NEEDLE-PUNCHED NONWOVEN GTX IN COASTAL ENGINEERING APPLICATIONS

NEEDLE-PUNCHED NONWOVEN GTX IN COASTAL ENGINEERING APPLICATIONS Georg Heerten NAUE GmbH & Co. KG, Management, Espelkamp-Fiestel Abstract: Coastal and hydraulic engineering applications were the starting point of the technical development of geotextiles. Because of economical, technical and ecological advantages, the use of geotextiles and geocomposites for filter and drainage functions is increasing worldwide and has a 40 years history already. Based on the application and project specific requirements corresponding products have to be designed and selected, but also installed without damage. Especially the installation without damage is important, because a potential puncturing of geotextiles makes the filter design unnecessary and needless. The paper will present the design of geosynthetic filters and drainage structures. For coastal protection measures nonwovens are proven being long-term resistant against ultraviolet radiation and saltwater [1]. High elongation behaviour provides the superior properties during the construction load case, which is determined as being the biggest risk for damaging the geotextiles. The paper will highlight case studies from embankment construction, hydraulic and coastal engineering structures with the use of needle-punched nonwoven geotextiles. I. INTRODUCTION In numerous structures of geotechnical and hydraulic engineering filter layers and drainage systems play an important role for the long-term service of the structure. Due to cost advantages and the easy installation traditional grain filters are very often replaced by geotextiles. The design specification, selection and installation of geotextile filters are not always carried out with the necessary care and attention with the possible consequences for the serviceability of the structure. Damage and repair will cause much higher costs and energy than a careful design and installation of geotextile filters and geosynthetic drainage components. II. FILTRATION BASICS A filter used in geotechnical and hydraulic engineering must meet the almost contradictory retention and permeability criteria. Still today most designers wrongly assume that retention and permeability are the only criteria which are required for a geotextile filter design and possibly tensile strength requirements or other typical product values out of data sheets are additionally specified. But it is important to point out that thickness, mass per unit area and elongation at break are very important requirements for filtration design and installation robustness, too. A designed or checked (field test) high installation robustness with no risks of puncturing of the geotextile filter during installation is the precondition with regard to the designed and specified "opening size" of the geotextile filter! Regarding bank protection and revetment design with geotextile filters a long-term stable geotextile/subsoil interface (intimate contact) is absolutely necessary. Especially fine cohesionless soils with a high amount of silt and fine sand are susceptible to "downslope migration". Silts, sandy silts and fine sands tend to encourage high mobility of small single grains at very low hydraulic gradients. As the underwater trimming of the revetment slope always creates a soil interface of loose density more or less uneven a lot of parameters including local liquifaction can lead to a downslope migration failure of the revetment as shown in Fig. 1. Normally this kind of revetment failure is a slow developing mechanism which can take years until the revetment deformation is discovered. On the other hand, the starting of downslope migration is the beginning of the total failure of the revetment structure. Figure 1. Revetment failure caused by downslope migration of soil particles III. STATE-OF-THE-ART GEOTEXTILE FILTER DESIGN A. Actual design and specifications The current practice of the geotextile filter design very often only asks for an opening size and a water permeability or permittivity to be determined. A thickness requirement for geotextile filters is still quite unknown and specification criteria to avoid puncturing during installation are widely missing, too. The openig size O 90,w (= the effective opening size of the geotextile where 90% of the well defined test soil is retained in the wet sieving test, DIN EN ISO 12956 Geotextiles and geotextile related products 27

Determination of the characteristic opening size) is usually required to be smaller than a factor x multiplied by a defined grain-size distribution parameter of the base material. O 90,w < x dy (1) This design criterion has no limits for the smallest size of pores to avoid filter cake formation and surface filtration by selecting a geotextile filter. As a consequence the geotextile filter may clog or block with a decrease in permeability up to 4 orders of magnitude as known and described also for grain filters [3]. A simple permeability criterion for geotextile filters that the permeability of the geotextile should be 10 to 100 times the permeability of the base soil is not enough for good design and performance of a geotextile filter. Therefore design and specification criteria for geotextile filters have to consider a limit for small pores thickness / filter length requirement safe installation criteria to avoid puncturing. B. Analogies grain filter / geotextile filter design Sufficient pore size and thickness requirements of the geotextile filter can easily refer to grain filter design by considering a pore size analogy and a thickness analogy comparing grain filters and needle-punched nonwoven geotextile filters. Only for this type of geotextile filters this comparison is possible, because of the three-dimensional pore structures in both alternatives, the grain filter and the needle-punched nonwoven geotextile filter (NP-NW-GTX filter). The basic idea of the pore size / thickness analogy approach simply is to offer "the same" pore structure for the filtration job by a grain filter or a geotextile filter, considering that the filter elements are the pores and not the elements (grains/fibres) forming the pores. Ref. [9] have investigated some "common" nonwoven geotextiles by using the mercury intrusion method showing pore size distributions with pores in the dimension of about 0.1 mm to 0.2 mm only. The pore size distributions refer to the pores of medium sand with d 50~ 0.6 mm. Based on this pore size the filter application of this "common" nonwoven should be limited to applications where the grain filter design is asking for medium sand! Compared to grain filters, also NP-NW-GTX filters can have a wide-spread pore size distribution and very different pore sizes, some special pore size distribution investigations were carried out [2]. The tested NP-NW-GTX filters were chosen from the wide range of actually available products based on the nonwoven staple fibre production. This prodcution technique is using cut fibres for the production of nonwoven geotextiles. The manufacturing process allows the use of a wide range of different fibres with regard to fibre size, raw material and crimping of the fibres creating a wide range of different products. The results with regard to the pore size distribution of the investigated staple fibre nonwoven are given in Table 1, including a comparison to the O 9wopening size. The results show that nonwoven (NW) needle-punched (NP) staple fibre (SF) geotextiles (GTX) can be produced in a wide range of pore size distributions to fulfill the demand for different pore sizes in analogy to the grain filter design for different filter applications. The pore size distributions refer to grain filters ranging from medium sand to medium gravel! Recently, the superiority of NP-NW-GTX filters produced of crimped staple fibres again has been demonstrated when the geotextile filters are exposed to turbulent water flow conditions like wave attack [8]. The new EN-ISO test method for determining the soil passing through a geotextile when exposed to turbulent water flow conditions has been used to study the different filtration properties of filament needle-punched nonwoven and staple fibre needlepunched nonwoven with regard to a big failure of a sea wall at Sydney's International Airport. This failure could have probably been avoided by using a staple fibre nonwoven filter geotextile [8]! 0,TABLE I. INVESTIGATED PORE SIZES OF NEEDLE-PUNCHED STAPLE FIBRE NONWOVEN GEOTEXTILES USING THE MERCURY INTRUSION METHOD COMPARED TO O 90,W DETERMINED BY WET SIEVING. Test specimen O 10 O 90 O 90,w (mm) (mm) (mm) Terrafix 600 0.06 0.3 0.08 Terrafix 601 S 0.075 0.8 0.15 Secutex 444 0.07 1.1 0.26 Depotex 755 GG 0.08 3.0 0.53 O 10 : 10 % pores are smaller O 90 : 90 % pores are smaller By using the "pore-channel-diagram" developed by Ref. [10] and the results of the pore size distribution of a NW-NP-SF-GTX filter [3] it is easy to receive an O 9wopening size criterion for NW-NP- 0,SF-GTX filters by considering d 5and U Iof the base soil (Fig. 2). Based on the diagram in Fig. 2 a range for selecting an acceptable opening size of a NW-NP- SF-GTX filter can be determined. The upper limit for the range of O 90,w,Dis given by the design value itself and the lower limit by the "as coarse as possible" approach in analogy to grain filters, with O 90,w.selected > 0.8 O 9w0I0,,D28

4 3 range for O 90,w,D C. Geotextile filter design With the pore size and thickness analogies derived for grain filters and NW-NP-SF-GTX filters a very simple geotextile filter design is possible: With d 50I and U I of the base soil out of Fig. 4 an opening size O 90,w,s of a suitable product can be determined O 90,w,D < O 90,w,s < 0,8 O 90,w,D (3) O 90,w d 50I 2 and the thickness t GTX of the geotextile filter should be limited by 25 O 90,w,D < t GTX < 50 O 90,w,D (4) 1 0 1 2 3 4 5 6 10 14 20 U I Figure 2. Range of design values of O 90,w,D as function of d 50I and U I of the base soil In addition, O 90,w,s > 0.2 O 90,w,D or O 90,w,s ~ d 50I should be considered as lowest limits. If selecting corresponding small opening sizes additional investigations or calculations are strongly recommended to avoid clogging or blocking phenomena of the geotextile filter with the danger of filter cake formation.the thickness analogy between required grain filter thickness and required geotextile filter thickness is based on the idea that one grain layer is one filtration slice for the grain filter and one opening size slice is one filtration slice for the NW- NP-SF-GTX filter. Therefore the grain filter thickness requirement of t F > 25 d 50II would lead to a geotextile filter requirement of t GTX > 25 O 90,w,D. The investigation of several NW-NP-SF-GTX filters dug up out of hydraulic structures showed that the reduction of permeability of these geotextiles after long-term service in the structure was in the frame of permeability reduction known from grain filter design with the "as coarse as possible" approach [3]. The thickness of these dug-up NW-NP-SF-GTX filters was 25O 90,w, D <t GTX <50O 90,w,D (2) demonstrating that a similar "as coarse as possible" approach for geotextile filters leads to the same safe long-term permeability for grain filters and NW-NP-SF-GTX filters under service conditions. Surprisingly it was found during numerous dug-up operations of NW-NP-SF-GTX filters that the thickness of the dug-up geotxtiles was "as produced" the thickness measured after production at the virgin fabric under a vertical load of 2 kn/m² according to DIN EN 964-1 Geotextiles and geotextile related products Determination of thickness at specified pressures Part 1: Single layers. Spontaneous pore/grain interactions during installation and compaction are stabilizing the "as produced" thickness. With these requirements regarding opening size and thickness of the NW-NP-SF-GTX filter a sufficient long-term permeability of the geotextile filter can be expected. Alternatively, a special permeability check is necessary for different geotextile parameters. D. Survivability criteria to avoid puncturing of the geotextile filter During installation a geotextile filter has to survive in the harsh environment of a construction site without being punctured. With the risk of puncturing any effort with regard to a safe filter design is useless! The installation stresses can vary in a wide range from filling up and compacting a drainage trench wrapped with a geotextile layer to heavy stone dumping on top of the geotextiles in e.g. revetment or breakwater constructions. Only NW-NP-GTX have an ability to deform (high elongation at tensile test, usually > 40 %) and provide high robustness. High elongation behaviour provides the superior properties during the construction load case, which is determined as being the biggest risk for damaging the geotextiles. This requires minimum strength and minimum mass per unit area, but maximum elongation [4]. Fig. 3 gives an indication of required mass per unit area of a nonwoven needle-punched geotextile as a function of weight and drop height of the stones/rocks based on special impact tests. The impact test has a long successful history and was developed by the Germany Waterway Authority Research Centre (BAW) already in the 1970s and is described in more detail in Ref. [5]. The abrasion impact by "rolling stones" in a revetment or the washing up and down of rough beach sand and sea shell mixture by wave action can attack and destroy a geotextile which does not have sufficient abrasion resistance. The effective test apparatus for geotextile abrasion resistance was developed by BAW in Germany also and has the same long-term history. It is used for international tender projects in hydraulic engineering (impact of 80,000 rotations of a water/gravel mixture should provide a minimum of 75 % strength retention) [4]. The impact test and the abrasion test will normally lead to nonwoven needlepunched geotextiles with m A> 600 g/m². But even in drainage ditches or other geotechnical and hydraulic engineering structures it is recommended not to use geotextiles with mass per unit area < 300 g/m² and elongation at break < 40 %. 29

max. drop height [m] 14 12 10 8 6 4 2 Max. drop height 0 0 500 1000 1500 2000 2500 3000 Mass ma per ss unit area of area Terrafix (g/m [g/m²] ) Rock w eigh weight t: 1200 kg 300 kg 240 kg 180 kg 120 kg 90 kg 60 kg 30 kg Figure 3. Maximum drop height of rocks onto needle-punched Terrafix nonwoven (BAW impact test) made of special crimped staple fibres IV. CASE HISTORIES A. Scour protection In rivers, waterways and open sea areas scour can be caused by natural currents or erosion effects due to drift currents, waves, backflow currents and ship's screw actions. Stability of waterfront structures, bridge piles and slopes of waterways at narrow passage openings can be endangered by progressive scour. Conventional flexible scour protection (mineral filter layers) in the vicinity of difficult accessible structures (e.g. bridge pile foundations or bottom edges of waterfront structures, Fig. 4) and in conditions of deep water, currents and/or tide influences is only possible with sophisticated technical equipment and time and cost expenditure. Under the above mentioned conditions the use of geotextile sand containers and local available sand provide significant advantages as they work as filter layer and concurrent as ballast. They can be placed accurately also in deep water under currents and waves by using a jib-crane placed on a pontoon or stone dumping barges. In cases it can be necessary to install a rip-rap cover, when abrasion resistant geotextiles are chosen and ship anchors are not expected. Figure 4. Scour fill installation by jib-crane / North Sea harbour List/Island Sylt (Germany) At German's North Sea and Baltic Sea ports scour fill and scour protection solutions with nonwoven sand containers, each with a fill volume of 1 m 3 have successfully been realised under currents of up to 2 m/s and in water depths of up to 20 m. Relating the necessity of dredging waterways for maintaining ship traffic a geotextile scour protection solution with sand containers provides additional benefit as they can be filled up with dredged waterway material. B. Sand dune barriers To stop erosion of the sandy beaches and dunes, and to save maintenance costs for beach nourishment at regular intervals, geotextile reinforced dune barriers are a simple and cost effective solution. The on site available beach sand can be installed in layers, with pre-defined layer thicknesses, compacted and finally secured by wrapping the geotextile back into the dune structure, so that the sand is incapsulated. The geotextile reinforced dunes absorb the energy of upcoming wave attacks due to the mobilisation of tensile forces, which are activated in the geotextile. To prevent the installed geotextiles against mechanical damage and environmental influence a final sand cover together with a beach grass protection is recommended. In addition the erosion of the fill material is excluded. Fig. 5 shows a sand dune barrier at the coast of Figuera da Foz, Portugal. The construction has been built with a maximum height of 8 m, 8 geotextile layers and vertical layer spacings of 0.6 m. Figure 5. Cross-section of a geotextile reinforced sand dune in Figuera da Foz (Portugal) For the protection of the Wangerooge Island more than 3000 sand containers type A Terrafix Soft Rock were installed in the year 2000. They build a 260 m long wall integrated in the natural form of the dune. After the installation the sandbags were covered with sand. The natural surface of the dune, with an inclination between 1:1 and 1:2, was covered by Terrafix 813 filtration geotextile up to 80 cm under the beach level. The first level of the sand container was installed in a distance of about 1 m from the dune foot (Fig. 6). The spare geotextile was wrapped around the first layer and finally the remaining containers were stapled in layers up to the final height. After the installation all containers were covered by 1 m up to 3 m of sand. The following winter storms in the year 2001/2002 removed all the sand cover. The waves were higher than the sandbag barrier. This overtopping was a major danger of destroying the dune behind the artificial barrier. In the year 2002 the height of the geosynthetical protection wall was increased using more than 6000 Soft Rock sand containers again. To increase the installation speed special filling equipment was used. 30

The excavator bucket was rebuilt to allow the loading of sand, filling the container and final compacting in one work step (Fig. 7). A clamp was used to fix the empty container. This change brought a saving of 50 % of the installation time and necessary personnel in comparison to the installation in the previous year. The construction time was reduced to less than 2 months only. Figure 8. Artificial barrier after the storm (28/29 October 2002) on the Island of Wangerooge / North Sea Figure 6. Construction of the artificial barrier on the Island of Wangerooge / North Sea The tender called for a groyne 2.5m high by 100m long which could withstand 3 m high waves. Another important criterion was that the geotextile should provide some form of vandal resistance. The first groyne, which was constructed using 2.5 m 3 containers (Fig. 9), proved to be a success. Figure 9. Maroochy Groyne No. 1 (Queensland, Australia) Figure 7. Construction of the artificial barrier on the Island of Wangerooge / North Sea At the end of October 2002 a strong storm hit the coast of the island with southwestern winds and wind speed up to 150 km/h. This was the strongest storm within the last 10 years. The sand covering the artificial barrier was removed, but the construction was not affected at all (Fig. 8). C. Groynes and Reefs Due to the success of the previous Maroochydore geotextile container sea wall constructed as emergency protection to a caravan park in Queensland/Australia, the council called for the design and construction of a groyne constructed from sand containers. The structure was stable under severe wave attack, was user friendly, aesthetically pleasing and the vandal deterrent geotextile performed beyond expectations. This allowed the council to approve the second phase of project which consisted of a further 3 groynes, constructed in April 2003, to protect the exposed headland. The areas between the groynes were nourished with 30,000 m 3 of sand. The Gold Coast area in Queensland / Australia is regularly affected by cyclones which generate deep water waves in excess of 12 m (H max ) and severe short term erosion of the beaches. In 1997 the "Northern Gold Coast Beach Protection Strategy" was initiated by the Gold Coast City Council to maintain and enhance the natural beach capacity by widening the beaches at Surfers Paradise [6]. Providing a long-term sustainable solution the strategy includes an initial 1.1 million m 3 of beach nourishment with additional ongoing beach nourishment of approximately 80,000 m 3 per year. In addition the construction of a submerged artificial reef at Narrowneck is designed as coastal control point within the open sand system so as not to disturb the sediment balance [7]. As a world-wide 31

pioneering feature two objectives have to be followed with this artificial geotextile "soft rock" reef constructed without any conventional hard elements like rip-raps, rocks and steel (Fig. 10): Stabilizing and enhancing the natural beach capacity by beach widening and creating a world-class surf break. This pioneering geotextile "soft rock" coastal protection solution as an alternative to hard rock structures has the following decisive benefits: able to achieve design shape 50 % cost of rock (hard rock) surface reduces risk of injury to surfers no rock transport and no traffic on roads flexible to cope with seabed movements no works on beach or impact on beach users able to be easily topped up, modified or removed if necessary (important for approvals). The reef is made of about 400 nonwoven needlepunched geotextile containers and more than 80,000 m 3 encapsulated sand. It fits into a square of 600 m 350 m, the cross-section profile ranges from about 1 m to 10 m below low tide sea level in distance of 150 m to the shoreline and it consists of two sides in V-shape with the northern part being the larger than the southern part. The northern part will form a right hand break, while the southern side will form left hand break on the Gold Coast. The reef is designed to be transparent for sediment movement and as a wave energy absorbing structure with a paddle channel in the centre. The shape of the reef jacks up the wave height by about 25 %. Surfers were found to be key stakeholders and the choice of sand filled geotextile containers for the reef construction, made of needle-punched staple fibre nonwovens, was heavily influenced by the improved safety for surfers. One major international surfing competition on the Narrowneck reef will generate 2.2 million $ of benefits to the community. V. SUMMARY The use of grain filters has a long tradition in geotechnical and hydraulic engineering structures. Due to cost advantages and the easy installation geotextile filters (GTX filters) and geosynthetic composite drains are more and more used today. By considering anologies between grain filter and GTX filter design which simply reflect on the fact that the pores are the filtering element and not the particles forming the pores, an easy and safe design is also possible for GTX filters. Based on the nonwoven staple fibres (NW-SF) technology, nonwovens with a wide spectrum of pores can be supplied, the pore size distribution referring to corresponding grain filter pore sizes of medium sand to medium gravel. In addition, sufficient installation robustness is necessary to avoid puncturing of GTX filters during e.g. stone dumping operations and a minimum strength, minimum thickness and high strain of the GTX filter need to be specified. On slopes with revetments the possible downslope migration of base soil particles needs to be considered in the revetment design, too. 5 0-5 -10 N bed profile artificial reef 100 200 300 400 500 600 700 Gold Coast City Council and Project managment: ICM Reef design: ASR Contractor: McQuade Marine Geotextiles: Soil Filters Australia Figure 10. Cross-section and plan view of reef design and involved project partners (position of sand containers taken from Gold Coast City Council design plan, 1999) Based on the decades of history and the present technology, GTX filters can improve construction, service time and the economic efficiency of geotechnical and hydraulic structures. Current investigations [8] show that staple fibre needle-punched nonwoven geotextiles have superior filtration properties and should be preferred for filtration in coastal and hydraulic engineering applications with turbulent water flow conditions. Geotextile nonwoven sand containers as "soft rock structures" for flexible coastal protection measures can provide significant advantages over "hard coastal structures" made of concrete, steel and rocks. Geotextile "soft rock structures" are variable or removable if necessary and there is always the possibility to combine geotextile structures with conventional elements like rip-rap or rock revetments. For coastal engineers it should be a challenge to design coastal and hydraulic structures balanced between the requirements of function, lowcosts, flexibility and considering the overall environmental impact. The applications of sand containers clearly demonstrate the advantage of elongation behaviour of a nonwoven geotextile. Since stresses are more or less important in the presence of flexible components on the one hand and - on the other hand - stresses are often the cause of failure, for coastal engineering applications, as a governing principle, from the authors' point of view coastal engineers should think in terms of elongation rather than in terms of stresses. REFERENCES [1] Heerten, G., "Long-term experiences with the use of synthetic filter fabrics in coastal engineering", Proceedings 17th International Conference on Coastal Engineering (ICCE), Sydney, Australia, 1980. [2] Heerten, G., "A contribution to the improvement of dimensioning analogies for grain filters and geotextile filters", Geofilters '92, Karlsruhe, 1992. 32

[3] Heerten, G., "Stand der Untersuchung und Bemessung des Filterverhaltens von Geokunststoff-Boden-Systemen", 3. Informations- und Vortragsveranstaltung über "Kunststoffe in in der Geotechnik", Technische Universität München, 1993. [4] Heerten, G., Werth, K., "Use of Geosynthetics in Hydraulic Engineering", Baltic Geotechnics X, Riga, Latvia, 2005. [5] Heerten, G., "The Challenge for the Use of Geosynthetic Construction Materials in Environmental, Coastal and Offshore Engineering Applications", Offshore Arabia, Dubai, United Arab Emirates, December 2006. [6] ICM, "Technical Report and Recommendations for North Gold Coast Beach Protection Strategy", International Coastal Management, Gold Coast, Australia, 1997. [7] Jackson, A. et al., "Strategy for Protection of the Northern Gold Coast Beaches", Australasian Coastal Engineering & Ports Conference, Christchurch, September 1997. [8] Maisner, M. and Myles, B., "Possible Culpability of Filter Geotextile in the Failure of a Sea Wall", The First Pan American Geosynthetics Conference & Exhibition, Cancun, Mexico, March 2008. [9] Prapaharan, S., Holtz, R.D. and Luna, J.D., "Pore size distribution of nonwoven geotextiles", Geotechnical Testing Journal, GTJODJ, Vol. 12., No. 4: pp 261-268, 1989. [10] Teindl, H., " Filterkriterien von Geotextilien", Bundesministerium für Bauten und Technik. Straßenforschung 153, Wien. 136 p., 1980. 33