Forterra Canada Provides Ottawa International Airport with new infiltration tank

Avia NG Airport Consultants reached out to Forterra Canada in December 2017 for assistance to design and build an infiltration tank. The Ottawa International Airport was expanding its apron area and needed to infiltrate water runoff back into the site as opposed to discharging it. Several design criteria needed to be met:

Completed infiltration tank install adjacent to existing Ottawa airport apron

1) Structurally must resist aircraft loading – Boeing B777-300ER
2) Horizontal seismic loading
3) Infiltration rate drop of 3.2” / hour as a minimum
4) Total storage capacity of 1400 m3 (49,400 cu. ft.)
5) Access points into the chamber

Using their XStream program, Forterra Canada was able to optimize the design and provide a cost-effective solution. We were able to drop the tank below the inlet pipe, thus maximizing the retention volume. This allowed us to minimize the footprint over the site.

The final Forterra design consisted of:

• 78 sections total, of 10’x8’ dry cast boxes aligned as a twin cell. Every 5th box had an interconnecting pipe.
• Manhole access points were located at each end of the tank. Bulkheads were cast into the boxes.

The manhole access points required structural distribution slabs to bedowelled to the XStream to support the aircraft load applied to the box sections.

The site was primarily native sand that was loose to very dense in nature. We proposed an 18” thick stone bed wrapped in filter cloth to allow the water to percolate into the native soil.

Specially engineered backfill was used on the sides of the box to resist the seismic loading. Native overburden completed the installation, eliminating the need for imported material. Once completed, 15” thick concrete pavement finalized the installation.

Each box was produced with a 12” opening in the bottom to allow for infiltration. During the installation process there was concern of adequate infiltration over time as silt would close up the open joints, and the water table could rise seasonally. In order to overcome this, 12” holes were provided in each third box above the springline with additional pipe to redirect the flow into the native soil. There was also concern that the compacted side-fill would not permit adequate drainage.

Infiltration tank install and bedding preparation

Greenbelt commenced the installation on June 18, 2019. Bernie Ingimundson, P.Eng. Senior Adviser for Greenbelt, headed up this project and was pleased with both the product and delivery.

Forterra Canada provided the precast concrete design and manufacturing expertise that helped Avia and Greenbelt address some of the challenges involved with the Ottawa International Airport apron expansion project.

By Ian Rowbotham, P.Eng., Principal with Avia NG Airport Consultants.