THE
BLAIR CROSSING BRIDGE.
5
ished till December 21st; air pressure was put on December 29th, andthe caisson was sunk to elevation 985.1 by January 6th, when work wassuspended and the pressure let off. The concrete filling of the cribwas completed on January 18th, and the laying of masonry began onFebruary 7th; air pressure was put on and sinking resumed on February16th, and on March 2d the caisson reached the rock at elevation 957.5.The work of filling the working chamber was completed March 5th, andthe masonry was completed March 31st.
On March 6th, 7th, and 8th, the machinery was transferred fromthe east side of the river to the west side, there being barely timeto get it across before the loss of the winter bridge, which occurred onMarch 9th.
The framing of the caisson for Pier IV. was begun January 23d, andthe erection of the caisson in the pit excavated for the purpose was begunFebruary 8th. This caisson was completed, including the concrete fillingabove the working chamber, March 14th; air pressure was put on March17th; the crib-work and concrete filling were finished April 2d; masonrywas begun April 5th; the caisson reached the rock, at elevation 954.9,April 14th; the concrete filling of the working chamber was begunApril 17th, and finished April 19th. The masonry of this pier was com-pleted May 9th.
The full details of the four piers, of the caissons, the air-locks, andthe appliances used in connection therewith, are given on Plates 4, 5, 6, 7,and 9. The rate of progress in sinking is illustrated graphically onPlate 8. Full records of the progress in detail of sinking these founda-tions were kept, and these records are given in Appendix E. The detailcost is given in Appendix F.
The concrete used was manufactured in a mixer consisting of a 9-inchspiral conveyer shaft running in a wrought-iron trough. Sand and cementwere mixed by this mixer, and stone was thrown in after depositing theconcrete mortar in position, the stones being thrown in separately byhand. The proportions used were generally as follows: three of sandto one of cement in the caisson above the working chamber; four of sandto one of cement in the working chamber and in the crib-work; roughstone of irregular size was worked into this concrete mortar, exceptingin the filling of the working chamber, which was entirely of concrete.Moreover, the working chamber was first made tight around the edges,and for about a foot over the whole bottom, with a mixture of two of sandto one of cement.
The stone used for masonry was quarried at Mankato , Minn. ; it isan excellent limestone of yellow color, which has shown evidences ofdurability in various structures around Mankato . The cut-waters of PiersI. and II. were made of blue granite quarried near St. Cloud , Minn . Thefaced stones are laid in Portland cement mortar, and the backing inMilwaukee cement, excepting that in very cold weather Portland cementwas used throughout.
A table showing the amount of cement used is given in Appendix D.
The specifications for masonry are given in Appendix B.
The total cost of the four piers in detail was as follows: —
Cost, excluding Freight.
Freight Charges.
Cost, including Freight.
Foundation I.
Caisson and filling . .
^9.596.51
$2,157.00
$”,753-51
5.594-38
1,773-78
7,368.16
Air-lock, shafts, etc. .
1420.51
61.09
1,481.60
Sinking caisson ....Erection and removal
5,004.01
4.80
5,008.81
of machinery ....
739-85
$22,355.26
23.86
$4,020.53
763.71
$26,375.79
Foundation II.
Caisson and filling . .
10,256.22
2,130-34
12,386.56
Air-lock, shafts, etc. .
1,522.54
44.88
1,567.42
Sinking caisson ....Erection and removal
4,989.17
•3°
4,989.47
of machinery ....
616.04
17,383-97
23.86
2,199.38
639.90
•9,583-35
Foundation III.
Caisson and filling . .
11,489.65
2,329.69
13, 81 9-34
Air-lock, shafts, etc. .
1,491.85
6,174.12
44-95
1,536.80
Sinking caisson ....Erection and removal
■3°
6,174.42
of machinery ....
689.88
19,845.50
23 86
2,398.80
7I3-74
22,244.30
Foundation IV.
Caisson and filling . .
9,236.06
2,016.39
11,252.45
4,762.22
1,541.24
6,303.46
Air-lock, shafts, etc. .
!,477-91
43- 1 7
1,521.08
6,104.83
Sinking caisson ....Erection and removal
6,104.55
.28
of machinery ....
822.81
22,403.55
156 62
3,757-7°
979-43
- 26,161.25.
Total of Founda-
TIONS .
$81,988.28
$12,376.41
$94,364.69
Masonry, Pier I. . .
20,938.72
4,581.05
25,5 T 9-77
“ “ II. . .
30,038.33
6,575-48
36,613.81
“ “ III. . .
3o,873°3
7,009.88
37,882.91
“ “ IV. . .
2i,395-°5
4,552.08
25,947-i3
Total of Masonry .
Grand Total of
103,245.13
22,718.49
125,963.62
Substructure. . .
$185,233.41
$35,'094-90
$220,328.31
In this table the freight charges have been stated separately, so as tomake a basis for comparison with other works, the cost without freightcharges practically representing what the cost of the work would havebeen at Chicago prices.
The shore ends of the approach spans were originally carried onCushing cylinders filled with concrete resting on piles. These Cush-ing piers are shown on Plate 3. The Cushing pier at the west endsettled badly during the grading of the approach; the weight of theapproach span was therefore transferred to a crib resting on the embank-ment, this crib being built up as the settlement continued. In June, 1885,a block of concrete 12' by 24' by 6 ' was built on the embankment 66 feetback of the Cushing pier, and this block of concrete now supports thewest end of the approach span which has replaced the shorter span origi-nally built, while the portion of the Cushing pier above ground hasbeen entirely removed. The east Cushing pier remains in its originalposition, having been pressed forward only a few inches by the buildingof the embankment.
The number of cubic yards of masonry and concrete in the entirework is as follows: —