Tunnel Project Helps Meet San Antonio Growth

By Jeff Griffin, Senior Editor | March 2009 Vol. 64 No. 3

"Alternatives analysis included evaluation of pipe materials, construction methods and routing," said Abdel Hamed, P.E., Weston Solutions. "Probable costs of each alternative were developed and compared including a net present value analysis. Non monetary considerations included utility locations, constructability, traffic concerns, environmental/permitting issues, community impacts, right of way/easements, operation and maintenance concerns and manhole junction structures.

"By dividing the 11,254 feet of sewer into 14 reaches' and specifying the acceptable construction methods for each reach, design documents allowed bidders flexibility to choose alternative construction methods."

Water, water and more water

The project's location in an area subject to serious flooding posed challenges from the beginning and delayed the start of construction.

The critical path at the beginning of any construction project is surveying and construction layout, said Dan Deemer, project manager for contractor BRH Garver Construction.

"This is especially critical when working in a flood zone such as Leon Creek," he continued. "Survey markers are no match for flood waters. They wash away or become covered by sediment and are unrecoverable. This was a major issue for us during our attempts to start the project. During the months prior to and after our Notice To Proceed in July 2007, San Antonio received record breaking rainfall amounts which caused dangerous flooding conditions along Leon Creek."

The ground was already saturated from the prior floods and to make matters worse, San Antonio received five times the average amount of rainfall for July 2007, the month construction was scheduled to begin.

"Leon Creek remained at dangerously high flood levels throughout July and August of 2007," said Deemer. "San Antonio received double the average amount of rainfall in August 2007 due to remnants of Tropical Storm Erin, preventing any chance of the flood waters receding. Survey stakes placed between flood events were washed away."

The last flooding came in September, causing 64 days to be lost on the project before work even started.

"We had to commit this shaft crew to a different project temporarily and then brought them to San Antonio at the first opportunity after the floods receded," said Deemer. "This had a ripple effect on the tunnel machine delivery to the site. We did not want to bring the tunnel machine out before the tunnel shaft was ready. The moment it was ready, we proceeded to mobilize the tunnel machine and place it in the shaft."

Tunneling