WE DESIGN SOAKAWAY & PREPARE STAMPED SOAKAWAY PERMIT DRAWING

 Installing a soakaway is one of the low-impact development (LID) practices that increases stormwater infiltration into the ground and supports filtration into underground aquifers to limit and prevent flooding, erosion, and sedimentation. 

A soakaway is an underground pit filled with stones and lined and covered with landscaping fabric, which helps stormwater soak into the ground faster. A soakaway is an infiltration trench that serves a single residential lot and which does not receive road runoff. Since the runoff is relatively clear, pre-treatment is not needed for a soakaway.

Soakaway is a rectangular or circular excavation lined with geotextile fabric and filled with clean, uniformly graded granular stone with 40% void capacity 

 or other void-forming material, which receives runoff from a perforated pipe inlet and allows it to infiltrate into the native soil. Soakaway typically services individual residential lots and receives only roof and walkway runoff, but can also be designed to receive overflows from rainwater harvesting systems.

 Typically, the soakaway should be located close to the ground surface; however, this will depend on the depth of storage in the soakaway, the potential for frost heave, and the stratification of the surrounding soil. The potential for frost heave is dependent on the native soils and the volume of water in the soakaway, which may freeze. 

Guidance on the recommended minimum soil cover for various subsurface soakaway depths and native soils is based on professional engineering opinion, the expansion of water because of freezing, and the potential availability of water to freeze. Ice lens formation is not anticipated to occur within the soakaway because of the size of the pores in the storage media.

The roof downspout is extended underground to an excavated soakaway. As per the Ontario Building Code, the base of a soakaway shall be a minimum of 1.0m below building footing, and at least 5m away from the foundation of the nearest building to prevent excessive foundation drainage.

Groundwater mounding calculations may be required to ensure that soakaway pits do not interfere with onsite sewage system leaching beds (septic beds). 

 Conveyance Pipe

The roof leader should extend into the soakaway for the full length of the soakaway. The extension of the roof leader should be perforated to allow water to fill the soakaway along the length of the pipe. The perforated pipe should be located near the surface of the soakaway, 75 mm to 150 mm from the top of the soakaway.  

Roof gutter guards can prevent leaves, sediment, and other debris from clogging the soakaway. An overflow pipe should be installed from the roof leader that discharges to a splash pad. 

A removable filter should be incorporated into the roof leader below the overflow pipe. The filter should have a screened bottom to prevent leaves and debris from entering the soakaway. It should be easy to remove so that a homeowner can clear the filter. Frequent use of the overflow pipe will indicate the need for filter screen maintenance.

The top of a soakaway can be planted with grass or plants, or can be designed as a rocky landscape feature.

When our licensed professional engineers design a soakaway, they choose appropriate dimensions for it to be effective to coping with design rainfall events without overflowing. 

Soakaway shall provide sufficient storage capacity to accept the stormwater from short, intense rainfall events and sufficient infiltration capability to disperse the stormwater from long, steady rainfall events. 

The performance of a soakaway depends on the size and shape of the soakaway and the site-specific hydraulic properties of the soil in which it is founded.

Common Soakaway 

A common soakaway may be viable in areas with compact built forms. Soakaways are ideal for small yards. Soakaways are usually more compact than rain gardens, which cover more land/surface. For many suburban properties, long trench soakaways often fit the narrow strips of land between driveways. 

The common soakaway can be located along the rear lot lines of residential lots. 

Water Table Depth

The depth from the bottom of the soakaway to the estimated seasonally high water table should be greater than or equal to 1.0m.

Depth to Bedrock

The depth from the bottom of the soakaway to the bedrock should be greater than or equal to 1.0m.

Soils

Soakaway can be used where soils have a percolation rate greater than 15 mm/h. This generally includes all soils coarser than a loam.

Storage Configuration

The length and width of a soakaway are dependent on the configuration of the development. The length of a soakaway (dry well) in the direction of inflow should be maximized compared to the width to ensure the proper distribution of water into the entire soakaway and to minimize the potential for groundwater mounding.

The permeability of the native soil will dictate the maximum allowable underground storage depth of a soakaway. Storage depths greater than 1.5m are generally not recommended for a soakaway from both a cost and a compaction perspective. The weight of the water in a deep soakaway will compact the surrounding native soil and decrease the infiltration capacity of the soakaway.

There are exceptions, however, to this maximum depth recommendation for a soakaway. In areas with deep sand lenses or significant horizontal soil stratification, a deep soakaway may be preferred. If, for example, a sand lens is located at a depth of 2m, it would be advantageous to construct a deep soakaway that drains into the lens. Soil investigations should be undertaken to determine whether these conditions exist.

 For a soakaway accepting only roof drainage, the potential for clogging is low, as is the risk of groundwater quality degradation. 

CONSTRUCTION INSTRUCTIONS

1. Mark off the area where the soakaway will be located, and do not allow equipment to travel over this area, which would compact the soil.
2. Install a soakaway after site construction has occurred and the site has been stabilized to prevent sedimentation and/or damage from construction activity. If the construction of the soakaway cannot be delayed, the soakaway location must be protected with a berm, silt fence, or a compost sock to prevent sediment from collecting in the area.
3. The soakaway bottom must be uniform, level, uncompacted, and free from rocks and debris. Do NOT compact subgrade. Excavation should be performed with the lightest practical equipment, and the excavation equipment should be located outside the limits of the soakaway.
4. Completely wrap the soakaway with nonwoven geotextile. If sediment and/or debris have accumulated in the soakaway bottom, remove prior to geotextile placement. 
5. Install continuously perforated pipe, observation wells, and all other soakaway structures. Connect the roof leader/downspout to the perforated pipe.
6. Place uniformly graded, clean-washed aggregate in 0.3m lifts. Compact lightly between lifts.
7. Fold and secure nonwoven geotextile over trench, with a minimum overlap of 0.6m.
8. Place 0.3m of topsoil cover over the soakaway.
9. Seed and stabilize topsoil.
10. Connect the overflow pipe to the downspout and position it over the splash block.

Storage Media

The soakaway is comprised of 19 mm diameter or larger clean uniformly graded granular stone with 40% void capacity or other void-forming material. Non-woven geotextile landscape filter fabric should be used to line the soakaway to prevent the pore space between the stones from being blocked by the surrounding native material.

Soakaway has both benefits and drawbacks compared to rear yard ponding. The benefits of a soakaway include greater recharge (less evapotranspiration) and less inconvenience to the homeowner (less surface water ponding). The drawbacks of a soakaway include regular maintenance and uncertain longevity.

Development standards allow foundation drains to be connected to the storm sewer. Ministry of Municipal Affairs and Housing's alternative standards allow the use of sump pumps to discharge foundation drainage to the surface or soakaway. Because foundation drainage is relatively clear water, the cost of stormwater management and sewage treatment can be reduced by keeping it separate from storm and sanitary sewers. The municipality should be contacted before recommending this type of control, as its use may not be permitted.

In areas where the seasonally high water table is within 1.0m of the building foundation drains, sump pumps should not be utilized. This requirement is imposed to prevent excessive sump pump operation and to prevent a looped system whereby the sump pump discharges maintain the foundation drainage. Where the use of sump pumps is not feasible, a "third pipe" may be used to convey foundation drainage to the receiving water.

In areas where the depth to bedrock is within 1.0m of the foundation drain elevation, foundation drainage by sump pumps is not permitted to prevent excessive sump pump operation and a looped system.

A soakaway should be located a minimum of 5m away from all building foundations to minimize the contribution of soakaway drainage to foundation drainage. 

If the foundation drains are being discharged directly to the surface, the discharge point at the ground surface should be located at least 2m away from all building foundations, and there should be sufficient grade from the foundation wall away from the building (≥ 2%) for 2 m to 4m to convey the foundation drainage away.

Discharges to the surface are usually directed to the rear yard to minimize the amount of surface drainage over sidewalks during the winter. Sump pumps discharging to the surface should discharge approximately 0.5 m above the ground surface to prevent blockages in the winter due to ice and snow.

Every soakaway must have one area drain. Soakaways in excess of 8m² must have additional drains on the surface to provide for greater surface drainage, a pump-out point, and to serve as a reminder for current and future owners that the facility exists.

 Stamped Engineering Report

A report, stamped, signed, and dated by a qualified professional engineer, is required for municipal approval of a soakaway. The report shall provide data from a percolation test indicating that the soils are suitable, a drawdown time of 48 hours, the sizing is correct for a 25mm rainfall event, and a suitable runoff coefficient and a factor of safety have been determined. 

It is recommended that a conservative drawdown time of 48 hours be chosen, recognizing that the percolation rates into the surrounding soil will decrease over time and that there will likely be a lack of maintenance in some cases. Longer drain-down times reduce the efficiency of a soakaway and can lead to anaerobic conditions, odour, and other problems.