Design Basics – Mound Design Refresher

Soils treatment mounds are pretty common in rural Saskatchewan and the new Saskatchewan Onsite Wastewater Guideline has brought into play some pretty significant changes that impact how mound systems are designed.

Mounds must be designed with Linear Loading in mind. Linear Loading determines how much water flows down the contour of the sand layer and penetrates the soil before breaking out. Properly designed mounds should not break out at the toe.

Here are some questions we get in the office on mound design:

  1. How deep should my sand layer be?
    a. Answer – The layer of clean, washed sand must be a minimum of 12 inches deep. If the vertical separation on the property is less than three feet, you must add an inch of sand for every lost inch of vertical separation. For instance, if you only have two and a half feet of vertical separation, your sand layer will have to be 18 inches in depth.
  2. How long will my sand layer be?
    a. Answer – The length of the sand layer is determined by dividing your peak flow from the residence being served by the Linear Loading Rate determined for the type of soil texture and structure, the per cent of slope, and the depth of vertical separation.
  3. How wide will my sand layer be?
    a. Answer – The width of your sand layer is determined by dividing the area of your sand layer by the length of your sand layer. (Hint – the width of your sand layer will be very close to the Linear Loading Rate.)
  4. How many orifices should I have in my mound?
    a. Answer – Under the new Guideline, you need one orifice for every 5.5 square feet of sand layer. The equation is simple: square footage of sand layer divided by 5.5 = minimum number of orifices. For example, if your sand layer is 520 square feet, then 520 divided by 5.5 = 94.54 orifices.  Round up to 95 orifices.
  5. What will the distance between my orifices be?
    a. Answer – Once you have determined how many orifices you need, divide them by the number of laterals you have to determine how many orifices will be in each lateral. Then take your lateral length and divide it by the number of orifices per later to get your spacing.

Need worksheets? Worksheets to calculate system size and pressure distribution are available on the SOWMA website in the member interface. Just log in with your username and password at http://www.sowma.ca