The John Hart Hydroelectric project is located on the Campbell River and close to the town of Campbell River on Vancouver Island, British Columbia.

The project team of SNC Lavalin, Aecon, and Frontier- Kemper Constructors have been selected to carry out this design-build project. The underground powerhouse is a concept that provides many benefits to the area, in particular the small project footprint, as the project is situated adjacent to a Provincial Park.

Combined with the efficiencies of modern drill and blast technology, the underground powerhouse will make the John Hart Generating Station Replacement Project (JHGSRP) a safe and cost effective alternative to traditional surface penstock and powerhouse options. The underground powerhouse is located where the rock cover is sufficient to prevent hydrojacking, thus obviating the need for approximately 700m of steel lined tunnel.

The underground excavation and support portion of the contract is scheduled for a duration of approximately three years. The entire generating station including power facilities, turbine installation and commissioning of the project is scheduled to be completed in approximately two additional years, for total project duration of five years.

The only lithological type encountered in the John Hart area is basalt (a mafic volcanic rock) from the upper sequences of the Karmutsen Formation, which is late Triassic in geological age (about 200M years old). Most of the basalts in the area have undergone low grade regional metamorphism.

A variety of original scale geological faults have been mapped in the region, but none appear to cross the study area. The closest previously mapped original scale geological fault occurs 1km to the south of the study area, it strikes west-northwest– east-southeast.

In spite of the observation that the closest "previously mapped geological fault occurs 1km to the south of the study area", there are fault zones which include zones of damaged rock, which are weaker than the rock mass outside of the fault zone and may contain polished or slickensided discontinuities and microfractures.

The bedrock along the tunnel alignment is expected to consist of basalt that has been subjected to low grade metamorphism, but is generally fresh.

Minor alteration (e.g., carbonate, epidote, and chlorite) is common within and adjacent to faults zones, but does not extend far from the fault boundaries. The altered rock, where present, generally exhibits lower strength and reduced rock mass quality values.

A cap of slightly weathered rock typically 0m to 3m thick was noted at the surface. Where present, this rock exhibited slightly lower strength.

Cognizance has been taken of the possibility that fault(s) may intersect the powerhouse and additional support allowed for this contingency.

In summary, the rock mass is relatively good quality, strong (UCS ave 122 MPa; Intact Modulus ave 64GPa) basalt with two sub-vertical joint sets, and the sub- flow contacts – described where encountered in borehole core as "welded".

With regard to the faulting: five "major" faults and several "minor" faults are anticipated.

The existing project consists of an intake structure in the John Hart Reservoir; surface penstocks (woodstave for the low pressure section and steel for the high pressure section) to a surface powerhouse with six units. Description of replacement project

The JHGSRP consists of the following:

  • A new intake through the existing concrete dam;
  • A 6.5m diameter intake shaft;
  • A power tunnel (8.1m by 8.1m, "D" shape);
  • An upstream surge shaft: 4m diameter; 109m deep;
  • An underground powerhouse 23m x 39m x 93m;
  • A gate house structure;
  • A downstream surge system;
  • A tailrace tunnel;
  • A tailrace outlet structure;
  • Two access tunnels; and
  • Various construction adits, from 6m x 6m to 9m x 6m.

Design Aspects
The design life of the power tunnel and powerhouse is 100 years, so all rockbolts are corrosion protected.

Twenty-one exploratory boreholes were drilled during the investigation of the proposed JHGSRP. All of these boreholes were logged and some were surveyed with optical and acoustic televiewers. In addition to the boreholes, surface mapping of the exposures of rock along the gorge between the dam and the existing powerhouse was carried out.

A further three boreholes are planned, specifically to investigate the major fault along the tunnel alignment and the powerhouse. The boreholes planned will all be logged and surveyed with an acoustic televiewer. The holes used for the investigation of the powerhouse will also be subjected to hydrojacking tests. The borehole logs and the surface mapping were used to assess the rockmass characteristics along the tunnel alignment and assign proportions for the various rock mass classes (Class I, Class II, Class III and Class IV) to the power tunnel. One borehole was drilled in the location of the powerhouse and this borehole was used in the preliminary design of the underground powerhouse.

As indicated, two more boreholes will be drilled and this information will be used to confirm or refine the design of the powerhouse support.

General design process
The design process for all the underground structures uses the following general procedure:

  • Break the tunnels into sections with a range of rock mass characteristics, as defined by the Q1, RMR2 and/or GSI3 rock mass rating systems.
  • Use the empirical Barton – Grimstad method (Grimstad et al. 1993) moderated with experience in other similar projects to get a preliminary support system. The support system would be for a given range of rock mass characteristics. For instance: for the power tunnel there would be four "Support Classes" – ranging from spot bolts only to pattern bolts and shotcrete to lattice girders and thick shotcrete support. See Figure 1.
  • Consider the joint sets and make sure any wedges/blocks are adequately supported by the support system.
  • Use finite difference (FLAC) and/or finite element (Midas GTS) methods to ensure that the rock mass and elements of support, the shotcrete, rockbolts and lattice girders are not overstressed.

Design of the powerhouse
A 3D rendering of the underground powerhouse is shown in Figure 2. The power tunnel can be seen approaching from the right; the rock trap can be seen at the extreme right, the manifold with three bypass valves and three intakes, the gate chamber and the tailrace tunnel. Sections of the two access tunnels can also be seen.

The location of the powerhouse was chosen so that the rock cover is sufficient to prevent hydrojacking of existing joints. Two criteria were used to evaluate this. The first was the Norwegian criterion: the factor of safety used was effectively 1.6. The second criterion will be used to confirm the location, which is a hydrojacking test carried out in a borehole. The factor of safety against hydrojacking for this test is 1.3.

Since only one borehole was available at the preliminary design stage, the borehole data was evaluated and a design value chosen for the tunnel and the same general method of design was used as described for the tunnels.

However, some conservative assumptions were made to allow for the following eventualities:

  • The rockmass is poorer overall. In this case, heavier support is required – a thicker layer of shotcrete and longer, more heavily loaded rockbolts; and
  • A fault runs directly through the powerhouse parallel to the main axis. For this case some additional support was required, particularly in the sidewalls and locally adjacent to the fault.

The FLAC analysis showing the loads in the rockbolts for the design case is shown in Figure 3. Where appropriate, FLAC 2D was used for the stress and deformation analyses of the tunnels and powerhouse. Where 3D analyses were required, for instance for the analysis of seepage between the power tunnel and the powerhouse and the analysis of the large span of the main access tunnel at the intersection with the gate chamber, Midas GTS was used.

Assumptions have been made regarding the hydraulic head losses, based on the roughness of the power tunnel. As head loss is a critical aspect of the project, measurements will be made, using precise survey techniques, of the crown of the powerhouse as the pilot drift is being driven to determine the overbreak characteristics.

These measurements will then be extrapolated to the power tunnel to calculate the head losses in the tunnel. If the head losses are too high, the dimensions of the tunnel can be adjusted to reduce these head losses to acceptable values. Method of construction – tunnels

Tunneling will start with a single crew working on the L40 heading, which will function as the service access for the powerhouse, and continue into the powerhouse crown. Shortly after the L40 heading is started, the L20 portal, serving as the main underground access, will be prepared and ready for tunneling to commence with a second crew. Three crews working concurrently on individual headings will be used during the peak mining phase.

The face drilling for tunnels and powerhouse crown level will be carried out with a two boom drill jumbo. A fleet of Atlas Copco Boomer E2C jumbos were selected and will come equipped with guidance software, COP 1838 drills, and 7.1m telescopic feeds. The drill jumbos have a 115kW Tier 4i diesel engine carrier, as well as two 75kW electric motors for drilling.

Tunnel Manager is the family of support software for planning, administration and evaluation of the drilling operation in mining and tunneling projects. It will be used in conjunction with Total Station navigation for guidance of the jumbos. The most noted features are designing tunnel alignment, face mapping, drill plan design, as well as evaluation of drilling results from logged data. This can be used for interpreting and documentation of ground conditions.

To ensure costs were minimised while complying with the strict environmental constraints of the project, bulk emulsion was selected as the primary blasting agent. While most areas will utilise traditional non-electric LP detonators, the powerhouse crown excavation and power tunnel trim holes will utilise electronic detonators for precision blasting.

A Normet Himec/Charmec series Explosive Charging Vehicle and Personnel Lifter will be utilised for underground transport and loading of the explosives. These units will be equipped with a bulk emulsion pumping system for easy transfer. Due to the sensitive nature of the surrounding existing structures that are still in operation, there are strict requirements on blasting controls. Electronic detonators will further assist the team in minimising charge per delay, as peak particle velocities will be closely monitored.

Mucking will be performed using a combination of an LHD (scoop) and mine truck. The Atlas Copco ST14 scoops were selected with a heaped capacity of 7m3, which will carry approximately 14 tons of material. A Cummins 250 kW Tier 3 diesel engine powers the scoops.

For mine haulage, Atlas Copco MT42 trucks were selected. These trucks have a heaped capacity of 21m3, which will carry approximately 42 tons of material. Cummins 388 kW Tier 3 diesel engine powers the trucks.

For the first few hundred meters of tunnel, solely the scoops will be used for mucking and hauling. Once the tunnels are advanced far enough, muck bays will be developed every 250m of tunnel. An example of a muck bay is shown in Figure 4. This will allow for quick mucking away from the face by moving the muck a limited distance. This clears the face quickly for ground support activities to occur simultaneously with the rest of the muck cycle. The scoop loads up to three heaped bucket loads into a mine truck at the muck bay for delivery to the surface.

Ground support
The ground support regime consists of shotcrete, rock bolts, and lattice girders depending on ground type classification. The contractor plans to use an in-cycle shotcrete approach, applying shotcrete as the primary ground support immediately following mucking and hydro-scaling. Shotcrete mix design will use admixtures that achieve early strengths approaching 1 MPa in the first hour, while maintaining long term strength requirements per contract. Bolting can be performed with the jumbo, providing primary ground support for miners to access the face safely. A double corrosion protection type bolt will be used, in which an expansion anchor can be set during initial installation. This provides increased robustness to the ground support regime, allowing post-grouting to occur later in the cycle simultaneously with other activities.

Method of construction – shafts
The intake shaft will be sunk conventionally from the surface using drill and blast methods. The surge shaft, which replaces the existing surge tower structures, will be developed using a raise bore machine located on the surface.

The 6.5m diameter, 60m deep intake shaft is situated in close proximity to the John Hart Reservoir and the existing dam and spillway structures. For this reason, the blast design, including the peak particle velocities will be closely controlled and monitored.

An air-powered tophammer jumbo will be used for all blasthole drilling. Explosives will be manually loaded. Once the round is detonated, an excavator or overshot mucker will be used to transfer the muck into a muck bucket. A crane will hoist the muck bucket to the surface, where it will be offloaded and then transferred to a haul truck and removed from site.

Once mucking is complete for that round, the ground support regime will commence. This consists of rock bolting with the possible addition of 50mm thick shotcrete as required for the ground type classification. The intake shaft will be completed before the power tunnel excavation reaches it.

The surge shaft will be excavated by a raise boring machine located on the surface. To begin this, the power tunnel excavation must reach the location of the surge shaft. The 350mm pilot hole will be drilled by the raise drill from the surface, connecting down to a stub tunnel off the power tunnel. When the pilot hole breaks through underground, the pilot drill bit will be removed with a specialized hydraulic wrench assembly. The 4.3m diameter cutterhead is brought in through the power tunnel, assembled in place, and connected to the drill string. The cutterhead will up-ream the shaft, letting the muck fall to the bottom. The muck will be removed with the scoop.

Once the shaft reaming is complete, the raise bore machine and cutterhead will be removed from the shaft collar. The ground support regime will commence, applied from a crane hoisted work platform. A combination of 75mm thick shotcrete, as well as patterned rock bolts will be installed as required for the ground type classification.

The powerhouse
The powerhouse will be constructed by driving a pilot tunnel along the alignment of the top of the crown of the powerhouse. When the L40 tunnel is driven down to the powerhouse, it will continue through the powerhouse as the pilot tunnel. This will be a 6m x 6m cross section tunnel. The remainder of the crown of the powerhouse will be excavated in the next pass of drilling and blasting.

Once the powerhouse crown is complete, the prime civil contractor will install the crane rails and the remainder of the powerhouse excavation can continue. It will be taken in bench blasts, approximately 6m – 7m deep per round. A muck pass will be excavated from the bottom of the powerhouse where the L20 intersects, up to the powerhouse crown section. The raise climbing method will be employed. Muck haulage will be primarily through the L20 tunnel during this phase of construction.

A construction adit will be driven towards the tailrace tunnel’s intersection with the Surge Chamber. From here, slot raises will be installed from the Gate Chamber down to the Surge Chamber level. The draft tubes and bypass tubes connect from here into the powerhouse, and will be driven in that direction.

Concluding remarks
At the time of writing of this paper, the underground works are undergoing final design. Tunnelling is scheduled to commence in November