Concrete Pipe Greatly Reduces Excess Soil Challenges

In December 2019, the Province of Ontario announced that they were phasing in a new regulation under the Environmental Protection Act that would result in a reduction of construction costs by making it easier to reuse excess soil generated during construction activities to minimize waste and to provide clear rules on managing and reusing excess soils.

On-Site and Excess Soil Management Regulation

The Ontario Ministry of the Environment, Conservation and Parks (MECP) estimates that 25 million cubic metres of excess soil is generated in Ontario every year. While most of this excess soil could be reused safely it is often moved off-site because it can’t or won’t be reused at the development site. The MECP implemented the On-Site and Excess Soil Management Regulation (O.Reg. 406/19) in January 1, 2021 to help reduce negative impacts and costs from improper management of excess soil such as:

Noise and dust

Truck traffic and road damage

Erosion and drainage

Other social, economic, health and environmental concerns

Best practice guides and calculation tools

While MECP has created best practice guides and calculation tools to assist with the excess soils regulation, the Greater Toronto Sewer and Watermain Contractors Association (GTSWCA) retained WSP Canada to help its members identify opportunities for the effective and beneficial use of both excavated soils and recycled materials for buried pipe construction.

A report published in August 2020, reveals that the production of aggregate in Ontario is approximately 45 to 90 million cubic metres annually with a growing demand over the next 25 years at unsustainable levels with an enormous carbon footprint associated with the mining and transportation of new aggregate sources.

The Executive Summary of the GTSWCA-WSP identifies the following issues associated with disposing of excess soils:

Extra costs on many infrastructure projects
Environmental impacts related to virgin aggregate extraction in existing agricultural and protected areas

Additional trucking distances to import virgin aggregate materials
Significantly larger carbon footprint related to the above issues

The GTSWCA-WSP report also proposes the following key areas of recommendations:

Performance requirements to ensure suitable performance of serviceability of engineered fill
Compaction methods to improve engineering properties of fill
Current uses and specifications on backfill material and compaction, and the need to explore reforms for more viable options

Guidelines to help assess the suitability of excavated native soils as trench backfill
Benefits of using recycled aggregates (or blended with RAP) as trench backfill for all weather placement
Recommendations for restructuring public works tenders to allow for greater use of native soils and recycled materials for backfill

Good for the environment and good for the economy

Many years prior to this new MECP excess soil regulation, the Ontario Concrete Pipe Association published an article Concrete Pipe Installed with Recycled Aggregate: Good for the Environment and Good for the Economy, in the Fall 2013 edition of the Concrete Pipe Journal to consider the opportunities to use recycled aggregate as a backfill material for concrete pipe which flexible plastic or corrugated steel pipe products do not allow. The same claims can be made about using select native soils to install concrete pipe.

Ontario Provincial Standard Specifications (OPSS.PROV) 401 used by the Ministry of Transportation, Construction Specification for Trenching, Backfilling and Compacting, clearly identifies the differences between the installation requirements of a rigid concrete pipe and flexible plastic or metal pipe for sanitary and storm sewers, pipe culverts, subdrains, forcemains, and watermains. The figure below illustrates the terminology used in Ontario, with the critical backfill soil surrounding each type of pipe highlighted, often referred to as the structural backfill for the pipe.

As highlighted in the figure, the structural backfill is critical for pipe-soil interaction to carry the loads imposed on the pipe and should consist of high quality material and proper compaction effort. For a rigid concrete pipe, this critical zone is limited to the pipe bedding which is from the bottom of the excavated trench to the springline of the pipe due to the inherent strength and load carrying capacity of the pipe. For a flexible plastic or metal pipe, the entire soil envelope from the bottom of the excavated trench to a minimum of 0.3m above the pipe is required to allow the pipe to transfer the loads into its surrounding soil.

An assessment of these two very different installation details results in a plastic or metal pipe requiring more high quality backfill material and generating more than twice as much excess soil than a concrete pipe.

Excess soil transport comparison: Concrete Vs. Flexible pipe

Concrete Pipe Generates 18 Less Truckloads of Excess Soil per 100m of 1200mm Pipe

A recent example of how concrete pipe can minimize excess soil generation and the construction costs related to meeting the new MECP regulation, the City of Ottawa along with local industry National Capital Heavy Construction Association (NCHCA) started their biennial construction specifications review in the summer of 2020. The review process divides up the City of Ottawa construction specifications and assigns working groups to each component. These working groups are comprised of City of Ottawa staff and key industry individuals. The Canadian Concrete Pipe and Precast Association (CCPPA) was asked to join the Water/Sewer Working group.

During the City of Ottawa specification review, the CCPPA proposed a change to the backfill requirement, for reinforced concrete pipe (RCP) only.

The proposed change for concrete pipe installation was to allow for select native soils to be used as the cover material above the pipe springline. Currently only Granular A or Granular B was required backfill for up to 300mm above the top of the pipe for both concrete and plastic pipe. It was explained that flexible pipe systems (plastic or metal) require this embedment up to 300mm above the top of the pipe for the flexible pipe system to have the capacity to carry the required loads. However, since concrete pipe does not rely on any structural capacity provided by the cover material above the springline, native soils could be utilized above the springline and thereby reduce the amount of excess soils generated.

This change was adopted by the City of Ottawa and is included in the 2021 City of Ottawa Construction Specifications dated March 1st, 2021.

With this continuing evolution in specifications, locally produced concrete pipe provides governments and developers with a tremendous opportunity to reduce the excess soils generated increase the use of properly processed recycled aggregates, and itself can be 100 percent recycled at the end of a very long service life.

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