In an effort to increase emergency water release capability, a new larger outlet was needed through the raised concrete dam. The outlet tunnel was to be bored from the downstream side of the dam and through the concrete to the water side. To hold back the water and provide a dry work space for tunnel boring operations, a special cofferdam was required.
The challenges for a cofferdam are structural in nature, in that is has to resist the water and gravity forces including earthquakes, and fit closely against an existing dam to prevent leaks and flooding of the work space. This particular project was especially challenging because the cofferdam had to be constructed in bolt-together pieces that would fit on trucks for transport to the jobsite.
The successful bidder on this project was Barnard Construction Company, Inc. (Barnard). Barnard requested that Gerwick design the cofferdam in shippable pieces that would fit on standard trucks, and would not require wide-load truck permits.
To hold its half-circle shape under hydrostatic load, each of the semicircular unit shells was stiffened with top and bottom bolt-together flanges and four box rings. Vertically, tee stiffeners were added and equally spaced to handle the vertical buoyancy loads. At the back of the shell arch on each side was a three-foot-wide stiffened plate, which was to set against the dam face to contain the side seals and anchor bolts. The mating half circle flanges were to be match fitted in the shop so that the field fit up would also match within tolerance at the jobsite.
In order to closely fit the fabricated side flanges of the cofferdam to the actual dam face in the planned location, it was necessary to map the face of the dam in the planned dam contact area. Gerwick determined that a fit of +/- two inches from the theoretical plane could be tolerated with a good seal design.
Four rubber lip seals were attached to the back of the flanges. These seals extended out about three inches beyond steel bearing bars and bearing blocks that would flush up against the dam. When pushed against the bearing bars and blocks by differential water pressure, the outward facing rubber strips would fit tightly against the dam, forming a watertight seal.
Since the cofferdam had to be transported in 8-foot sections, a total of 14 units were designed to be bolted together on site. Except for the drilling and installing of the side and bottom anchor bolts, this goal was accomplished. Most unit bolting was done on land at the assembly site before float out.
From the onset of the job, the concept of lowering the cofferdam to its final position against the dam would require a large crane on a barge, or a crane on top of the dam. Two large beams fixed to the top of the dam were provided by Barnhart as support for two sliding beams and jacks to lift and position the cofferdam. This operation completed the sealing on the sides and bottom of assembly, and the cofferdam was now ready to be dewatered. Barnard placed a dewatering pump in the bottom unit and was able to dewater the entire assembly in about one day.
While the cofferdam was being set, the tunnel subcontractor was busy boring into the dam from the downstream side of the structure. Within a short time, they were able to bore through into a completely dry space inside of the cofferdam as planned. They completed the pipe installation through the dam, placed concrete around the pipe, and then installed a bulkhead over the outlet in preparation for the future placement of the sliding gate valve.
Throughout the process of designing this complex cofferdam, our engineers delivered innovative design and installation solutions that solved the many challenges faced on this project. Our expert construction technical support and timely calculations ensured the successful completion of this vital structure that San Diego residents rely on to deliver a continuous and reliable source of water to their homes and businesses.