 The linking pipes, generally made of steel, are blind in the linking length and perforated or windowed in the length corresponding to the well.The wells have a concrete bung at the bottom and are filled, after withdrawal of the temporary sheathing pipe, with dry draining material and are closed with an impermeable clay bung.In normal conditions, these wells reach a depth of 20–30 m, but, in especially favourable cases, may reach 50 m. Some of these wells have drainage functions across their whole section and others can be inspected.