Geosystems, Possibilities and Limitations for Applications - CiteSeerX

EuroGeo4 Paper number 282 GEOSYSTEMS, POSSIBILITIES AND LIMITATIONS FOR APPLICATIONS Adam Bezuijen1 & Erik Vastenburg2 1 2

Deltares and Delft Universty of Technology. (e-mail: [email protected]) Deltares. (e-mail: [email protected])

Abstract: Geotextile containers and geotextile tubes (with geomattresses and geobags called geosystems) offer the possibilities to construct coastal structures and bank protection structures without the use of rock and rip-rap. In situations where rock is not available close by or not desired in a structure (for example because the structure is close to a surfing or swimming area or the construction is temporarily and has to be removed), this is a decisive advantage. This has led to quite a number of structures with geosystems all over the world, a number that presumably will grow in the near future. However, recently also a number of structures with geosystems were proposed, but were abandoned in the later design stages and a different construction method was used. There were various reasons to change the construction method, such as: speed of construction, costs of containers, uncertainty with the construction method, subsoil stability etc. The paper analyses various projects and produces some guidelines in which applications a geosystems structure can be a strong alternative and for which construction conditions it is a less likely alternative. It also shows what research is needed to expand the applicability of geosystems solutions in coastal structures. Keywords: Geotextile containers, Geotextile tubes, Stability, Costal. INTRODUCTION The first application of geosystems dates from the 50-ties of the last century. An early example is shown in Figure 1 where an estuary in the South-Western part of the Netherlands is closed with geotextile bags filled with sand. This application is based on the much older application of sand bags that were carried by hand, mostly used in an emergency case, to prevent failure of a dike or dam and also used on a large scale in repair works, after for example the flood of 1953 in the South-Western part of the Netherlands. Since the first applications, the geotextile technology improved leading to stronger geotextiles, geotextile containers and tubes were developed that allow for less labour intensive application and more information was obtained on the durability of these products. Consequently the number of applications has grown. Sand-filled geotextile bags were used successfully as bottom protection, groynes. Sand filled geotextile tubes were used to protect mangroves against wave attack, temporarily partition of a water basin, groynes, dune toe protection. Sandfilled geotextile containers have been applied as erosion protection, closure dams, artificial reefs etc. Figure 1: Closure of the Estuary "Pluimpot" with sand bags in 1957. Yet not all applications where geosystems were envisaged or used were successful. In some cases it was decided on the design table that an alternative solution was more attractive, in other cases it appeared after preliminary tests or during building or even after the construction that significant changes were necessary in the design. In the paper we describe some of the projects that were not successful in various stages of the project. We do not take this point of view because we do not ‘believe’ in geosystems, but because we think that describing those projects that were not a direct success can be useful for future projects. All projects mentioned are real projects, but since it is always easier for the owner of the project to tell about its success than its troubles, we will not present the exact locations of most of the projects. PLANNING STAGE Core of breakwater The breakwater was planned in the Atlantic Ocean at 2.7 km out of the South American coast. The total length of the breakwater was circa 2 km. The average water depth at the project location is nearly MSL – 15 m. With respect to 1

EuroGeo4 Paper number 282 the wave climate it can be mentioned that nearly 90% of the time the waves are higher than 1 m, 48% higher than 1.5 m and 13% higher than 2 m. These conditions can be called rather rough for construction of the breakwater. The goal of the feasibility study was to investigate two possible methods to build the breakwater. The first method was the conventional method using rock to build the core of the breakwater. The alternative was a core built up using geotextile containers and tubes to save on the total amount of rock needed to build the breakwater. From the seafloor till MSL – 4 m the core would be built up using geotextile containers. Above this level it was planned to use geotextile tubes. For this project there was plenty of rock available, but the transport over land was foreseen as being complicated. Here price became a decisive issue. The costs of the imported geotextile containers and some estimates on the filling costs in the rough sea conditions led to a significant higher price per cubic metre for the geosystems solution compared to the rock solution. In this application costs of the geotextile containers and tubes were significant and therefore the degree of filling of the containers and tubes was important. It was foreseen that a split-barge with only a limited opening width of the barge was available. This resulted for the containers in a degree of fill of only 35% of the theoretical maximum given circumference of the container and therefore the price per cubic metre for the filled and placed geotextile containers was at least 50% higher than the price of the geotextile tubes. Compared with the alternative where the core consists of rock the price was 50% higher. Changing the barge, so that a 45% degree of fill could be used, appeared to lead in this project to a multimillion dollar saving possibility, but as mentioned, another solution was used. Temporary breakwater In the same area as the first example a temporary breakwater was envisaged at 8 m water depth to have a sheltered area against wave attack during the construction stage. Due to restrictions coming from natural value of the surroundings this breakwater had to be removed several years after construction when the sheltered area was not necessary anymore. The obligation to remove the breakwater after some time made the geosystems solution an ideal solution. Just cutting the geotextile would be sufficient for removal. However, only in a later stage of the project after the execution of the first hydraulic tests on stability, did it become clear that the soil conditions below the breakwater were very poor. This would not lead only to settlement but to stability failure of the breakwater due to squeezing of the soft layer underneath the breakwater, see Figure 2.

depth (m)

5 Y breakwater y mud stab. under water stab. above water

0 -5 -10

Pa τ τ

Pp -20

-10

0

τ τ 10

τ τ 20

Pa 30

40

50

60

70

80

X (m) Figure 2: planned breakwater and possible maximum slope (the thick line) before squeezing of soft layers occurs Assuming that the maximum passive resistance is mobilized in the soft layer at the toe of the breakwater (indicated as Pp in the soft layer in Figure 2) the active loading from the breakwater on the subsoil Pa is equal to this passive force at the toe of the breakwater, but increases going more to the centre of the breakwater due to the friction between the soft soil and the firm layers underneath and the geotextile between the soft layer and the breakwater structure. This lead to the formula:

q = 4S u + 2

τx

(1)

h

Where q is the maximum loading from the breakwater, Su the strength of the soft soil layer, τ the friction between the soft layer and subsoil and the soft layer and the geotextile between the soft layer and the breakwater, x the distance from the toe of the breakwater and h the thickness of the soft layer. With this formula it is possible to calculate the maximum slope that is possible to prevent squeezing. This slope is indicated with the thick line, dotted for the underwater situation and full for the situation above the water line. It is clear that only gentle slopes could be applied or a trench would need to be dredged to remove the soft layers and refill with sand. Both solutions are much more costly. It was decided to go for another solution without this breakwater. So here it was the subsoil that prevented the application of geosystems.

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EuroGeo4 Paper number 282 Shore protection The port of Rotterdam is in need of extra space. To fulfill this need the choice has been made to reclaim new land in the North Sea. The total required amount of sand for this reclamation is more than 325 million m3. The total area will be 2000 ha, of which 1000 ha will be used for industrial activities like container trans-shipment; distribution activities, and the chemical industry. To protect the sand against erosion and the new land from flooding, seawalls need to be built. There are different types of shore protection planned. A way to characterize these is by dividing them into hard and soft shore protection. The hard protection solutions are methods where concrete and rock are used. The soft methods have the character of a natural dune landscape. For the new port, a total length of hard seawalls of 4 km is needed. The required length for the soft shore protection is 8 km. To build the new port area a lot of material is needed and the costs are enormous. To save on the total amount of materials and to lower the costs, geosystems could be an alternative. For the project, several options with geotextile containers and tubes have been outlined by Deltares (Bezuijen e.a. 2005) Figure 3 shows an alternative for the hard seawall. The slopes, as well as the top layer of the berm, are built up using geotextile containers. This design results in lower hydraulic forces on the armour layer. Due to the redundancy of the hydraulic loads, the armour layer can be dimensioned lighter. This will influence the total costs. NAP + 19 m

1:2 Hs = 8,0 m

Armor layer 5 ton d = ca. 2,5 m

NAP + 5 m

Design water level NAP – 3 m

1:50 Fiter layers d = ca. 3 m

ca. 200 m 1:3 Geotextile containers 8mx2m

NAP -21 m

Figure 3: Alternative hard seawall; armor layer combined with geosystems. The primary design of the soft shore protection consists of a natural dune profile as shown in Figure 4. The profile is very gentle and occupies a great part of the project area. By using geosystems the outer slope can be built up steeper and the profile can be build shorter (‘hanging beach’ principle). This saves a lot of sand, approximately 9,000 m3/m1. In addition less space (circa 60%) is needed to build the shore protection. This could lead to a financial and an environmental benefit. 200 m

300 m

500 m

100 m

50 m

End profile when using geosystems

109 m 10,6 m 1:3 1:4

NAP 1:25 1:75 1:100

1:50

6,1 m 3,0 m 1,0 m -1,0 m -5,0 m -10,0 m

1:20 -20,0 m

Figure 4: Primarily design of the soft seawall. Although geosystem solutions have been discussed, they do not feature in the current design at the time this paper was written (the final design is not yet known). This was a design and construct contract, so the contractor has to chose the design within certain boundaries. The reasons for the design choice were not known when this paper was written. This can be costs or the potential risk under the possibly severe conditions. Looking at the applications realized so far, such a huge project is possibly at the moment simply ‘a bridge too far’. More research on the consequences of upscaling our existing knowledge to these dimensions and wave conditions is needed. This cannot be a matter of laboratory testing only, but should be accompanied by field trials to obtain practical knowledge on larger projects and projects in more severe conditions. This knowledge should include more than the stability of the structures, although this is also an issue but include the logistics of filling the geotextiles, dropping the container, and localization of geotextile tubes.

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EuroGeo4 Paper number 282 CONSTRUCTION STAGE Underwater dam An old sand mining pit had to be partly refilled with sand, for the construction of a railway tunnel underneath the adjacent river. The mining pit had a water depth of approximately 20 m. To accomplish this part of the route a dam was built, through which the tunnel was bored. It was planned to use geosystems to construct the side slopes of the dam. By using geotextile containers the side slope could be built up much steeper (1:3) than a conventional sand sprayed slope (1:10). This would save a lot on the total amount of sand needed to build the dam. During the construction phase it was shown possible to build the side slope, using geotextile containers, with a slope ratio of 1:3. However, dumping the geotextile containers took more time than planned and the tunnel boring was going faster than scheduled. With respect to the project planning and the project risks, the authorities decided to continue the work by using conventional sand spray techniques. In retrospect it can be stated that for this project the logistics involved in filling and closing the containers, sailing to the dumping side and dumping the containers was underestimated. Further it appeared that at water depths of 20 m the placing accuracy is less than was anticipated based on experience in water depths up to 15 m. The location of each container was located after dumping and was compared with the location of the dumping barge. From these measurements it could be derived that the placing accuracy decreases significantly at larger depth, see Figure 5 and Figure 6. 7

standard deviation (m)

4 3

10 m

2

6 5 4 3 2 1 0

10 m

0

1

5

10

15

20

water depth (m)

Figure 5: Top view of barge positions and locations of dumped containers for the first 4 geocontainers.

Figure 6: Standard deviation in the difference between barge position during dumping and final location as a function of depth.

Waste disposal Cheek and Yee (2006) reported on field tests where the dredged sediments were placed in a geotextile container and dumped at the sea bottom. The sediments were polluted and the aim of the placement in the geotextile container was to avoid spreading of this pollution over the sea bottom. Tests were done with 3 types of containers: a 600 m3 container using a geotextile with a tensile strength of 120 kN/m, a 300 m3 container using a geotextile with a tensile strength of 120 kN/m and a 300 m3 container using a geotextile with a tensile strength of 200 kN/m. It was shown that the first two containers ruptured and that good results were obtained with the third one. It is the opinion of the authors of this paper that especially the test with the first container (and the costs involved) should have been skipped, since based on literature available at that time it was clear that this could only lead to a failure. Unfortunately, no dimensions and parameters are presented in the paper, but based on the pictures we assume dimensions and parameters in the order of magnitude as follows: Length of barge (L) 30 m, cross-sectional area perpendicular to the length axis (A) 20 m2. Density of the fill (ρ) 1400 kg/m3, stiffness of the geotextile (E’) 800 kN/m, strength of the seams (70 percent of the tensile strength of the geotextile) (Ts) 56 kN/m. Pilarczyk (2000) and Bezuijen et al (2004) present the following relation between these parameters and the falling velocity, where E’ and Ts have to be presented in the dimension N/m :

Ts = v

AρE ' S ⇒ v =T S AρE '

(2)

Using this equation the geotextile container will rupture at a falling velocity of 2.3 m/s. This velocity is reached after a free fall of only 1 m. For the last container tested the rupture velocity according to this relation is 3.4 m/s. This velocity is reached after a free fall of 1.5 m. The results show that even the last geotextile container tested was probably close to failure, depending on the falling height. The formula presented here is rather conservative, because all kinetic energy is assumed to be transformed to elastic energy in the geotextile during impact. In reality there will also be energy losses in the deformation of the fill and friction between the geotextile and the bottom. A calculation method taking into account the friction between the geotextile and the subsoil led to higher possible impact velocities of 3.9 and 5.0 m/s for the geotextile containers respectively. Still the allowable free falling height of the first container according to this

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EuroGeo4 Paper number 282 calculation is low: 3.6 m while being 9 m for the last container. The falling velocity for the last container is close to the final velocity (5.6 m/s). This means that if a bit more strain is allowed than the 10% that is now assumed or the density of the fill is a bit less, the geocontainer will survive the impact at all depths. Based on these results the rupture of the first container could be expected, so here a change of design was necessary because the first design was not worked out well enough. Groyne A groyne structure was made in Japan using geotextile containers. During construction, the structure was badly damaged by a typhoon. In the back analysis it appeared that the factor Hs/(ΔD) was larger than 1 during the storm conditions. In this factor, Hs is the significant wave height, D the height of the geotextile tube and Δ the relative density ( (ρ-ρw)/ρw), where ρ is the density of the fill and ρw the density of the water. According to literature (Pilarczyk, 2000) this would lead to failure. So this failure was caused by inadequate design or not taking into account the possibility of a typhoon. The consequence of Δ being relatively low (normally 1), while it can be up to 1.9 in traditional coastal defenses using rock, is that during extreme conditions even the largest geotextile containers and tubes are not stable. Large scale model tests are necessary to evaluate the stability in critical situations, since Vinjé (1968) has shown that small scale model tests can over-predict the stability of the structure leading to unsafe results. DISCUSSION The potential of geosystems in hydraulic engineering is that it is possible to save on building materials as rock and concrete. Especially in delta areas rock is quite often scarce and geosystems can be an interesting alternative. Quite large projects are realized in, for example, the Markermeer in The Netherlands. The average depth of the lake is circa 3.5 m and the surface area is 700 km2. Geotextile tubes are successfully used as a core for breakwaters with a total length of more then 10 km. In addition, new plans using geosystems are on the drawing boards. Figure 7 shows a typical construction with geotextile tubes suitable for this area. Another location where geosystems were used to save on the used amount of rock was in the Mississippi delta in the USA. With the present day material and construction prices it seems not possible to compete with traditional solutions when sufficient rock material is present close to the construction site. In such cases geosystems are only applied in special cases as when rock material could damage an adjacent structure or the structure has to be removed after some time. In The Netherlands more and more the total lifecycle of the used materials is taken into account. When comparing rip-rap with geosystems, it seems to be plausible that the geosystems score better. When the importance of these comparisons increases (expressed in money), this can lead to an increase in the use of geosystems.

Figure 7: Typical construction of a small breakwater with a core of geotextile tubes and an armor layer of rip-rap. Up to now successful applications have been mostly realized in relatively calm wave conditions, e.g. in rivers, estuaries and lakes, although some beach projects at sea are also realized with success. Nowadays the limitations of geosystems are quite often not the weak points of the system itself, but of the logistics and quality control around the project. The barges that drop geotextile containers should have smooth edges where the containers leave the barge. Although this seems rather obvious, ruptures due to sharp edges are reported regularly. Filling is crucial since an even filling for geotextile containers is necessary to avoid damage (Bezuijen et al 2002) and geotextile tubes can burst due to too high pressure when they are filled too fast. An important aspect is the speed of placement since, especially for geotextile containers, quite some equipment is needed and therefore the time necessary for the cycle: filling the container, closing it, sailing to the dumping side, dumping it and sailing back to the site where the next container is loaded is of importance. The underwater dam described in this paper was not really optimized for a fast turnaround times which made it necessary to choose another method to construct the dam. Contractors who work with geotextile containers and geotextile tubes all agree that you need to develop a feeling about working with these elements. It is a form of craftmanship. To increase the applicability of geosystems the following design issues have to be resolved by research or practical experience: • The stability of geosystems under significant wave conditions, for the individual elements as well as the complete mound. Most experiments to test the stability under wave attack are performed on a relatively small scale and, as 5

•

• •

EuroGeo4 Paper number 282 mentioned, Venis (1968) has shown that there are scale effects where small scale model tests can over-predict the stability of the structure. This has to be investigated further. The placing accuracy of geotextile containers at larger depths is still an issue. The accuracy seems rather limited at depths larger than 15 m, but field experiments where containers were dumped to such a depth are only reported from one construction site. Another placement technique can possibly be developed when a higher accuracy is needed. For example lowering the container close to the bottom before releasing it. The loading on the geotextile during impact of a container on the sea bottom is described (Bezuijen et al. 2004) and can be used in a design as is shown in this paper, but its accuracy has never been tested for sludge filled containers. The stability of geotextile tubes during filling is an issue especially when these are stacked.

Reading the examples in this paper it has also to be mentioned that most geosystems projects are completed successfully. As mentioned in the introduction these projects are chosen to show what can go wrong and we hope that these experiences will be used as a guide to improve the design and execution of projects with geosystems in the future. CONCLUSIONS It is shown that geosystems are not different from other new techniques. Problems did occur in various stages of different projects. As is shown in the examples the reasons for the problems are quite different, ranging from soft soil, to underestimating the forces during installation. The situation is not different from other constructions in hydraulic engineering: careful design, execution and planning is necessary for a successful project. Uncertainty on the behaviour under wave attack with waves of several metres hampered application in sea conditions. The procedures developed until now for placing geosystems require calm conditions. These aspects need some development to use these systems in open sea conditions. The impact of a geotextile container on the sea bottom is an important loading condition in the design, furthermore dumping at large depth seems to be inaccurate, so there is room for improvement of the placing method for containers (guided placement?). A lot of successful applications all over the world have shown the potential of geosystems. In addition, the experience with geosystems grows with every project executed; this will make it much easier for authorities and contractors to take geosystems into account as a possible alternative during the design stages. We hope this paper helps to focus on critical design and execution issues and in this way lead increases the success rate of future projects. Acknowledgements: We want to thank the CUR commission F42 for stimulation discussion and Delft Cluster for funding the research on geosystems. Corresponding author: Mr Adam Bezuijen, GeoDelft, P.O. Box 69, Delft, 2600 AB, Netherlands. Tel: +31152693785. Email: [email protected]. REFERENCES Bezuijen A., M.B. de Groot, M. Klein Breteler, E. Berendsen.2004, Placing accuracy and stability of geocontainers, Proc. EuroGeo 3, Munich, 2004. Bezuijen A, R.R. Schrijver, M/ Klein Breteler, E. Berendsen.,K.W. Pilarczyk. 2002 Field tests on geocontainers. Proc. 7th Int. Conf. on Geosynthetics, Nice. Bezuijen A., L.N. Booster, M. Klein Breteler, M.B. de Groot, J.L. van de Velde, H. Verheij, Maasvlakte 2, Research proposals for innovative coastal protection with geosystems (in Dutch), Deltares report, Deltares, 2005. Cheek P.M. and T.W. Yee 2006. The use of geosynthetic containers for the disposal of dredged sediments, a case history. Proc. 8th Int. Conf. on Geosynthetics, Yokohama pp 753-756 Pilarczyk K.W. (2000), Geosynthetics and Geosystems in Hydraulic and Coastal Engineering, A.A. Balkema, Rotterdam Venis, W.A., 1968, Closure of estuarine channels in tidal regions, Behaviour of dumping material when exposed to currents and wave action (in Dutch), De ingenieur, 50, 1968

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