Design and Implementation of Innovative, Openable, In-Trench Rockfall Catchment Fences for Improvement of Worker Safety in Steep Slope Pipeline Construction

ABSTRACT: This paper presents the implementation of an in-trench rockfall catchment system on a steep slope in the Coast Mountains of British Columbia. Using a combination of drone photogrammetry, digital geological mapping, change detection tracking, and numerical rockfall modeling, Stantec identified a rockfall hazard presenting a high risk to the workers and equipment in the trench downslope. Using numerical modeling and field observations of rockfall trajectories, the construction team sought to design a rockfall mitigation system to control the rockfall hazard while satisfying a tight construction timeline and the need for an openable, gate-like design. The solution was a series of 100 kJ rock catchment fences- each 3 m high and spanning the 5 m wide trench, secured to the granodiorite trench walls with grouted dowels. The gate-style rockfall mitigation design is, to the team's knowledge, the first of its kind to be implemented in pipeline construction. The design was successfully used throughout the approximately 6-week period of pipe installation on the slope, marking a successful solution to a unique construction challenge. 1. BACKGROUND Coastal GasLink is a 670 km (420 mi) long pipeline which will transport natural gas from Groundbirch to Kitimat, British Columbia, across some of the most rugged alpine terrain in Canada. The approved Coastal GasLink route was determined by considering Indigenous, landowner and stakeholder input, the environment, archaeological and cultural values, land use compatibility, safety, constructability, and economics. The slope of interest for this paper is within Section 8 West of the pipeline alignment- a 13 km (8 mi) long span, fraught with geohazards, across the highest passes of the Coast Mountain Range, see Fig 1. Further details of the Coastal GasLink are provided in Coastal GasLink (2023). The slope designated Steep Slope 6B (SS6B) by the project team is the focus of this paper, and the site at which the gate-style rockfall catchment fence system was implemented. SS6B is 250 m (820 ft) long and is sloped at approximately 30 degrees. It is located near the junction of two converging glacial valleys in the headwaters of the Kitimat River. At an elevation of 860 m (2820 ft), SS6B is approximately halfway between the glaciofluvial and colluvial debris in the valley bottoms and the 2000 m (6500 ft) high, glacially-eroded mountain peaks which surround both valleys.