Field research on the performance of various drainage materials in Lithuania
Abstract Field evaluation of the performance of a drainage system or of various drainage materials is a long-lasting research, very often with disappointing results. These disappointing results stem either from a too wide a spread of the field data, or one or more necessary parameters which were not measured, or an insufficient knowledge of the flow occurrence in the vicinity of the drain resulting in a wrong interpretation and evaluation of the research findings. Yet, a correct evaluation of the performance of a drainage system is of primary importance to ascertain the real value of a drainage system or to compare various drainage materials. As a result of the wide spread of field data within a measuring season, a large amount of data from several seasons is required to obtain reliable results. From discharge and total head loss measurements a second-degree polynomial results which only permits a determination of whether the design criteria are met. The similarity with Hooghoudt’s equation allows the determination of soil hydraulic conductivity and the equivalent depth of the impervious layer. With this data the drain spacing can be checked. Too wide a spacing can be the cause of malfunctioning of a drainage system, but the drainage materials can also affect its performance. To evaluate or compare drainage materials it is necessary to measure the so-called approach-flow head loss in the vicinity of the drain. The relationship between discharge and approach-flow head loss should theoretically be linear and the regression coefficient gives the approach-flow resistance. However, the relationship depends to a large extent on the approach-flow occurrence in the vicinity of the drain. For non-linear relationships, the approach-flow resistance can be estimated from the slope of the tangent line, which tends to a constant value at greater discharges for flow patterns covering the whole drain circumference. The approach-flow resistance depends on the hydraulic conductivity of the soil and does not permit a direct comparison of various drainage materials. Therefore, the approach-flow constant or the dimensionless approach-flow resistance is calculated. This parameter is related to the material itself and to the installation conditions reflected in the hydraulic conductivity of the trench backfill. Consequently the comparison of the various drainage materials includes the trench backfill. The approach-flow head loss fraction is that part of the total head loss which is dissipated by the approach-flow. The relationship between approach-flow head loss and total head loss should theoretically be and is, indeed, linear so that the slope of the regression line gives the approach-flow head loss fraction. This parameter combines the influence of drainage material and flow pattern so that real flow situations in the vicinity of the drain, which are not necessarily the same for all materials, can be compared.
Electrolytic analogue study of the effect of openings and surrounds of various permeabilities on the performance of field drainage pipes
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.
Linguistic experts as semantic tour guides
This article examines the phenomenon of linguists testifying as experts on meaning in legal disputes over the interpretation of statutes, contracts, transcripts of tape-recorded conversations, and other important legal texts before courts in the USA. It concludes that there is an important role for linguists in such cases -- the role of the tour guide. It suggests that judges need not be concerned about linguists usurping the traditional roles of the judge and the jury as ultimate interpreters provided that the linguist's testimony is appropriately circumscribed.