By Greg Keller
How can a large community pool complex be constructed on land that is slightly unstable? In the case of the Horace Mann Pool in Rapid City, S.D., designers used 63 underground concrete piers (along with specially designed cardboard boxes under the floor slab) to accomplish this feat. This project was an engineering and construction marvel, to say the least. As recently as 30 years ago, this type of pool design and construction was not possible. Today, however, when posed with this challenge, these construction techniques are available to builders to make such a project a reality.
The existing site
There was an existing pool on-site; however, it was 43 years old and derelict at the time of decommissioning. It had outlived its average 30-year lifespan, as the pool suffered from an abundance of large cracks that were no longer feasible to fix, thus the need for the pier-design system to install the new pool. The facility had also outgrown its design, as pool goers today want more options than a basic rectangular basin of water, which is what was previously in place.
Therefore, the new design was to have an abundance of play features and modern pool amenities that would be sure to impress the people of Rapid City.
As previously mentioned, to design and construct this type of aquatic facility the pools had to be built on top of 63 underground concrete piers which reach down to solid ground. The piers range in diameter from 610 to 1067 mm (24 to 42 in.) depending on the needs of the specific location. The geotechnical report and design plans helped determine the size of the drilled piers required for each specific location.
For instance, the drilled piers around the perimeter of the lap pool are 610 mm (24 in.), while some are 1067 mm (42 in.) in diameter towards the centre of the pool. In total, 24 piers were required to build the lap pool: 18 at 610 mm (24 in.), five at 914 mm (36 in.), and one pier at 1067 mm (42 in.) in diameter. Thirty-seven piers needed to be installed to build the recreation pool: 20 at 914 mm (36 in.), 12 at 1067 mm (42 in.), and seven at 610 mm (24 in.) in diameter.
Each pier is created by drilling into the ground at the desired location to the required depth. If the soil is loose (i.e. caving in) a support/casing tube or drilling fluid is used. Then, a prefabricated steel reinforcement cage is dropped in place. Finally, concrete is poured into the hole and left to cure before construction can continue.
Next, a layer of cardboard forms are placed on areas that do not have piers. These forms allow the earth to expand up into them without cracking the concrete should there be any movement in the earth. These systems are not just any ordinary cardboard box. In fact, they are specifically manufactured for this use, as they have a precise wall thickness and construction type, along with specific thicknesses of the void in between the cardboard walls. They must be placed on a dry day to maintain their structural integrity, and then concrete must be poured on top of them immediately to avoid potential rain damage. If concrete pours cannot be performed on the same day as the cardboard forms are installed, they can be covered to protect them from any impending storms.