GEOSYNTHETICS AND GEOSYSTEMS IN COASTAL ENGINEERING

- A REVIEW -

Pilarczyk Krystian, (former)Rijkswaterstaat, Hydraulic Engineering Institute, Delft, The Netherlands

HYDROPIL, Nesciohove 23, 2726BJ Zoetermeer, The Netherlands, krystian.pilarczyk@gmail.com

ABSTRACT

Geosynthetics/geotextiles became a common component in filter construction (Giroud, 1987). Geosystems, however, as geomattress, geotubes, geocontainers, geocurtains, etc., are relatively new construction systems in civil (hydraulic/coastal) engineering applications. Till recently, these systems were designed based mainly on the experience from the previous (usually small) projects. Information provided to the potential users is usually restricted to general folders. Pilarczyk (2000) has reviewed most of the systems available on the international geosynthetic market. Actually, international research and 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 relevant websites), and some recommendations for future use and/or improvements are discussed.

1. INTRODUCTION

In 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 as

geotextiles/geosynthetics, natural (geo)textiles, gabions, waste materials, etc.

Geosynthetics/Geotextile systems utilize a high strength synthetic fabric as a form for casting large

units by filling them with air, water, sand or mortar, as a screen for guiding flow or a curtain for

collecting sand, etc. Structures made of flexible, high-tensile strength geosynthetics have the

advantage of simple manufacturing, lightweight transportation and usually an easy construction

process; strength and durability can be chosen according to the purpose. This all has prompted a

demand for cheaper, less massive and more environmentally acceptable engineering as it is using

geosystems. A large number of ideas has been born on the use of geosynthetics in civil/coastal

engineering. Pilarczyk (2000) has listed a number of actual and/or potential applications. Some of the

recent applications are shown in Figure 1.

These new (geo)systems (geomattresses, geobags, geotubes, seaweed, geocurtains and screens) were

applied 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 and

breakwaters 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 protective

structures in hydraulic and coastal engineering both in developed and developing countries. The main

obstacle 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 vague

experience than on the general valid calculation methods. More research, especially concerning the

large-scale tests and the evaluation of the performance of projects already realised, is still needed. The

recent 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 in

various geosystems with applications in hydraulic and coastal engineering, to present relevant data

gained from various studies, and to record data from projects where geotextiles and geosystems were

installed. The following geosystems will be reviewed in more detail: geotextiles in revetment

structures, 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. Geomattresses

are 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 GEOMATTRESSES

2.1. Functional design

The function of a revetment is to protect the slope against hydraulic (and other) loadings, such as waves

and currents. To evaluate the stability, information is required about the hydraulic design conditions, the

structural 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 the

revetment is only a part of the total project. Geotextiles primarily contribute to criteria on filtration and

retention, but also to criteria on stability of revetment, and must also satisfy the other criteria resulting

from 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 pore

sizes small enough to prevent the migration of the larger particles of the base soil. Moreover, concerning

the permeability, not only the opening size but also the number of openings per unit area (Percent Open

Area) is of importance (Pilarczyk, 2000). However, it has to be stressed that geotextiles cannot always

replace the granular filter completely. A granular layer can often be needed to reduce (damp) the hydraulic

loadings (internal gradients) to an acceptable level at the soil interface. After that, a geotextile can be

applied 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 filters

The 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 applications

The soil tightness of the initial situation and permeability requirements can be checked by means of the

well-known criteria for geometrically tight (geotextile) filters. However, the large number of this criteria

and often unclear limits of their application are very confusing for designers. Especially, the definition of

permeabilities, treatment of unstable soils and proper interpretation of index tests vs. performance tests are

still a problem for designers .

In revetment structures geotextiles are mostly used to protect the subsoil from washing away by the

hydraulic 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 other

components in the bank protection structure (i.e. internal stability of the subsoil at the interface with a

geotextile). 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 (Klein

Breteler et al. 1990, 1994, 1998, Pilarczyk, 2000, 2002). Also, the requirement that the permeability of

the cover layer should be larger than that of the underlayers cannot be met in the case of a closed block

revetment. The cover layer is less permeable, which introduces uplift pressures during wave attack. In

the case of a geotextile situated directly under the cover layer, the permeability of the cover layer

decreases drastically. Since the geotextile is pressed against the cover layer by the out flowing water, it

should be treated as a part of the cover layer. In this case the permeability ratio of the cover layer and

the base or filter layer and its thickness, represented in the leakage length (Λ), is found to be the most

important 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 and

geomattresses) should be design based on definition of their leakage length which provides a kind of

optimization between all design requirements, and which leads automatically to application of the

concept of geometrically open geotextiles (Klein Breteler et al., 1990, 1994,1998, CUR, 1995).

2.3 Geomattresses

The permeability of the mattress is one of the factors that determine the stability. It is found that the

permeability given by the suppliers is often the permeability of the geotextile, or of the so-called Filter

Points. In both cases, the permeability of the whole mattress is much smaller. A high permeability of the

mattress ensures that any possible pressure build-up under the mattress can flow away, as a result of

which the differential pressures across the mattress remain smaller. The stability is therefore the largest

with a large mattress permeability. In the long term, however, pollution of the Filter Points or the clogging

of the geotextile can cause a decrease in the permeability. The susceptibility for blocking can be reduced

by increasing the gradation of the subsoil. To reduce the susceptibility for clogging it is recommended to

reduce the sludge content of the subsoil. Due to the lack on proper information on the total permeability of

the mattresses, the indicative permeability of mattresses can be calculated based on the knowledge of

placed block revetments and the collected information from the literature and company information. By

introducing the concept of leakage length the indication of stability for various geomattresses can be given

as shown in Figure 3 (Klein Breteler et al., 1998, Pilarczyk, 2000, 2002). To obtain more accurate results

it 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.

(Λ=leakage length [m], D = thickness of top layer b = thickness of the filter layer (m), k = permeability of the filter layer or subsoil (m/s), and k′ = permeability of the top (cover)layer (m/s))

Figure 3 – Calculation results for concrete mattresses

Hydraulic performance of geomattresses. About 20 years of experience with concrete mattresses all over

the world provides an evidence that these systems, if properly designed and taking into account all

possible 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 equal

or milder than 1 on 1.5 with wave heights up to 1.5 m and current velocities up to 7 m/s. Application of

concrete mattress placed directly on sand is usually limited to a wave height of about 1.0 m due to the

possible geotechnical instabilty of subsoil when exceeding this value. Often, other failure modes can be

decisive for the damage of structure, therefore, especially in case of a severe wave attack, it is important to

put 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 prospective

users. 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 will

provide the additional protection. However, the top layer of fabric forming the concrete units will

gradually loose its strength. The bottom fabric is not subject to UV degradation and therefore does not

suffer loss of strength. The geotextile is principally treated as a fabric form for the containment of

concrete; 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 dilute

colored acrylic emulsion, at about 5 year intervals, which provides also the additional protection of the

fabric against ultraviolet degradation.

Figure 4 – Example of functioning of a block mat attached to the geotextile

On long term, especially when no UV-protection for geotextile is applied, the surface-geotextile will

deteriorate and the concrete filled-mat will function as a block mat (Figure 4); a block mat with concrete

'k

bDk=Λ

'/

Dk

bkD=Λ

units connected to the lower sheet of geotextile by existing binders, which normally are used as spacers to

provide a required thickness. These binders should have a proper strength to compensate the weight of the

concrete element. That also means that for structures with a long lifespan the stability of the mattress

should be controlled for this situation.

The system illustrated in Figure 4 is based on a geotextile to which the blocks are attached. Since the

geotextile allows blocks to be displaced slightly a considerable interactive force can be mobilized which

may induce the movement of the filter material. The system only works properly if washed-in material is

applied. If the system does not satisfy the above specifications the cover layer should be designed as loose

blocks.

Additional cabling. Where severe wave action is anticipated possibly with danger of severe scour, or

where soil cannot be properly compacted and extensive settlement is expected (i.e. soft soils), concrete

mattresses may be constructed (strengthened) with cabling. Nylon or polyester cables (ropes) are usually

used instead of steel cables to avoid any possibility of damage by corrosion. Cables are inserted between

the two layers of fabric prior to concrete injection. Synthetic cables are lighter than the grout slurry that is

utilized to fill the mats and therefore semi floats within the grout mass. In addition the cable pass through

the grout ducts rise in every block and through the edge narrow section interconnecting the blocks. When

filling 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 in

the concrete-filled compartments (blocks) to enable the mattress to resist tension.

Recommendations

For the future study on the design guidelines for the concrete mattresses the following items can be

recommended:

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 aspects

Filter 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 the

geotextiles by falling stones should not be underestimated. The following factors should be taken into

consideration:

- 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 if

followed. Therefore the proper supervision during execution is of great importance.

3. GEOSYSTEMS

3.1. General information

Geotextile systems utilize a high-strengt synthetic fabric as a form for casting large units by filling with

sand 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 examples

are shown in Figures 1 and 5. Mattresses are mainly applied as slope and bed protection. Mattresses were

already discussed under Revetments (Chapter 2). Bags are also suitable for slope protection and retaining

walls or toe protection but the main application is the construction of groynes, perched beaches and

offshore breakwaters (Heerten et al., 2000, Lawson, 2003). The tubes and containers are mainly

applicable for construction of groynes, perched beaches and (offshore) breakwaters, and as bunds for

reclamation works (Figure 6). They can form an individual structure in accordance with some functional

requirements for the project but also they can be used complementary to the artificial beach nourishment

to 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 isolate

contaminated materials from harbour dredging, and/or to use these units as bunds for reclamation

works.

a) Geotube as a breakwater b) BEROSIN system (inclined curtain)

as bed protection

Figure 5 – Examples of application of geosystems

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 after

placing by divers near the shore and anchoring to the bed catches the sand transported by currents and

waves providing accretion on a shore and preventing the erosion. The horizontal curtain (sheet) can be

easily 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. The

sea-side edge is kept in position by the workboat. By ballasting some of the outside pockets at the lower

edge with sand or other materials and with help of divers, the lower edge is sinking to the required

position. The proper choice of permeability of geotextile creates the proper conditions for sedimentation

of suspended sediment in front/or under the curtain and at the same time allowing the water to flow out

without creating too high forces on the curtain and thus, on the anchors. In case of Pilot project at the

coast of Vlieland (NL), some of the horizontal curtains placed in the intertidal zone have provided a

growth 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 rather

vague experience than on the general valid calculation methods. However, the increased demand in

recent years for reliable design methods for protective structures have led to the application of new

materials and systems (including geotextile systems) and to research concerning the design of these

systems. Contrary to research on rock and concrete units, there has been no systematic research on the

design 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 some

useful results which can be of use in preparing a set of preliminary design guidelines for the geotextile

systems 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 and

http://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 or

mortar) and their design aspects are briefly discussed below. For more detailed information on these and

other coastal protection systems and measures applied nowadays throughout the world, together with

recommendations and guidelines, the reader will be guided to the relevant manuals and publications (see

references and websites).

Offshore breakwater at design water level CD+3.5 m

50 m

3.2. Geobags

Geobags can be filled with sand or gravel (or cement, perhaps). The bags may have different shapes and 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 for temporary 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. A training groyne is a good example. The working of a groyne is difficult to predict in advance. That is

why it is a good procedure to make such a construction using a relatively cheap product first, to see how one thing and another works out, and subsequently either make improvements or, after some time, a permanent structure. Above a flow velocity of 1.5 m/s, the geosystems filled with sand cannot be used because the sand in the systems is no longer internally stable.

Figure 7– Application of geobags Sandbags 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 rough foundation. Sharp elements may easily damage the casing of the element. Geosystems must not be filled completely. With a fill ratio of approximately 75% an optimum stability of the elements is reached. A sound surface protection is necessary if sandbags are used in circumstances where they are

under heavy attack of flow or waves. Stability criteria are discussed in Pilarczyk (2000). The first indication on stability against waves, based on small-scale and large-scale tests, is given below: - Stability on slopes

ξop

s

cr

2.5 =

D

H

∆ (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 a structure)

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 as

toe protection of wind-farm foundation in the sea can be found in Grune et al.(2006).

Figure 8 – Large-scale tests Wave Flume Hannover

Stability 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 of

deformation 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 rubble

material. Although the effect of the deformations of the sand containers is significant, no stability

formula is available to account for those deformations. To achieve a better understanding of the

processes that affect the stability of GSC-structures several types of hydraulic model experiments and

analyses 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) internal

movement of sand in the containers, (v) variation of contact areas among neighbouring GSCs during

wave action, (vi) types of displacement of GSCs within a coastal structure and finally (vii) the effect

of 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 the

processes 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 of

observed displacement are developed. Stability formulations for each type of coastal structures made

of geotextile sand containers such as breakwaters, revetments, dune reinforcement and scour

protection systems are proposed.”

3.3. Tube system

Geotube 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 and site conditions. The tube is delivered to the site rolled up on a steel pipe. Inlets and outlets are regularly spaced along the length of the tube. The tube is filled with dredged material pumped as a

water-soil mixture (commonly a slurry of 1 on 4) using a suction dredge delivery line (Figure 9). The choice of geotextile depends on characteristics of fill material. The tube will achieve its desired shape when filled up to about 80%; a higher filling grade is possible but it diminish the friction resistance between the tubes.

Figure 9 – Filling procedure of Geotube Tube can be filled on land (e.g. as dikes for land reclamation, bunds, toe protection or groyns) or in water (e.g. offshore breakwaters, sills of perched beaches, dikes for artificial islands or interruption of

gullies caused by (tidal)currents). The tube is rolled out along the intended alignment with inlets/outlets centered on top. When a tube is to be placed in water, the effects of buoyancy on the tube geotextile prior to filling as well as on the dredged material's settling characteristics must be considered. In order to maximize inlet/outlet spacing, an outlet distant from the inlet may be used to enhance the discharge of dredged slurry and thereby encourage and regulate the flow of fill material

through the tube so that sufficient fill will flow to distant points. The tube will achieve its desired shape when it is filled up to about 70 to 80% of the theoretical circular diameter or a height equal to about a half of the flat width of the tube; a higher filling grade is possible but it diminishes the frictional resistance between the tubes.

Commonly, the filter geotextile (against scour) and flat tube are fully deployed by floating and holding them in position prior to beginning the filling operation. The filter geotextile is often furnished with small tubes at the edges when filled with sand holds the filter apron at place. This apron must also extend in front and behind the unit, commonly 2 times the height of the entire structure or 2 to 3 times

the local wave height. The wave transmission can be treated as for smooth impermeable structures.

3.4. Container system

Geocontainer is a mechanically-filled geotextile and "box" or" pillow" shaped unit made of a soil tight geotextile. The containers are partially prefabricated by sawing mill widths of the appropriate length

together and at at the ends to form an elongated "box". The "box" is then closed in the field, after filling, 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 bottom dump hoppers scows, or split barges. The containers are filled and fabricated on the barge and placed

when securely moored in the desired position. Positioning of barge for consistent placement - a critical element of constructing "stacked" underwater structures - is accomplished with the assistance of modern surveying technology. The volume of applied geocontainers was up to 1000 m

3.

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 Geosystems

When applying geobags, geotubes and geocontainers the major design considerations/problems are related 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 large

depths), 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, and f) 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 and calculation of a required number of tubes for a given height of structure, knowledge of the real shape of the tube after filling and placing is necessary. The change of the cross-section of the tube depends on the static head of the (sand)slurry. Depending on this static head, the laying method and the behaviour of the fill-material inside the tube, it is possible that the cross-sectional shape of the filled

tube (with a theoretical cross-section with a certain diameter, D) will vary from a very flat hump to a nearly 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 of sand- or mortar-filled tubes based on some experiments with water. The Leshchinsky's method (PC-

program) combines all the previous developments and can be treated as a design tool. The design of the shape of the geotube is an iterative process. To obtain a proper stability of the geotube and to fulfil the functional requirements (i.e. required reduction of incoming waves/proper transmission coefficient the width and the height of the tube (= a certain crest level) must be calculated. If the obtained shape of geotube does not fulfil these requirements a new (larger) size of a geotube must be taken into

account or a double-line of tubes can be used.

Dumping process of containers and practical uncertainties. A summary of various forces during the 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 through the 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 required minimum 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 friction forces along the bin side are retarding this process. Due to these forces the tension in geotextile is developing at lower part and both sides of the geocontainer. The upper part is free of tension till the moment of complete releasing of geocontainer. The question is how far we are able to model a friction

and the release process of geocontainer. 2. Geocontainer will always contain a certain amount of air in the pores of soil and between the soil and the top of (surplus) geotextile providing an additional buoyancy during sinking. The amount and

location of air pockets depends on soil consistency (dry, saturated) and uniformity of dumping. The air pockets will exert certain forces on geotextile and will influence the way of sinking. The question is how to model in a proper way the influence of soil consistency and air content on shape and stresses in geotubes/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 geocontainers

During the impact the cross-sectional shape of geocontainer will be undergoing a continuous reshaping; from cone shape, first probably into a transitional cylindrical shape, and through a certain

relaxation, into a semi-oval shape or flat triangular/rectangular shape dictated by soil type, perimeter and elongation characteristics of geotextile. The question is how far we are able to model this impact phenomena and resulting forces/stresses in geotextile. The impact forces with the bottom are a function 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 or as 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 and

internal loadings.

Practical note: The prototype experience indicate that geocontainers with volume up to 200 m3 and

dumped 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 the

geotextile with tensile strength equal or more than 150 kN/m. This information can be of use for the

first selection of geocontainers for a specific project. The placing accuracy for depths larger than 10 m

is still a problem (Bezuijen et al., 2004). However, it should be realized that craftsmanship is very

important for a successful geocontainer project and can hardly be described with formulas.

3.6. Durability

Durability of geotextiles is frequently coming question especially concerning the applications where a

long life-span is required. Geotextile is a relatively new product. The first applications are from 60's.

Recently, we have tested in the Netherlands some 30-years old geotextiles used as a filter in revetment

structures (K&O, 1979). In general, these geotextiles were still in a good conditions (still fulfilling

properly the prescribed filter and strength functions). It was also confirmed by recent international

experience (Raymond et al., 1993, Mannsbart et al. , 1997, CEN, 1998). The technology of geotextiles

is improved to a such extend that the durability tests under laboratory condition indicate the life-time

of 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 armour

layers. However, in case of exposed geosystems the UV radiation and vandalism are the factors, which

must be considered during the design. All synthetics are vulnerable to UV. The speed of UV

degradation, 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 to

various environment have provided the following results (see Table) concerning the strength of the

surface 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) and

UV stabilized. As the period in which the fabric is exposed is short (in terms of months), no serious

problems are to be expected. In case of more or less permanent applications under exposed conditions

the fabric must be protected against direct sunlight. There is a number of methods of surface

protection for geosystems. To provide additional UV and abrasion protection to the exposed sections

of tubes, a coating of elastomeric polyurethane is often used. This coating, however, has a tendency to

peel 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 ebb-flood

50 m under the sea level

PET: stabilised

PA : stabilised PA : no PP : stabilised PP : no

63%

33% 14% 41% 1%

62%

8% 6% 46% 16%

94%

85% 80% 93% 92%

93%

91% 71% 95% 95%

Note: the geosynthetics under water and in the intertidal zone show very little degradation in strength in comparison with geosynthetics placed on land; in the inter-tidal zone the geosynthetics are covered very soon by algae which provide very good UV protection.

The permanent surface protection by riprap or blockmats is a rather expensive solution and it will normally be applied only when it is dictated by necessity due to a high wave loading or danger of vandalism or other mechanical damage ie. boating, anchoring, etc. In other cases it will be probably a cheaper solution to apply a temporary protection of geotextile tubes by an additional layer of a strong

geotextile provided with special UV-protection layer. This extra protection can be realized by adding the highly U.V.-stabilized nonwoven fleece needled onto the main fabric. The function of this felt layer is also to trap the sediment particles and algae, which give again extra U.V.-protection.

4. CONCLUSIONS

Geosynthetics and geosystems constitute potential alternatives for more conventional materials and

systems. They deserve to be applied on a larger scale. However, doubts among specifying authorities

and 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 the

literature search (Pilarczyk, 2000) and recent international research will help to overcome some of

these doubts and will be used as a basis for learning and promoting in the application of geosynthetics

and geosystems.

The basic material for geosystems are geotextiles or, more generally, geosynthetics. Proper knowledge of

these materials (technological properties, design specifications, test methods, etc.) is essential for a proper

choice of material needed to fulfil the functional requirements of geosystems resulting from the specific

requirements 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 that

geotextiles and geosystems are only a part (or a component) of the total project and that they have to be

treated and integrated in the total perspective of a given project

Systematic (international) monitoring of realized projects (including failure cases) and evaluation of

the prototype and laboratory data may provide useful information for verification purposes and further

improvement of prediction methods. It is also the role of the national and international organizations to

identify this lack of information and to launch a multiclient studies for extended monitoring and

testing programmes, to provide users with an independent assessement of the long-term performance

of 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 of

design 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 of

geosystems. The IGS in cooperation with other international organisations (i.e., PIANC) should

undertake actions in this direction.

Finally, there is a rapid development in the field of geosynthetics and geosystems, and there is always a

certain time gap between new developments (products and design criteria) and publishing them in

manuals or specialistic books. Therefore, it is recommended to follow the professional literature on this

subject (Journal of Coastal Engineering, Journal of Geotextiles and Geomembranes, Geosynthetics

International, Geotechnical Fabric Report, and Proceedings of Geosynthetic Conferences) for updating the

present knowledge and/or exchanging new ideas.

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