Cracking Up

Renovation of McMicken Dam Mitigates Growing Fissures

By Scott Blair

The 10-mi long McMicken Dam, an earthen flood control structure in Phoenix's West Valley, began developing earth fissures due to subsidence. Engineers immediately went to work rehabilitating the sections of the dam that were most affected. In the process, ground-breaking monitoring and construction techniques were developed with wide-reaching applications.

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Located northwest of Phoenix, the 10-mi-long McMicken Dam was built in 1955 to protect Luke Air Force Base and farmland. During a rehabilitation in the 1980s, earth fissures were discovered by Tempe-based AMEC Earth & Environmental, an engineering firm retained by the dam's owner, the Flood Control District of Maricopa County.

Since that time, the 200-sq-mi area protected by the dam, which also includes the city of Surprise, has undergone explosive growth.

"In 2000 we decided to take a second look at it and found that the fissures were now in close proximity to the dam at the south end," says Michael Greenslade, dam safety engineer with the district. "We went into an immediate rehab of that south end to mitigate that whole part of the dam for the effects of the fissures."

The district again hired AMEC for both the study and to lead the engineering design of the solution. The deep fissures, some of which are over 100-ft-long but less than one-in.-wide, are caused by the strain put on the ground as a result of long-term withdrawal of groundwater throughout the Phoenix valley, according to AMEC senior geologist Ralph Weeks.

"Since the early 1950s, there has been subsidence at [nearby] Luke Air Force Base upwards of 17 ft. It's a widespread problem, and this is just one of the secondary results of that subsidence," Weeks adds.

Since it is impossible to see the extent of an earth fissure with the naked eye, a variety of conventional and high-tech techniques were used to identify them, including aerial photography, trenching and satellite-based radar data acquired through a partnership between AMEC and the European Space Agency.

"After a series of studies look at what is the most economical and best-engineered structure that we can put out there, we ended up choosing to remove the last 3/4 mi of the dam and build a new dam segment," says Lawrence Hansen, senior vice president of AMEC. This replaced the most at-risk section of the entire dam.

The new dam is comprised of solid soil cement, a highly compacted mix of local soil with a small amount of Portland cement and water, which has rarely if ever been used in this type of structure, according to Hansen. "It's a hardened structure that would be able to resist whatever fissures that might develop, and whatever openings might develop underneath the dam, it could span them."

The replacement dam is just 1,450 ft long, necessitated by the move from the high-fissure zone to a moderate zone. A large basin was constructed in order to collect water from upstream for the area that no longer has a dam in front of it. The Phoenix office of Stantec Consulting designed the basin to contain flows up to 50-acre ft.

Aggregate obtained during the basin's excavation was used in the soil cement.

The dam reaches up to 28 ft high and has a 20-ft-wide crest, which was then buried in landscape fill.

Phoenix-based DBA Construction was the general contractor on the project, and construction on the nine-month project was completed in late 2006.

More than 110,000 cu yds of dirt was moved during construction, both in creating the basin and in preparing the foundation for the dam. "There was an elaborate effort taken to prepare the foundation before the dam was reconstructed," Weeks says. In some spots, crews excavated more than 10 ft.

At the district's request, the new dam has a more natural 'S'-curve shape with gentle grading. Stantec worked with landscape architect Environmental Planning Group, also of Phoenix, on the project's aesthetics.

The planting and seeding portion of the landscaping package was also extensive. "A lot of native plants were used so that it blends into the existing environment," says Tim Ginter, PE, project manager with DBA Construction.

"In time you won't be able to see anything was ever there."

A gel-like product called drywater was installed near some plants. "It allows the plant to mature a little bit as the gel disperses the water slowly, making it hardy enough to survive the Arizona summers," Ginter says. "It was unique that a landscaping project this large has no irrigation system."

An ongoing facet of the project is the continued study and monitoring of developing earth fissures at the site. "There are not many places on the planet that this amount of monitoring is being executed," Weeks says. "Some of it is conventional, proven technology, but there are some prototype systems that the flood control is investigating to consider whether this should be the long-term monitoring for a variety of structures around the Valley."




Key Players

Owner: Flood Control District of Maricopa County
Engineers: AMEC Earth & Environmental; Stantec Consulting
General Contractor: DBA Construction
Other: FNF Construction; EPG; Morrison & Maierle