Drainage with wellpoint system is used, as we have seen, in all those soils that are permeable due to porosity, such as gravels, sands, silts and clays. The wellpoint system in its most schematic form consists of a series of wellpoints, installed in the ground where the water table must be lowered, connected to one another and to a high-vacuum pump through a series of manifolds, fittings and connection joints. The table on the following page shows the various installation systems in relation to the characteristics of the excavations. The operating principle is based on the deviation of the groundwater flow in the direction of filtering elements (wellpoints) put in vacuum by the pump. The gradient created between the pressure of the water outcrop in the ground (atmospheric pressure) and the filtering elements (wellpoints) directs the water table flow towards the latter with a speed, in the various types of soil, characterized by permeability. When in a specific section of soil, the wellpoint system – which can be compared to a well with the diameter of the area circumscribed by the system – releases the quantity of water that filters through the interstices. The groundwater level in the soil begins to depress forming a cone-shaped surface.
Continuing pumping increases the section of soil that remains without water until the flow rate emitted by the wellpoint system is equal to the filtration flow rate. At this point, the groundwater level stabilizes forming the “cone of influence”.
Of course, the width of this cone of influence is closely linked to the permeability of the affected soils and therefore will be wider in soils with high permeability.
The figure shows the amplitude values of the cone of influence, measured on some types of soil during the use of wellpoint systems. The filter drawn represents the quantity of filter tips needed for the total extraction in a specific section of soil.