Page 15 - Industrial Plants

he existing NPS 24 Trans Mountain “Frequently affected by snow
Pipeline (TMPL) system, built in the avalanches, making the bottom
early 1950s, stretched approximately inaccessible for most of the year,
1,176 kilometres from a storage geologically and geotechnically Dry
terminal in Edmonton, AB to the Gulch is unique, due to the contact
T Westridge Marine Terminal in between two stratigraphic units,
Burnaby, BC. resulting in brittle faulting and
Trans Mountain proposed the Trans Mountain fracturing of the rock mass
Expansion Project (TMEP), involving the expansion of
the existing TMPL system. Spread 5B of TMEP entailed
constructing approximately 89 kilometres (from KP Another important aspect is the unknown thickness of
987+100 to KP 1,076+000) of 914 mm (NPS 36) OD accumulated talus and depth to the groundwater table
pipeline. Bonatti, in a joint venture with Kiewit at the bottom of the gulch. There is unconfirmed
Corporation, was responsible for pipeline construction moderate metal leaching and acid rock potential
activities from KP 987+100 to KP 1,038+100. associated with the stratigraphic unit to the southwest
of Dry Gulch. Depending on the chosen crossing
method, management of rockfall hazard and snow
avalanches prior to and during construction could have
Dry Gulch Geomorphological, required extensive temporary and/or permanent
Geological, and Geotechnical retention measures.
Setting

Dry Gulch is a steep V-shaped canyon near the Construction Schedule
Coquihalla Summit, approximately 39 kilometres Restriction & Method Selection
northeast of Hope, BC. The pipeline route crossed
the canyon near KP 993+000, about 200 metres One of the key considerations for the construction of
northwest of the Dry Gulch bridge on Coquihalla the crossing was the schedule for completion of the
Highway 5. The distance between the top of the works and the available time window. The completion
northeast wall and the top of the southwest wall is of the project had been forecasted for the end of 2023,
approximately 225 metres. The invert of the gulch is necessitating the completion of the Dry Gulch crossing
at an elevation of 1,101.3 metres above sea level on by summer 2023. Additionally, work conditions in this
the pipeline alignment. area were very challenging during the winter season
The northeast wall of Dry Gulch is about 94 metres due to low temperatures, snow precipitation, and high
high, with the upper portion featuring a rock outcrop avalanche risk.
with a slope angle of about 57 degrees (locally as
steep as 78 degrees) and extending for about 45 “One of the key considerations for the
metres. Further below, there is a blocky rock construction of the crossing was the
colluvium lower slope with a slope angle of about 37 schedule for completion of the works
degrees extending for about 94 metres to the and the available time window, with
bottom of the gulch. the completion of the project forecast
The southwest wall of the gulch is 97 metres high, for the end of 2023
mostly featuring a rock outcrop with a slope angle of
about 42 degrees (locally as steep as 58 degrees) Considering the complex geomorphological, geological,
and extending about 121 metres downwards. This is and geotechnical settings and the schedule restrictions,
followed by a rock block colluvium lower slope with the feasibility of the crossing, related to the construction
a slope angle of about 29 degrees and about 32 methodology, was carefully assessed. Initially, “Open
metres long up to the bottom of the gulch. Cut” was considered as the default option; however,
Dry Gulch is frequently affected by snow avalanches, after a detailed study, it was deemed too complex in
making the bottom inaccessible for most of the year. terms of safety, environmental impact, durability, and
Geologically and geotechnically, Dry Gulch is unique schedule. After a wide comparison of various
due to the contact between two stratigraphic units technologies (HDD, Direct Pipe, Microtunnel, Raise
(Eagle Gneiss to the northeast and Falls Lake Suite boring), the TMEP Project Management team identified
granitic rocks to the southwest), resulting in brittle the HDD methodology as the most sustainable option
faulting and fracturing of the rock mass. for the crossing execution.

IndustrIal Plants - May 2024
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