RE:RE:TMX Files NEW VARIANCE APPLICATION With CER On 12/14/202315. Trans Mountain also completed a geotechnical investigation program at the HDD entry and exit locations, prior to starting a geotechnical assessment pilot hole with the HDD rig. The findings of the investigation and assessment confirmed that the Mountain 3 HDD crossing would be completed primarily in bedrock conditions.
16. Trans Mountain utilized the pilot hole from the geological assessment to continue the construction of the Mountain 3 HDD crossing. Due to the length of the crossing, a second drilling rig was utilized from the exit side, to intersect the pilot hole drilled from the entry side. Due to the hard rock conditions of the crossing, which limit the ability to take larger steps between each ream pass, Trans Mountain has completed 24-inch, 30-inch, 36-inch and 42-inch ream passes, and a partial 48-inch ream pass. The hole is currently large enough to accommodate the pullback of NPS 30 pipe.
17. Hard rock is a difficult medium in which to complete the Mountain 3 HDD crossing because hard rock wears out tooling at a faster rate than weaker formations. Changes or fractures in the formation can also cause the reamer tooling to “chew through” the zone erratically, causing increased torsion or eccentric loading on the downhole tooling. Crossings underneath mountains, such as the Mountain 3 HDD crossing, also have a high probability of encountering water inflow zones underground. While Trans Mountain has experienced water inflow on other hard rock mountain crossings, in those cases the water inflow was not as severe as that experienced at the Mountain 3 HDD crossing.
18. During the geotechnical HDD pilot hole, three fractured zones within the bedrock were noted that produced water inflow into the HDD borehole. The water inflow was variable and measured above 30 cubic metres per hour (“m3/hr”) at its peak. Trans Mountain completed a series of grouting applications to mitigate the inflow to facilitate the HDD installation. At the time the geotechnical HDD pilot hole and grouting program were executed, there was no indication that the rate of inflow on successive reaming passes would not be feasibly mitigated through the initial grouting program.
19. However, Trans Mountain’s experience has been that the rate of water ingress has increased with each ream pass, indicating that the grouting is becoming less effective. With each ream pass, the rate of water ingress has gotten closer to the previous, unmitigated, measured rate above 30 m3 /hr.
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