The planning for the construction of a 2,500m tunnel adjacent to Canada’s most recently active volcano presented some unique challenges during the development of the Upper Lillooet River run-of-river hydro facility. The project is located in an environmentally sensitive and mountainous area near Pemberton, British Columbia. The area has historically severe restrictions imposed by unpredictable weather, landslide hazards, and the presence of sensitive wildlife species. Additionally, a major forest fire required the construction site to be evacuated for two months during the 2015 season. The Upper Lillooet Tunnel Project is a prime example of how to successfully complete a very challenging project in a remote, mountainous, environmentally sensitive area.

THE PROJECT

The tunnel is located on the north-east side of the Upper Lillooet River and directly across the valley from the Mt. Meager volcanic complex. Water from the river is diverted through a side draft intake into the tunnel, which connects to a 3.6m diameter, 1,600m-long steel penstock to the powerhouse. The facility is capable of generating 81.6MW of renewable power. The tunnel is a 6m wide x 5.5m high “D” shaped tunnel that passes through varying geology. The most recent eruption from the Mt. Meager volcanic complex (2,360 years B.P.) left varying layers of geological materials beginning in welded breccia, and through unconsolidated pumice and quaternary deposits that contained burned vegetation and trees, and into the underlying tonalite bedrock.

As the tunnel portion of the project allocated geotechnical risk onto the owner, on-site engineers, reporting directly to the owner, were required. Because tunnelling conditions were anticipated to be very challenging, as well as to address the numerous technical challenges including the geology, groundwater inflows, tunnel support, schedule, winter shutdowns and safety at the remote site, engineers were on-site full time.

The ultimate challenge was excavating through a 70m section of tunnel that passed through a transitional zone comprising a loose, water-bearing pumice layer and unconsolidated quaternary deposits, requiring a detailed cover-consolidation grouting program. Following the completion of grouting activities, umbrella supports were installed, and excavation was completed by a combination of roadheader, hydraulic hammer, and drill and blast. This section alone represented nine months of focused, innovative work.

GEOLOGY AND EXCAVATION

Travelling upstream from the downstream portal, the tunnel crosses through metamorphic and igneous basement rocks, through a contact – transitional zone consisting of buried soil and a pumice horizon and finally into the young volcanic breccia rocks (associated with the most recent eruption of the Mount Meager volcanic complex) as the tunnel approaches the upstream portal at the intake. The tunnel was excavated as two separate headings, from the upstream and downstream portals at a consistent grade of 0.4 per cent (Figure 1).

The vertical cover over the tunnel was typically greater than 50m, with a maximum of 220m. The upstream portal had the lowest cover at approximately 30m but increased to more than 50m within 100m of the portal. Solid rock cover was significantly less at the upstream end of the tunnel, particularly through, and immediately after, the transition zone between the recent volcanic rocks and the basement rocks. In this zone, less than 6m of rock cover was observed through probe drilling (although approximately 120m of ground cover was noted above).

The thinning rock cover nearing the intake was inherently risky and would have required significant support for the excavation. The Contractor proposed a 230,000 cubic metre open cut excavation in lieu of tunnelling for the upstream most 65m section, joining the intake to the tunnel portal with backfilled precast concrete arches. This fixed the cost and improved schedule and safety.

The upstream welded breccia was excavated by drill and blast methods, in approximately 6m-long blast rounds. Excavation proceeded at a rate of approximately one-and-a-half to two rounds per day, under ideal conditions. As excavation continued from the upstream portal towards the transition zone, delays were experienced due to challenges caused by the water inflows which, made loading blast holes difficult and required increasingly larger pumping capacity. Prior to the initiation of the cover and consolidation grouting, average production had slowed to one round per day.

Probe and grout hole drilling from the cover grouting program identified the location of the start of the transition zone, and it was known that the unconsolidated materials would first be encountered in the invert, with the contact rising up the face as the heading advanced. As the excavation approached the transition zone, the excavation sequence was adjusted, the rounds were shortened, and the blast patterns modified for the strong welded breccia.

When the transition zone sediments were encountered, the upper portion of the face was still strong welded breccia, while the lower face was grouted sediments. For mixed face conditions, the lower face was excavated using a roadheader attachment mounted on an excavator for a 1m to 1.5m advance, and the remaining strong rock in the upper face was blasted. When only small amounts of strong rock remained in the upper face, the rock was hammered out using a hammer attachment on the excavator. Depending on the ground conditions and the spacing of the support to be installed, 1.0 to 1.5m length rounds were excavated through the transition zone. In some sections with loose sediments, half of the face was excavated by roadheader, shotcrete was applied, and then the remainder was excavated before installing the full support.

The roadheader tool on the excavator proved to be an appropriate and efficient method for excavation of the grout consolidated sediments.

Between the transition zone and the downstream portal, the basement rock was generally more competent and required less engineering support. Drill and blast with typical patterned rock bolts advanced the excavation, averaging just over 6m a day. Occasionally faults and shear zones were encountered requiring additional bolt and shotcrete support.

Breakthrough was achieved from the downstream heading almost two years after the beginning of excavation. Excavation was postponed during the 2014/2015 winter season for five months and again halted during the summer of 2015 for two months due to a large forest fire. Additionally, during this time, the project also had to adhere to strict environmental regulations surrounding hours of work, noise and travel restrictions near mountain goat migration corridors and other sensitive habitats in the spring and fall.

After the completion of excavation, a final lining in the form of poly-fibre reinforced shotcrete was applied in designated zones of the tunnel to meet the project design life. An invert slab was poured for the entire length of the tunnel as it was more expedient than cleaning the invert and applying localised invert treatment where required.

TRANSITION ZONE GROUTING PROGRAM

Prior to construction, the transition zone was expected to have potentially high localized inflows, while for the remainder of the tunnel the water inflows were anticipated to be quite low and not expected to require significant management. However, this was not reality, and during excavation of the welded breccia in the upstream tunnel, high water inflows were encountered. The high groundwater inflows in the breccia and at the rock soil interface effectively brought the tunnel advancement to a stop. Balancing between production and stopping to grout was a challenge. Lost production due to excess water had to be balanced against the cost of starting to grout sooner.

The flows increased significantly as the excavation approached the transition zone. Rough measurements of the total inflows to the upstream tunnel heading before grouting began were between 7,000-8,000L/minute along the 400m excavated length of the tunnel. Meeting compliance for environmental regulations on the project required a robust water management program. This meant pumping to immense settling ponds and detailed treatment prior to release back into the Upper Lillooet River.

The water inflows were observed to originate primarily from discontinuities in the welded breccia rock mass prior to reaching the transition zone. Inflows were monitored at specific locations throughout the tunnel, both near the portal and further into the tunnel. For monitoring points near the portal, (closer to the river), it was observed that the inflows increased with higher river levels, indicating a connection to the Upper Lillooet River. During the summer, monitoring points further into the tunnel were found to have reduced inflows as the local snowpack receded, with some inflows reducing to the point of being unmeasurable; suggesting a correlation with snowmelt and rainfall infiltration rather than river levels. Generally, inflows were high within approximately 150m of the river, and within approximately 100m of the transition zone, but were more consistent and manageable through the middle length of tunnel excavation.

As the excavation proceeded, the water inflows reached the point of becoming unmanageable, and the decision was made to start a cover grouting program. Because cover grouting had not been carried out concurrently with, and in advance of tunnel excavation, it was acknowledged that total tunnel inflows would not necessarily be decreased, but likely be deflected, potentially flowing back into the tunnel behind the advancing face.

In preparation for the excavation through the transition zone underlying the welded breccia, a consolidation grouting program was designed to work in conjunction with the cover grouting program.

The cover and consolidation grouting program was designed with the following considerations: tunnel dimensions, anticipated ground conditions, ground support requirements, and limitations of the available equipment. The grouting hole layout (Figure 2) at each grouting face included:

¦ Seventeen, 42m deep grout holes around the perimeter of the excavation, angled outwards from the tunnel profile to cover the designed installation length of the ground support;

¦ Twelve, 27m deep grout holes down the core of the perimeter holes to consolidate the transition zone soils through which the tunnel would be excavated.

The grouting program was used for three consecutive overlapping campaigns, resulting in an overall envelope length of 90m. The cover grouting in the welded breccia reduced the inflows from greater than 6,000 L/minute as measured in probe holes, to less than 6 L/minute over the 16m between the first and second grout programs. The consolidation grouting was very successful at strengthening the full range of soils in the transition zone so that the tunnel face could stand vertically which allowed full face excavation through what otherwise would have been loose, ravelling soils in places.

INNOVATIONS AND TEAMWORK

With the anticipated challenges of excavating through geologic conditions that were highly varied and difficult to investigate before the start of excavation, the goals of the project team were: to develop designs for tunnel excavation, rock support and groundwater inflow control that could be adapted to the conditions at the tunnel headings; and to foster a relationship of cooperation and collaboration on site within the owner, engineering, construction, environmental monitoring and safety groups. The level of communication and understanding within the team allowed for design adjustments to occur as the conditions were exposed at the tunnel headings during tunnel excavation.

Due to the nature of the unconsolidated zone and the unconformity being largely unknown, innovative solutions to challenges were frequently required. To assist with excavation through the unconsolidated materials and immense water inflows in the transition zone, the cover and consolidation grouting program was designed to be used in conjunction with umbrella support.

Each advance of the tunnel through the unconsolidated zone required intensive grouting initially, followed by the installation of umbrella pre-support (or canopy tube support), then excavation by a combination of roadheader, hydraulic hammer, and drill and blast, and, finally, ground support by lattice girders and shotcrete.

CONCLUSION

The 2,500m-long tunnel was constructed partially through the most recent volcanic deposit in BC, including a zone of unconsolidated soils. An originally unplanned grouting program was designed and implemented to manage unexpected water inflows and consolidate largely unknown materials, which differed greatly from deposits encountered during various initial project investigations. From challenging geology, to some of the strictest environmental requirements faced by a run-of-river project in the province of British Columbia, the owner, engineer and contractor collaborated as a team to come up with solutions and manage each, and sometimes daunting challenge. The tunnel was delivered in time for commercial operations and overall considered a great achievement and success.

LESSONS LEARNED

¦ Designs and construction methods for the tunnel excavation, support and lining must be flexible enough to cover the full range of tunnel conditions that are expected and sometimes unexpected;

¦ The owner must have an engineering team on-site that has the experience and the authority to select and adapt the designs for the groundwater control and ground support to suit the actual conditions that are encountered at the tunnel headings;

¦ Close cooperation is essential among the owner, the engineer and the contractor throughout tunnel excavation in order to avoid costly delays;

¦ Recent advances in grouting technology and state-of-the-art grouting methods were proven successful in controlling high initial groundwater inflows. It was critical that the grouting was directed by an experienced grouting team on site;

¦ The team should be prepared to use all types of ground support, even if they are not commonly used in the region. In the case of Upper Lillooet, the use of umbrella pre-support proved to be efficient and effective, even though it is not commonly used in North America.

Innergex Renewable Energy Inc., owner of the project, enlisted tunnel designer, Golder Associates to be responsible for the design and construction engineering of the tunnel in collaboration with in-house personnel.

The cooperation between the tunnel designer, contractor (CRT-EBC Inc.) and owner allowed for adjustments to the design as the excavation progressed, and the encountered conditions became better understood.