The bruce nuclear complex is located on the east shore of Lake Huron, 300km northwest of Toronto. A concrete-lined tunnel excavated beneath the lake in 1979 forms part of the station’s cooling water system, which includes the intake channel, forebay and pump-house. The tunnel is 900m long, with an excavated diameter of 9.2m, and was excavated using drill and blast method. The tunnel was lined with 250mm non-reinforced concrete. A required 193 m3/ second cooling water flow is conveyed into the station via the intake system and then returned to the lake through an open discharge channel. As part of the cooling water system, an offshore shaft, 30m deep, was constructed, circular in shape with a diameter of 9.5m. A rock plug, 14m thick, at the top of the intake shaft, was left in place until the tunnel lining was completed. After tunnel flooding, the upper 10m of the plug was blasted and removed by a marine operation. The lower 4m of the plug was blasted and fell into the sump below tunnel invert level.
DESIGN
The primary design consideration for the tunnel was that the tunnel should be aligned to minimize its length, while also satisfying hydraulic and geological conditions. Furthermore, for the given alignment and length, the tunnel diameter was to be based on the minimum sum of construction cost and capitalized cost of pumping over the life of the plant. The selection of the length and horizon of the tunnel was based on two primary criteria:
a. To locate the intake shaft at a sufficient lake depth (minimum 15m) to ensure an adequate supply of cold water of reasonably constant temperature to the generating station and to prevent ice clogging of the intake structure as well as to minimize the entrance of fish; b. To maintain an adequate distance between the intake and the discharge channel so that recirculation of heated water is avoided.
Provision for seepage control
The geological investigations indicated that water control would be a major part of the construction work. To control seepage during excavation, a provision was made in the technical specifications for probe hole drilling and consolidation grouting. An 11m3/minute inflow provision was included in the unit price of the excavation. Seepage in excess of this was to be treated as an additional pay item. It was estimated that one-third of the excavation time would be spent on grouting. In order to meet the specification criteria, the contractor proceeded to perform a comprehensive program of probe drilling and grouting. Construction experience demonstrated that the provisions stipulated in the contract documents proved valid considering that probe drilling and grouting implemented during construction were satisfactory in controlling the water flows. A typical mining cycle included drilling and pre-grouting to a distance of 30m beyond the tunnel face and then drilling and blasting to a distance of 24m. This provided 6m of grouted rock beyond the tunnel face. A total of 49 grout cycles were required to grout the entire tunnel length. As per Figure 1, typical groutcycle consisted of 15 to 46 grout holes, depending on local conditions. The entire operation resulted in a total of 40,000m of probe and grout hole drilling and injection of 5,300m3 of cement grout. Water seepage was minimized by grouting, while excess water was pumped from the tunnel via a weir box to the ground surface. Progressive estimates of the potential water inflow indicated a cumulative total of 112 m3/minute for the full tunnel length; pressure grouting reduced this by about 93 per cent.
Unusual conditions
The first, unusual ground conditions developed when tunnelling encountered an open, high pressure, water-bearing feature at the bottom of the decline ramp. The flows out of some of the probe holes were estimated to be up to 3.5m3/minute. Attempts to grout this zone after injecting about 140m3 of cement and sand proved unsuccessful. After a review and evaluation of the exploratory findings, grouting attempts, and assessment of the hydraulic requirements, it was decided that it would be more economical and safer to divert the tunnel around this zone rather than mine through this feature. The second unusual condition occurred in the mid-portion of the tunnel when an inflow 22m3/minute developed. The inflow caused the erosion of 30m3 of brecciated material into the tunnel. Due to the sudden rush of water, the tunnel was temporarily abandoned. Remedial measures to control the flow consisted of constructing a 4.5m-thick concrete bulkhead, placed directly against the tunnel face. A grout cut-off wall extending a full 360 degrees around the tunnel and a grout cone was made to consolidate the surrounding rock. Final grouting of the major flow was done through a 300mm pipe installed through the concrete, directly into the flow zone. A total of 240m3 of grout was required to seal this open zone.
Lake Ontario
The Darlington nuclear station is located on the north shore of Lake Ontario approximately 60km east of Toronto. A required 153m3/s water flow is conveyed into the station via a 1,000m-long, 9.5m-diameter intake tunnel and then discharged into the lake through a tunnel and a series of vertical diffusers extending approximately 1,800m offshore. The discharge tunnel consists of two main components; the discharge portion and the diffuser portions, each approximately 900m long. The main section of the discharge tunnel has a semi-flat roof with a finished span of 9m and a height of 7.2m. This shape was considered to be most suitable for excavation in the horizontally-bedded sediments. The first and mid portion of the diffuser tunnel have the same span as the discharge tunnel. The height of the mid portion, however, decreases to 5.7m. The outer portion is 5.2m wide and 4.7m high. The three sections of the diffuser tunnel are connected by two, 12m-long transition zones. As per Figure 2, a total of 90, 1,220mm-diameter vertical shafts were drilled to penetrate the tunnel roof in a staggered fashion on a 10m pattern. With the diffuser system, the warmer discharge water is immediately mixed with colder lake water, which limits the temperature difference between the mixed water and surrounding lake water to less than 2°C at the lake surface. The shafts were steel lined with a 9mm-thick casing with an inside diameter of 1,000mm. Drilling of the diffuser shafts was done from a barge platform raised on spuds, about 2.5m above lake surface. Both tunnels were excavated in the early 1980s, using drill and blast method.
Special provisions
Both tunnels were positioned within massive, thickly-bedded limestone formation. A comprehensive program to measure initial stress in the rock was carried out at the site. In-situ stress measurements indicated the presence of high horizontal stresses up to 12 MPa. The laboratory test results, conducted on rock samples, indicated that the compressive strength varied from 62 to 92 MPa, while the elastic modulus ranged from 39 to 61 GPa. Rock-structure-time interaction analyses were performed to ascertain the time delay required between excavation and installation of concrete lining. The analyses indicated that the 90-day delay between excavation and lining would be adequate. The installation of the concrete liner went as planned.
Lessons Learned
While tunnelling under Lake Huron, an important consideration was the control of water seepage into the excavations. Prior to construction, it was estimated that approximately one-third of all mining time would be related to seepage control. Upon completion of the project, it appeared that this initial assessment was valid. The cement grouting programs were successful in reducing the initial water inflows into the excavations by over 90 per cent.
Although the exploratory drilling provided a reasonably good picture of the ground conditions; a large-scale pumping test would have provided valuable information on overall hydraulics of the water-aquifers. The owner contemplated such tests but never executed them.
In contrast to the Huron Lake experience, completely different tunnelling conditions were encountered under Lake Ontario. The ground conditions were more predictable; the tunnel exhibited no water inflows during construction. The rock formations were tight as a result of the in-situ stress condition. A monitoring program conducted during construction confirmed that the recommended three-month waiting period between excavation and concrete lining was adequate.
Experience from the intake tunnel (excavated first) provided high confidence in regards to ground conditions. As a result, the owner decided not to invest money on performing exploratory drilling for the discharge tunnel (second excavated). This decision proved to be correct.
