A recent Noticed in Nova Scotia post touched on the aesthetics of the Robie Street Reservoir. Stephen Archibald mentions that the reservoir was built in 1913, and it has received new roofs twice – once in 1946, and again in 1999. Curiously, this simple tank has been quite the source of innovation.

Halifax water works on a high pressure and a low pressure system. Water is gravity fed in 24″ and 27″ mains for the low pressure system, and in 15″ mains for the high. In the Map above, the Yellow line divides the high and Low Service Areas.

The Robie Street High Service Reservoir was built in 1913 to provide supply and additional pressure to the high service. The site selected was known as Hungry Hill, and was the highest locally available spot.  The reservoir has an interior diameter of 160′, and is 25′ deep. The walls are 3′ thick at the bottom, tapering to 18″ at the top. The roof was almost flat, and was a 4″ slab supported by radial concrete girders and 53 18″x18″ posts.

By the mid 1940’s the Halifax water system had just about lived its normal life and was due for replacement. The population had increased from 70,000 to 130,000, and there were again issues with water pressure for firefighting.  An engineering report of the system was produced, and the Halifax Water commission was formed and set out fixing the system. The Robie Street reservoir included.

The reservoir was in bad shape. By 1945 the roof was close to failing completely, and chunks had fallen into the reservoir. The girders supporting the roof were failing at the shear points with the columns. The inside wall was worn down to rebar due to ice action. Ten inches of slime covered the floor of the reservoir. The outside wall suffered from spalling. The Water Commission decided to undertake repairs.

The old roof was removed, but the columns were retained. Loose material was chipped away from the inside, and the areas sand and water blasted.  Steel wire mesh was then anchored in place, and multiple layers of gunite were shot in place. Once the inside was done, the same process was done on the exterior.

Gunite was, at one time, a trademarked name that specifically refers to the dry-mix shotcrete process. In the dry-mix process, the dry sand and cement mixture is blown through a hose using compressed air, with water being injected at the nozzle to hydrate the mixture, immediately before it is discharged onto the receiving surface. The concrete mixture is by pneumatic pressure from a gun, hence “gun”-ite.

The roof dome at the time was the largest pre-stressed concrete shell in the world. At 3 inches thick, it was cast in place on forms, after the edge ring was cast. The dome took 5 days to pour, and work progressed around the clock, with two 12 hour shifts. The pre-stressing was accomplished by applying 180,000psi of force on a 0.162″ steel wire, and wrapping it around the ring 360 times, in 5 layers. A coat of gunite was applied between each layer, and in all 42 miles of wire were used.

The outside of the reservoir was finished with a white cement brush coat.

By 1999, it was time to replace the roof again. This time a lightweight aluminum geodesic dome was chosen. The existing roof was removed with explosives, which were also used to release the pre-stressed ring beam. Charges were set around the perimeter.

With the roof replaced, the Robie St. High service Reservoir can continue to serve the people of Halifax.