Difference between revisions of "Permeable pavements: Sizing"
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==To calculate the invert of the underdrain from the base of the reservoir== | ==To calculate the invert of the underdrain from the base of the reservoir== | ||
For designs that include an underdrain, the maximum depth of the stone reservoir below the invert of the underdrain pipe (''d<sub>s, max</sub>'') can be calculated as follows: | For designs that include an underdrain, the maximum depth of the stone reservoir below the invert of the underdrain pipe (''d<sub>s, max</sub>'') can be calculated as follows: | ||
<math>d_{s, max}=\frac{f'\times | <math>d_{s, max}=\frac{f'\times t}{0.4}</math> | ||
{{Plainlist|1=Where: | {{Plainlist|1=Where: | ||
*''f''' = Design infiltration rate (mm/hr), and | *''f''' = Design infiltration rate (mm/hr), and | ||
Revision as of 18:01, 26 September 2018
The following calculation is used to size the stone storage bed (reservoir) used as a base course for designs without underdrains. 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 stone reservoir (all graded layers)[edit]
The equation for the depth of the stone bed is as follows:
![{\displaystyle d={\frac {[Q_{c}\times R+P-f'\times t]}{V_{R}}}}](https://wikimedia.org/api/rest_v1/media/math/render/svg/d315c8f9ca97b2ad1afce9fd03b3282b8330c9e6)
Where:
- d = Stone bed depth (m)
- Qc = Depth of runoff from contributing drainage area, not including permeable paving surface (m)
- 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.
- P = Rainfall depth (m)
- f' = Design infiltration rate (m/day)
- t = Time to fill stone bed (typically 2 hr)
- VR = Void ratio for stone bed (typically 0.4 for 50 mm dia. clear stone)
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 stone reservoir 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
Where:
- dmax = Stone reservoir depth (m)
- f' = Infiltration coefficient for native soils (m/hr)
- VR = Void space ratio for aggregate used (typically 0.35 for 50 mm clear stone)
- t = Time to drain (design for 96 hour time to drain is recommended)
The value for native soil infiltration rate (f') used in the above equations should be the design infiltration rate that incorporates a safety correction factor based on the ratio of the mean value at the proposed bottom elevation of the practice to the mean value in the least permeable soil horizon within 1.5 metres of the proposed bottom elevation. 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.
When sizing the area of permeable paving based on the contributing drainage area, the following equation may be used: Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\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.