Unique design approach and innovative use of materials address structural design challenges for the Residuals Treatment Facility at the Bearspaw Water Treatment Plant

Bearspaw Water Treatment Plant - Residuals Treatment Facility

The City of Calgary water system provides safe, reliable drinking water to over 260,000 residential customers, 20,000 industrial, commercial, and institutional customers, as well as customers in the surrounding communities of Airdrie and Chestermere.

Calgary’s population has increased steadily for the last 25 years. The projected population for Calgary in 2033 is 1.5 million people. As the City continues to grow, so does the demand for water.

In 2003, the City of Calgary engaged Associated Engineering to design upgrades of its Bearspaw and Glenmore Water Treatment Plants that would not only meet growing demands, but also be innovative and sustainable.

The Associated Engineering team met this challenge with the design of the new Residuals Treatment Facility at the Bearspaw Water Treatment Plant. The new 54 million litres per day facility treats wastewater generated by the potable water treatment process, and recycles treated water back into the main plant process. Operating since late 2006 this facility allows the City to successfully eliminate discharges previously sent to the Bow River and save approximately 6% of water taken from the river, thus preserving this pristine water resource and protecting the environment.

Facility Layout
The new Residuals Treatment Facility integrates several treatment processes, which are hydraulically connected with pipes, conduits, and channels. These processes also feature various size tanks and other hydraulic structures, centrifuge and conveying equipment, chemical storage tanks, electrical and mechanical equipment, and areas for operation and maintenance. These intricate process requirements resulted in a complex facility layout with structures of varied footprint, height, and vertical alignment.

Corner benching within thickener tanks were designed with cellular concrete fill overlaid with floor topping and shotcrete

Design and Construction
To determine the most appropriate materials for this project, the Associated Engineering team considered several important factors including process and functional requirements, architectural impact, geotechnical conditions, structural requirements, 100-year design life, low maintenance, cost, and constructability.

Concrete was selected as a preferred construction material because of its suitability for construction of water retaining and conveying structures, and its ability to meet unique geometric requirements. In addition, concrete was used for architectural finishes and for building structures where adequate support and rigidity was necessary for proper equipment operation, vibration control, and heavy vehicle access.

Due to varied loading requirements and vertical alignment of structures, the Residuals Treatment Facility was founded on two types of piles. At grade or shallow-buried structures were founded on ‘Compacto’ piles using ‘zero-slump’ concrete and an expanded base terminated within the gravel layer. The Thickener Area structure, with the base slab some 8 metres below grade and relatively close to native bedrock, was founded on 900 millimetre diameter cast-inplace, rock-socketted piles.

Benching and floor slopes, required within the thickener tanks, were designed with cellular concrete fill having a density of only 475 kilograms per cubic metre to reduce weight and allow for easier removal in the future. Cellular concrete fill was overlaid with floor topping and shotcrete at the corner benching to provide a durable, watertight finish.

A maximum crack width of 0.2 millimetres was used as the design criteria for water retaining structures, taking into account early age thermal and restraint factors, as well as long-term temperature differentials. Attention to reinforcement detailing using smaller diameter, closely spaced bars ensured that cracks were evenly distributed and tight. As a result, very few cracks developed, and their limited width promoted self-healing and ensured water tightness.

Construction of thickener tanks

Concrete was a key material that enabled the design and construction of structures required by the water treatment process. Specially formulated concrete mixes, using minimum 20% and up to 40% fly ash, reduced permeability and cracking potential, improved concrete workability and provided durable, watertight structures. Cement replacement with fly ash also affirmed the City’s commitment to sustainable design and LEED™ principles.

The Bearspaw Residuals Treatment Facility represents leading-edge technology in water plant residuals handling and enables the City of Calgary to improve the security and sustainability of their future water supply.

Lead Structural Engineer, Risto Protic advises, “A unique approach to the design and detailing of concrete water retaining structures, use of specially formulated concrete products and their innovative application, combined with quality construction contributed to the performance of structures and success of this project.”