 Generally, the minimum internal diameter is greater than 3.5 m for drains with a length of 20 to 30 m. Longer drains require wells with a diameter of up to 8–10 m. To determine the network of microdrains planners take into consideration the makeup of the subsoil and the hydraulic regime of the slope.The drainage in these wells is passive, realised by linking the bottom of adjacent wells by sub-horizontal perforations (provided with temporary sheathing pipes) in which the microdrains are placed at a gradient of about 15-20° and are equipped with microperforated PVC pipes, protected by non-filtering fabric along the draining length.Once the drain is embedded in the ground, the temporary sheathing is completely removed and the head of the drain is cemented to the well.