ANDRIEUX & ASSOCIATES GEOMECHANICS CONSULTING DEPLOYS BUILDERA CRACKMON® 3D TO ADVANCE THE SAFE PUBLIC OPENING OF A QUÉBEC REGIONAL-PARK CAVERN

Case Study

TREVOR DALTON, BUILDERA

Summary

Parc Régional de la Chute-à-Bull—a public land of scenic waterfalls and hiking trails in Saint-Côme, Québec—is home to a cavern network composed of multiple large, accessible chambers. Eager to open the cave for tourism, park development coordinator Simon Degrandpré tapped a reputable geomechanics consulting firm, Andrieux & Associates Geomechanics Consulting, L.P. (“A2GC”), for expertise and guidance. A2GC launched an ongoing stability analysis project in May 2019, employing 3D scanning technology and multiple Buildera CRACKMON® 3D crack monitors to document chamber block motion over time, helping to facilitate a safe public cave opening. 

Background

A2GC (www.a2gc.ca) is a rock-mechanics consulting firm based in Montréal, Québec. Under the direction of mining engineer Patrick Andrieux, Ph.D., P.Eng., Eng., the experienced A2GC six-member engineering team provides technical support, research, and development in numerous disciplines:

• Geomechanical mine design

• Advanced numerical modeling

• Drilling and blasting engineering

• Ground-control design and practice review

Since 2015, A2GC has offered these comprehensive geotechnical services to mining and construction professionals, contractors, government agencies, and other consulting firms [1]. At the time of writing, the company’s personnel have amassed over 55 years of technical and consulting experience with more than 15 years of working at active mine sites.

Parc Régional de la Chute-à-Bull is a historic public territory that promotes environmental protection and safe, sustainable tourism. This picturesque natural site attracts visitors from across the globe, featuring an 18 meter waterfall, six kilometers of winding trails, and a pine and spruce forest on the bank of the Boule river [2]. Down the hill from Refuge du Belvédère (one of the park’s three campsites), a large pile of rocks give way to a hidden cave network, which holds two relatively easy-access chambers in an expansive network of solid igneous blocks. Simon Degrandpré sought structural advice, recruiting A2GC to conduct a long-term investigation of the chambers’ stability. This study was an essential precursor to safe public access.

Challenge

• A2GC typically works with commercial mining companies, so this regional park project presented an unusual set of challenges:

  • No full-time scientists on park staff

  • No access to electricity and advanced technology

  • A notably smaller budget, and

  • A stronger emphasis on security, as public safety is paramount when opening the cave to family tourism.

• Loose blocks and debris occupied the cavern at the start of the project. To ensure safety, A2GC recommended scaling work. In July 2019, Degrandpré hired a scaling company to remove the insecure rocks and clean the cave roof and walls.

• Little sunlight penetrates the caves where the ambient air temperature varies throughout the seasons, ranging from as cold as -30 ºC in the winter to a moderate +20 ºC during the the summer. Its ventilated configuration invites water infiltration, particularly as snow melts during the spring. 

• Additionally, the A2GC monitoring plan accounted for Matawinie regional environmental guidelines, which prohibit cave access throughout the winter to protect hibernating bats. 

Solution

• To pinpoint the magnitude and direction of rock displacement, if any, A2GC project engineer Amélie Ouellet, Eng., M.A.Sc. (civil engineering, rock mechanics), specified and mounted a series of stainless-steel Buildera CRACKMON 3D crack monitors in four critical and accessible locations along the cave walls and overhangs [3]. Crack separations in these areas could indicate larger structural shifts within the caverns.

• The 3D crack monitors measure movement along X, Y and Z axes with millimeter precision, providing quick visual assessment of rock movement in all dimensions. A2GC employed a recommended combination of thixotropic 5-minute epoxy (Buildera STRUPOXY®) and stainless-steel fasteners to secure the monitors in place. Flush-mount crack monitor versions (Model 2412-FSS) were used on generally co-planar cracks, whereas inside-corner versions (Model 2412-ICSS) were installed on corners or where walls intersected ceilings and perpendicular overhanging masses.

• A2GC tracks cavern rock movements with CRACKMON 3D crack monitors and another civil engineering company’s 3D laser surveys. To build a more accurate understanding of the cave stability, they plan to combine this data into a comprehensive report. By correlating the laser surveys with the crack-measurement instruments, A2GC could measure the movement of blocks with high confidence and precision. 3D laser-monitoring systems are extremely sensitive to differential motion on a broad scale but are relatively intricate and expensive to deploy on a regular basis. Buildera CRACKMON 3D devices, however, are a compact and affordable solution for continuous localized monitoring of rock displacement over time.

• According to A2GC Founder and Principal Engineer, Patrick Andrieux notes, “While these instruments measure displacements smaller than what the eye can discern, laser surveys offer higher precision. The objective is to provide a convenient visual tool for the park guides and managers to observe whether significant displacements have occurred.”

• Compared to traditional thermoplastic crack gauges that are more sensitive to thermal expansion, the use of 18-gauge type 304 stainless steel for this application mitigates X-Y measurement errors due to the metal’s low coefficient of linear expansion of just 17 μm/m-ºC. For each 10 ºC change in ambient cave temperature, this equates to 0.17 mm per meter of differential expansion and contraction (per plate). Accounting for the short unconstrained length of each CRACKMON 3D plate (< 88 mm, typ. from fasteners to floating end), and factoring that each plate is mounted in opposing directions, this computes to a negligible 0.03 mm measurement error from thermal expansion and contraction per 10 ºC temperature change.

• According to Amélie, Buildera CRACKMON crack monitors’ simple, battery-free design makes for an “easy, inexpensive way to monitor displacement and flag movement.”

Results 

• To date, A2GC has two sets of measurements—spring 2020 data and baseline data from 2019—and plans to collect additional data during summer 2020. Parc Régional de la Chute-à-Bull does not provide official guidance on what particular movements are acceptable, thus A2GC focuses on observing differential trends in chamber-block motion. 

• Although the measurements have not indicated any significant movement thus far, there is not yet enough data to confirm a trend. The firm is awaiting results from a 2020 laser survey, and will correlate its data with the earlier measurements to make a conclusion with high confidence. 

• Initially, A2GC and park coordinators planned to open the cave in August 2020, but complications and delays due to COVID-19 slowed the operation. If 2020 3D surveys correlate with the most recent crack-monitor measurements, this will increase A2GC confidence in the data, helping to corroborate the final decision to open the site to the general public.

Buildera CRACKMON 3D crack monitors are efficient and affordable monitoring tools that A2GC will continue to leverage for the next several years, optimizing public safety at Chute-à-Bull.

Acknowledgments

Buildera is grateful to Amélie Ouellet for her time and invaluable contributions to this case study, including installation photographs and other essential technical project information. Buildera also expresses its appreciation to Patrick Andrieux and Simon Degrandpré for their support and approval to author and publish this case study.

ABOUT THE AUTHOR

Trevor Dalton is a writer and filmmaker at Wesleyan University with a background and interest in film production, environmental sciences, and statistics. He is a frequent contributor of case studies and customer-success stories for Buildera.

References

[1] A2GC – Andrieux et Associés Consultation Géomécanique, L.P. 2020. A2GC | Firme De Consultation Géomécanique | Ingénierie | Montréal. [online] Available at: https://www.a2gc.ca [Accessed 8 July 2020].

[2] Parcs régionaux de la MRC de Matawinie. 2020. Parc Régional De La Chute-À-Bull - Parcs Régionaux De La MRC De Matawinie. [online] Available at: https://www.parcsregionaux.org/parcs-regionaux/parcs/parc-regional-de-la-chute-a-bull/ [Accessed 8 July 2020].

[3] Buildera. CRACKMON® 3D XYZ-Axis Crack Monitor. [online] Available at: https://www.buildera.com/crackmon-3d-xyz-crack-monitor [Accessed 8 July 2020].