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.
Materials for subsurface land drainage systems
Reliable subsurface drainage systems for groundwater table and salinity control are needed to maintain or enhance the productivity of irrigated lands and to contribute to the rural development of lowlands in the humid tropic. This publication presents guidelines to assess the need for envelopes and for the selection of appropriate materials (i.e. pipes and envelopes) for the proper and lasting performance of subsurface drainage systems. In addition, it also contains guidelines for adequate installation and maintenance of drainage materials as well as the required specifications and standards of such materials, which may be used in tender documents for implementation of subsurface drainage works. The purpose of this technical paper is to provide this practical information to drainage engineers and contractors who are in charge of drainage projects.
Long-term performance of geosynthetics in drainage applications
This report documents and presents the results of a multifaceted field exhuming and laboratory study that focused on the long-term performance of geosynthetic drainage systems. During the field phase of the project, 91 sites involving six types of drainage systems were exhumed and evaluated. Eighty-four of the sites involved some type of highway edge drain system. Problems were mainly encountered with construction/maintenance items and the geotextile filters surrounding the various drainage materials, mainly soil retention problems. Recommendations are made to address the construction/maintenance problems as well as the soil retention problems with the geotextile filters. Notably, recommendations to change the method of installation of prefabricated geocomposite edge drains are made whereby the product is to be moved to the shoulder side of the trench and is to be backfilled against and beneath the pavement slab using sand puddled in place with water. Three laboratory test methods were utilized and/or developed. The long-term flow tests were instructive in illustrating various problematic soils. The fine-grained soils, either in the upstream soil or in the permeating liquid, resulted in very low flow rates. This test method should be utilized for selecting geotextiles for fine cohesionless soil applications of a severe and/or critical nature. The fine fraction filtration and dynamic fine fraction filtration tests were not particularly relevant insofar as the thrust of this study is concerned. They are not recommended for further evaluation. Using the field exhumed sites as "ground-truth", insofar as acceptable or nonacceptable performance of the various geotextile filters was concerned, a critique of the various geotextile design criteria was undertaken. This involved analyzing numerous criteria for permeability, soil retention and excessive clogging. It was determined that the currently used Federal Highway Administration criteria are excellent predictors (i.e., design methods) in all cases. These criteria should be disseminated to the widest possible audience and utilized accordingly. The findings of this study strongly support that the state of the art in long-term performance of geosynthetic drainage systems is very positive. Geosynthetics are indeed providing the lifetime performance behavior that they suggest and promise. However, continued vigilance so as to obtain intimate contact of geotextiles to their adjacent soil and an ever-alert awareness of maintenance problems must be assured.