Permeable pavements: Sizing
The following calculation is used to size the stone storage bed (reservoir) used as a base course. It is assumed that the footprint of the stone bed will be equal to the footprint of the pavement. The following equations are taken from the ICPI Manual [1]
To calculate the total depth of the foundations (all aggregate layers)[edit]
The equation for the depth of the stone foundation (df, max, mm) is as follows:
Where:
- RVCT = Runoff volume control target (mm)
- R = Ac/Ap; the ratio of contributing drainage area (Ac) to permeable paving area (Ap). Note that the contributing drainage area (Ac) should not contain pervious areas. R should not normally exceed 2.
- f' = Design infiltration rate (mm/hr)
- t = Time to fill stone bed (typically 2 hr)
- VR = Void ratio for stone bed (typically 0.4 for 50 mm dia. clear stone)
On highly permeable soils (e.g., infiltration rate of 45 mm/hr or greater), a maximum stone reservoir depth of 2 metres is recommended to prevent soil compaction and loss of permeability from the mass of overlying stone and stored water.
To calculate the invert of the underdrain from the base of the reservoir[edit]
For designs that include an underdrain, the maximum depth of the storage layer below the invert of the underdrain pipe (ds, max) can be calculated as follows:
Where:
- f' = Design infiltration rate (mm/hr), and
- t = Drainage time (hrs), e.g. 96 hours, check local regulations for drainage time requirements.
- 0.4 = Void ratio of clear stone
When sizing the area of permeable paving based on the contributing drainage area, the following equation may be used: Failed to parse (syntax error): {\displaystyle A_p= \frac{Q_c\times A_c}{V_R \times dp – P + q'\times t}}
- ↑ Smith, D. 2006. Permeable Interlocking Concrete Pavements; Selection, Design, Construction, Maintenance. 3rd Edition. Interlocking Concrete Pavement Institute. Burlington, ON.