Electrolytic analogue study of the effect of openings and surrounds of various permeabilities on the performance of field drainage pipes

Studie van de invloed van de porositeit en permeabiliteit op de werking van drainbuizen op grond van een model, waarbij het watertransport wordt gesimuleerd m.b.v. metingen van potentiaalverschillen in een elektrolyt(zout-)oplossing

The effect of various openings and surrounds of various permeabilities on the performance of field drainage pipes was studied by means of an electrolytic analogue. The results obtained were compared with these of analytical solutions. Rather simple and sufficiently accurate solutions exist to determine the entrance resistance of pipes with smooth outer surface. These theoretical solutions cannot be applied to pipes with corrugated cuter surface provided with perforations in the valley of the corrugations for which the corrugations are filled with soil. The shape of the corrugations and the boundary of soil and corrugation present additional difficulties in obtaining an exact theoretical solution for such drains.

From the investigations performed it follows that the smallest entrance resistance is obtained at the greatest subdivision of a given perforation area or perimeter per unit drain length. The most favorable perforations which confer the lowest entrance resistance are these with the smallest area or perimeter such as circular perforations and, for rectangular slits, those with the smallest length. Except for circular perforations, an increase of the actual perforation area of 20 - 25 cm 2/m to about 50 cm 2/m will considerably reduce the entrance resistance.

Using permeable envelopes. the entrance resistance decreases considerably up to an envelope thickness of about 5 mm after which a constant value is obtained. The effective radius, however, continues to increase with increasing envelope thickness due to the decrease in radial resistance. Increasing the permeability of the envelope reduces the entrance resistance and increases the effective radius up to a permeability ratio of 20. Any further increase of the permeability ratio is of less significance. For a constant value of pipe radius plus envelope thickness much the same effective radius is obtained if the thickness of the envelope is at least 5 mm.

A less permeable drain surround increases the entrance resistance enormously and inadmissible values are quickly reached. A constant entrance resistance is obtained for thicknesses of about 10 mm and upwards. The effective radius decreases due to the increase of radial resistance and exceptionally small values are obtained. The increase of entrance resistance and decrease of effective radius are particularly marked for permeability ratios less than 0,2.

The entrance resistance only changes slightly when a drain is surrounded by an envelope which has a reduced permeability over a certain percentage of its original thickness. Due to the effect upon radial resistance. the effective radii will decrease with decreasing permeability and increasing thickness of the blocked zone. The effective radius never assumes such extremely small values as are obtained with a drain directly surrounded by a wholly less permeable layer.

The entrance resistance of a drain pipe is constant and depends only on the geometrical characteristics of the pipe itself. It is, however, important to give an exact description of the flow pattern, since its omission can result in faulty conclusions being drawn about the entrance resistance.

Due to the entrance resistance, the hydraulic gradient in the vicinity of the perforations can reach high values and massive invasion of soil particles may occur. These gradients are markedly reduced when the drain is surrounded by a permeable envelope.

Although the approach flow conditions are mere favorable if water is standing above the drain, the entrance resistance which causes a certain water level above the drain will raise the water table midway between drains more then an ideal drain operating with the game head.