That Dam Job
That Dam JobBoyer, Inc. of Houston has assumed the combined role of general contractor, pumper and ready-mix supplier during the construction of a tailwater weir for the Trinity River Authority of Texas.
TAMS engineers designed the new tailwater weir, or so-called mini-dam, to establish a permanent pool at the base of the Lake Livingston Dam stilling basin. It will act as a buffer by taking the energy out of the water when it is released from the gates and slowing the flow into the Trinity River, thereby controlling erosion.
As the special project called for time and labor-saving methods, the traditional crane and bucket approach was not used for placing concrete. Instead, the established 16-year old Boyer organization purchased all its own concrete placing equipment and modified various aspects of it to meet the specialized needs of the job. The equipment will also be used to handle future jobs within the company's expanding inland marine division.
Owning equipment makes a difference
Brad Boyer P.E., head of engineering for the family-owned company, said, "Having our own equipment on this particular job has improved efficiency by 20 percent. It has also eliminated risk, as we can work around the clock if we know floodwaters are approaching."
In particular, Boyer selected Putzmeister products - a MXR 24/28 separate placing boom mounted on hydraulic legs and an electric-powered BSA 1408E trailer-mounted concrete pump.
Brad Boyer said, "We've worked on rehabilitating dams before but never anything as involved as this project. That's why we needed reliable, top-of-the-line equipment by a reputable manufacturer. Putzmeister was the obvious choice, especially as we were most comfortable with the technology and performance capabilities of their electric-powered trailer pump."
The 760-foot-long weir is divided into concrete cells sized 20 feet wide by 50 feet long. They are pumped in 5-foot-high increments using 250 yards of concrete per pour. As a result, four pours make up the 20-foot depth needed in each section.
Water as ballast
The placing boom's 79-foot horizontal reach can access three cells before it needs repositioning to another spot. As a result, the equipment setup would need to be relocated about 15 times during the project. Trying to avoid labor-intensive moves, Brad himself developed the idea of using water instead of concrete as ballast. This avoided the setup and dismantling of heavy concrete ballast blocks during each move and took advantage of the water in plentiful supply at the dam.
Consequently, a 15-foot-tall donut-shaped ballast tank that encircles the placing boom's pedestal was developed. Boyer's structural engineer, Ben Anderson P.E., prepared the final design for the structure, which was fabricated at Boyer's Houston fabrication facility. The base weighs 25,000 pounds alone and 65,000 pounds when filled with 5,000 gallons of water. The water is simply emptied when necessary to move the base and refilled when relocated to a new spot. The approach is proving highly economical, as it is both time and labor efficient.
Special mix design
Boyer also set up it's own batch plant on site, stockpiling all materials required. Ready-mix trucks wouldn't have been able to access the steep incline of the dirt road leading to the weir and the below-grade placement of concrete. However, more importantly, their own batch plant would ensure reliable delivery of concrete when needed, and they could maintain strict quality control standards of the special mix design.
The mix, although unusual for most pours, is fairly common for dam projects. It is comprised of 337-pound cement, 112-pound fly ash, 1650-pound course, 1400-pound fine, 1.5-ounce Daravair for durability during freeze-thaw cycles and 38-ounce Daracem, a high range water reducer.
At the batch plant, a high-pressure Putzmeister BSA 1408E concrete trailer pump is positioned next to the mixer. Originally purchased to handle larger projects in the future, the trailer-mounted unit easily pumps the required 30 yards an hour for this job, although it could easily handle four times that amount.
The trailer pump is electric-powered, chosen for its clean air benefits and because the use of diesel power is limited on other sites. Its quieter operation also makes it easier to communicate on site during a usual 10-hour working cycle.
To deliver the concrete mix, 5-inch pipeline will stretch over 1,000 feet to reach the farthest end of the project. This accounts for a 760-foot dam length and a steep 250-foot path from the batch plant to the mini-dam.
Keep it Cool
Another highly unusual aspect of the job includes cooling the 5-foot thick concrete, to remove the heat during hydration. As temperatures above 1600F damages concrete, it would take about 30,000 pounds of ice to pre-cool this huge mass. However, specially designed one-inch black steel coolant pipes run in a zigzag pattern within each cell. Over a 14-day period, they continuously carry river water through the pipes to cool the concrete. Afterwards, grout is inserted in the pipes and they are left as reinforcement. "A similar method was utilized when building Hoover Dam, and this approach still proves effective today," said Brad Boyer.
The weir is divided into concrete cells that are pumped in 5-foot-high
increments using 250 yards of concrete an hour. Four pours make
up the 20-foot depth required.
Watch out for floodwaters
Situated only 200 feet downstream from the floodgates, the crew doesn't go beyond pumping or excavating five to six cells in width at a time because frequent floodwaters can quickly destroy work not already poured. Fortunately, a river monitoring system alerts the crew up to 10 days in advance, which allows time to finish what's started and move equipment to higher ground.
Boyer started the project in November 2001 by building access roads from the east and west bank to river bottom, strengthening them to hold the weight of construction equipment when it progressed across the channel. In June 2002, the first concrete was placed. Upon completion in November 2002, over 14,000 cubic yards concrete will have been placed on the $6.8 million job.
Boyer, Inc. specializes in rehabilitating and replacing infrastructure that over the decades has deteriorated, outlasted its design or must perform to a higher level. The firm has divisions specializing in civil, mechanical, electrical, and inland marine. For more details about the capabilities of Boyer, Inc., access the company's web site at www.boyerinc.com.
Owner: Trinity River Authority of Texas - Fort Worth, TX
Engineer: TAMS Consultants, Inc. - New York, NY
General contractor: Boyer, Inc. - Houston, TX
Pumping contractor: Boyer, Inc. - Houston, TX
Ready-mix supplier: Boyer, Inc. - Houston, TX
Equipment: Putzmeister BSA 1408 E trailer pump and Putzmeister MXR 24/28 separate placing boom