Page:Final Report - The Columbia River Interstate Bridge.pdf/34

 transversely and a portion of the roadway and the sidewalk is carried on cantilever beams, and there are other minor details of differences, such as necessary trolley poles, etc. These spans rest on concrete piers supported by piles of the same general type as described, for the main river. The piles were sunk to about the same depths, but as the likelihood of scour is much less in the sloughs, the bases are not carried as far below the river bed.

The bridge over the Oregon Slough consists of ten deck girder spans 100 ft. long, and one span 115 ft. long, making the total length 1,137 ft. 6 ins. At the present time there is no navigation in the Oregon Slough above the site of the bridge. but as there is possibility of navigation being developed above the site, it was necessary to provide for some type of movable span to furnish the necessary channel openings for vessels. Permission was secured from the War Department to build the structure with all fixed spans at the present time, with the provision that one span should be so arranged that at any time in the future when it may become necessary it can be converted into a movable span by providing towers at each end, and by equipping it with machinery, counterweights, etc., so that it may be operated as a vertical lift span. The 115 ft. span, above mentioned, is arranged for such possible use and will afford a channel 100 ft. wide. This provision and arrangement saves the Commission an expenditure at the present time of about $30,000 and permits a movable span to be provided when necessary without discarding any parts of the present structure and without interrupting traffic.

The bridge over the Columbia Slough consists of four deck girder spans of the same general type as described for the Oregon Slough, each 75 ft. long. After the first tentative approval of the plan for the Columbia Slough bridge was given by the engineers of the War Department, the status of this slough was changed so that there has become a remote possibility that it may at some time be considered navigable. It has therefore been agreed that should such development occur, one of these 75 ft. spans will be made to lift in the same manner as above described, providing a clear channel opening of 60 ft. in width.

The embankment over Hayden Island has a total length of about 1,480 ft., the embankment of the main approach to Union Avenue has a total length of about 10,500 ft., and the embankment in the secondary approach to Derby Street has a total length of about 5,800 ft. The embankment for the two approaches in Vancouver has a total length of about 500 ft.