In practice, the characteristic flow rate of the filters considerably differs from that obtained in free suction because it is conditioned by the peculiar properties of the soil (permeability and porosity) and the frictions that occur between the soil and the filter and between the filter and the suction circuit. It was possible to graph the variation in the flow rate of the filters in the various types of soil in relation to their diameter. The flow rates were measured at constant depth and prevalence by choosing those soils characterized by greater uniformity.
In soils with fine grain sizes and low permeability, the increase in the diameter of the filters does not lead to particular variations in the flow rate. In soils with high granulometry and high permeability, on the other hand, the increase in the diameter of the filters leads to a considerable improvement in their unit flow rate, as long as the uniformity of the soil – on which the permeability coefficient directly depends – is respected. The experiments carried out have demonstrated the variability of the values reported below as a function of the mixing percentage of the soils. Soils with large granulometries (gravels) have been found very often, showing flow rates very similar to those of fine soils because they are mixed with a matrix of silty sands or silts.