THE BLAIR CROSSING BRIDGE.

7

VI.

APPROACHES.

The east approach is 10,175 feet long from the connection with themain track of the Sioux City and Pacific Railroad to the east end of theiron-work. The west approach is 6,208 feet long from the western endof the iron-work to its connection with the Sioux City and Pacific Railroad .The total length of main line belonging to the Missouri Valley and BlairRailway and Bridge Company, including the bridge itself, is 17,717 feet.The alignment and grades are shown on Plate 3.

The 3,000 feet of the eastern approach adjoining the bridge werebuilt in the form of a timber trestle; this trestle was divided into threesections of 1,000 feet each, called respectively Trestles I., II., and III.All the trestles were built in 16 feet spans. Trestle No. I. was built ofpine timber, resting on oak piles, with a floor of 12 feet oak ties. TrestleNo. II. was built of pine timber on pine piles with 8 feet ties; its con-struction being generally somewhat cheaper than that of Trestle No. I.Trestle No. III. was built of cottonwood timber on cottonwood piles, butwith pine stringers and ties. The trestles were built by Mr. N. Desparois,of Sioux City , the company furnishing the timber. The part of theapproach east of Trestle No. III. is an earth embankment, the materialfor which was borrowed from the sides, the higher portion adjoiningTrestle No. III., however, being left below grade till after the openingof the bridge, the track being carried in part on a temporary blocking.After the opening of the bridge, the embankment was filled out with earthhauled by trains from the west side. In the fall of 1883, the lower partof Trestle No. III. was filled by scraping in sand from the sides. Duringthe winter of 1883-4 the steam-shovel was kept at work in the big cuteast of Blair, and the filling of the trestle was begun. The portion ofTrestles II. and III. which is across the slough channel (shown on Plate 3)was filled to an elevation of about 1040 during this winter. The breakingup of the ice in March, 1884, carried away the whole of Trestle No. I.and 600 feet of Trestle No. II., being all of the trestle-work which hadnot then been rendered secure by filling. The filling, however, of theslough channel had the effect of permanently closing that channel, andwhen the flood receded, the sand-bar was left in good condition for work.The 1,600 feet of trestle which had been destroyed were replaced at once

with a trestle similar to Trestle No. II., except that it was built in 20 feetspans. During 1884, the whole of Trestle No. III. and all that was leftstanding of Trestle No. II. were filled to grade by trains; the base ofthe new trestle was filled to elevation 1021 by scraping material fromthe sides, and to elevation 1030 by trains. The whole timber trestlewas filled to grade in 1885, though some widening remains to completethe bank.

The west approach consists of a graded railroad in cut and fill froma connection with the Sioux City and Pacific track eastward to FishCreek; then of a trestle 1,184 feet long, with a short span of Howe trussat the east end, from the west side of Fish Creek to the east side of theold Sioux City and Pacific track; and of a solid embankment from thistrestle to the western end of the iron-work. The approach west of FishCreek was graded by contract in the fall of 1882. The trestle was builtby Mr. N. Desparois, on the same terms as the trestles on the east side.The embankment between the trestle and the bridge was built of earthexcavated by the steam-shovel and unloaded from a light cottonwoodtrestle built for the purpose. On July 19th, 1883, this embankment hadbeen carried about to grade to a point as far as Station 4. During thatnight a sudden settlement of 6 feet took place, the ground at the sidesbeing forced up about 6 feet. An examination by borings revealed thefact that a layer' of soft mud existed below the surface, which evidentlymarked the course of an old river channel; the filling was continued, andthe embankment finally brought to grade, though not until the groundat each side had risen about 14 feet and the settlement so far as it couldbe ascertained was over 40 feet. The old channel crossed the line of theapproach diagonally, and entered the river about 300 feet above the bridgeline. If the original location selected for the bridge had been adhered to,this trouble would have been much less serious. The eastern end of theapproach was leveled off to the full width of the final embankment at anelevation of 1039, and on this a temporary timber trestle 220 feet longwas erected, so as to complete the approach in season for the opening ofthe bridge. During the interruption of 1884 this embankment was raisedto the level of 1054, and in November of the same year it was broughtup to grade. Only a slight settlement occurred until after the embank-ment was brought to grade, when a most rapid settlement, which amountedto nearly 10 feet, took place at the end of the iron-work, the cylinderswhich carried the approach span being carried down with the bank, but itcaused no interruption of train service. A slight movement in the top

of Pier IV., though not more than could be explained by a very moderatecompression in the timber and crib-work of the foundations, showed thatthe bank was exerting a pressure against this pier. As the short approachspan could be used elsewhere, it was thought best to replace this by alonger span, and slope off the end of the embankment, which wasaccordingly done in the summer of 1885.