Design factors in geotextile structures for control of water movement.

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1 Design factors in geotextile structures for control of water movement. Keith Slater, M.Sc., Ph.D., C. Text., F.T.I. School of Engineering, University of Guelph, Ontario, Canada. Abstract The current rapid expansion of geotextile uses is taking place because of the appreciable advantages, in both versatility and cost, which these materials enjoy in comparison with more traditional ones. One of their more potentially valuable applications is as constraining devices for the movement of water. For this purpose, it is important that the actual material selected should be capable of achieving long life expectancy. In order to achieve this aim, the design of the geotextile, in terms of fibre properties and construction parameters, must be carefully planned. This paper examines the conditions of use likely to be encountered in typical applications and recommends manufacturing parameters that will ensure optimum possible effectiveness of the geotextile over an extended period of time. 1. Introduction Geotextile structures are currently becoming of ever-increasing interest in many applications, especially in civil engineering. Their recent rapid rise to increased prominence has come about because they are able to impart unusual characteristics to a structure simply, quickly, with much more flexibility than traditional materials, and at a relatively low cost. One of the more useful roles they play is in the control of water movement. Their suitability for the purpose is outstanding, and they do not have any real rivals in their effective abilities in this use. In fact, other materials, such as concrete, wood, stone, asphalt and similar substances are slow, expensive and difficult to install, so that they are currently becoming less and less frequently adopted. The major disadvantage of geotextiles is their comparatively short life span, when compared with those of the more robust materials. If they are to continue to compete satisfactorily, then, it will be necessary to ensure that they achieve their maximum effectiveness for each situation involving their various uses for water control. Currently, geotextiles are selected for a given end-use on two major criteria, initial cost and initial properties. The choice is scarcely influenced by another, more crucial, criterion, that of ability to continue to function over a long period of time. The long-term effectiveness is governed by resistance to degradation, but this cause of potential destruction may be vastly different in specific cases. A cost-benefit analysis needs to be performed to determine the optimum choice of geotextile to use in each case. 2. Water movement The first task to be carried out in this aim is to establish what the possible uses of geotextiles in water control might be. These may be subdivided into two broad categories, those uses in which the actual movement of water is channelled and those in which any water present is prevented from washing away other substances that should remain in position. The first of these includes primarily such applications as water course stabilisation, in which a stream, river, canal, etc., bank is prevented from overflowing its boundaries, so that the water flows only where it is supposed to flow. A further use, similar in principle, is as a tidal barrage, so that the shore is protected from large surges of water in heavy storms. Yet another application is the presence of geotextiles in roofing structures, where their function is to ensure that water flows along surfaces to the ground instead of falling on the people sheltered by the structure.

2 In considering the prevention of removal of other substances, the first two of the above-mentioned uses also carry out this function, in that the soil of the river bank or shoreline suffers reduced erosion. In addition, there is one further application that is intermediate between the two categories, the prevention of water loss from a landfill site. The need for this arises because containment of the water is essential to prevent dangerous substances, chemical or microbiological, which may be dissolved in it, from entering drinking water supplies, as they would if free escape was allowed. In the second category proper, the most obvious example is perhaps the stabilisation of a grassy slope. A geotextile structure is laid across the slope and remains in place until grass or other plantings have established a root structure that will prevent erosion of soil, washing away nutrients and destabilising the geometry of the slope, from taking place. Another example is the stabilisation of road beds, in which a geotextile fabric is laid over the sub-soil to make sure that the stones, concrete, etc. used in making the road cannot be washed away to create a dangerous possibility of subsidence. In each of these applications, there are conditions to which the geotextile is exposed that might hasten its degradation, and hence shorten its life. In order to counteract this undesirable occurrence, it is essential to identify the potentially harmful sources of fabric destruction and design the structure to remove or minimise the effects of the degradative source. 3. Sources of degradation By the very nature of their application locations, geotextiles are likely to be subjected to many potentially harmful actions. They must clearly be in virtually constant contact with water, and with anything the water happens to have dissolved in it. If they touch the ground, biological agents are almost certain to be present and may cause damage when they contact the fabric. If they are exposed to daylight, especially in tropical countries where ultraviolet levels are high, they may undergo actinic destruction. If they are near sea water, they may be degraded by salts. If they move in relation to their surroundings, they may suffer mechanical forces that bring about degradation. Even if they are not in contact with other solid surfaces but are subjected to high mechanical forces (which may be tensile, compressive, flexural, abrasive, tearing or bursting in nature), they can still be damaged. Thus, it is crucial to identify which degradative sources are in contact with the geotextile in any specific case before design of the optimum material for that application can be attempted. 4. Geotextile uses For the purposes of this paper, discussion will be restricted to the use of geotextile fabrics in the six applications mentioned above. Other uses exist, and many more will doubtless be developed, but the approach adopted in considering them to establish the principles of procedure will be the same in these other cases also. Table 1 shows to what level of each potentially harmful source of degradation each of the applications is likely to be exposed. A designation of high means that resistance to a specific source of degradation is crucial, one of mod(erate) implies a relatively minor need for resistance, and one of low means that the factor is almost, or completely, irrelevant. As can be seen in the Table, geotextiles (except when they are used in road beds or slope stabilisation) must be highly impermeable to liquid water transfer. Water vapour resistance, however, is never of major concern in any of the listed applications, a fact that eliminates the usual dilemma (crucial in rainwear and other protective garments, for instance) of how to prevent liquid water transmission from taking place while still allowing vapour to permeate. As might logically be expected, resistance to chemical, biological or ultraviolet sources of degradation is very much dependent on whether a

3 particular fabric is exposed to a particular source, a matter entirely governed by the location in which the material must be placed in order for it to be used. Similarly, resistance to mechanical action depends on the stresses applied to the geotextile product, again a matter of where it is installed and the conditions of contact with adjacent surfaces. A low or moderate need for resistance is achieved when the surrounding substances provide resistance to movement or stress application, or give structural support to the geotextile, while a high designation reflects a case where the fabric is left to provide resistance to the applied stresses without any aid from its surroundings. A good example is the comparison between uses in preventing leaching and in making roof structures; in the former case, the soil of the landfill holds the fabric firmly, and may even tend to crush it if the site is improperly established, while the latter use exposes the geotextile completely to the vicissitudes of wind and solar radiation. In order to provide characteristics that meet the needs of a specific situation to the best possible extent, fibre type and fabric design factors must be carefully considered. 5. Fibre selection and construction factors In considering the type of fibre to use, cost is always an important factor. In theory, many of the newer materials initially developed for space-age applications would probably be far superior to some of those used in routine geotextile fabrics, but their cost is prohibitive for a textile product that is often regarded as almost akin to scrap. If one material lasts, say, for five years in use and the life of the geotextile can be extended by using a more expensive one, then the increased cost may or may not be justified. If the cost of the substitute fabric is twice that of the original one but the life is increased to fifteen years, then the added expense may be warranted. If life expectancy is only increased to ten years, though, then it may not be regarded as worth paying the extra cost, since the only added expense in five years time is the cost of labour for replacement. If the cost is twenty times as large, though, there is probably no point in considering the substitution, even if the increase in life span is expected to be thirty times. There are good reasons for this; first, a life expectancy of one hundred and fifty years is not worth taking the risk of financial ruin because of the initial high cost and, second, during the lengthy period in question there is every likelihood that new technology may well produce vastly superior materials at a fraction of the cost. As a result, the choice of fibre content is usually restricted to polyester, polypropylene, polyethylene or polyamide. The decision of which one to choose is therefore simplified considerably, though the possible usefulness of a natural fibre should also not be ignored. The requirements of the fibre type preferred are again heavily dependent on the use envisaged. In applications where exposure to microbiological agents is likely to reduce effectiveness, for instance, natural fibres are normally unacceptable. The obvious exception, in the case of a geotextile used for slope stabilisation, arises from the fact that a natural fibre, usually jute or hemp, is needed only for a relatively short period of time to allow root structures to become established, after which decomposition of the geotextile used may often be desirable for the purpose of visual aesthetic reasons. It is thus entirely reasonable to add natural fibres to the list of four materials provided above for use in such specialised cases. Where exposure to solar radiation is heavy during plant growth, though, a natural fibre may be destroyed before enough root structure is established for good stabilisation. In that case, one of the four above-mentioned synthetic fibres would still be preferred. The actual one selected depends on other factors; if mechanical stresses are high because of a need to stretch the fabric tightly, or because the stabilised slope is in a high-traffic area, as in a spectator embankment, then polypropylene or polyester would be better than nylon, because the latter tends to be degraded more easily in sunlight and is then subject to failure as a result of force application. A relatively open-weave structure is

4 usually desirable to allow the access of rain for watering the plants and so that roots can spread without hindrance. In terms of human safety and environmental protection legislation, the most stringent requirements are probably those exercised in preventing toxic substances from leaching out of landfill sites. Because ultraviolet exposure of a buried fabric is negligible, the actinic degradation effect from this source does not need to be considered, and nylon may well be acceptable. If there is constant movement of the geotextile as a result of water flow or soil shifting beside or below the barrier, though, then it may be preferable to choose polypropylene because of its superior resistance to fatigue flexing. In either case, the use of a coating to enhance water flow resistance is advisable; coatings are usually of PVC or polyurethane, though these materials may decompose slowly over a long period of time. To counteract this, the more stable PTFE may be used if it can be sandwiched between fabric layers to prevent problems arising from its inherent fragility and potential for delamination. The most crucial application, from the viewpoint of diversity and magnitude of challenges, is the use of geotextiles as river bank reinforcement structures. Ultraviolet resistance is categorised as a moderate criterion, but all others (except for moisture vapour penetration) are rated as high in importance. Ultraviolet resistance may in fact become a significant factor in times of drought, or of low water flow for other reasons, when the stabilising structure is exposed above the surface of the water for a considerable time and subjected to solar radiation. Drought conditions usually coincide with hot weather when radiation levels are often highest, so the moderate classification reflects a variation in need over a period of time rather than a consistently moderate probability of degradation occurring as a result of solar exposure. In such a case, the fibre selected, which must also be highly resistant to mechanical stresses, would probably be polypropylene or, if movement was restricted but tensile stress constant and high, polyester. In this application, resistance to liquid flow would be of paramount importance because seepage would defeat the purpose of the containment, so a tightlywoven structure with a coating on the surface would be able to provide best results. Road bed applications provide a different set of constraints. Here there is no concern about solar exposure, nor is there any need to provide resistance to liquid water transfer. In fact, good permeability to water movement is an advantage, since it encourages drainage away from the foundations of the road to reduce the risk of flooding, frost movement, or displacement of structural components in heavy rain. As long as the sub-surface is properly installed, there will be little or no movement of the geotextile, and hence no concern about tensile stress resistance. What is important is the ability to resist compressive and, if the subsurface is not hard, burst stresses. Nylon, with its high elasticity and breaking elongation, is probably the optimum choice in this case, and a loosely-woven structure would meet the needs of the situation well as long as the fabric is not distorted badly during initial installation. For roofing uses, a totally different set of criteria is needed. The fabric will inevitably be placed under high tensile and flexural stress, and may be subjected to abrasion or bursting stresses in unusually high wind situations. In addition, ultraviolet degradation is likely to be high, and the need to resist liquid water penetration is paramount in order to provide protection in the shelter of the roofed area. Coating will almost inevitably be needed, and should be chosen to reduce ultraviolet degradation. A dyestuff able to absorb well in the ultraviolet region of the electromagnetic spectrum may also be advantageous in prolonging the life of the roofing fabric. The preferred fibre content will probably be polyester, in a high-tenacity form, since this material has high acid resistance that will enable it to last well in the presence of acidic components in rain or near an industrial site. Polyester also resists well, in terms of mechanical stability, the changes in temperature that occur during seasonal variations in much of the world. A closely-woven structure, to reduce the possibility of mechanical deformation during use, would be the best substrate fabric on which to deposit the coating, and the fabric weight or thickness would be selected to provide the optimum compromise between high strength and low weight.

5 Polyester may also be the optimum choice for tidal barrage protective devices, not only for the same reasons of resistance to solar radiation and mechanical stresses, but with the added ability to resist salt solutions. In this case, though, it will be necessary to incorporate a buoyant filling to make sure that the barrage never sinks. A foam such as polyurethane or expanded polystyrene is the best suggestion in this case, so that it will resist salt and retain its impervious nature over a long period of time. Thus, with care paid to the long-term needs of any specific geotextile use, it is theoretically possible to choose the single best combination of fibre type and construction technique to ensure satisfactory operation. There may, though, be a need for compromises to be made. If, for example, slope stabilisation is needed on the banks of a river that frequently overflows, then the need for a natural fibre with open structure for optimum stabilisation may be superseded by the need for a durable synthetic fibre with close-woven structure for bank stabilisation. The plant roots may then be stunted as a result of over-watering and lack of drainage, so it may be necessary in such cases to change the type of plant used on the banking to one able to tolerate high water levels around roots. Similarly, it may be beneficial to choose different fibres, of a slightly less satisfactory nature, to compensate for unusual environmental conditions. A tidal barrage subjected to frequent battering that causes high movement, for instance, may require polypropylene, with its lower density (and hence better buoyancy properties) and flexural resistance, for durability. The same fibre may be better suited for roof structures in high wind conditions, and again has the advantage of lower density to reduce the weight loading on the support structure. A road bed in which acid spills occur frequently (in an industrial setting, for instance) may have a longer lifetime if polyester is used instead of nylon. Such compromises, of course, will be made at the discretion of the designer after analysing all the potential hazards to which the geotextile is likely to be exposed. 6. Conclusions Geotextile products are finding increased use in many modern applications as a result of their ability to provide enhanced performance over more traditional materials at a lower cost and with much simpler application procedures. Their usefulness can be augmented if careful attention is paid to the choice of fibre content and construction techniques, to match the degradative conditions to which they are exposed. Chemical, microbiological, actinic and mechanical stresses can be a problem, and the design of fabrics for successful operation over long periods of time needs to have account taken of all these factors before final selection of fibre and fabric parameters is made.