Medium and large diameter drainage wells are deep drainage structures, used in areas where there is a heterogeneous lithostratigraphic situation, characterized by alternating horizons of different permeability and complex hydrogeology. The construction of these structures involves the use of complex equipment and drilling techniques commonly used for excavating water wells.
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The wells can be built in isolation or placed next to each other to form a drainage sheet pile wall. The wells can consist of an outer ring of bored piles and internal concrete wall lining cast in steel formwork, although sometimes there are walls made entirely of reinforced concrete. Medium and large diameter drainage wells are often combined with sub-horizontal drains and drainage trenches. They are used effectively in consolidation and stabilization interventions of landslide slopes where it is necessary to intercept groundwater and significantly lower the water table level, while at the same time reducing negative effects on the environment, to ensure the stability of important structures or human settlements.
Drainage trenches are linear structures generally arranged parallel to the line of maximum slope of the hillside, with limited depths, they can reach 4 – 6 m, and widths in the order of a meter (from 80 cm to 1.20 m). However, it is possible to reach even greater depths, up to approx. 10 m, by proceeding with excavations in successive phases, without having to resort to the formation of tunnels and galleries. The methods of constructing drainage trenches vary depending on the depth and the different local hydrogeological situations. The trenches must be built with attention to the stability of the excavation walls, in small sections, proceeding from downstream to upstream so that, even if partially constructed, they can already exert their action.
The bottom of the excavation can have a uniform slope in the case of slightly inclined slopes (10°-15°), while in the case of more steeply inclined slopes or very long trenches, the bottom of the excavation is stepped and catch basins are positioned. At the bottom of the trench, filtering pipes, micro-slotted, in polyethylene, steel or concrete are placed. Of more recent application are spiral pipes consisting of a harmonic steel spiral reinforcement covered with non-woven geotextile and waterproofed at the base by an HDPE geomembrane.