DAMS: Great Falls Sluiceway

The 12-MW Great Falls hydroelectric plant has provided hydroelectricity to a corrugated paper mill in Bathurst, New Brunswick since 1921. Smurfit-Stone Container (Canada), which owns both the mill and the hydro plant, found deficiencies in the dam’s stability and spilling capacity measured against modern engineering criteria and embarked on a comprehensive rehabilitation program.

The plant is located on the Nepisiquit River, approximately 35 kilometres from the town of Bathurst in northeast New Brunswick. The dam consisted of a stoplog sluiceway spanning the south half of the river and a curved overflow spillway, with flashboards, along the south side of the forebay.

Both the sluiceway and the overflow spillway discharge into a narrow gorge of the river. The original sluiceway had nine stoplog bays with wooden stoplogs, ranging in width from 3.6 metres to 4.4 metres. In the winter, it was necessary to de-ice the stoplogs using steam from a coal-fired steam plant built for that purpose on the south abutment. The overflow spillway, a 91-metre long, 6-metre high gravity structure, is equipped with 1.2-meter-high flashboards which are used to raise the reservoir level between May and November. In the winter, the reservoir level is lowered to provide the freeboard needed to accommodate floods caused by ice jam breakups.

Rehabilitation alternatives

The consulting firm MAJM Corporation of Toronto was contracted by Smurfit-Stone to supervise the investigations into rehabilitation requirements and to evaluate alternatives. MAJM identified two major areas to be addressed. First, the lower section of the sluiceway structure needed to be strengthened as it did not have acceptable structural and stability reserves to meet current criteria. Second, the dam facilities had to be upgraded to accommodate the inflow design flood of 1,750 cubic meters per second determined by earlier flood studies.

After evaluating several rehabilitation alternatives, the field was narrowed to two. The first consisted of replacing the stoplog bays with two radial gates and one submerged gate.

The second sluiceway alternative was to replace the bays with a rubber dam and one submerged gate. The 5-metre diameter rubber dam would be one of the largest of its kind. Rubber dams up to 6 metres in diameter have been installed at other facilities, although not in harsh northern environments such as northern New Brunswick. Both alternatives included a second phase to install a new 1.2 metre diameter rubber dam on the existing overflow spillway to replace the current flashboards.

Consulting firms RSW of Montreal and ADI of Fredericton were commissioned to finalize the rehabilitation concept, carry out detailed design engineering and supervise construction. The retrofit included objectives to:

achieve 50 years of service life without major repairs

conform to current criteria for stability and spillway capacity

increase hydroelectric generation, if possible

minimize ice problems

minimize environmental impact during construction

be cost effective in construction and operation

maintain access to the south abutment over the sluiceway, and

prevent powerhouse flooding.

The alternative of installing a new rubber dam and submerged gate on the sluiceway proved to be the best way to meet these objectives. The rubber dam has the ability to be deflated, even in the event of power failures. This would allow the dam to pass ice, especially during floods caused by ice breakups. The cost of the rubber dam (10 to 15 per cent less than the next best alternative) was also an important factor.

Most rubber dam operators accept the risk of losing reservoir storage due to acts of vandalism. However, the risk of vandalism is not as high as it might seem, as the dam material is highly resilient and can be quickly repaired without special tools or training. Furthermore, the Great Falls complex is a run-of-river facility and the rubber dam retains only a small storage volume. The water volume could be replaced relatively quickly if the dam were to deflate.

The final rehabilitation included the removal of eight existing sluiceway piers, foundation and preparation/ grouting, and installation of a new concrete ogee and abutments. The new sluiceway rubber dam is 36 metres long with airlock access. The dam’s rubber sheeting is 28 millimetres thick and it has a pear-shaped profile in section, with the larger downstream portion round to hold back 5 metres of water. A new control building was constructed to house blowers for inflating the dam. A new bridge deck was added, and a submerged steel gate was installed in the sluiceway to allow draining of the reservoir below the ogee crest.

Construction and installation

Construction lasted approximately 150 days from July 1999 to March 2000. The contractor, Groupe CRT of Lvis, Quebec, was responsible for design and construction of the dewatering and cofferdam installations. To construct the cofferdam, the contractor hauled fill from a nearby quarry and placed it in a horseshoe pattern on the riverbed from the south embankment to the north abutment of the sluiceway. For the upstream seal, the contractor chose to overlap a geomembrane along the upstream face of the cofferdam. When construction was completed, the owner and consultants chose the least environmentally intrusive option: leaving the cofferdam directly on the riverbed with a significant layer of rockfill in place.

The original spillway piers were demolished within a few weeks of dewatering, thanks to round-the-clock efforts by the contractor, and owing to the poor condition of the concrete which made demolition easier than expected. Approximately 5,500 cubic meters of new spillway concrete were poured. The general contractor installed the rubber dam with the manufacturer, Bridgestone, supervising the installation.

The rubber dam arrived on site, wound around a large spool to facilitate installation. Two large cranes were needed to move the spool from the flatbed trailer and later to install the dam. During installation one crane supported the spool shaft, attached to a spreader beam, and the second crane was used to handle the unfurled length of the dam.

A steel frame supporting rubber dam anchors was embedded in the last concrete pour on the ogee crest. Installation included precisely cutting holes at 200 mm intervals in the rubber sheet to match the pattern of the embedded anchor bolts. Installation was relatively straightforward for the horizontal anchor bolts but proved to be more of a challenge on the sloping surfaces. About two weeks after installation was complete, the air blower system serving the dam went into operation.

The sluiceway design provided an airlock to the core of the rubber dam to give access for inspection and repairs. The custom-designed airlock doors can easily be dismantled and removed for repairs if required. Two 4-HP blowers located in a control building adjacent to the dam maintain the design air pressure of 60 kiloPascals.

Automatic control equipment includes a programmable controller in the powerhouse, which receives continuous monitoring information on water levels, dam inflation, and major equipment. A second controller located at the mill in Bathurst has remote control capabilities through a radio modem communication link.

Completed within budget, the project has achieved the objectives of improving the structure’s stability and spilling capacity, and reducing demand for on-site operation. There was no significant loss of generation and construction was within the time initially allotted.

One important contributor to the success of the project was the extensive data provided by the owner, which allowed the consultants to make informed choices early in the design process. Another key element, particularly important when the contractor and the major supplier are not the same, was close monitoring of the construction and installation work. Although one cannot expect major rehabilitation of an 80-year-old structure in a difficult work environment to be uncomplicated, strategies such
as these can help to control costs and ensure the timely completion of the project.

When phase 2 is completed and the 1.2 metre diameter rubber dam has replaced flashboards on the curved overflow spillway, the full extent of the benefits of raising the forebay during the winter in terms of generating power will be known.

Alvin Sonier is with Smurfit-Stone Container (Canada) of Bathurst, N.B., Michael A. Scribner, P.Eng. is with ADI of Fredericton, N.B. and Claude M. Chartrand (ing.) is with RSW of Montreal. All were managers on the project.

Owner: Smurfit-Stone Container (Canada)

Prime consultants: RSW, Montreal (engineering); ADI (construction management), Fredericton, N.B.

Other consultants: MAJM, Toronto (investigations)

General contractor: Groupe CRT, Lvis, Que.