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City says voids found in rock formations during WTP4 construction

Monday, March 7, 2011 by Mark Richardson

Workers testing core samples for areas at or near the Water Treatment Plant 4 construction site discovered voids in limestone rock formations, two in an area of the main plant site on Bullick Hollow Road that will house the upflow clarifier and another along the planned route for the Jollyville transmission main.


City staff working on the WTP4 site made their quarterly report to the Environmental Board last week, briefing members on the project’s progress since December. Staff outlined the status of various major parts of the project, including progress on the raw water intake, clarifiers, and the filter complex, and well as a review of the project’s environmental commissioning.


Environmentalists are particularly concerned about voids in the limestone rock formations at the main site and along the planned route to transmission tunnels because they could be habitats for endangered animals and insects that the city has vowed not to disturb in the process of constructing the facility. There are a number of endangered and threatened species living near the construction site, including the threatened Jollyville Plateau salamander.


Robyn Smith, environmental commissioning officer with the city’s Watershed Protection Department, said the discovery of both voids caused an immediate work stoppage at the sites until biologists, geologists, and other environmental staff could investigate them.


“The void we found along the Jollyville transmission main route was along River Place Road, near the site of the original WTP4 location,” Smith said. “We called in a staff biologist and immediately put a downhole camera in the void, basically looking for bugs and other animals. Over a period of three weeks, we set traps down there …  but there were no endangered species recovered.”


Smith said staff sought and received federal Fish and Wildlife Service approval before allowing the work to resume. She said the voids discovered in the excavation area of the main plant where the upflow clarifiers are planned was in an area in which the city has provided environmental mitigation, meaning that other land has been preserved to offset any environmental problems found at the site.  However, no endangered species were found in those areas either.


The upflow clarifiers are two 150-foot circular structures where raw water pumped up through tunnels from Lake Travis will be mixed with chemicals to begin the treatment process. Five massive pumps will be installed to draw water through a 48-inch tunnel up 320 feet to begin feeding it into the treatment process.


Project engineer William Stauber reported to the board that most of the activity at the site up to now has been excavation for parts of the treatment plant such as the raw water intake site, the clearwells (where sediment is separated from the water), and the filter complex (where water will go through a series of six filters before being distributed). He said that the stormwater ponds to filter rainwater and runoff from the site have been completed.


Stauber said the next step would be to finalize engineering plans for various parts of the treatment plant and put them out for bids. He said a large construction crane is being shipped to the site from Austria to assist moving many of the large components into place. 


He said he expects the final design for the Jollyville Transmission main project to be completed by April and put out for bid. The next public meeting on the water treatment plant is scheduled for March 29.

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