Page:Final Report - The Columbia River Interstate Bridge.pdf/47

 periods of a few hours at a time the dredge pumped as much as 1,000 cubic yards per hour. There was, of course, a very considerable run-off of sand from the embankment, as well as a certain amount of ﬁne material which ﬂowed away with the waste water, and it is estimated that about 250,000 cubic yards more than the above net amount was transported. The discharge pipe line was extended to a length of about 5,500 ft., working from the dredge alone. For the greater distances a booster pump was installed in the line to give additional impetus. This pump was operated by a single 1,000-horsepower motor operating with considerable overload. The dredge and booster pump together transported through a maximum length of 9,000 ft. of pipe. Such long distance dredging into an embankment so comparatively narrow and high is believed to mark a record for work of this character. The pipe was of the ordinary riveted variety with slip joints made of 7 gauge material on the pontoons and of 10 gauge material elsewhere. It was moved about by teams and wagons.

The embankment was formed by the use of timber bulkheads. These were built of 6 by 8 ins. posts, about 10 ft. centers, ' supporting 2 by 12 ins. sheathing, surfaced both edges. The sides of the embankment were built up by these means in steps 8 ft. wide and 4 ft. high. The ﬁrst bulkheads were placed upon the natural ground surface by driving in the 6 by 8 ins. posts with a hand maul and setting the lower plank into a small trench so that the bulkhead sheathing extended perhaps 8 to 12 ins. below the ordinary ground surface. When the sand had been ﬁlled in about the top of such ﬁrst bulkheads, posts for succeeding bulkheads were set in place and the lower plank placed so that it extended about 12 ins. below the top of the ﬁrst bulkhead below. These posts were tied back into the embankment by 2 by 6 ins. ties spiked on near the top of each post and extending back to a short post, in front of which were placed a few pieces of lagging to offer additional resistance. The pipe was laid to discharge into the middle of the embankment and was carried forward from the river, bringing the embankment up to the ﬁnal grade and working away from the dredge. A framework of baffleboards was placed under the discharging end of the pipe, causing the water to spread out and spill over the ground below and run forward, distributing the different sizes of material as the velocity decreased. At some convenient low point there was provided an outﬂow down the side of the embankment, for which the steps of the embankment were paved with plank to prevent wash.