Complications On The Rio Grande

Storm Sewer Trunk Line Is Successfully Rehabilitated Using Two Trenchless Methods
By Jeff Griffin, Senior Editor | May 2009 Vol. 64 No. 5
The access pit for FRP insertion.

All FRP was installed from one insertion pit in one week and then grouting operation began.

"Due to the large annular space, a special grout technique was borrowed from large diameter tunneling experience," Williamson said. "This technique used gravel to fill the void on each side of the liner holding it in place and then using grout to complete the operation." Williamson added that the ease and speed of insertion and grouting helped assure the project's success by relieving concerns about another unexpected rise in the level of the Rio Grande.

For the upper portion of the project, approximately 800 feet of Sekisui RibLoc spiral wound HDPE liner was used. This segment of host pipe had more than 25 feet of cover.

A continuous strip of highly stiff, low-weight plastic liner was spirally wound into the host pipe by the hydraulically self running winding machine that interlocks the strips of the liner as it advances forward, forming the structural liner. The winding machine rotates around a frame that bends the steel reinforcement which is built into liner. After the host pipe has been lined, the winding machine is disassembled and removed through the access pit. With this process, an exit pit is not needed.

"The spiral wound pipe process provided the ability to slipline the pipe's difficult vertical transitions and horizontal bends and transitions without the use of fittings," explained Williamson. "In addition, the use of spiral wound pipe reduced the number of access shafts that would have been necessary with other methods of construction, allowing the hotel in the project area to remain open."

Mega-void complication

During construction of the upper segment, “a large 20 by 20 by 15 foot void was discovered above the host pipe," he said. "It was caused by erosion of a 60 inch CMP connected to the host pipe from above by a vertical 90 degree bend. The lower section of the 60 inch pipe had completely corroded away, allowing the storm water to erode the bedding around the pipe and creating the giant void."

The void was near the hotel under a street intersection which could have potentially caused a catastrophic collapse of the street above.

"The intersection was immediately closed and the pavement was removed to fill the void and replace the lateral based on a design developed by LAN's engineering team," said Williamson. "Lining of the host pipe was completed to keep the pipe structurally secure while the void was filled with backfill and the lateral replaced.”

The void repair required about two weeks.