<p>Both modular systems require a lower bedding layer of angular stone to permit infiltration, and provide a foundation for the installation:
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*Plastic modules usually have a parabolic shape to resist the load above. The spaces between the rows of the gallery are then filled with more clear stone to support the overlying infrastructure.
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*Concrete modules are often box shaped and can, in some circumstances, be employed without any additional cover. However, a minimum of 20 cm cover is for most applications. Where this cover is (irrigated) topsoil this can support turf grass. Deeper soil is required to support more complex plants.