Gilboa Dam work ahead of schedule
Rehabilitation work expected to be complete by summer
GILBOA A lot can go wrong during a large-scale construction project like the $400 million restoration of the Gilboa Dam.
But contractors, engineers, construction workers and nature itself were all in sync this year, putting the massive effort two years ahead of schedule.
And according to project managers, ideal conditions this year led to concrete capable of withstanding pressure greater than they’d hoped for. It’s been measured to withstand at least 1,000 additional pounds per square-inch than necessary.
Since work began in 2009, workers added about 87,500 cubic yards of concrete to the structure — 54,000 cubic yards of which was done this year alone.
“This has been a particularly productive year,” said engineer John Vickers, chief of the New York City Bureau of Water Supply’s Western Division.
Vickers, engineer Mark Suttmeier of the New York City Department of Environmental Protection, project engineer Todd Yanoff and DEP deputy press secretary Adam Bosch gathered at the southern Schoharie County project earlier this month for a tour and inspection of the project, which is being handled by Barnard Construction Co. and D.A. Collins Construction Co.
Millions of people rely on the Gilboa Dam, including residents in New York City who drink the water stored in the 17.6 billion gallon Schoharie Reservoir the dam holds back.
Residents downstream, meanwhile, count on the 87-year-old structure’s condition. A sudden failure would send catastrophic amounts of water and debris down the Schoharie Creek into the Mohawk River west of Amsterdam and all the way to Schenectady.
Engineers studying the concrete dam in 2005 realized it didn’t meet modern design standards. They feared in its condition, flooding at a level greater than the 1996 disaster might push the dam right out of the way.
The 1996 flood sent water cascading over the dam at a rate of about 78,000 cubic feet per second. That pushed the DEP to start an emergency stabilization project, which entailed drilling holes in the dam and connecting 80 steel cables to the bedrock beneath. The project, completed in 2006, was designed to give engineers time to design the current modernization project.
The cables likely saved the Schoharie Valley below the dam when tropical storms Irene and Lee inundated the region in 2011, dropping rainfall that surpassed the 1996 flood. Floodwater gushing over the Gilboa Dam was so great in 2011, it’s been compared to the volume that goes over Niagara Falls, measured at about 110,000 cubic feet per second.
Well-coordinated labor, an on-site concrete production plant and favorable weather were all factors putting the project ahead of schedule.
For Vickers, one of the biggest factors has been the absence of another disaster in the Northeast after 2012’s Superstorm Sandy and 2011’s Irene and Lee.
And the project’s status as ahead of schedule is even more remarkable because Tropical Storm Irene flooded the work site, causing a $22 million setback.
By the time work is done, there will be an estimated an additional 1 million pounds of concrete holding back the Schoharie Reservoir, in addition to more cables mounted to the bedrock to strengthen the dam’s western abutment.
The weather — and lack of water gushing over the dam — enabled contractors to continue working this year unabated.
Engineer Todd Yanoff said they try to produce concrete for the face of the dam with a compressive strength of 4,000 pounds per square inch. After mixing, inspectors draw out a cylinder of the concrete before it’s poured into place; those cylinders have tested with an average strength of 5,000 psi.
On the spillway floor, where concrete takes the greatest pounding as water crashes over the dam, the concrete tested at 6,000 psi.
Strength isn’t a concern in terms of rigidity — the dam is designed with some flexibility. It isn’t a single structure, but rather several, 75-foot-long sections, called monoliths, that enable it to give a little under stress, Vickers said.
New features are also being added, including a new inspection tunnel that stretches the entire length of the dam. Beginning at the bottom of a 60-step staircase, the tunnel includes small tubes inserted more than 100 feet below the dam which allow groundwater to seep and flow safely to the spillway. That feature prevents groundwater, affected by the amount of water in the reservoir, from putting upward pressure on the dam.
At about 80 percent complete, the project initially planned for “substantial completion” in 2016 should meet that target by next summer.
Though the rehabilitation should be complete in 2014, another major phase of another project is in the planning stages. Engineers are devising a low-level outlet that will enable operators to release water from the Schoharie Reservoir without impacting the spillway. That work will require drilling a tunnel nine feet in diameter from the reservoir’s bottom, beneath an adjacent mountainside and roadway and leading back to the Schoharie Creek.
DEP spokesman Adam Bosch said the successful work isn’t due to people rushing, either. The record of safety for workers on the job has been impeccable, Bosch said.