This kind of drainage is installed with the aim of controlling waters emerging from earthworks, which includes not only the water that appears at the base of the excavations but also the flow coming from excavated slopes. Five types of systems can be used:
These consist (see Sections 13.3.4 and 13.3.6) of a layer of granular material with constant thickness (normally between 0.40 and 0.60 m) that is spread at the base of the excavation along the formation, or, for an embankment situation, at its foundation (Fig. 13.19). This layer is placed between geotextiles having separation and filter functions.
Fig. 13.19 Drainage layers
At the base of large embankments crossing deep valleys, the main aim may be to keep the water table at its original position in the old valley bottom. For this purpose it may be more desirable to use a series of trenches let into the original valley profile which lead water away before it can rise into the base of the embankment. Due to their appearance in plan these are colloquially known as “Christmas Tree” drainage systems (Fig. 13.20).
