The Exploitation of Aquifers - Hydrogeology: Objectives, Methods, Applications - page 183

pumping, and construction is facilitated by intermediary vertical wells
enabling a better long-term maintenance.
2.3 Wells and wells with radial collectors
A well is a catchment construction penetrating vertically into an aquifer.
It generally has a large diameter (from 1 to 5-6 m) and a depth no greater
than a few meters to a few tens of meters, although certain wells can reach
a hundred meters in rocky terrain.
The main construction diffi culty originates from the need to evacuate
water as soon as the well reaches the water table. This is why the oldest
wells did not extend far below the water table, and collected water only
through their base.
Today, well construction favors the opening of drainage channels in the
walls of the well below the water table, and continued excavation all the
way to the impermeable substratum in order to draw from the entire cross-
section of the aquifer and thereby increase productivity (Figure 97a).
In such cases, and when conditions permit it, care must be taken to
surround the well with a layer of gravel, which serves as a sand fi lter, around
the walls in the permeable zone, and to protect the upper part of the well
against surface water infi ltration, by ringing it with cement.
When the permeable zone is not very thick, wells are frequently
dug with an impermeable shaft lining until they reach the impermeable
substratum, and then completed with horizontal drains radiating into the
most productive section. Each drain is equipped with a sluice gate as soon as
it is completed, which facilitates drainage during construction (Figure 97b).
This method results in a noticeable increase in the well's productivity.
Figure 97 Typical diagrams of a well (a) and a well with radial collectors (b).