Award of Excellence: Little Mountain Reservoir

The Little Mountain Reservoir, in Vancouver’s Queen Elizabeth Park, east of downtown, is a crucial link in the Greater Vancouver Water District (GVWD) supply system, which serves two million people. One of the biggest reservoirs in Canada, Little Mountain is also the largest local source for emergency post-earthquake drinking water and firefighting needs, capable of storing some 175 million litres.

The original reservoir built in 1911 was an open basin contained by an earth embankment. In 1963, a precast concrete roof was added which supported parking, a plaza, and hundreds of daily users, but within years the roof exhibited deterioriation. After studies showed that the embankment might fail in even a moderate earthquake, it was decided that the entire reservoir needed to be replaced.

Sandwell as the prime consultant, with main sub-consultant Associated Engineering, led a team of 12 other specialist sub-consultants to carry out the pre-design, detailed design and construction management. Partnering between the owner, consultants and contractor helped the project to be completed on schedule and within budget.

The reservoir had to be designed with: a 100 year design life; resistance to the maximum credible earthquake; 25% increased emergency storage volume; roof capability to support large buses and landscaping without affecting water quality; two separate cells able to operate independently; and optimized water quality. Half of the reservoir’s capacity had to be useable within nine months. Special post-9/11 security features were included and sustainability principles were to be followed throughout the construction.

The new concrete “box”

Following workshops, public meetings and open houses that gathered important feedback used in the design, a radical but simple concept was developed. The structure is a monolithic concrete “box,” 180m x 120m x 10m in size, with no expansion joints and incorporating a central dividing wall to create two cells.

Seismically, the solution provides a robust structure, however, one that could be prone to cracking due to the size of the box. Innovative measures were therefore developed to make it “crack free.”

The box was constructed within the perimeter of the existing reservoir, but with excavations into the embankment for new vertical side walls. Also the roof level was raised. The reservoir no longer relies on the embankment to contain water during an earthquake and there is no more potential for a catastrophic release of water.

A joint-free structure

To design what may be the largest joint-free concrete structure and the largest water storage reservoir in B.C., two features were necessary. First, the roof needed insulation to eliminate potentially damaging thermal effects once the structure was complete; the insulation also needed to sustain loading from buses in the future.

Second, the construction sequence required the structure to be “completed” within a strict temperature tolerance to ensure there were no unacceptable built-in thermal strains. This “completion” was successfully achieved by infilling a 1-m x 400-m perimeter gap to structurally tie the roof to the walls, while the temperature of the concrete was continually controlled using cooling water and over 20,000 m2 of temporary insulating tarps.

Many other technical innovations were used. For example, this is the first use of an earthquake detection sensing panel in a water storage facility in B.C. that can automatically close valves on all main pipes in an earthquake. This was also the first use in B.C. of “multiple orifice pressure-reducing valves” in a municipal application to provide operational reliability at significant cost savings. Submerged marine access doors, another first for a water reservoir, connect the valve chamber to the cell interiors, giving easy access for cleaning and maintenance.

The rebar design, plus specified construction sequencing and curing, was critical to meet the “crack free” criteria that all cracks be less than 0.2 mm wide (to enable self-healing). State-of-the-art fluid computer simulations confirmed the locations and sizes of inlets and outlets needed to produce the required water mixing and chlorine residuals. A unique double membrane roofing system met the GVWD goal to protect the stored water from future rooftop usages.

The main scheduling challenge was to complete 80% of the construction contract in a nine month period on a site with very limited access, set in the middle of Vancouver’s second most popular park.

The contractors developed innovative procedures to meet the scheduling challenge such as using custom-made rolling roof forms and specialty steel reusable wall forms. Unforeseen difficult ground conditions encountered during the critical nine-month phase increased the scope by nearly 10%. Thanks to rapid redesigns and inspections and the contractor’s commitment to the $37-million project, Cell 1 was in use by May 2003 as planned, in time for that year’s hot summer. Substantial completion of Cell 2 was in November 2003 as scheduled.

Ecosmart concrete

All concrete debris from the demolition was crushed, and all reinforcing steel was separated, with both materials transported to recycling plants. Ecosmart concrete that uses flyash, a by-product of coal-fired plants, in place of cement was used for all 27,500 cubic metres placed. The project represents the highest average flyash replacement of cement (some 44% overall) to date in B.C. The result was concrete of excellent quality and a reduction of approximately 4,100 tonnes of C02 emissions. The amount of cracks exceeding the “crack free” criteria was 1/10 of that allowed for, contributing to cost and schedule savings.

Spill prevention, and sediment control plans were closely monitored during construction. Construction noise was controlled with noise barriers, and landscaping was used to hide the reservoir from public view. Three times as many trees were planted as were removed, and heritage stone was used on the face of the valve chamber.

The project won a 2003 APEGBC Sustainability Award and 2004 Consulting Engineers of B.C. Award. Its innovations features are now being applied to another GVWD project.

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Name of project: Little Mountain Reservoir Reconstruction, Vancouver

Award-winning firms: Sandwell Engineering with prime subconsultant Associated Engineering (BC), Vancouver (John Sherstobitoff, P.Eng., Minoo Colah, P.Eng., Frank Szeto, P.Eng., Dave Winter, P.Eng., Sean Bolongaro, P.Eng., Dale Harrison, P.Eng.)

Owner: Greater Vancouver Water District

Other key players: Golder Associates (geotechnical); Levelton (concrete and quality assurance); Enkon Environmental (environmental); Arbortech Consulting (arborist); BKL Consultants (noise); Colborne Architectural Group (architecture); Graham Industrial (contractor)

Suppliers: GSE Lining (roof membrane); Soprema (roof membrane)