 908 000 “3164 000 “
Total weight on foundation.....
Deduct for buoyancy 257 500 cu. ft. below elevation 182 at
62.5 lbs..
Immersed weight...
45 424 250 lbs. = 10 505 lbs. per sq. ft.16 093 750 lbs.
29 330 500 lbs. = 6 783 lbs. per sq. ft
It is expected that at least 40 feet of this foundation will beperpetually buried in the sand, and to obtain the in-creased pressure on this area of foundation the weightof this sand in water should also be deducted, this weightbeing 172 960 cu. ft. at 58 lbs. ...... 10 032 000 lbs.
Fatigue weight on bottom of foundation. 19 298 500 lbs. = 4 463 lbs. per sq. ft.
From this may be deducted the skin friction on 40 verticalfeet of caisson, which is assumed to be at least 400 lbs.per square foot on 11 120 sq. ft. 4 448 000 lbs.
Actual probable fatigue measure. 14 850 500 lbs. = 3 434 lbs. per sq. ft.
It would, therefore, appear that with no allowance whatever forbuoyancy the pressure on the foundation is within the strain borne bythe unsupported cubes experimented on without material compressionand with no deformation of shape; while the actual fatigue pressure,without allowance for skin friction, is less than one half what these cubesbore. Furthermore, it must be remembered that this foundation is 40feet below the bottom of the river, and that the mats around the piersare expected to maintain the bottom where it now is.
During the progress of the work the river at the bridge line showeda gradual tendency to deepen. This is illustrated by the seven differentlines of river bottom shown on Plate 8. It will be observed that thetendency to deepen has been much greater in the western half of the riverthan in the eastern, and it will also be noted that elevation 125, or about40 ft. above the bottom of the foundation, is the probable limit of scour.Of course this scour may be very greatly increased by local disturbances,but it is hardly conceivable that even with great neglect the bottom ofthe river should ever be less than 20 feet above the bottom of the founda-tions. Should this deep scour occur, the value of the breadth of thefoundations will be evident. The soundings show the effects of the matsaround the pier sites.
The working of the mats was so satisfactory that it was thought bestto reinforce them with an additional mat of the same dimensions as oneof the original ones, and to make this mat the foundation of a heavy rip-rap protection. This foundation mat was put in in September, 1892,weaving having been begun on the 4th of that month, and the sinking ofthe mat completed on the 15th of September; 1228 cords of brush and879 tons of riprap were used in it. The riprap was put in in February,1893. It was of a comparatively heavy character and was brought fromquarries on the Ohio River near Rose Claire. The total amount of rip-rap placed was 2504 gross tons. This additional work is included in thestatement of cost given above.
PIER III.
The plans for Pier III were worked up in connection with thosefor Pier II. Soundings had shown the bottom of the river to be atelevation 160 at the site of Pier III, and borings had found clean riversand at elevation 108 where clay was reached.
The caisson for Pier III is shown on Plate 17. It is 39.6 feet high,47 feet wide, and 92 feet long, and in construction was similar in allrespects to the caisson for Pier II. The outer pockets were left empty for