SOAKAWAY - DRYWELL - ENGINEERED DESIGN

A soakaway (dry well) is an underground pit filled with stones and lined and covered with landscaping fabric which helps stormwater soak into the ground faster. A soakaway (dry well) is an infiltration trench which 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 (dry well.) 

Soakaway (dry well) is a rectangular or circular excavation lined with geotextile fabric and filled with clean granular stone or other void forming material, that receive runoff from a perforated pipe inlet and allow it to infiltrate into the native soil. Soakaway (dry well) typically service individual residentail lots and receive only roof and walkway runoff but can also be designed to receive overflows from rainwater harvesting systems.

 Typically, the soakawAy (dry well) should be located close to the ground surface; however, this will depend on the depth of storage in the soakaway (dry well), 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 (dry well) which may freeze. Guidance on the recommended minimum soil cover for various subsurface soakaway depths and native soils is based on professional 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 (dry well) because of the size of the pores in the storage media.
The roof leader is extended underground to an excavated soakaway (dry well). The soakaway (dry well) should be located at least four metres away from the foundation of the nearest building to prevent excessive foundation drainage.
Groundwater mounding calculations may be required to ensure that soakaway (dry well)pits do not interfere with sewage system leaching beds. 

 Conveyance pipeThe roof leader should extend into the soakaway (dry well) for the full length of the soakaway (dry well). The extension of the roof leader should be perforated to allow water to fill the soakaway (dry well) along the length of the pipe. The perforated pipe should be located near the surface of the soakaway (dry well) (75 mm - 150 mm from the top of the soakawy (dry well).  
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(dry well). 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.

 If deemed necessary, the stamped engineering design of an infiltration gallery (soakaway) would cost $1,295+HSTand if inevitably required a stormwater management report would cost $1,895+HST.
If the soakaway pit is being proposed, a test pit may be required by the municipality  to confirm the clearance of the water table and soil percolation. Our fee to visit the site  to observe the depth of the groundwater table and one soil sample analysis would be $695+HST.
 

 Common soakaway (dry well)Common soakaway (dry well) may be viable in areas with compact built forms. The common soakaway (dry well) can be located along rear lot lines of residential lots. 

Water Table Depth

The depth from the bottom of the soakaway (dry well) to the estimated seasonally high water table should be greater than or equal to one metre.

Depth to Bedrock

The depth from the bottom of the soakaway (dry well) to the bedrock should be greater than or equal to one metre.

Soils

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

Storage Volume

A minimum storage volume of 5 mm over the rooftop area should be accommodated in the soakaway (dry well) without overflowing. The maximum target storage volume should be 20 mm over the rooftop area since 90% of all daily rainfall depths are less than this amount.

Storage Configuration

The length and width of a soakaway (dry well) 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 (dry well) and to minimize the potential for groundwater mounding.

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

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

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

It is recommended that a conservative drawdown time (24 h) 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.

Storage media

The soakaway (dry well) is comprised of clear stone (50 mm diameter). Non-woven filter cloth should be used to line the soakaway (dry well) to prevent the pore space between the stones from being blocked by the surrounding native material.

Soakaway (dry well) has both benefits and drawbacks compared to rear yard ponding. The benefits of a soakaway (dry well) include greater recharge (less evapotranspiration) and less inconvenience to the homeowner (less surface water ponding). The drawbacks of a soakaway (dry well) include greater 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 (dry well). 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 one metre 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 one metre of the foundation drain elevation, foundation drainage by sump pumps is not feasible. This requirement is imposed to prevent excessive sump pump operation and a looped system.

A soakaway (dry well) should be located a minimum of four metres away from all building foundations to minimize the contribution of soakaway (dry well) 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 two metres away from all building foundations, and there should be sufficient grade from the foundation wall away from the building (≥ 2%) for two metres to four metres to convey the foundation drainage away.

Discharges to the surface should be 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.