2024 Ontario Building Code Guide: Fill Based Absorption Trenches & Construction Standards
Sarah's Expert Insight
OBC Compliance Specialist
"The 1:4 slope requirement and the 15-meter horizontal extension are often overlooked in rural London and Muskoka projects, leading to premature system erosion or permit failures. At empcontracting.ca Inc., your construction contractor in London, Ontario, we emphasize that proper lift-by-lift compaction is the only way to prevent distribution pipe shifting, which is a common cause of effluent breakout in fill-based systems."
Understanding Section 8.7.4 of the Ontario Building Code (OBC) is critical for ensuring the longevity and compliance of private sewage systems. When local soil conditions are insufficient for standard leaching beds, fill-based absorption trenches provide a viable solution, provided they meet strict loading and construction criteria. As empcontracting.ca Inc. your construction contractor in London, Ontario, we prioritize these technical standards to protect your property and the environment.
Loading Rates for Fill Based Absorption Trenches
The loading rate of a leaching bed is determined by the percolation time (T) of the soil. According to Table 8.7.4.1., the maximum daily loading rates per square metre are as follows:
- 1 < T ≤ 20: 10 (L/m²)/day
- 20 < T ≤ 35: 8 (L/m²)/day
- 35 < T ≤ 50: 6 (L/m²)/day
- T > 50: 4 (L/m²)/day
OBC 8.7.4.2. Construction Requirements
Soil and Fill Depth Standards
Except for shallow buried trenches, fill-based systems require unsaturated soil or compliant leaching bed fill to extend at least 250 mm over the entire bed area. Furthermore, this fill must extend at least 15 metres beyond the center lines of the outer distribution pipes in any direction where effluent moves horizontally.
The 75% Percolation Rule
If the underlying soil has a percolation time greater than 15 minutes, any added fill must have a percolation time of at least 75% of that underlying soil. This ensures hydraulic compatibility between the native soil and the engineered fill. If this cannot be met, specific depth increases (minimum 900 mm) or adjusted pipe lengths are required to maintain system efficiency.
Site Preparation and Stabilization
Proper site prep is non-negotiable. The area must be cleared of vegetation and the added fill must be compacted in layers to prevent uneven settlement of distribution pipes. To ensure long-term stability, the side slopes of the added fill must not exceed a 1:4 ratio (one unit vertical to four units horizontal) and must be stabilized against erosion. For professional implementation of these complex requirements, contact empcontracting.ca Inc. your construction contractor in London, Ontario.
Disclaimer: This content is generated from the Ontario Building Code for educational purposes. Always consult with a licensed professional engineer or your local building department before starting any construction project in London, Ontario.