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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
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Although more labor intensive than tunneling, a slide-rail trenching system kept the Fort Collins, Colo., stormwater-pipeline project within its $35 million budget.

Editor's note: Trenchless construction is a great way to avoid digging up environmentally sensitive areas. More contractors are bidding on jobs, increasing competition and lowering anticipated project costs almost to the level of open-cut construction.

But location isn't the only variable that influences pipe installation. Up to half the cost of trenchless construction is associated with mobilizing the equipment and excavating the entry pits. Mobilizing may require additional ramp-up time, potentially affecting project schedules.

That said, on alignments exceeding 100 feet, some techniques — such as horizontal directional drilling — actually entail less time and disruption than open-cutting.

There's also pipe length, width, and depth to consider. Substrate conditions and water levels may prohibit trenchless entirely, or require that a different pipe material be specified. Budget and politics play a role.

The following scenarios show how two public works departments determined what would work best for their unique challenge.

KNOW THE GEOTECHNICAL AND HYDROLOGICAL CONDITIONS

When the Westminster Department of Public Works & Utilities decided to place an 8-foot stormwater pipe under one of Colorado's busiest highways, a jack-and-bore system was the first choice for installation.

Preliminary designs called for 240 feet of reinforced concrete pipe (RCP) that connected to grouted, boulder-lined channels on each side of the interstate. The pipe would carry 750 cubic feet of water per second, serving as a key link in a larger, $7.5 million stormwater improvement project.

Subsurface conditions soon affected their plans.

During the preliminary feasibility study, geo-technical consultants CTL|Thompson Inc. explored conditions along the alignment and recommended one tunnel. But the subsequent analysis — consisting of 40-foot borings adjacent to and in the middle of the road — revealed that the upper part of the soils, to a depth of about 18 feet, was clay. Beneath the clay was wet, sandy, and gravelly soils and claystone bedrock.

These “split face” conditions restricted the use of a jack-and-bore system. Instead, a tunneling contractor would need to use a much more expensive “earth pressure balanced” boring system.

To eliminate split-face conditions, CTL recommended running two 66-inch pipes, each through a tunnel of its own, built 4 feet higher than originally estimated. Raising the elevation of the invert and using smaller-diameter pipes reduced the need for dewatering, which in turn reduced overall project costs.

“The conditions for the final project were ideal,” says Mike Galuzzi, lead civil engineer at WHPacific Inc., design engineering firm for the project. “But if we hadn't collected detailed geotechnical information and stayed with the original design, we would've met with increased costs, delays, and disruption to interstate traffic that would have caused public outcry.”

As Westminster learned, ground conditions such as hard bedrock, clay, sand, or shallow groundwater determine whether open-cutting is even possible. If it isn't, they determine what type of trenchless technology to use.

“Understanding subsurface conditions is the most important consideration when analyzing trenchless projects,” says Chris Knott, project manager and estimator for specialty contractor BTrenchless. “You need to know exactly what you're getting into, and at what depth, and in what location.”

Geotechnical consultants recommend performing a series of borings along the proposed alignment and, for complex projects, digging test pits at select points.

The presence of groundwater is also critical.

In wet conditions, open-cut contractors must dewater excavations before and during construction, which increases costs. That's also true for trenchless construction. If the site can't be dewatered because, say, the water is contaminated, a more advanced, and thus more expensive, trenchless method may be required.

“Dewatering issues aren't considered as thoroughly as they should be,” Knott says. “Incorporate dewatering into the project design schedule and budget, or you'll face significant project delays resulting in additional costs and change orders.”

Groundwater conditions should be identified during the geotechnical investigation along the alignment. If shallow groundwater exists, several options are available for dewatering, depending on the sub-surface conditions: sloping trenches, pumping the water from the excavation, and a series of well points or even cutoff walls.