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City Engineering: Chestnut Street bridge rebuild could take a year

The Chestnut Street sinkhole closure is viewed from the north. The street to the left is Ellston Street, which allows a detour to Fillmore Street. A runner can be seen crossing Chestnut at Ellston. The Sinton Trail uses Ellston Street going east from Chestnut and parallels South Douglas Creek going west.
Westside Pioneer photo
       The city has provided a rough estimate for replacing the Chestnut Street bridge over South Douglas Creek.
       A year at the most.
       “We could luck out, and it could be six to nine months,” said Mike Chaves of City Engineering, “but it's safe to say one year. If we can shorten it, we're going to try.”
       A complicating factor is the generally higher volume of stormwater that occurs during rainfalls, as a result of the burn scar from the 2012 Waldo Canyon Fire, he elaborated. South Douglas flows out of that part of the mountains, through the Mountain Shadows and Holland Park subdivisions and eventually into Monument Creek.
       Chestnut is a two-lane frontage road west of I-25 that connects Fillmore Street and Garden of the Gods Road. The bridge consists of compacted dirt. The creek runs about 30 feet under the road in a corrugated metal pipe.
       A sinkhole appeared on the paved surface Aug. 10 and rapidly grew in size. A city contractor has since torn out all the pavement, removed utility lines that were under the road and stabilized the drainage through there. Traffic is being detoured at Vondelpark Drive on the south side of the bridge and at Ellston Street on its north side.
       Chaves said the cause of the sinkhole, initially uncertain, has been determined to be a hole in the pipe, which allows water to seep out and “create a cavity” in the compacted dirt. The stabilizing includes a temporary pump system to prevent water build-up on the site, he added.
       A design for a new bridge is being developed by City Engineering with HDR Inc., a private contractor. Chaves said two options are under consideration.
       - One option would install box culverts for storm drainage to replace the metal pipe in compacted-dirt layout similar to the current one.
       - The other option is for a more traditional type of bridge. This would allow room underneath for the creek as well as the Sinton Trail. The trail currently follows the water through the city's Douglas Creek Open Space (upstream). However, at Chestnut, the trail has to be routed over to Ellston.
       According to Chaves (who was interviewed Sept. 9), city staffers expect to make a decision on the bridge design within about a month. A public meeting will be scheduled around that time - at either Coronado High School or the El Paso County Service Center - to explain the plan, he said.
       At the recommendation of city staff, voters in 2012 approved the storm pipe's replacement as a $1.8 million A-list item for the Pikes Peak Rural Transportation Authority (RTA). Along with falling apart, the pipe has been found to be undersized in terms of how much flood water it can carry, Chaves said. The original schedule had called for its replacement in 2019.
       Chaves does not know yet if the budgeted $1.8 million will be enough to cover the cost of an entire new bridge at Chestnut, or whether additional RTA funds may be needed.
       To date, since the sinkhole problem forced the road closure, the RTA board has approved $500,000 (in August) to be used for the bridge. From that, there have been allotments of $176,000 and $210,000. But Chaves said that the remedial work done to date, prior to building a new bridge, would have been necessary anyway and is not wasted money.

Westside Pioneer article
(Posted 9/9/15; Transportation: Major Roads)

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