Salt River Project - Horse Mesa Dam

The project required approximately 24 men full-time for nine months in the extreme Arizona environment, working 300 feet above the river using rappel lines and lifelines constantly. During the entire project there were no lost-time accidents or injuries resulting from the work at this site. This is truly a safety accomplishment!
Recipient of the prestigious 2003 Award of Excellence presented by the International Concrete Repair Institute (ICRI)

Site Logistics
The dam site, being 300 feet above the river below, surrounded by rock cliffs and more than and hour and a half from civilization created challenges to even getting to the site. The only dry access to the site is a 20-minute mountain trail which traverses back and forth through a steep canyon in the one of the most rugged, arid and rocky environments in Arizona. Regular semis refused to travel the road, requiring all deliveries of equipment, scaffolding and material to be brought in on smaller flat bed trucks. An old school house, from the original construction campground 60 years ago, was renovated to house the on-site cook, food supplies and four team members. Four camping trailers were acquired (room for only four) to house an additional 16 of the 24 men required to man the project. The remaining 4 men roomed at the marina motel, accessible only by boat from the site. Accessing the work areas safely required building scaffolding in areas only accessible by professional rigging crews. Rhino, the subcontractor selected for rigging, was able to utilize rappel lines with all of the appropriate fall protection, to gain access and install the initial leg supports on the rock cliffs and old concrete spillway below the work area. Action Scaffolding was selected to provide and erect the system scaffold around each pier, to include stair towers, staging and debris netting. Although site is actually a power generation station on a river, there was not adequate power or water for use in construction. Generators were used to make the appropriate 240 3 phase power. The lake water was determined to have too many chlorides for use in anything other than the 7 day wet cure. Potable water was brought in via water wagon from the spring location a few miles away.

Demolition Method
Removal of deteriorated concrete included a few surface repairs only an inch or two deep and mostly larger repairs from six inches up to a foot deep. The concern that large pneumatic hammers might cause undue stress and vibration through the remaining structure, creating additional damage, was the initial reason for restricting demo hammers to less than 90# hammers. The demolition process was therefore performed by hand using smaller pneumatic chipping hammers. To address the large amount of compressed air that would be needed and the inability to locate large equipment near the site, a 1000 cfm air compressor was staged at the road. The air was piped to the site where it was allocated to the various tools with a custom-fabricated, high-pressure manifold. The debris fell to the various levels of scaffolding, where it was collected and placed into buckets and lowered down the 800 foot cable stretched across the river, from the spillway to the road below. The debris bucket emptied directly into the dump trailer for disposal. To achieve the desired removal of concrete behind existing reinforcing steel, chipping hammers and bush hammers were utilized.

Surface Preparation
After the demolition was completed, the host concrete and the steel were sandblasted. To thwart off any new ASR, perhaps regenerated by the application of new materials, the surface of the repair areas were treated with the lithium nitrate prior to placing any material. Following the specification and manufactures' recommendation, the substrate was saturated surface dry (SSD) using potable water, prior to material application.

Application Method Selection
Various methods of placing material were evaluated including: form and pour, wet process shotcrete, and dry process shotcrete. Dry process shotcrete was determined to be the most efficient, cost effective method while being able to deliver quality repairs. The shotcrete pump (dry process gun) would have to be located on the top of the dam, while the repairs were being shot 200 feet away and 75 feet below the top of the dam. Wet process would have required slower set materials and a clean-out area. A clean-out area was feasible but the wet process method would never allow for temporary shut-downs due to mobilizing to new areas or even lunch breaks. Forms would not have allowed for the thorough inspection of surface prep, the timely application of protective coatings on the steel nor the consistent application of the lithium. Dry process shotcrete allowed for temporary shut downs of the gun, just-in-time ordering of full truckloads of bagged materials (over 10,000 bags used), reduced the waste from clean-out, and allowed for visual ongoing inspection of the repairs as they was being placed. Air pockets, voids and honeycombs were non-existent. Bagged materials were able to be delivered in full pallets to the gun location on the top of the dam. The addition of lithium to the dry bagged goods was performed at the mix station just prior to placing the material in the gun hopper.

 

 


Repair Process Execution
Although the bulk of the work was dry-process shotcrete, other repair methods were incorporated as required. Epoxy injection of crack repair was performed to over a half mile of cracks including removal of surface sealer. Urethane foam injection was performed on all moving joints to prevent moisture intrusion. Due to the 30 feet of head pressure behind the dam, urethane foam injection was also used to stop the seeping of water onto the spillway and into the repair areas. The control joints on the spillway were all cleaned and caulked with reservoir grade urethane sealant. Deteriorated reinforcing steel (square bar) was removed and replaced as needed. Once the repairs were completed on the top of the spillway, the entire surface was treated with the topical version of the lithium and portions of the slabs were then sealed with an epoxy flood coat. For aesthetics and safety reasons, the epoxy flood coat was seeded with sand. Unforeseen conditions found During the repair process, certain locations were deemed to be unsafe due to the hazard of rocks falling. The size of the rocks were sufficient to crush a large truck. (Or the powerhouse below) Repairs needed to be done, however, and the contractor was requested to install the appropriately engineered rock netting and rock anchors. This occurred more than once, each time with successful results from the remedial work.

 


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