Tunneling under a river is never cheap or easy, but Sundt found a less expensive way to do it without compromising quality while installing a new water transmission main beneath the American River for the City of Sacramento.
Normally, before you bore under a river you first have to sink deep shafts on either side – the jacking shaft and the receiving shaft. A boring machine placed at the bottom of the jacking shaft pushes against the inside wall to force pipe under the river to the receiving shaft.
Because it is being pushed against during boring, the jacking shaft must have much stronger walls than the receiving shaft. It’s usually constructed by driving large metal sheet panels (called sheet piles) into the ground in a box configuration, removing the dirt inside, then adding the cross bracing.
Sundt opted for an innovative, faster approach to create the tunnels for the 6,500 linear feel of new, 54-inch transmission main. We drilled deep holes around the circumference of a 22-foot-diameter circle. Then crews constructed a 22-foot-diameter caisson with 24-inch-thick concrete walls on top of the circle to a height of about 12 feet. After the concrete hardened, we removed the forms and excavated the dirt from inside the caisson until it slid down into the earth from its own weight. The process was repeated again and again, until the caisson reached a depth of 80 feet.
Both shafts were completed in far less time than usual, allowing crews to begin the process of tunneling under the river sooner than a more traditional method would have allowed.
City of Sacramento
Howe Avenue Water Transmission Main