The choosing of screen slot dimensions for water wells depends essentially on the gravel pack grain-size distribution and this latter dependes on the grainsize distribuzion of the water-beraing soil.
In case of wells where it is not possible to have an artificial gravel pack (rotary drilled wells) it is neessary to choose the screen so that 30-40%of the water-bearing soil can be extracted in order to make an area of high permeability around the screens; therefore, the size of the slots must be nearly equal to the valve d30 - d40 on the soil grain-size distribution.
In case of artificial gravel pack, there are various methods, but the easier one is the following:
multiply the value d30 by 4 if the soil is fine and homogeneous (U < 2,5), by 6 if the soil is big and heterogeneus (U> 2,5) and by 9 if the soil is fine and heterogeneus; dans the value obtained on the same diagram in corrispondence with d30 and the trace a grain-size line passing throught this point and having a value of U< 2,5 for soil U>2,5 or U=2,5 for soil U>2,5.
The screen slots for water wells must be such as to retain not less than 90% gravel, that means it is necessary to have b<= D10