More than two tons of high explosives set off in July 2007 shattered the top 10 ft of the Marmot Dam, located along the Sandy River and 40 miles east of Portland, Ore. The project is the state’s largest dam removal project in decades.
The removal of the dam is the first step in the $17-million decommissioning of the Bull Run Hydroelectric Project that will involve the removal next year of a 16-ft-high dam downstream across the Little Sandy River, a nearly 3-mile-long wooden box flume and the 160-acre Roslyn Lake above the 22-megawatt power plant. The infrastructure diverts almost all of the flow from the Little Sandy River, prime habitat for several species of migrating salmon, for power production. Portland General Electric (PGE) and 22 government entities and conservation groups struck an agreement to restore free-flowing waters through the Sandy River Basin.
The 47-ft-high concrete dam originated as a timber structure in 1913. Following a flood, Portland-based Natt McDougall Co. rebuilt the dam in 1989 using roller compacted concrete (RCC), which at the time was a relatively new no-slump material.
Ironically, the same contractor that built the dam was hired to remove it. “It’s a little tough” emotionally to go back and destroy it, said Natt McDougall, the company’s CEO.
The project was running six weeks ahead of schedule by the end of August 2007 when work ceased at the site as part of the removal plan. The bottom 20 ft were left in place until October 2007 when the last of the 20,000 cu yards were to be removed, along with a lined diversion channel and a coffer dam breached so that river flow would flush out the streambed to the original elevation. The stockpiled chunks of concrete from the dam will be recycled into road surfacing.
At midnight on July 1, 2007, the contractor began building the 18-ft-high earthen coffer dam, a temporary bridge and diversion channel. Thereafter, the dewatering plan for the coffer dam was critical because the 10,000 cu yards of borrow used to create the coffer structure rested only on sediment without any reinforcement or significant compaction. It was here during construction of the Marmot Dam that Natt McDougal Company mastered its dewatering methods. While building the replacement structure for the timber dam in 1989, the well drilling subcontractor discovered that the layout of bore holes specified by an engineering firm had inadequate capacity.
“That forced us to install an unforeseen second row of wells on our own,” McDougal said. “Every since we’ve developed expertise in dewatering our projects. When this job came out, the engineers had copied the same dewatering scheme with the deeper 12-in. dewatering wells originally called for in the 1989 plans. Knowing beforehand that wouldn’t work, we convinced them to let us substitute 36-in.-diameter wells with 18-in. HDPE casings packed in ½-in. gravel instead of 12-in. holes.”
The crew drilled 10,000 holes in a pattern on the well casings that were inserted into the 10 wells spaced at 30-ft intervals atop the coffer dam. Working with Rich Epperly, a pump specialist with the Portland office of ITT Flygt, another revised plan emerged that initially called for pumps capable of 288 gal per minute (gpm) at 70 ft total discharge head to dewater the coffer dam. Epperly recommended that each well receive an ITT Flygt Model 2640 submersible pump to collectively deliver approximately 3500 gpm. Model 2640 is one of the six new 2600 series submersible dewatering pumps that ITT Flygt has recently introduced.
The pumps feature innovative hydraulics, fewer components, new materials and an ergonomic slim-line design. The DuraSpin hydraulic system improves wear resistance and sustains performance over time three to six times better than traditional open type impellers. A single screw enables easy adjustment for maximum pump performance. The results of all of these features are unmatched wear resistance, consistent long-term performance and thus the lowest cost of ownership.
The plan finalized in May originally envisioned 20 wells but was amended to ten, after the drilling crew reached bedrock beneath the channel at only 40 ft. The shallower wells, only one of which required two pumps, increased the discharge volume of each unit. The pumps worked flawlessly and helped the crew remove the RCC much faster than anticipated.
After moving back onto the project in October, the crew dug out the remaining 20 ft of the Marmot Dam, pulled the diversion channel liner, re-graded the banks and removed the pumps and well casings across the coffer dam. This allowed the river to saturate the temporary structure, and have it gradually degraded and flushed downstream once the Marmot Dam was gone.
