Three-quarters of downtown Pueblo, Colo., lies within a floodplain along the Arkansas River, protected by the 90-year-old, 2.8-mile-long Pueblo levee. Repairing the 45-foot-tall, concrete-faced levee offered unanticipated complexity – from a lack of historical records and funding to preserving public art – but the engineers at CTL|Thompson and NorthStar Engineering and Surveying have made significant progress.
The levee needed to meet the Federal Emergency Management Agency’s post-Hurricane Katrina certification rules, which was easier said than done in a tightly constrained work area with steep slopes, a narrow crest, and an adjacent active railyard. Funding issues were challenging, and the political climate was touchy because the levee has long been a community icon that holds the Guinness World Record for the world’s largest outdoor mural. The list of stakeholders was disparate and extensive, including historians, city staff and elected officials, artists’ groups, and the team of contractors.
During the last three years of work, the team has learned a great deal, including resolving the project’s most significant challenges. The remainder of the work – including lowering the rest of the levee’s length and constructing a levee to the north, along Wild Horse Creek – is expected to span three to four more years.
Solving the challenge of steep slopes and tight margins makes construction simpler – and less costly. The steep levee faces on both the wet and dry sides prevented the team from walking them, not to mention driving equipment along the slopes and accessing the levee’s crest for investigation. The top of the levee varied from 4 to 8 feet in width. The river on one side and a major railroad switching yard on the other side along most of the levee’s length limited access to the work area.
Four major roadways – including Interstate Highway 25, Colorado’s main north-south artery that extends throughout the West – cross the Arkansas River. Additionally, the site’s situation amid an old residential neighborhood, with homes built up to the toe of the levee, requires treading with care. And because of river flows, work can proceed only from November through March.
NorthStar found through hydraulic evaluation that lowering the height of the levee – and consequently widening it to 20 feet at the top – could still achieve the required level of flood protection. Doing so also helped facilitate construction and lowered construction costs.
Narrow track rig drilling enables soil sampling and cuts costs by two-thirds. Initial plans called for craning a mini drill rig from the bridges at the top of the levee to drill holes, but the team feared the light-duty machine might not yield the depths desired. Work also had to take place next to active railroad tracks, calling for coordination with the railroad – and presenting significant potential safety concerns for the crew. The drill area was limited to where the team could access the toe on public streets or alleys.
Drilling at the crest of the levee using a narrow track rig enabled the team to climb the face of the slope to get the job done. In addition, the equipment was better suited for the expected soil conditions, allowing the team to obtain the desired depths. Use of the 4-foot-wide track rig also cut drilling costs to one-third of those expected with the light-duty, crane-set rig. A conventional, truck-mounted rig was used to drill test holes on the dry side of the levee toe.
The levee’s historical significance requires additional approvals. Although the original levee is not on the historic registry, the means and methods used to build it were considered “of the time,” requiring approval by the state historic preservation officer.
SWCA Environmental Consultants of Denver performed a cultural resources inventory in December 2014, enabling the Phase 1 contractor to proceed. To achieve signoff on the project, NorthStar is advising the U.S. Army Corps of Engineers – which had alerted the team to the additional approval – and other agencies of progress being made. Provided those requirements are met, approval will be granted when construction is finished.
An updated levee needs updated funding. The Pueblo Conservancy District’s original funding assessment for levee improvements stopped sometime in the 1950s. During the ensuing 90 years, as a result of zoning changes, demolition and rebuilds, new ownership, and all the challenges of such a long time span, it became extremely difficult to determine an equitable format for the maintenance fund assessments.
In the end, all parties involved recognized the benefits of the levee repair. The team decided that everyone in the county benefits, at least indirectly, from the existence of the levee. The conservancy district reinitiated the assessment, enacting a three-tiered approach that changed the boundaries from the floodplain itself to all of Pueblo County.
A Guinness World Record-holding work of art demands intense community engagement. Lowering the height of the levee, and essentially replacing the existing concrete facing, created political concerns because the water side of the structure holds the Guinness World Record for the largest outdoor mural, created by contributions from thousands of artists over four decades.
To facilitate a new iteration of artists’ efforts, the team has held ongoing meetings with the artist community to understand its concerns and seek a reasonable solution. To preserve the historic record, the district had high-resolution photos taken of each mural along the levee. The original work unfortunately was removed, but as this engineering project proceeded and concrete facing was replaced, the team created a way for artists to safely access the water side and create new murals. A groundswell of interest has emerged in redesigning a long-term solution that will beautify the community and give artists a showcase for their work.
Historical documentation can be difficult to track but is vital to avoid surprises. Maps, charts, and other potentially helpful documents were in short supply during the first phase of the project. Because the levee essentially had no recorded history, the team recommended a test program for the first 2,000 feet that would expose major soil conditions, confirming what the borings disclosed and providing insight into how the levee was built.
The team found the toe of the levee at the upstream end to be 8 feet below the active river bottom, which was a surprise and a challenge from an engineering standpoint. As work on the first phase progressed and team members saw the “as-built” conditions, sheet pile was brought in to aid in dewatering the toe to create a dry working environment. Gathering information early and often decreases the likelihood of surprises once groundbreaking begins on future phases.
The team attributes its initial successes in addressing a professional challenge of this magnitude to more than just sound engineering expertise. Working well together, communicating early and often, and identifying areas of collaboration have been crucial. As one example, the team found an area to stockpile 50,000 cubic yards of dirt hauled off the site. The City of Pueblo then could reuse that material for a bridge project in the immediate vicinity.
Each day the team works on the Pueblo levee project shows that foresight, creativity, and collaboration enable substantial challenges to be overcome – and reveal valuable lessons in that process. New approaches and opportunities for learning are always ahead.
Bill Hoffmann, P.E., is vice president and senior principal engineer at CTL|Thompson (ctlt.com), a full-service geotechnical, structural, environmental, and materials engineering firm. CTL|Thompson is headquartered in Denver and has offices in Fort Collins, Pueblo, Colorado Springs, Glenwood Springs, and Summit County, Colo.; and Cheyenne, Wyo. NorthStar Engineering and Surveying Inc. (northstar-co.com) is a privately owned, full-service civil engineering and surveying firm located in Pueblo, Colo.