ASCE Publishes Technical Paper by Erinn Johnson

Senior Staff Geologist, Erinn Johnson wrote a technical paper titled, “Using Directional Drilling Techniques to Intersect the American Tunnel”, that was recently published by the American Society of Civil Engineers (ASCE).

Erinn shared a synopsis and abstract about the paper. Download Erinn’s full article on ASCE’s website to get more details on the methods and lessons learned.

(Image of job site at Bonita Peak Superfund near Silverton, Colorado)

At the Bonita Peak Superfund site near Silverton, Colorado, the Deere & Ault team of engineers and geologists worked with specialty drilling contractors to successfully intercept the American Tunnel and installed a monitoring well. Why is this special you ask?

The American Tunnel was the main drainage tunnel for several mines in the area and contains three bulkheads. The accessible bulkhead was not installed with a pressure gauge, and prior to 2019 there was no way to confirm how much water was being held behind it. This was a cause for concern.

The difficult part of the project: drilling an inclined boring precisely enough to intercept the 11-foot diameter tunnel from over 600 feet away.

How did we do it? The team used a combination of historical research, careful digitization of hand-drawn maps (ca. 1970s), detailed ground surveys, gyroscopic and magnetic surveys within the borehole, and conventional and directional drilling techniques to plan and intercept the tunnel. The drilling was completed through intensely fractured, altered and faulted andesitic rock, which did not make it easy to reach the interception point. Different drilling techniques were utilized for the project to avoid the wandering that can occur with conventional drilling methods. The introduction of a steerable drill bit (with downhole surveying) allowed for orientation corrections of the borehole. This ultimately allowed for the successful interception and installation of the monitoring well – on the first try!

Original Abstract:
In 2019 a team of geologists, engineers and specialty drilling contractors worked for the EPA at the Bonita Peak Mining District Superfund Site near Silverton, Colorado. The team successfully installed a monitoring well in an inclined borehole drilled into the American Tunnel. A combination of historical research, careful digitization of hand-drawn maps (ca. 1970s), detailed ground surveys, gyroscopic and magnetic surveys within the borehole, and conventional and directional drilling techniques, were utilized to intercept the anticipated location of the 11-foot-diameter tunnel from over 600 feet away. Drilling was completed through intensely fractured, altered and faulted andesitic rock.

Conventional geotechnical drilling methods can result in significant “wandering” of the borehole due to deflection of the drill stem within the rock. The introduction of a steerable drill bit (with downhole surveying) allowed for orientation corrections of the borehole. The program was successful, allowing for construction of a monitoring well into a sealed portion of the tunnel to determine the water pressure behind the existing bulkhead. The well will provide groundwater levels and water quality data to augment understanding of the regional hydrology, assess the safety of the bulkhead, and the evaluate the hydrologic impact of work on other local abandoned mine workings.

Article by Senior Staff Geologist, Erinn Johnson

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