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Terraced Transitions: Multi-level Retaining Wall Enables Urban Renewal Project Next to Greenway Trail

In 2002 in Colorado Springs, CO, work began on a plan to transform a blighted property into a premier retail center – University Village Colorado. The project required assembling 30 parcels, creating an Urban Renewal district, and securing funding through sources including the City of Colorado Springs Urban Renewal Authority and the University of Colorado at Colorado Springs.

North Nevada Avenue, where University Village Colorado is now located, was designated by the city council as an urban renewal site. This meant millions of dollars in sales tax revenue from development in the area could be used to widen and improve North Nevada Avenue, install curbs, add traffic signals, bury a major electrical power line, improve drainage in the area, and complete other necessary upgrades.

However, combining the properties into a single development presented numerous engineering challenges. The site contained thousands of cubic feet of undocumented fill. The project also required importing and placing more than 750,000 cubic feet of select fill to resolve grade changes between the east and west site boundaries (formed by the adjacent Fountain Creek on the west and North Nevada Avenue on the east). Terracon was retained to provide environmental, geotechnical, and construction materials engineering and testing services, led by Greg G. Souder, P.E., LEED AP, project manager.

A soil retaining wall was required to maximize the usable development area, address the grade change between Fountain Creek and the west property boundary, and retain unstable fill material previously placed in that area. The type of retaining wall and its alignment were crucial since it would determine the overall usable site area for the retail center and the location of the anchor stores. The wall would need to be almost one-half mile long and more than 30 feet tall.

A mechanically stabilized earth (MSE) retaining wall was selected. MSEs are popular because of the low cost and flexible construction methods.

However, for this project, the design of the MSE wall system was complicated by several factors. The City of Colorado Springs required that the wall be aesthetically pleasing and that no segment (or tier) of the wall be more than 15 feet high.

MSE products typically include small block products. However, in large wall construction such as the one for University Village Colorado, such block facings are often disproportionately small compared with the overall size of the wall. The City indicated it did not want a small-block-faced wall along the popular recreation trails of Fountain Creek. In addition to the City’s requirements, each big-box anchor store had design and construction specifications for retaining walls.

The developer and Terracon selected a large segment block face for this wall with a multi-tier design. The larger facing provided a higher level of stability at the wall face, minimized localized deformation often observed in small segment block walls, and included an integral drainage system to reduce lateral pressure on the wall face. However, because the use of a largeblock product was not pre-approved by some of the anchor stores, significant evaluation and testing was required to obtain approval from their design engineers. The requirements included additional testing of connection strength of the selected geogrid reinforcement. At the time, only two geogrid products from one supplier had been tested with the selected block. The design of this wall required greater geogrid strength at the lower levels of the tallest wall sections. Testing of the third (and stronger) geogrid products was required to verify the design assumptions.

An independent laboratory built a test frame for the large segment block, tested that frame for reliability, and performed the necessary test set-ups and iterations. Terracon collaborated with the large-block manufacturer and the geogrid manufacturer to build the initial testing frame, supply material for testing, and pay for the testing. Fast-tracking the testing was critical to the development to avoid further delays that could have potentially resulted in the loss of funding, loss of the anchor stores, and loss of the outlying retailers. The test results confirmed the original design assumption.

Fountain Creek presented its own challenges since it represented a 100-year-old flood plain along the property line and would require addressing impacts to the creek in the event of a failure caused by stormwater runoff or other mechanisms. Site development required that stormwater runoff from a large portion of the property be drained toward the wall. Classic stormwater inlet structure designs presented several potential problems. Inlet structures often become blocked during precipitation events and this could have resulted in water flowing over the wall, compromising its structural integrity. Terracon addressed this problem by designing a concealed spillway to minimize potential overflow and protect the lower wall tier. The spillway was designed with a vegetative cover and would only be visible during an overflow event. This was a simple and relatively inexpensive safety feature intended to prevent a potentially catastrophic consequence.

Another common cause of retaining wall failure is water infiltration caused by leakage of underground utility pipes within the backfill near the wall. A storm drain line, which could not be relocated outside of the critical backfill zone, was restrained to minimize the potential for joint slippage and subsequent leakage. This restraint design could significantly reduce failures in future walls if instituted as a standard.

The retaining wall follows the popular Pike’s Peak Greenway Trail. Terracon and the owner worked closely with the City, the Parks and Recreation Department, and the City’s Forestry

 

Division to preserve as much native vegetation as possible surrounding the wall. The wall’s winding face creates a soft transition for trail users from the gentle flows of Fountain Creek to the urban scene 30 feet above. One of the many terraces created by the winding tiers was developed as a park-like open space for the retail employees and consumers to enjoy.