Geotechnical drilling projects in the Pacific Northwest region of the United States have become increasingly complex as “developable” sites become more problematic because of severe geologic problems and topographic constraints. Projects become more complicated too as the design and building codes attempt to account for stronger earthquake ground motions and “safer” completed projects. Redevelopment of older sites also adds to the geo-construction challenges. In short, the easy sites are gone.

Geo-construction practices and tools are being developed at a rapid pace to address the geotechnical construction challenges. Innovations in drilling technology from both Europe and the United States have been instrumental in providing the tools and techniques necessary to conquer these new challenges.

The city of Duvall, Wash., located on the glacially formed side slopes above the Snoqualmie River basin, was faced with an infrastructure dilemma due to rapid expansion and topographic constraints. Duvall is located approximately 40 kilometers east of Seattle. As the Seattle metropolitan area has expanded and outlying areas like Bellevue and Redmond (home of Microsoft) have developed and expanded, these more distant areas like Duvall, surrounding Seattle, have become increasingly popular as more affordable and quieter areas of residential refuge in the country. Planning and road construction was more utilitarian in the early days when the city did not foresee the requirements for expansion.

Cherry Valley Road, which services the area schools and rapidly expanding residential areas, is a heavily used intersection joining with the Duvall-Woodinville Road. The intersection needed to be widened to install sidewalks and a bike lane to accommodate the pedestrian and bicycle traffic. The problem was that the widening required a 9-meter-high, near vertical cut into glacial till, with houses located above the top of the slope with setback less than 6 meters. The project was further complicated by the need to maintain street operations adjacent to the slope while construction progressed.

A permanent soil nail and shotcrete wall system was selected by the city as the best alternative to provide slope reinforcement and support for the widening of the intersection. A sculpted faux rock finish was selected for the finish shotcrete to minimize the industrial appearance of the infrastructure improvements.

The site construction constraints virtually eliminated the use of classic dedicated soil nail drilling equipment, as there was no way to create benches to access the slope cut. In addition, soil conditions and soil nail design diameter and drilling depth required a minimum 6-inch diameter hole penetrating up to 20 feet into the very dense glacial soil, comprised of a cemented sandy silt with gravel and cobbles and a few boulders the size of a small car.

Basically, the wall would have to be constructed by reaching from the base of the slope. The size of the soil nails and drilling conditions limited the installation methods to basket mounted equipment and down-the-hole hammers, or extended reach equipment with moderate or large size top hammers.

As the top hammer system would clearly be the fastest installation method, Northwest Soil Stabilization (NW Soil) chose a TEI excavator mast and TE560 top hammer drifter to install the more than 250 soil nails for the project. The TEI mast was mounted on a John Deere 225 excavator with more than 25 feet of vertical reach to be able to build the wall from the base of the cut.

Soil nail holes were drilled through the very dense glacial till soils utilizing the TE560 drifter and carbide cross cut bits. Air was used to flush the cuttings from the holes. A relatively low air pressure and volume was used to minimize the particle blowout from the holes, and avoid impacts to vehicles and pedestrians on the active street adjacent to the site. Drillers were also very particular about plunging and stroking the holes to flush cuttings, and extra time was taken to assure the holes were adequately cleaned while not allowing blowouts.

Because of the inherent strength of the glacial, the general contractor was able to cut from top to base in preparation for the drilling. This allowed complete access to face with the excavator mounted drill, reducing the wait time between lifts for convention soil nailing. It also allowed the deletion of the temporary shotcrete, which provided a cost savings to the owner. 

The TE560 has an impact energy of 43 Nm and 400 daNm of rotary torque. The top hammer running the carbide bits cut rapidly through the cemented silt and cobbles/boulders. The high torque capacity was also useful, twisting the larger particles through the drill holes past the bits and avoiding lockups while drilling. (Button bits were also attempted but maintaining low airflow volume and pressure made plugging a problem, as the penetration rates tended to produce cuttings too quickly for the air stream capacity).

The TEI mast also performed admirably with the radio remote controls allowing the driller complete control over all drill functions from the cab of the excavator, while maintaining all kinematic controls of the equipment. The system also allowed safe operations for the helper in a man basket suspended up to 7 meters in above ground. 

Production ranged from 15 to 20 nails per day depending on the elevation of the nail installation and the variability in the soil. Nail installation was completed in just 14 working days, with nearly 4,500 total feet of drilling and 10 proof load tests. Shotcrete application required another four days to complete, including carving and staining. Again, work was completed from the ground level utilizing man lifts.  

Advanced TEI drilling equipment allowed the project to be completed safely, effectively and ahead of schedule. The equipment worked flawlessly in very difficult drilling conditions. The city was pleased with both the process and the outcome, with NW Soil receiving complements on both our work and our equipment. The city’s engineer and project manager was very impressed with the TEI excavator mounted mast and drilling system, saying, “That is a cool piece of equipment.” We couldn’t agree more.

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