GEOSYNTHETICS AND GEOSYSTEMS IN COASTAL ENGINEERING - A REVIEW -Pilarczyk Krystian, (former)Rijkswaterstaat, Hydraulic Engineering Institute, Delft, The NetherlandsHYDROPIL, Nesciohove 23, 2726BJ Zoetermeer, The Netherlands, firstname.lastname@example.orgABSTRACTGeosynthetics/geotextiles became a common component in filter construction (Giroud, 1987).Geosystems, however, as geomattress, geotubes, geocontainers, geocurtains, etc., are relatively newconstruction systems in civil (hydraulic/coastal) engineering applications. Till recently, these systemswere designed based mainly on the experience from the previous (usually small) projects. Informationprovided to the potential users is usually restricted to general folders. Pilarczyk (2000) has reviewedmost of the systems available on the international geosynthetic market. Actually, international researchand experience from the projects contributed to improvement of design techniques and their reliability.The overview on the use of geosynthetics and geosystems, sources of information (also relevantwebsites), and some recommendations for future use and/or improvements are discussed.1. INTRODUCTIONIn recent years traditional forms of hydraulic and coastal works/structures have become very expens-ive to build and maintain. Various structures/systems can be of use in hydraulic and coastal enginee-ring, from traditional rubble and/or concrete systems to more novel materials and systems such asgeotextiles/geosynthetics, natural (geo)textiles, gabions, waste materials, etc.Geosynthetics/Geotextile systems utilize a high strength synthetic fabric as a form for casting largeunits by filling them with air, water, sand or mortar, as a screen for guiding flow or a curtain forcollecting sand, etc. Structures made of flexible, high-tensile strength geosynthetics have theadvantage of simple manufacturing, lightweight transportation and usually an easy constructionprocess; strength and durability can be chosen according to the purpose. This all has prompted ademand for cheaper, less massive and more environmentally acceptable engineering as it is usinggeosystems. A large number of ideas has been born on the use of geosynthetics in civil/coastalengineering. Pilarczyk (2000) has listed a number of actual and/or potential applications. Some of therecent applications are shown in Figure 1.These new (geo)systems (geomattresses, geobags, geotubes, seaweed, geocurtains and screens) wereapplied successfully in number of countries and they deserve to be applied on a larger scale. Recently,geotubes and geocontainers filled with dredged material have been used in groynes, dikes andbreakwaters in a number of projects around the world, and their use in this field is growing very fast.Because of the lower price and easier execution these systems can be a good alternative for protectivestructures in hydraulic and coastal engineering both in developed and developing countries. The mainobstacle in their application, however, is the lack of proper design criteria (in comparison with rock,concrete units, etc.). In the past, the design of these systems was mostly based on rather vagueexperience than on the general valid calculation methods. More research, especially concerning thelarge-scale tests and the evaluation of the performance of projects already realised, is still needed. Therecent research on these systems has provided better insight into the design and applications.The aims of this paper are to review the pros and cons for the use of geotextiles/geosynthetics invarious geosystems with applications in hydraulic and coastal engineering, to present relevant datagained from various studies, and to record data from projects where geotextiles and geosystems wereinstalled. The following geosystems will be reviewed in more detail: geotextiles in revetmentstructures, geomattresses, geobags, geotubes and geocontainers.
The main points of the comment concern: availability of (proper) design criteria of the systems,functional design and durability, execution aspects, and limitations in application. NB. Geomattressesare discussed under Revetments (Chapter 2) because of similarity in design approach. Figure 1 – Some concepts on the application of geotextiles and geotextile systems
2. REVETMENTS: GEOTEXTILE FILTERS AND GEOMATTRESSES2.1. Functional designThe function of a revetment is to protect the slope against hydraulic (and other) loadings, such as wavesand currents. To evaluate the stability, information is required about the hydraulic design conditions, thestructural properties and the possible failure mechanisms. It is stressed that, when designing revetments,the designer should bear in mind that the geotextile is only one of the components involved, and that therevetment is only a part of the total project. Geotextiles primarily contribute to criteria on filtration andretention, but also to criteria on stability of revetment, and must also satisfy the other criteria resultingfrom functional requirements. The fulfilling of all theses criteria can even be conflicting.The geotextile can serve three functions, of which only the first two will be considered here:- separation; prevention of erosion of the subsoil through the structure,- filtration (permeability),- reinforcement of the subsoil against sliding.In principle, the geotextile must always remain more permeable than the base soil and must have poresizes small enough to prevent the migration of the larger particles of the base soil. Moreover, concerningthe permeability, not only the opening size but also the number of openings per unit area (Percent OpenArea) is of importance (Pilarczyk, 2000). However, it has to be stressed that geotextiles cannot alwaysreplace the granular filter completely. A granular layer can often be needed to reduce (damp) the hydraulicloadings (internal gradients) to an acceptable level at the soil interface. After that, a geotextile can beapplied to fulfil the filtration function. In respect to the filters for erosion control (granular or geotextile)the distinction can be made between (see Figure 2), (Klein Breteler et al., 1990, 1994, also on website): • geometrically tight filters • geometrically open filters, and • transport filters (when a limited settlement is allowed). Figure 2 – Principles of filtersThe main performance benefits of geotextiles are:- reduction in the number of layers and volume of granular filter materials,- providing consistent quality filtration characteristics properties,- effective filters for fine silty soils subject to turbulent variable flow conditions,- reduction of maintenance of erosion control systems and cost,- maintaining and enhancing stability of protection structures,- providing easier installation than conventional filter layers, particularly for underwater installation.
The important properties of geotextiles and criteria in respect to the functional requirements are:- suitable filtration qualities and high permeability,- stable fibre network,- resistance to damage during construction,- installation flexibility,- soil - geotextile friction angle, and- ultraviolet light resistance.2.2. Design criteria for geotextiles in revetment applicationsThe soil tightness of the initial situation and permeability requirements can be checked by means of thewell-known criteria for geometrically tight (geotextile) filters. However, the large number of this criteriaand often unclear limits of their application are very confusing for designers. Especially, the definition ofpermeabilities, treatment of unstable soils and proper interpretation of index tests vs. performance tests arestill a problem for designers .In revetment structures geotextiles are mostly used to protect the subsoil from washing away by thehydraulic loads, such as waves and currents. Here the geotextile replaces a granular filter. Unfor-tunately, the mere replacing of a granular filter by a geotextile can endanger the stability of othercomponents in the bank protection structure (i.e. internal stability of the subsoil at the interface with ageotextile). Therefore, an additional criterion concerning the necessary total thickness (or unit weight)of revetment (top layer plus sublayer) to avoid internal instability of soil should be defined (KleinBreteler et al. 1990, 1994, 1998, Pilarczyk, 2000, 2002). Also, the requirement that the permeability ofthe cover layer should be larger than that of the underlayers cannot be met in the case of a closed blockrevetment. The cover layer is less permeable, which introduces uplift pressures during wave attack. Inthe case of a geotextile situated directly under the cover layer, the permeability of the cover layerdecreases drastically. Since the geotextile is pressed against the cover layer by the out flowing water, itshould be treated as a part of the cover layer. In this case the permeability ratio of the cover layer andthe base or filter layer and its thickness, represented in the leakage length (Λ), is found to be the mostimportant structural parameter, determining the uplift pressure, see Figure3; (Klein Breteler et al.,1998, Pilarczyk, 2000, 2002). In general, all low-permeable top layers (block revetments andgeomattresses) should be design based on definition of their leakage length which provides a kind ofoptimization between all design requirements, and which leads automatically to application of theconcept of geometrically open geotextiles (Klein Breteler et al., 1990, 1994,1998, CUR, 1995).2.3 GeomattressesThe permeability of the mattress is one of the factors that determine the stability. It is found that thepermeability given by the suppliers is often the permeability of the geotextile, or of the so-called FilterPoints. In both cases, the permeability of the whole mattress is much smaller. A high permeability of themattress ensures that any possible pressure build-up under the mattress can flow away, as a result ofwhich the differential pressures across the mattress remain smaller. The stability is therefore the largestwith a large mattress permeability. In the long term, however, pollution of the Filter Points or the cloggingof the geotextile can cause a decrease in the permeability. The susceptibility for blocking can be reducedby increasing the gradation of the subsoil. To reduce the susceptibility for clogging it is recommended toreduce the sludge content of the subsoil. Due to the lack on proper information on the total permeability ofthe mattresses, the indicative permeability of mattresses can be calculated based on the knowledge ofplaced block revetments and the collected information from the literature and company information. Byintroducing the concept of leakage length the indication of stability for various geomattresses can be givenas shown in Figure 3 (Klein Breteler et al., 1998, Pilarczyk, 2000, 2002). To obtain more accurate resultsit is recommended to perform (by manufacturers) permeability tests for mattresses as a hole (as a system)and some model/prototype tests for verification of stability; see also website (insert Breteler for author):http://www.wldelft.nl/rnd/publ/search.html.
bDk bk Λ= Λ / D= k Dk (Λ=leakage length [m], D = thickness of top layer b = thickness of the filter layer (m), k = permeability of the filterlayer or subsoil (m/s), and k ′ = permeability of the top (cover)layer (m/s)) Figure 3 – Calculation results for concrete mattressesHydraulic performance of geomattresses. About 20 years of experience with concrete mattresses all overthe world provides an evidence that these systems, if properly designed and taking into account allpossible failure modes, may function as originally designed and with a minimum or no maintenance(Spraque et al., 1992). Documented measurements exist for application of these systems on slopes equalor milder than 1 on 1.5 with wave heights up to 1.5 m and current velocities up to 7 m/s. Application ofconcrete mattress placed directly on sand is usually limited to a wave height of about 1.0 m due to thepossible geotechnical instabilty of subsoil when exceeding this value. Often, other failure modes can bedecisive for the damage of structure, therefore, especially in case of a severe wave attack, it is important toput due attention to design of the filter, the flanks, the crown (crest) and the toe of the mattress (Pilarczyk,2000).Durability. Ultraviolet (UV) strength degradation of geosynthetics is always a concern to prospectiveusers. The fabric for geomattresses is usually manufactured with nylon and polyethylene yarns which are,on short term (a few years), quite UV resistant. Often, silt which is accumulated in textured surface willprovide the additional protection. However, the top layer of fabric forming the concrete units willgradually loose its strength. The bottom fabric is not subject to UV degradation and therefore does notsuffer loss of strength. The geotextile is principally treated as a fabric form for the containment ofconcrete; the reduction of strength in the top fabric does not necessarily effect the stability of the system.Where appearance is an important consideration, the upper surface may be spray coated with dilutecolored acrylic emulsion, at about 5 year intervals, which provides also the additional protection of thefabric against ultraviolet degradation. Figure 4 – Example of functioning of a block mat attached to the geotextileOn long term, especially when no UV-protection for geotextile is applied, the surface-geotextile willdeteriorate and the concrete filled-mat will function as a block mat (Figure 4); a block mat with concrete
units connected to the lower sheet of geotextile by existing binders, which normally are used as spacers toprovide a required thickness. These binders should have a proper strength to compensate the weight of theconcrete element. That also means that for structures with a long lifespan the stability of the mattressshould be controlled for this situation.The system illustrated in Figure 4 is based on a geotextile to which the blocks are attached. Since thegeotextile allows blocks to be displaced slightly a considerable interactive force can be mobilized whichmay induce the movement of the filter material. The system only works properly if washed-in material isapplied. If the system does not satisfy the above specifications the cover layer should be designed as looseblocks.Additional cabling. Where severe wave action is anticipated possibly with danger of severe scour, orwhere soil cannot be properly compacted and extensive settlement is expected (i.e. soft soils), concretemattresses may be constructed (strengthened) with cabling. Nylon or polyester cables (ropes) are usuallyused instead of steel cables to avoid any possibility of damage by corrosion. Cables are inserted betweenthe two layers of fabric prior to concrete injection. Synthetic cables are lighter than the grout slurry that isutilized to fill the mats and therefore semi floats within the grout mass. In addition the cable pass throughthe grout ducts rise in every block and through the edge narrow section interconnecting the blocks. Whenfilling the fabric with the grout slurry the grout ducts rise to the approximate center line of the blocks.Therefore, the cables are nominally located in the center of the blocks. The cables become embedded inthe concrete-filled compartments (blocks) to enable the mattress to resist tension.RecommendationsFor the future study on the design guidelines for the concrete mattresses the following items can berecommended: There is still a need for a proper testing and specification of permeability for various geomattresses, and for verification of calculation method on the effective permeability of systems. It is still necessary to determine more precise values of internal friction between the mattresses and various subgrades (including geotextiles) both, in dry conditions as well, in wet conditions (under water). It is necessary to investigate/collect information concerning the real contact (cavities) between the mattress and the subsoil for various types of mattresses, and various quality of execution. This is important in respect to the possible soil migration along the slope leading to the deformation of subsoil surface, and also in respect to the uplift forces on the mattress. This aspect likes the most uncertain point in the design procedure, and may affect the quality of design. The existing stability criteria for the wave attack should be verified on large scale, especially, for applications with wave height larger than 1 m; More attention should be paid to the stability of the edges of the mattress to avoid overturning by current action. When using concrete mattresses for pipeline protection, especially in the surf zone, some additional studies are needed on their stability under the combined action of currents and breaking waves. Due to the fact that the mattresses, on the long term, must function as block mats the number and strength of binders should be carefully examined for this purpose. The already existing constructions and the future applications should be systematically documented and evaluated in respect to their performance under various soil and hydraulic conditions. These information can be used for further improvement and/or extension of the range of application of these systems.2.4 Execution aspectsFilter or revetment constructions with geotextiles for riverbank and shore protection, bottom protection,and other applications are vulnerable during installation. Especially, the risk of damage caused to thegeotextiles by falling stones should not be underestimated. The following factors should be taken intoconsideration:- weight and shape of stones;
- the fall height;- type and strength of geotextile;- type, compaction grade and saturation level of the subsoil.The following situations can be distinguished:- the tensile strength of a geotextile is exceeded due to the falling stone. It can happen because of the stone penetrating the geotextile or by punching when the stone laying on the geotextile is hit by another stone;- the tensile strength of a geotextile between the stones is exceeded due to the tensile forces exerted by stones, resulting in the geotextile tearing.Although the existing damage criteria are not yet fully examined they are usually adequate for practice iffollowed. Therefore the proper supervision during execution is of great importance.3. GEOSYSTEMS3.1. General informationGeotextile systems utilize a high-strengt synthetic fabric as a form for casting large units by filling withsand or mortar. Within these geotextile systems a distinction can be made between: bags, mattresses,tubes, containers and inclined curtains. All of which can be filled with sand or mortar. Some examplesare shown in Figures 1 and 5. Mattresses are mainly applied as slope and bed protection. Mattresses werealready discussed under Revetments (Chapter 2). Bags are also suitable for slope protection and retainingwalls or toe protection but the main application is the construction of groynes, perched beaches andoffshore breakwaters (Heerten et al., 2000, Lawson, 2003). The tubes and containers are mainlyapplicable for construction of groynes, perched beaches and (offshore) breakwaters, and as bunds forreclamation works (Figure 6). They can form an individual structure in accordance with some functionalrequirements for the project but also they can be used complementary to the artificial beach nourishmentto increase its lifetime. Especially for creating the perched beaches the sand tubes can be an ideal, low-cost solution for constructing the submerged sill. They can also be used to store and isolatecontaminated materials from harbour dredging, and/or to use these units as bunds for reclamationworks.a) Geotube as a breakwater b) BEROSIN system (inclined curtain) as bed protection Figure 5 – Examples of application of geosystems
Offshore breakwater at design water level CD+3.5 m 50 m Figure 6 – Example of reef structure composed with geotubes (Fowler et al., 2002), http://geotecassociates.com/; (also: Pilarczyk, 2003, www.tawinfo.nl)An interesting application for shore erosion control is the geocurtain known under the name BEROSIN(Figure 5b). The BEROSIN curtain is a flexible structure made of various woven geotextiles, which afterplacing by divers near the shore and anchoring to the bed catches the sand transported by currents andwaves providing accretion on a shore and preventing the erosion. The horizontal curtain (sheet) can beeasily spread (at proper sea conditions) by a small workboat and two divers. The upper (shore-side) edge,equipped with some depth-compensated floaters, should be properly anchored at the projected line. Thesea-side edge is kept in position by the workboat. By ballasting some of the outside pockets at the loweredge with sand or other materials and with help of divers, the lower edge is sinking to the requiredposition. The proper choice of permeability of geotextile creates the proper conditions for sedimentationof suspended sediment in front/or under the curtain and at the same time allowing the water to flow outwithout creating too high forces on the curtain and thus, on the anchors. In case of Pilot project at thecoast of Vlieland (NL), some of the horizontal curtains placed in the intertidal zone have provided agrowth of a beach/foreshore of 0.5 to 1.0 m within a week while others within a few weeks (Pilarczyk,2000). Geocurtains can also be applied for construction of submerged reefs and sills.In the past, the design of geotextile systems for various coastal applications was based mostly on rathervague experience than on the general valid calculation methods. However, the increased demand inrecent years for reliable design methods for protective structures have led to the application of newmaterials and systems (including geotextile systems) and to research concerning the design of thesesystems. Contrary to research on rock and concrete units, there has been no systematic research on thedesign and stability of geotextile systems. However, past and recent research in The Netherlands, USA,Germany and in some other countries on a number of selected geotextile products has provided someuseful results which can be of use in preparing a set of preliminary design guidelines for the geotextilesystems under current and wave attack (Pilarczyk, 2000). The results from the recent, large scale tests,with large geobags, can be found on the website: http://www.fzk.uni-hannover.de/286.html?&L=1 andhttp://www.delftcluster.nl/website/files/files_org/consortium/Final_H4595_low.pdf.The main (large) fill-containing geosystems (geobags, geotubes and geocontainers filled with sand ormortar) and their design aspects are briefly discussed below. For more detailed information on these andother coastal protection systems and measures applied nowadays throughout the world, together withrecommendations and guidelines, the reader will be guided to the relevant manuals and publications (seereferences and websites).
3.2. GeobagsGeobags can be filled with sand or gravel (or cement, perhaps). The bags may have different shapesand sizes, varying from the well-known sandbags for emergency dikes to large flat shapes or elongated"sausages" (see Figure 7). The most common use for sandbags in hydraulic engineering is fortemporary structures. Uses for sand- or cement-filled bags are, among other things: · repair works; · revetments of relatively gentle slopes and toe constructions; · temporary or permanent groynes and offshore breakwaters; · temporary dikes surrounding dredged material containment areas.Because this material is easy to use and cheap, it is extremely suitable for temporary structures. Atraining groyne is a good example. The working of a groyne is difficult to predict in advance. That iswhy it is a good procedure to make such a construction using a relatively cheap product first, to seehow one thing and another works out, and subsequently either make improvements or, after sometime, a permanent structure. Above a flow velocity of 1.5 m/s, the geosystems filled with sand cannotbe used because the sand in the systems is no longer internally stable. Figure 7– Application of geobagsSandbags can be placed as follows:1. As a blanket: One or two layers of bags placed directly on the slope. An "interlocking" problem arises if the bags are filled completely. The bags are then too round. A solution is not to fill the bags completely, so that the sides flatten out somewhat, as a result of which the contact area becomes larger.2. As a stack: Bags stacked up in the shape of a pyramid. The bags lie half-overlapping with the long side parallel to the shoreline.When installing geobags/geosystems, one should see to it that this does not take place on a roughfoundation. Sharp elements may easily damage the casing of the element. Geosystems must not befilled completely. With a fill ratio of approximately 75% an optimum stability of the elements isreached. A sound surface protection is necessary if sandbags are used in circumstances where they areunder heavy attack of flow or waves. Stability criteria are discussed in Pilarczyk (2000). The firstindication on stability against waves, based on small-scale and large-scale tests, is given below:- Stability on slopes
Hs 2.5 = ∆ D cr ξ op (1)For sand-filled bags the relative density is including water in pores (∆=1); ξop= breaker parameter.- Stability of crest elements of breakwaters around SWL (for units lying parallel to the axis of astructure) Hs/(∆ b) = 0.5 to 1 (2)In which ‘b’ is the width of the unit (perpendicular to the axis of the structure).More recent, large scale tests, with large geobags on slopes (Figure 8), can be found on the website:http://www.fzk.uni-hannover.de/286.html?&L=1 (Oumeraci et al., 2003). Application of geobags astoe protection of wind-farm foundation in the sea can be found in Grune et al.(2006). Figure 8 – Large-scale tests Wave Flume HannoverStability criteria Hannover (Oumeraci, 2003, Restall et al., 2004):- on the slope (3)where D= L.sinα, with L= the length of geobag placed perpendicularly to the structure face.-stability of the crest elements of slope structure (4)where Rc is the distance between the crest and the Still Water Line.Recently, Recio (2007) studied the hydraulic stability of sand-filled containers and the influence ofdeformation on their stability in the scope of PhD-thesis: http://www.digibib.tu-bs.de/?docid=00021899. The scope of his study is summarized below.“Softer and low cost protection alternatives, such as structures made of geotextile sand containers(GSC) are often used instead of more expensive and hard coastal structures made of concrete or rubblematerial. Although the effect of the deformations of the sand containers is significant, no stabilityformula is available to account for those deformations. To achieve a better understanding of theprocesses that affect the stability of GSC-structures several types of hydraulic model experiments andanalyses were performed. The processes investigated were: (i) permeability of GSC-structures, (ii)wave-induced loads on the sand containers, (iii) wave induced flow on GSC-structures, (iv) internalmovement of sand in the containers, (v) variation of contact areas among neighbouring GSCs duringwave action, (vi) types of displacement of GSCs within a coastal structure and finally (vii) the effectof the deformations on the stability of GSC-structures. In addition, a flow model and two structural
dynamic models were further developed, validated and applied towards a better understanding of theprocesses involved in the wave structure interaction. Based on the experimental and numerical results,analytical stability formulae that account for the deformation of the individual GSCs for each type ofobserved displacement are developed. Stability formulations for each type of coastal structures madeof geotextile sand containers such as breakwaters, revetments, dune reinforcement and scourprotection systems are proposed.”3.3. Tube systemGeotube is a sand/dredged material filled geotextile tube made of permeable but soil-tight geotextile.The desired diameter and length are project specific and only limited by installation possibilities andsite conditions. The tube is delivered to the site rolled up on a steel pipe. Inlets and outlets areregularly spaced along the length of the tube. The tube is filled with dredged material pumped as awater-soil mixture (commonly a slurry of 1 on 4) using a suction dredge delivery line (Figure 9). Thechoice of geotextile depends on characteristics of fill material. The tube will achieve its desired shapewhen filled up to about 80%; a higher filling grade is possible but it diminish the friction resistancebetween the tubes. Figure 9 – Filling procedure of GeotubeTube can be filled on land (e.g. as dikes for land reclamation, bunds, toe protection or groyns) or inwater (e.g. offshore breakwaters, sills of perched beaches, dikes for artificial islands or interruption ofgullies caused by (tidal)currents). The tube is rolled out along the intended alignment withinlets/outlets centered on top. When a tube is to be placed in water, the effects of buoyancy on the tubegeotextile prior to filling as well as on the dredged materials settling characteristics must beconsidered. In order to maximize inlet/outlet spacing, an outlet distant from the inlet may be used toenhance the discharge of dredged slurry and thereby encourage and regulate the flow of fill materialthrough the tube so that sufficient fill will flow to distant points. The tube will achieve its desiredshape when it is filled up to about 70 to 80% of the theoretical circular diameter or a height equal toabout a half of the flat width of the tube; a higher filling grade is possible but it diminishes thefrictional resistance between the tubes.Commonly, the filter geotextile (against scour) and flat tube are fully deployed by floating and holdingthem in position prior to beginning the filling operation. The filter geotextile is often furnished withsmall tubes at the edges when filled with sand holds the filter apron at place. This apron must alsoextend in front and behind the unit, commonly 2 times the height of the entire structure or 2 to 3 timesthe local wave height. The wave transmission can be treated as for smooth impermeable structures.
3.4. Container systemGeocontainer is a mechanically-filled geotextile and "box" or" pillow" shaped unit made of a soil tightgeotextile. The containers are partially prefabricated by sawing mill widths of the appropriate lengthtogether and at at the ends to form an elongated "box". The "box" is then closed in the field, afterfilling, using a sewing machine and specially designed seams (Figure 10). Barge placement of the site-fabricated containers is accomplish using a specially configured barge-mounted crane or by bottomdump hoppers scows, or split barges. The containers are filled and fabricated on the barge and placedwhen securely moored in the desired position. Positioning of barge for consistent placement - a criticalelement of constructing "stacked" underwater structures - is accomplished with the assistance ofmodern surveying technology. The volume of applied geocontainers was up to 1000 m3. Figure 10 – Filling and placing of geocontainer
The advantage of these large barge-placed geocontainers include:* Containers can be filled with locally available soil, which may be available from simultaneous dredging activities;* Containers can be relatively accurately placed regardless of weather conditions, current velocities, tides, or water depths;* Contained material is not subject to erosion after placing;* Containers can provide a relatively quick system build-up;* Containers are very cost competitive (for larger works).3.5. General Design Considerations of GeosystemsWhen applying geobags, geotubes and geocontainers the major design considerations/problems arerelated to the integrity of the units during filling, release and placement impact (impact resistance,seam strength, burst, abrasion, durability), the accuracy of placement on the bottom (especially at largedepths), and the stability under current and wave attack.The geotextile fabric used to construct the tubes is designed to:* contain sufficient permeability to relieve excess water pressure,* retained the fill-material,* resist the pressures of filling and the active loads without seams or fabric rapture,* resist erosive forces during filling operations,* resist puncture, tearing, and ultraviolet light.The following design aspects are particularly of importance for the design of containers:a) Change of shape of units in function of perimeter of unit, fill-grade, and opening of split barge,b) Fall-velocity/equilibrium velocity, velocity at bottom impact,c) Description of dumping process and impact forces,d) Stresses in geotextile during impact and reshaping,e) Resulting structural and execution requirements, andf) Hydraulic stability of structure.Some of these aspects are briefly discussed hereafter.Shape and mechanical strength of geotubes. For the selection of the strength of the geotextile andcalculation of a required number of tubes for a given height of structure, knowledge of the real shapeof the tube after filling and placing is necessary. The change of the cross-section of the tube dependson the static head of the (sand)slurry. Depending on this static head, the laying method and thebehaviour of the fill-material inside the tube, it is possible that the cross-sectional shape of the filledtube (with a theoretical cross-section with a certain diameter, D) will vary from a very flat hump to anearly fully circular cross-section. More recently, Silvester (1990) and Leshchinsky (1995, 1996)prepared some analytical or numerical solutions and graphs allowing the determination of the shape ofsand- or mortar-filled tubes based on some experiments with water. The Leshchinskys method (PC-program) combines all the previous developments and can be treated as a design tool. The design ofthe shape of the geotube is an iterative process. To obtain a proper stability of the geotube and to fulfilthe functional requirements (i.e. required reduction of incoming waves/proper transmission coefficientthe width and the height of the tube (= a certain crest level) must be calculated. If the obtained shapeof geotube does not fulfil these requirements a new (larger) size of a geotube must be taken intoaccount or a double-line of tubes can be used.Dumping process of containers and practical uncertainties. A summary of various forces duringthe dumping and placement process is given in Figure 11 ( Pilarczyk, 2000).1. The required perimeter of geotextile sheet must be sufficient enough to release geocontainer throughthe given split width bo for a required cross-sectional area of material in the bin of barge Af (or filling-ratio of fill-material in respect to the max. theoretical cross-section). The derivation of the requiredminimum length of perimeter of geotextile sheet is given in (Pilarczyk, 2000). After opening of the
split of a barge the geocontainer is pulled out by the weight of soil but at the same time the frictionforces along the bin side are retarding this process. Due to these forces the tension in geotextile isdeveloping at lower part and both sides of the geocontainer. The upper part is free of tension till themoment of complete releasing of geocontainer. The question is how far we are able to model a frictionand the release process of geocontainer.2. Geocontainer will always contain a certain amount of air in the pores of soil and between the soiland the top of (surplus) geotextile providing an additional buoyancy during sinking. The amount andlocation of air pockets depends on soil consistency (dry, saturated) and uniformity of dumping. The airpockets will exert certain forces on geotextile and will influence the way of sinking. The question ishow to model in a proper way the influence of soil consistency and air content on shape and stresses ingeotubes/geocontainers.3. The forces due to the impact with the bottom will be influenced by a number of factors: * consistency of soil inside the geocontainer (dry, semi-dry, saturated, cohesie, etc.) and its physical characteristics (i.e. internal friction); * amount of air; * permeability/airtightness of geotextile; * strength characteristics of geotextile (elasticity/elongation vs. stresses, etc.); * fall-velocity (influenced by consistency of soil; saturated soil diminish amount of air but increa- ses fall speed); * shape and catching surface of geocontainer at impact incl. effect of not horizontal sinking (i.e. catching of bottom with one end); * type of bottom (sand, clay, soft soil, rock, soil covered with rockfill mattress, etc.) and/or type of sublayer (i.e. layer of previous placed containers). Figure 11 – Development of forces during dumping of geocontainersDuring the impact the cross-sectional shape of geocontainer will be undergoing a continuousreshaping; from cone shape, first probably into a transitional cylindrical shape, and through a certainrelaxation, into a semi-oval shape or flat triangular/rectangular shape dictated by soil type, perimeterand elongation characteristics of geotextile. The question is how far we are able to model this impactphenomena and resulting forces/stresses in geotextile. The impact forces with the bottom are afunction of the fall velocity (dump velocity) of a geocontainer (Pilarczyk, 2000, CUR, 2006).
4. In final situation the geocontainers will perform as a core material of various protective structures oras independent structure exposed to loading by currents and waves, and other loadings (ice, debris,ship collision, vandalism, etc.). In most cases the geocontainers will be filled by fine (loosely packed)soils. The question is how these structures will behave in practice under various types of external andinternal loadings.Practical note: The prototype experience indicate that geocontainers with volume up to 200 m3 anddumped in water depth exceeding 10 m have been frequently damaged (collapse of seams) using geo-textile with tensile strength lower than 75 kN/m, while nearly no damage was observed when using thegeotextile with tensile strength equal or more than 150 kN/m. This information can be of use for thefirst selection of geocontainers for a specific project. The placing accuracy for depths larger than 10 mis still a problem (Bezuijen et al., 2004). However, it should be realized that craftsmanship is veryimportant for a successful geocontainer project and can hardly be described with formulas.3.6. DurabilityDurability of geotextiles is frequently coming question especially concerning the applications where along life-span is required. Geotextile is a relatively new product. The first applications are from 60s.Recently, we have tested in the Netherlands some 30-years old geotextiles used as a filter in revetmentstructures (K&O, 1979). In general, these geotextiles were still in a good conditions (still fulfillingproperly the prescribed filter and strength functions). It was also confirmed by recent internationalexperience (Raymond et al., 1993, Mannsbart et al. , 1997, CEN, 1998). The technology of geotextilesis improved to a such extend that the durability tests under laboratory condition indicate the life-timeof geotextiles at least of 100 years (when not exposed to UV radiation).There is no problem with durability of the geosystems when they are submerged or covered by armourlayers. However, in case of exposed geosystems the UV radiation and vandalism are the factors, whichmust be considered during the design. All synthetics are vulnerable to UV. The speed of UVdegradation, resulting in the loss of strength, depends on the polymer used and type of additives.Polyesters (PET) are by nature more light stable than, for example, polyamide (PA) and polypropylene(PP). As an example, the Dutch tests with geosynthetic ropes (stabilised and not stabilised) exposed tovarious environment have provided the following results (see Table) concerning the strength of thesurface yarns after 3 years (in %) in comparison with the original strength.To avoid the problem with light degradation the fabrics must be properly selected (i.e. polyester) andUV stabilized. As the period in which the fabric is exposed is short (in terms of months), no seriousproblems are to be expected. In case of more or less permanent applications under exposed conditionsthe fabric must be protected against direct sunlight. There is a number of methods of surfaceprotection for geosystems. To provide additional UV and abrasion protection to the exposed sectionsof tubes, a coating of elastomeric polyurethane is often used. This coating, however, has a tendency topeel after about a number of months and therefore, has to be re-applied. Table 1- Results durability tests on geosynthetic ropes fabric type land-climate sea-climate intertidal zone 50 m under ebb-flood the sea level PET: stabilised 63% 62% 94% 93% PA : stabilised 33% 8% 85% 91% PA : no 14% 6% 80% 71% PP : stabilised 41% 46% 93% 95% PP : no 1% 16% 92% 95%Note: the geosynthetics under water and in the intertidal zone show very little degradation in strengthin comparison with geosynthetics placed on land; in the inter-tidal zone the geosynthetics are coveredvery soon by algae which provide very good UV protection.
The permanent surface protection by riprap or blockmats is a rather expensive solution and it willnormally be applied only when it is dictated by necessity due to a high wave loading or danger ofvandalism or other mechanical damage ie. boating, anchoring, etc. In other cases it will be probably acheaper solution to apply a temporary protection of geotextile tubes by an additional layer of a stronggeotextile provided with special UV-protection layer. This extra protection can be realized by addingthe highly U.V.-stabilized nonwoven fleece needled onto the main fabric. The function of this feltlayer is also to trap the sediment particles and algae, which give again extra U.V.-protection.4. CONCLUSIONSGeosynthetics and geosystems constitute potential alternatives for more conventional materials andsystems. They deserve to be applied on a larger scale. However, doubts among specifying authoritiesand design engineers about the quality of the design criteria for some of the products, and the long-term performance, are still limiting factors in the increased use. It is hoped that the results of theliterature search (Pilarczyk, 2000) and recent international research will help to overcome some ofthese doubts and will be used as a basis for learning and promoting in the application of geosyntheticsand geosystems.The basic material for geosystems are geotextiles or, more generally, geosynthetics. Proper knowledge ofthese materials (technological properties, design specifications, test methods, etc.) is essential for a properchoice of material needed to fulfil the functional requirements of geosystems resulting from the specificrequirements of a project under consideration. Information on which can be found in a number of publica-tions, textbooks, manuals and design guidelines. Moreover, the designer should bear in mind thatgeotextiles and geosystems are only a part (or a component) of the total project and that they have to betreated and integrated in the total perspective of a given projectSystematic (international) monitoring of realized projects (including failure cases) and evaluation ofthe prototype and laboratory data may provide useful information for verification purposes and furtherimprovement of prediction methods. It is also the role of the national and international organizations toidentify this lack of information and to launch a multiclient studies for extended monitoring andtesting programmes, to provide users with an independent assessement of the long-term performanceof geosynthetics and geosystems.The final conclusion on the use of the fill-containing geosystems can be formulated as follows:- There are still much uncertainties in the existing design methods. Therefore, further improvement ofdesign methods and more practical experience under various loading conditions is still needed.- There is an urgent need for internationally accepted guidelines for design and application ofgeosystems. The IGS in cooperation with other international organisations (i.e., PIANC) shouldundertake actions in this direction.Finally, there is a rapid development in the field of geosynthetics and geosystems, and there is always acertain time gap between new developments (products and design criteria) and publishing them inmanuals or specialistic books. Therefore, it is recommended to follow the professional literature on thissubject (Journal of Coastal Engineering, Journal of Geotextiles and Geomembranes, GeosyntheticsInternational, Geotechnical Fabric Report, and Proceedings of Geosynthetic Conferences) for updating thepresent knowledge and/or exchanging new ideas.REFERENCESBezuijen, A., de Groot, M.B., Klein Breteler, M. and Berendsen, E., 2004, Placing accuracy and stability of geocontainers, 3rd EuroGeo, Munich, Germany, also Proc. 7th IGS, Nice; see also: http://www.library.tudelft.nl/delftcluster/ (reports Delft Cluster 1, Coast and River).
CEN, 1998, Guide to durability of geotextiles and geotextile related products, European Normalization Committee (CEN), CEN/TC189/WG5/N210, Paris.CUR, 1995, Design manual for pitched slope protection, CUR report 155, ISBN 90 5410 606 9, P.O. Box 420, Gouda, the Netherlands.CUR, 2006, Ontwerpen met geotextile Zandelementen (report CUR 217) (Design of sand-filled geo- elements; actually under translation), Centre for Civil Engineering Research and Codes (CUR), P.O.Box 420, Gouda, NL.Davis, J.E. and M. C. Landin, 1997, Proceedings of the national workshop on geotextile tube applica- tions, Technical Report WRP-RE-17, U.S. Army Engineer WES, Vicksburg.Groot, M.B. de; Klein Breteler, M.; Berendsen, E., 2004, Feasibility of geocontainers of the sea shore, Proc. 29th Int. Conf. on Coastal Engineering (ICCE), Lisbon.DELOS, 2005, Environmental Design of Low Crested Coastal Defence Structures; D 59 DESIGN GUIDELINES, EU 5th Framework Programme 1998-2002, Pitagora Editrice Bologna; www.delos.unibo.it.Fowler, J., Stephens, T., Santiago, M. and De Bruin, P., 2002, Amwaj Islands constructed with geotubes, Bahrein, CEDA Conference, Denver, USA; http://geotecassociates.com/.Giroud, J.P., 1987, Geotextiles and Related Products, in Geotechnical Modelling and Applications, Sayed M. Sayed editor, Gulf Publishing Company, Houston.Grüne J., Sparboom, U., Schmidt-Koppenhagen, R., Wang, Z., Oumeraci H., 2006: Scour Protection for Offshore Wind Energy Monopile Structures with Geotextile Sandcontainers, Offshore Wind and other marine renewable Energies in Mediterranean and European Seas, Civitavecchia; http://www.fzk.uni-hannover.de/uploads/tx_tkpublikationen/ OWEMES__Rom__2006_Gruene_final_manuscript_Corrected_03.pdf.Heerten G., Jackson, A., Restall S., Saathoff F., 2000, New developments with mega sand containers of non-woven needle-punched geotextiles for the construction of coastal structures. Proc. 27th ICCE, Sydney.Klein Breteler, M.; Verheij, H.J., 1990, Erosion control by hydrodynamically sandtight geotextiles, 4th Int. Conf. on Geotextiles, Geomembranes and Related Products, The Hague, The Netherlands, Also: Delft Hydraulics publication 446, 1990.Klein Breteler, M.; Smith, G.M.; Pilarczyk, K.W., 1994, Performance of geotextiles on clay and fine sand in bed and bank protections, 5th Int. Conf. on Geotextiles, Geomembranes and Related Products, Singapore. Also: part of Delft Hydraulics Publication 488, 1995.Klein Breteler, M.; Pilarczyk, K.W.; Stoutjesdijk, T., 1998, Design of alternative revetments, 26th International Conference Coastal Engineering, Copenhagen, Denmark; (see also website: http://www.wldelft.nl/rnd/publ/search.html (insert Breteler for author).K&O, 1979, Shore and bank protection works in practice and theory; investigation of performance of geotextiles in practice (in Dutch), 25 years of Dutch Association for Coastal and Bank Protec- tion Works (K&O), Rotterdam.Lawson, C.R. (2003) “Geotextile containment: international perspectives”, Proceedings Seventeenth GRI Conference, Geosynthetic Institute, Philadelphia, USA, December, pp. 198-221.Leshchinsky Dov and Ora Leshchinsky, 1995, Geosynthetic confined pressurized slurry (GeoCoPS); supplement Notes for Version 1.0, US Army Corps of Engrs. and Nicolon Corporation.
Leshchinsky, D., O. Leshchinsky, H.I. Ling, and P.A. Gilbert, 1996, Geosynthetic Tubes for Confi- ning Pressurized Slurry: Some Design Aspects, J. of Geotechnical Engineering, ASCE, V. 122, No. 8.Mannsbart, G. and B.R. Christopher, 1997, Long-term performance of nonwoven geotextile filters in five coastal and bank protection projects, Geotextiles and Geomembranes, Vol. 15, no. 4-6.Oumeraci, H., M. Hinz, M. Bleck and A. Kortenhaus, 2003, Sand-filled Geotextile Containers for Shore Protection, Proceedings ‘Coastal Structures 2003’, Portland, Oregon, USA; http://www.fzk.uni-hannover.de/uploads/tx_tkpublikationen/sf_geocont_03.pdf; http://www.fzk.uni-hannover.de/286.html?&L=1Pilarczyk, K.W., 2000, Geosynthetics and geosystems in hydraulic and coastal engineering, A.A. Balkema Publ., Rotterdam; http://books.google.nl/books?q=Pilarczyk%2C+Geosystems&btnG=Pilarczyk, K.W., 2002, Design of Revetments, Hydraulic Engineering Institute, Delft; www.tawinfo.nl (select: english, downloads).Pilarczyk, K.W., 2003, Design of low-crested (submerged) structures: An overview, 6th COPEDEC, Sri Lanka; www.tawinfo.nl, (insert: english, downloads).Recio Molina, Juan Antonio, 2007, Hydraulic Stability of Geotextile Sand Containers for Coastal Structures - Effect of Deformations and Stability Formulae -, PhD-thesis, Leichtweiß-Institut für Wasserbau, Technical University of Braunschweig, Germany; http://www.digibib.tu- bs.de/?docid=00021899.Raymond, G.P and Giroud, J.P., eds., 1993, Geosynthetics Case Histories; thirty five years of experi- ence, International Society for Soil Mechanics and Foundation Engineering, Bitech Publishers Ltd., Richmond, Canada.Restall. et al, 2004, Australian&German experiences with geotextile containers for coastal protection, 3rd EuroGeo, Munich, Germany.Silvester, R., 1990, Flexible Membrane Units for Breakwaters, in Handbook of Coastal and Ocean Engineering, John B. Herbich, editor, Vol. 1, pp. 921-938.Sprague, C.J. and M.M. Koutsourais, 1992, Fabric formed concrete revetment systems, published in Geosynthetics in Filtration, Drainage and Erosion Control, R.M. Koerner, ed., Elsevier Applied Science (reprinted from Geotextiles and Geomembranes, Vol. 11, Nos. 4-6).Van Steeg, P. and Klein Breteler, M., 2008, Large scale physical model tests on the stability of geocontainers, , Report H4595, Delft Hydraulics and Delft Geotechnics for Delft Cluster; http://www.delftcluster.nl/website/files/files_org/consortium/Final_H4595_low.pdf