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Rehab Of A 100-Year Old Brick Storm Sewer
Although this was a relatively straight forward job, there were several challenges identified.
• This was a very busy construction site with numerous subcontractors and trades working on different aspects of the overall job. This was primarily a building project and rehab of the existing storm sewer was just a necessary evil required to get the project approved. Therefore the rehab contractor was viewed as the “low man on the totem pole” when it came to priority to access certain parts of the site. The lay down and work areas were greatly restricted;
• The existing storm sewer discharged to the Potomac River through a flap gate. Due to a combination of age and debris, the flap gate did not seal well. This is common and to be expected. However the tidal influence of the river resulted in two – 7 feet of standing water always being in the pipe;
• Even medium size rains could cause the pipe to fill up with storm water with only 10 - 15 minutes warning; and
• There were no plans of the existing sewer but a limited inspection determined that the sewer was in relatively good condition and the curve had a radius roughly equal to 125 feet. The curve was partially a smooth radius and partially made of chords with angle points.
The project site is at K Street & 4th Street SW a little less than a mile southwest of the capitol.
The first order of business after mobilization and site move-in in mid July 2009, was to control the tidal river backwater and the storm water flow. During the preliminary site investigation the depth of water in the sewer was measured at about two feet. However, it was soon discovered that this measurement was taken during an extremely low tide time. The actual depth of standing water in the sewer varied from two feet to as much as 7 feet. Several attempts to control this backwater were made using a large off-road construction tire tube and then a sand bag weir. The final solution was to use a fabric-type inflatable pipe plug that was flexible enough that it could be rolled up and inserted through the manhole. The edges were reinforced and the plug chained to the top of the downstream manhole. This plug also served to prevent short-circuiting of the ventilation system. Even during dry weather there was a small base flow in the storm sewer. A small sand bag weir and submersible pump was installed at the upstream manhole which was piped to discharge at the downstream manhole.