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Drainage layer

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The crushed stone layer is evenly interposed between the drainage network with the pipes arranged in trenches and the sandy growth layer, in accordance with USGA regulations to create the "suspended water table" subgrade system. In this type of subgrade, the water remains suspended in the sandy growth substrate due to capillary forces between the sand grains until it is completely saturated. In the case of heavy rainfall, however, the excess water seeps into the gravel layer by gravity and drains away rapidly through the continuous flow in the drainage pipes below. A few hours after the end of the rainfall, the water required for the growth of the grass remains in the plant substrate and the so-called "field capacity" of the substrate is reached, which keeps the water-air ratio in the rhizosphere in balance.

The USGA method is still the best known method for creating highly drained, footprint-resistant golf greens and premium sports fields. The system manages to maintain the ratio of water to air in the growing medium more evenly across the surface than any other system.

The choice of grit seems trivial, but it also depends on the characteristics of the sand. The selected crushed stone can be placed in contact with the top layer of sand as long as the conditions in the following table are met.

Performance Factors Recommendations
Bridging Factor D15 (crushed stone) ≤ 8xD85 (top soil)
Permeability Factor D15 (crushed stone) ≥ 5xD15 (top soil)
Uniformity Factors D90 (crushed stone) / D15 (crushed stone) ≤ 3
  No particles greater than 12 mm
  Not more than 10% less than 2 mm
  Not more than 5% less than 1 mm

Draining layer made of crushed stonesDraining layer made of crushed stones

The table is based on engineering principles according to the criterion that relates the largest 85% of the top soil particles to the smallest 15% of the crushed stone particles. In practice, a kind of "linking bridge or junction" between the larger component of the top layer (which has an overall fine particle size) and the smaller component of the bottom layer (which has an overall coarser particle size) is evident. This prevents the migration of finer particles into the coarser lower layer and at the same time ensures good permeability.

D85 (top soil) can be defined as the particle diameter below which 85 % of the particles (by weight) are smaller.

D15 (crushed stone) can be defined as the particle diameter below which 15 % of the particles (by weight) are smaller.

D15 (crushed stone) must be less than or equal to 5 times D85 (top soil) for the "junction" to occur.
To maintain adequate permeability through the layers (from the top soil to the underlying crushed stone) D15 (crushed stone) must be greater than or equal to 5 times D15 (top soil).
Finally, to ensure good uniformity of the layers, the crushed stone must have a uniformity coefficient (D90 crushed stone / D15 crushed stone) less than or equal to 2.5.
Note: If it is not possible to comply with all the parameters in the table, an intermediate layer must be placed between the grit layer and the sandy topsoil.

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