Cobble-Filled Soils Challenge Idaho Contractor

Any trenchless installation contractor that has ever attempted to bore through the sedimentary stuff will likely tell you that of all the ground conditions they encounter, cobble is among the most difficult.

For Grant Durtschi however, owner of Forest Lake LLC, headquartered in Driggs, Idaho, conquering cobble has become his livelihood.

Located just a cobblestone’s throw west of the Grand Teton mountain range near the Idaho/Wyoming border, Forest Lake LLC was established by Durtschi in 1995. The company focuses on trenchless installations  —  primarily fiber optic cable, sewer and power lines — and has successfully navigated underground bores in a variety of challenging conditions; although most often through cobble. A Wyoming native, Durtschi is intimately familiar with the terrain and soil composition, and although the majority of the company’s work takes place in the west and northwest regions of the U.S., Forest Lake LLC has ventured as far as Indiana and Illinois to tackle trenchless projects.

“There are a lot of things that we do well, but if I have to hone in on a specialty, it would be cobble,” Durtschi says. “Drilling in difficult conditions has been our bread and butter, and it doesn’t get much trickier for a trenchless installer than cobble. We encounter it so often and in so many different formations that we’ve really become cobble experts.”

The characteristic that makes cobble so challenging for trenchless installers is its inconsistency. Chris Fontana, cutting edge/attachment parts sales manager with Vermeer, explains.

“Cobble can define many different conditions,” Fontana says. “It may be a loose conglomerate of varying size glacial stone — anywhere from pea-sized to a basketball — mixed with sand and soils to create the condition, or packed tightly from thousands of years of compression, a formation also referred to as cemented cobble. Maintaining a pilot bore plan in cobble can be difficult as the bit tends to deflect off oversized stones, hence changing the bore path. The longer the bore, the greater the risk of borehole collapses.”

Not only is cobble an extremely hard substance, making it difficult to cut, it’s dicey. During drilling, cobble pieces move about making it difficult for tooling (bits) to penetrate due to deflection. So instead of being cut, cobble deflects drill bits and steering. The amount of cobble in a bore path will also limit the effective bore length as the longer bores tend to increase the potential for borehole collapse. Most difficult are cobble formations that have little substance material, e.g., soil, sand, etc., that can help hold drilling fluid within the confines of the bore path. And while drilling fluid is important to maintaining the integrity of any bore, there is likely no application where the fluid mixture plays a more critical role than cobble.

On Top of Grand Teton

In 2011, Lake Forest LLC was selected as the installation contractor for a major fiber-optic broadband expansion project right in their own backyard. The Teton Broadband Project — brainchild of Silver Star Communications, a Wyoming-based communications and technology company, based in Thayne, Wyo. — will expand existing broadband service to improve communication needs to schools, government offices, fire/EMS departments, hospitals and medical facilities as well as Wyoming power companies. At the recommendation of officials at Silver Star, Lake Forest LLC was chosen by general contractor Tetra Tech, based in Denver to play a major role in the two-pronged project. 

“We had done a lot of work for Silver Star in the past so they were familiar with our expertise, especially drilling through difficult conditions,” Durtschi says. “Silver Star recommended us to the general contractor and it’s been a good arrangement. The bulk of the job is through the city of Jackson, Wyo., on Teton Pass, through Grand Teton National Park and beyond. We refer to the area as cobble heaven, although most contractors would likely have quite the opposite descriptor for it. I think you know what I mean.” 

After consulting with the U.S. Forest Service (USFS), National Park Service (NPS), U.S. Army Corps of Engineers (USACE), Teton County, the Town of Jackson and others to create the most convenient, practical and environmentally sensitive construction route and method, Silver Star officially kicked off the project April 15, 2011. It specifies a 4-in. HDPE pipe be installed for the fiber-optic cable via two routes — extending some 109 miles — through some of the most challenging drilling conditions known to trenchless contractors.

The Tooling Difference

As if cobble weren’t enough, Durtschi’s crews also faced some rugged surface terrain as the route was mapped over the Rocky Mountains at Teton Pass. A Vermeer D24x40 Series II Navigator and D50x100A horizontal directional drill combo was selected by the Forest Lake LLC trenchless team to carve out an underground bore path of more than 40,000 ft to accommodate the pipe. A heavy drilling fluid mix composed of a bentonite polymer combination was used to help maintain the integrity of the bores. Most shots were back-reamed with a 6-in. standard reamer.

“We did a series of bores up one side and down the other of Teton Pass, including six bores at the top, or within a quarter of a mile of the top, of the mountain pass,” Durtschi says. “We were actually drilling beside areas that were walking and bike paths for recreationalists. We are always aware of our footprint, but for this job we paid very close attention to any potential degradation of the environment.”

As noted earlier, because cobble is very non-homogeneous, i.e., angular and irregular with very sharp corners, it’s very difficult to maintain borehole integrity, making tooling selection even more critical. Durtschi selected the Armor Drilling System, manufactured by Vermeer.

Of the dozens of bores completed by Forest Lake LLC on the project thus far — several beneath streams, rivers and environmentally sensitive areas — there was one 700-ft shot beneath the scenic Buffalo River, a tributary of the famous Snake River located near the unincorporated village of Moran, Wyo., that was especially challenging. The Buffalo fork is one of Wyoming’s finest locations for fishing, offering an array of fish species in clear waters that drain the Teton Wilderness. The drill team was required to maintain a minimum of 3 ft clearance beneath river and stream beds.

“The cobble and riverbed combination was especially challenging here,” Durtschi says. “The Armor tooling was really a lifesaver. There’s no margin for error when you’re boring underneath rivers. The D36x50 drill, along with the design of the tooling system, was instrumental in helping keep the bore on course, away from trouble.”

Having had extensive experience drilling in conditions present along the installation route of the Teton Broadband Project, Durtschi and crews didn’t encounter anything all that unexpected underground; but rather, a condition on the surface that they hadn’t anticipated.

Given that the bulk of the project was completed within Grand Teton National Park, a conglomerate of federal, state and local regulatory and environmental agencies were represented on site daily; most notably the U.S. Forest Service, Environmental Protection Agency (EPA), Army Corps of Engineers and National Park Service, along with local concerns, including representatives from Teton County and the Town of Jackson, among others.

“We didn’t run into anything that we haven’t faced before when drilling in these conditions,” Durtschi says. “What we didn’t anticipate, however, was the level of environmental oversight we’ve had. It’s not unusual for there to be a minimum of two groups represented at the drill launch site. It’s not that they are really looking over our shoulder, but more just to support the effort in minimizing any environmental effects.”

Long-term plans for Internet or other communications services are in the strategic planning phase, but ultimately, project owner Silver Star Communications hopes to provide additional services to the Jackson community beyond increasing broadband capacity and reliability for Jackson residents. Immediate priorities include providing services to community anchor institutions (emergency services, educational establishments, etc.). Officials expect the project to be completed and connected on or before October 2012.

Randy Happel is a features writer for Two Rivers Marketing, Des Moines, Iowa