Successful permeable interlocking concrete pavements (PICP) rely on an adequately designed sub-surface retention system, properly sized to handle the amount of water both falling on and running into the PICP system. Proper depth and open-graded material selection (void space) are essential. The formula listed below is used to determine base/subbase depth.
Factors that influence that depth include: rainfall, runoff area, area of permeable pavement, void ratios and infiltration rate of the native soil. Later in the exercise portion of this lesson, you will control these variables and test whether your selected base depth will handle water loads or flood.
This equation is intended for full exfiltration designs where all water is captured from a given design storm and completely infiltrated into the soil. Water volumes exceeding the design storm will need to drain from (1) pipes near the surface or (2) out to the surface at the lowest end of the pavement. Partial exfiltration designs include some infiltration and some outflow. In such cases, perforated drain pipes are placed a specified distance above the soil subgrade to allow for some infiltration. When the water level rises and reaches the perforated drain pipes, excess water drains out of the base. This approach infiltrates less water than if the entire base is used for storage and infiltration as with a full exfiltration design.