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SURPRISE, Ariz.— McMicken Dam is an approximately 30-foot-tall earthen flood control structure spanning more than 10 miles through the city of Surprise. The dam is maintained by the Flood Control District of Maricopa County (District) and is a crucial flood control facility that protects thousands of homes and tens of millions of dollars' worth of property in Surprise, Sun City and other communities in the northwestern Phoenix metropolitan area. Recent repairs by the District's operations and maintenance crew to the dam's principal stormwater outlet, along with the installation of a earth fissure monitoring system under the guidance of District engineering and dam safety staff, are two of the ongoing efforts to preserve the facility's flood control benefits for years to come. Concurrent with maintenance of the dam, the McMicken Dam Project component of the Wittmann Area Drainage Master Plan is a District initiative that is currently analyzing and developing alternatives for the overall rehabilitation or replacement of McMicken Dam.
Comprehensive Repair Completed to Strengthen Main Stormwater Outlet
A repair project completed in July 2007 put the spotlight on the skilled maintenance crew of the District's operations and maintenance (O&M) division.
McMicken Dam was originally built by the U.S. Army Corps of Engineers in the mid-1950s to protect Luke Air Force Base and surrounding agricultural land from flooding. Over the years, the dam has received regular routine inspections to monitor its structural integrity. One area of concern has been the principal outlet, a concrete passageway through the dam near the north end that releases stormwater collected behind the dam into a channel which carries the water out to the Agua Fria River. Several years of investigation of this dam section culminated with a February 2006 inspection by a District dam safety engineer who determined cracking in the dam near the outlet walls was cause for a comprehensive repair.
The repair commenced in April 2007 with the District's O&M crew performing the construction repair work. The scope of the 11-week project was significantly larger than O&M normally handles; maintenance on a flood control structure is usually a two- or three-day project. Nevertheless, the crew of 15 tackled the job with confidence, working 10 or more hours each day. The construction site was near the county sheriff's gun range, which was active on Mondays and Tuesdays, therefore safety considerations prompted the O&M crew to work Wednesday through Saturday. Earthmovers, graders, bulldozers and backhoes converged on the site, stripping away six feet of the upstream dam slope in 125-foot sections on either side of the principal outlet concrete structure. The excavated soil was replaced with specialized granular filter material and a sheet of "geotextile" fabric, with a top layer of compacted soil. This new slope structure was designed to prevent further destabilization, erosion and cracking on the dam. Additional construction work added a concrete-filled trench underneath and around the entrance slab of the principal outlet to prevent water seepage and erosion.
"Without flood protection from McMicken Dam, the quality of life we enjoy in the West Valley and the vital mission of Luke Air Force Base would not be possible," said District 4 County Supervisor Max Wilson. "I applaud the Flood Control District's maintenance staff for their diligence and hard work in keeping the dam an integral part of flood control in our area."
The repair project provided numerous opportunities for the District O&M staff to learn new construction techniques and refine existing processes. Varying soil conditions at the toe, or base, of the dam prompted on-site changes to the execution of the engineering plans. To comply with strict air quality regulations and minimize dust creation, the District employed unique high-pressure water jet/compressed air blowers to clean debris off the dam slopes. Some O&M crew members received on-the-job experience which allowed them to earn equipment operation certifications. Recycled rock was used both for structure and aesthetic value. Most importantly, a comprehensive manual was produced detailing the steps used during the project, with copious notes, photographs and charts providing an historical account that can be used as a guide for future maintenance projects.
"This site-specific repair at McMicken Dam is just one example of the important, high-quality work performed by the dedicated staff of the Flood Control District's operations and maintenance division," said Tim Phillips, P.E., chief engineer and general manager of the District. "The behind-the-scenes activities of our maintenance crews ensure the District's 22 dams and more than 50 other flood control facilities function safely for the benefit of the citizens of Maricopa County."
High-Tech Earth Fissure Detection System Installed
Early in the 2007 Arizona monsoon season, heavy rains contributed to the opening of a 1/4-mile-long, 12-foot-wide and 10-foot-deep crack, or fissure, in the ground through a rural neighborhood in Queen Creek. According to the Arizona Land Subsidence Group, a technical advisory committee of geological and engineering professionals, earth fissures are a common feature in the south-central Arizona landscape. One cause of fissures is ground subsidence, when soil below the surface shifts or collapses, commonly due to a combination of unstable soil types and groundwater pumping for irrigation and development. Fissures are a threat to public infrastructure such as roads, dams, canals and bridges. To maintain the integrity and safety of McMicken Dam, the District has developed an advanced ground subsidence monitoring and earth fissure detection system near the south end of the dam.
During maintenance on McMicken Dam in the 1980s, the existence of fissures was detected in the area of the southern segment of the dam. In 2002, the District commenced another examination of the area and found fissures to be in very close proximity to the dam. Plans were developed to protect the dam, and by the end of 2006, the District realigned the southernmost 1.5 miles of the dam away from the known fissure zone. The McMicken Dam Fissure Zone Remediation Project constructed a new dam segment with an erosion-resistant soil cement core designed to withstand damage from potential future fissure activity in the area.
Subsequent to the completion of realignment work at the southern end of the dam, the District installed its comprehensive monitoring system as a kind of early warning system to alert maintenance and engineering staff to the presence of earth fissures. The system incorporates a combination of established techniques along with prototype solutions. From the air, sophisticated satellite-based radar imagery and cost-effective, high-resolution color aerial photography is utilized to monitor regional ground deformation and potential fissures. On the ground, ground inspections as well as several monitoring techniques installed and monitored by District staff are used. Rods are anchored into the ground down to stable soil at selected locations where continuing soil strain is suspected. A sensitive stainless steel tape is then stretched between anchor points to detect horizontal shifts in the ground. Survey monument points on the top of the dam and at the base are monitored by GPS and optical leveling equipment. A prototype technique, currently in the testing stage, sends an electromagnetic signal along a coaxial cable resting in a concrete trench. If the cable deforms due to a shift in the soil, the signal is altered. Observations based on the signal can determine the location of the ground shift and potential fissures. These fissure detection techniques are being phased in at other District flood control dams throughout Maricopa County where the presence of earth fissures is suspected.
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