Willow Creek Detention Pond

Project Focus

This $850,000 roller compacted concrete (RCC) project was for improved the Willow Creek channel for the Rampart Range Metropolitan District #1, Lonetree, Co. The work consisted of 39,000 cy of grading; 6000 cy of borrow; an extension of a 7' X 5' RCBC to a single pre-cast 7' X 5' RCBC including new headwalls and wingwalls; 3 concrete box flow control structures; 3,570 sf of sheet piling and cap for the weir structure; and 3,000 cy of RCC for 3 drop structures and access roads.

The design of the RCC drop structures created several challenging construction problems. The geometry of the stepped facing for the drop structures was not straight line as is typical. The stepped faces were all curved with radii as short as 50'. Entry points to the placement sites were difficult to access by traditional means. In addition, the existing drainage channel was highly susceptible to flooding.

Project Details

Construction & Innovation
Typically, RCC is mixed in a pug mill but IHC modified a Hagan ribbon mixer and successfully mixed a quality product using a non-traditional method. Access to the drop structures was difficult so the plant was located proximate to the site and trammed to the placement site with wheel loaders. This allowed IHC to provide a steady supply of RCC to the placement sites, which were often confined by steep slopes not passable with conventional haul units. Placement was by a D-5 Cat dozer using GPS guided instruments. The soil-cement was off-loaded by the Cat wheel loaders and placed in front of the D-5 by a Cat 325 crawler backhoe. Compaction was achieved by a dual drum vibratory roller. The curved vertical stepped face of the RCC made compaction and alignment on the face difficult. IHC used a unique, fabricated 4' concrete block form system with picking eyes to form the tight radii. The forming system also provided for hand compaction on the stepped vertical face assuring that all compaction requirements were met. The forms were leapfrogged ahead by a rough terrain crane as needed to accommodate production demands. The vertical face achieved all density requirements, was well within all dimension specifications and never affected the productivity of the project.

Quality
All RCC was well within the designed parameters. The contract called for a 90-day strength of 2000 psi unconfined compressive strength. The average for the project was approximately 2150 psi with all tests within the minimum limits. Specifications called for a maximum dry density of 98% as determined by ASTM D558. An average density of 99.5% was achieved for the project. The RCC was water cured and covered with insulating blankets to ensure proper cure. To provide risk management and to ensure quality, IHC developed and executed an internal CQC program that included testing, inspection and thorough documentation.

Schedule & Complexity
Preventative measures to protect the site against flooding were critical in order to meet the 120-day schedule. The project was bid in early 2003 and had to be complete within 120 calendar days. Even with flood protection in-place, the project was ravaged by a 100-year storm when the project was only 30% complete. IHC was able to repair the damage and still complete the project on time without being assessed any liquidated damages.