St. Mary's Spillway
|Alberta Environmental Protection in Alberta, Canada, was facing a huge problem. What were they going to do with an aging and failing dam? The St. Mary's Dam, located approximately 25 miles (40 km) southwest of Lethbridge, Alberta, was built in 1951 and officials felt the structure could no longer be relied upon in its present condition.
"The old one was not up to the new dam safety standards. The under-slab drainage system was not performing properly, a lot of freeze-thaw action was occurring and it was becoming a big maintenance headache for Alberta Environmental Protection," Dave Lapins, resident engineer for AGRA Earth & Environmental, said. "We went through a lot of investigations to determine if we should rehab it or if it was cheaper just to build a new one. Ultimately, the decision was made to build a new one adjacent to the old one."
Alberta Infrastructure administered the $47 million project and hired NAC Constructors in Campbellford, Ontario, as prime contractor. NAC turned to an experienced spillway paver, Getkate Construction Ltd., to slipform the flip bucket and chute slab.
"They hired us because of our experience on the Old Man River Dam project from 1990," John Getkate, president of Getkate Construction, said. "They looked us up and asked me to look at the profile of the spillway to see if I thought our equipment would work and we went from there."
Getkate had previously used a GOMACO C-450 on the Old Man River Dam project. For the St. Mary's job though, due to the length of the spillway, Getkate used his C-700.
"We went to the C-700 because the flip bucket was approximately 108 feet (33 m) long," Getkate said. "The C-450 would have provided the same finish on the regular chute slabs, but since we had the equipment on site already, we just used the C-700 for the whole job."
Concrete had to be placed in front of the C-700 with a crane and bucket. The 5076 psi (35 MPa) mix with 1.5 inch (38 mm) aggregate couldn't be pumped. The slump of the concrete was approximately three inches (76 mm). Concrete was placed over heavy steel reinforcing.
"There were two mats of reinforcing," Getkate said. "It was anywhere from .6 to one inch (15 to 25 mm) rebar and ranged from six inches (150 mm) on centers each way to 12 inches (300 mm) on center each way between the two mats."
The concrete slab averaged 24 inches (610 mm) thick.
The flip bucket was poured longitudinally and the slope and all the chute slabs were poured transversely at a 3:1 and a 5:1 slope.
"The design engineer determined where he wanted the construction joints and we put our machine transverse on the long length pours for the chute slabs," Getkate said. "Then we had to go parallel with the pour length for the flip bucket."
Paving on the spillway started from the bottom and worked its way up. Joints determined the width of the slabs.
"The paving widths were determined by the consultant and we poured no wider than 20 feet (6.25 m)," Getkate said. "We had some tapering pieces where we had to use the self-widening feature because it would taper out to a wider section at the ends. It was all variable.
"The biggest concern was keeping the tolerance on the flip bucket... and everything else for that matter."
Tight tolerances of .12 inch in 10 feet (3 mm in 3 m) had to be met. Getkate had no problems meeting the strict requirements.
The most difficult part of the project, according to Getkate, was slipforming the flip bucket.
"The flip bucket is on a radius and we had to set our C-700 to that curve," Getkate said. "We had to bend the carriage rail to that profile and since we were coming from a 3:1 slope and then flipping up, we had to make sure the legs on the machine were plumb.
"There was a lot of ingenuity on the part of my guys to make that profile work. It took a while to set it up, but once everything was in place, it went well."