Subsurface drainage is made up of different parts but all are linked directly with the surface drainage system and all are, fundamentally, taking care of groundwater or water that infiltrates through the pavement surface.
Part of the rainfall-runoff infiltrates into the ground and continues as subsurface flow. Part of this may, in turn, continue as a sub-horizontal subsurface flow, depending on the permeability of different soil layers. This can lead to increases in the moisture level under the pavement, reducing the bearing capacity. To decrease this phenomenon, the scheme design should note that:
• it is a good practice in embankments in the Mediterranean countries either to keep the thickness between the underside of the pavement and the natural soil to at least 1.0 m, or, if necessary, a drainage layer (see below) as well as other measures should be used, depending of subsoil characteristics; and
• in a cutting, the depth of the lateral drains should allow for an adequate drainage depth to the groundwater level, normally more then 1.0 m.
Because of the different types of subgrade and pavement construction, it is important to be able to differentiate between those pavements where water flow will be largely vertical, wetting the subgrade (with the associated loss of subgrade support strength) and those situations where vertical water flow will be arrested by impermeable layers in the sequence, forcing the water sideways and necessitating different drainage measures. Figure 13.4 shows three types of pavements, A B and C, which are now described.
A. Subgrade layer with low permeability — infiltrated water will flow above the subgrade, at the bottom of granular base/sub-base layers, according to the maximum crossfall;
B. Permeable subgrade layer and impermeable subsoil — infiltrated water flows on the layer between the subgrade and impermeable subsoil;
C. Permeable capping layer and subgrade — the water percolates vertically through every layer.
The basis for choosing whether approach Case A, B or C applies is set out in Fig. 13.5.
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