Split-face conditions on the lower and upper quadrants of a stormwater tunnel alignment prompted a Colorado public works department to specify tunneling, which could save the department $750,000 to $1 million. Photos: CTL|Thompson
Although more labor intensive than tunneling, a slide-rail trenching system kept the Fort Collins, Colo., stormwater-pipeline project within its $35 million budget.
WHEN ONLY OPEN-CUTTING WILL DO
Six times over the past 70 years, storms have ravaged the 5-square-mile area of west-central Fort Collins, Colo.
In 1997, floodwaters were deep enough for college students to kayak down the main road, causing hundreds of millions of dollars in damage and killing five people.
The city pledged in 2001 to supplement its existing stormwater control system with a $35 million system of detention ponds, conveyance canals, and pipelines. The project hit a snag, however, when the utilities department realized one 102-inch reinforced concrete pipeline would run through a residential area that already hosts numerous underground utilities.
Using an alternate product-delivery management system, the department convened a team of civil engineers, geotechnical consultants, and construction contractors to collectively evaluate detention and conveyance alternatives.
The team determined that the best route was through the neighborhood. But it was risky: Without adequately shoring the excavation, open-cutting could undermine the foundations of two multifamily structures in the pathway. Furthermore, contractors would encounter saturated soils including clays, caving sand, and gravel at a depth of 35 feet.
“We exceeded a depth that even a triple-stacked trench box couldn't handle,” says Andrea Faucett, manager of municipal engineering for Ayres Associates, the lead engineering firm.
Geotechnical consultants CTL|Thompson Inc. excavated test pits and drilled numerous borings to evaluate soil, bedrock, and groundwater conditions. Based on the results, the design team decided open-cutting was possible. Residents, however, were less than thrilled with the idea. They didn't want the area torn up for six to eight weeks and have to park half a block from home in the interim.
The other viable option was a $1.5 million micro-tunneling system. But it was more than twice the cost of open-cutting.
A resolution came with the discovery of the slide rail system, a little-used dynamic trench box that allowed the contractor to open-cut through the neighborhood for roughly $500,000.
Issues like impact on the community, parking, and traffic can halt projects. To keep from setting off a flurry of complaints, managers reached out to the residents at community meetings, discussing the project and explaining what to expect, before announcing their decision.
They'd learned the importance of public buy-in when a stormwater drain had to be placed in the city's historic downtown area. Open-cutting would have disrupted the pedestrian-heavy area, which was also adjacent to a heavily trafficked state highway. It was sure to cause an uproar. Managers specified a jack-and-bore with hand mining instead.
“Put all these factors on the table, and work it through with your design and construction team early on to avoid any complications,” says Chris Knott, project manager and estimator for specialty contractor BTrenchless.
The neighbors were comfortable with the project, which proceeded on schedule.
—Dornfest (firstname.lastname@example.org) is a certified professional geologist who serves as the geotechnical department manager and senior engineering geologist in the Fort Collins, Colo., office of CTL|Thompson Inc.
For an overview of trenchless options, visit here.