The saga of Confluence Park construction
Ebbs and flows of water and coal tar remediation at Denver’s Confluence Park
Ebb and flow, words usually used to describe water movements, serve a dual purpose in describing the actions necessitated by the construction team for the Confluence Park Soil and Water Remediation project.
In order to overcome unforeseen site conditions and weather fluctuations, the construction team was forced to ebb and flow its resources and processes to bring the project to fruition.
History of Confluence Park
Early Colorado settlers first gathered at the convergence of Cherry Creek and the South Platte River, searching for gold in 1868. The ensuing industrial boom led to factories and landfills disregarding the integrity of the South Platte River. With non-existent environmental oversight, the South Platte remained an eyesore and afterthought until the Flood of 1965.
This catastrophic event killed 21 people in June 1965, with river flows up to 40,000 cubic feet/second. According to the Colorado Department of Natural Resources, the river’s average flow is about 800 cfs. The Flood of 1965 served as a turning point in the development of the South Platte River, leading the way for the construction of the nearby Chatfield Dam and further riverside development in Denver.
Confluence Park was then created in 1974, after a public-private partnership comprised of the City and County of Denver and the Greenway Foundation formed to reclaim the South Platte River from a dumping ground to a recreational area. Since the official dedication in September 1975, the area has grown through several additions, the latest of which includes the visioning of the Confluence Park 2013 Master Plan. This multi-phased master plan was designed by Wenk Associates, and includes an area known as Shoemaker Plaza, along 15th street, adjacent to the REI Building in Denver.
Discovery of coal tar
The construction of the Shoemaker Plaza began in early 2015, with ECI Site Construction Management hired as the general contractor at a contract for $4.2 million. The construction team was first forced to ebb and flow with the project based on the discovery of coal tar in May 2015. Coal tar is a dark liquid formed by the distillation of coal and contains polycyclic aromatic hydrocarbons (PAHs). According to the Environmental Protection Agency, PAH’s are known to carry human carcinogens and are toxic to aquatic life.
This contamination shut down the project for 15 months, and caused the team to seek various alternatives to keep the highly publicized project on course.
“I was impressed with how well the City of Denver handled the issue. Despite this being a high-profile project with a set budget and timeline, Mike Bouchard (assistant director of design and construction at Denver Parks and Recreation) managed the contracting process and public interface, so the construction team could weigh all the alternatives and not be forced into a rushed decision,” commented Ian Mestdagh, senior project manager at ECI Site Construction Management.
As a result, the construction team was able to offer the City and County of Denver several means to re-route this project by minimizing shut-down time, securing the additional necessary resources and expediting the construction process.
Dewatering and coal tar removal
A critical part of managing the ebb and flow of the Confluence Park project included the addition of Earth Services & Abatement Inc. (ESA) to the construction team. ESA has a 19-year history of working with the City of Denver and is known for handling complicated remediation projects. ESA was hired to dewater the site and, once dry, remove the contamination, taking on the added risk of the environmental impacts of the project. “There is always a concern when dewatering a project, especially with high levels of contamination. We took a conservative approach at Confluence Park with three water treatment trailers on-site at one point, running an elaborate dewatering and contamination removal system,” commented Kory Mitchell, president of ESA.
It paid off – of the 50 million gallons of water treated, none of the water samples were above the regulatory limits for water discharge, according to Mitchell. He explained that ESA engaged in a value-engineering process with the City as well as the State health departments to develop an agreement for discharging the water.
“Discharging water is a sensitive issue because no one wants to take on the responsibility of possibly discharging contaminants back into the river, which is why we put together a top notch team and worked closely with regulators to stay ahead of any complications.” Mitchell said.
Time and money saving measures
The Confluence Park project progressed with ECI and ESA working side-by-side, implementing several methods to save time and money on the project. Metsdagh commented that due to the tiny footprint of the project and the additional contamination issue, the team worked in double shifts. This 24-hour operation included moving soil at night to create space to work the next day, all while having excavators and the dewatering system running during the day to keep the project moving forward.
ESA worked with ECI to keep the project site dry by pumping and filtering the water so that it was clean enough to discharge back in the river. This allowed ECI to keep working and pour concrete. The use of sheet walls versus concrete walls alongside the river helped minimize the amount of soil needed to be moved, and saved the construction team work-time and costs. Another timesaving measure included the use of pre-cast concrete for the terracing, which only takes about seven days, versus the more traditional cast-in-place method, which would take 1.5 months, according to Metsdagh.
The construction team also had to overcome extreme weather. With the project recommencing in September 2016, the team performed extensive concrete work during the most challenging time of year – winter. ESA took precautions to heat-wrap all the piping to prevent pipe freezes.
According to Metsdagh, the South Platte River experienced some of the highest water flows on record in 2015. He explained while that the average water flow in the South Platte is between 500 and 800 cfs, during this project, the water flow tapped out at 3,500 cfs for 3.5 months and even as much as 16,000 cfs during a major thunderstorm.
The most recent water flow challenge for the project was caused by a water main break in the Highlands by Denver Water that completely flooded the site. This took place January 28, 2017, when a two-foot wide water main broke at West 29th Avenue and Zuni Street.
ESA overcame this unforeseen issue by responding immediately and had the site cleaned out in one day, according to Mitchell.
The Confluence Park project presented unique challenges and unforeseen issues the construction team had to overcome. Moving with the ebb and flow of the South Platte River and the conditions therein, the construction team devised and implemented several innovative tactics, and effectively used their resources to help transform the South Platte River from a dumping ground into a recreational area that will represent the next generation of Denver’s parks. ECI will continue the transformation of Confluence Park with estimated completion in Fall 2017.