FEASIBILITY STUDY OF THE CONTINUOUS GEOTUBE®

P.M.N. VAN ZIJL, W.J. VLASBLOM, J.G. DE GIJT, E.J. BROOS AND J. DE BOER

Feasibility Study of the Continuous Geotube® 19

ABSTRACT

Geotextile tubes have been applied for anumber of years in hydraulic and coastalengineering such as shore protection andbreakwaters. Based on a concept originallysuggested by the Dutch marine contractorACZ (now Van Oord), the so-calledGeotube® was further developed by Ten Cate Nicolon, and has provensuccessful on land as well as in shallowwater. Using Geotubes® in rough and deepwater however provides another set oftechnical difficulties.

Discussions with a commission from CUR(Centre for Civil Engineering Research andCodes) -- comprising several companies andorganisations amongst which Van OordACZ, Boskalis, IHC, Rijkswaterstaat (RWS),Rotterdam Public Works, the manufacturersof geotextile and TU Delft -- about thepotential problems resulted in further studyof the feasibility of a continuous Geotube®that could be used in the open seas insteadof just along the shoreline. Now that scaletests presented here have provided goodinformation about possible solutions anda conceptual design of the equipment,a prototype test is necessary.

INTRODUCTION

For several years geotextile tubes orGeotubes® (a registered trademark of Ten Cate Nicolon b.v.) have been applied inhydraulic and coastal engineering such asshore protection and breakwaters. They have been used on land as well as inshallow water. With the present techniquesit is very difficult to apply Geotubes® inrough and relative deep water.

In 2002 the CUR committee C129presented a report that describes theexpected problems in the concept device tofabricate a continuous Geotube® that canbe used in open sea conditions. The report“Praktijkproef continue geotube” is basedon the Dutch patent NL 1011043. One ofthe main problems foreseen by thecommittee was the simultaneous filling andstitching of the geotextile. This paperdescribes a folding method (patented) forstitched Geotubes®, which makes itpossible to separate the filling process fromthe fabrication of the tube, themathematical modelling of a Geotube®during the operation, the performed scaletests and a way the overcome theremaining problems.

The following processes take place duringinstallation of the continuous Geotube®:- On a pontoon a tube is formed from a roll

geotextile.- The edges of the geotextile are sewed

together.- The tube is secured to the pontoon by a

clamping system.- The Geotube® is filled with sand trough

the opening.- During placing the tube is (partly)

supported by a slide

The concept of the continuous Geotube®makes it possible to place a Geotube® indeep and rough water (Figure 1). Becausethe tube is filled before it is placed, the tube is stable after placing. There areseveral technical difficulties, but the twomain difficulties in the concept are:- The stresses in the geotextile and a clamp

system to transfer the forces to thepontoon.

- The integration between sewing of thegeotextile and filling of the tube withsand.

Above, Geotube® units were used to outline an area

for a wetlands, creating a barrier sufficient to allow

the area to develop (Courtesy of Ten Cate).

20 Terra et Aqua | Number 102 | March 2006

MATHEMATICAL MODEL

To understand the static stresses acting inthe geotextile, a mathematical model wasmade. The main simplification in the modelis that the influence of the sand particleswas neglected. The tube is filled with aslurry with a density twice as high as thedensity of water. The model describes theshape of the Geotube® after placing aswell as the tangential and axial stresses inthe geotextile when it is placed over a slideto the bottom. The bottom section of theslide is modelled as a quarter of a circlewith a 5 metre radius and the top sectionas straight vertical (Figure 2). Only the bottom section is considered inthe model. This part supports the tube fromto vertical position to horizontal position.With the stresses acting on the geotextile,the total axial force on the clamp systemcan be calculated. For 5 metrecircumference Geotube® which is filled at5 metres above the bottom, the final heightafter placing will be 1.40 metres and thetotal static axial force is 85 kN. The dynamic forces caused by external andinternal influences are difficult to describe.A safety factor which includes an aspect fordynamic forces is used to calculate therequired geotextile strength. A general usedvalue for the safety factor is 3.

FILLING AND SEWING OF THEGEOTUBE®

The integration between the processes ofsewing and filling could not be realisedaccording to the CUR committee. To guarantee the quality of the seam it isnecessary that:- There is no tension differential between

the two edges of the geotextile.- The working area of the sewing

equipment is free of sand and water.

These two demands cannot beaccomplished onboard a pontoon. The whole process of fabricating andplacing the Geotube® is dependent on theprocess of making the seam. To guaranteethe quality of the seam the tube has to bepre-fabricated. A pre-fabricated tube whichis rolled up can only be filled through inletports. Filling through inlet ports is not anoption for the continuous Geotube®. The solution is to fold the tube in a specialmanner, so a package with a filling openingis formed (Figure 3). As seen in Figure 3,the first step in the method is folding thetube inwards from two sides, so twoparallel folds (4) are made. The size of thefilling opening is dependent on the distancebetween the folds (length of 5). Hereafterthe tube is folded inwards (6) from twosides perpendicular to the first step. Then the first step can be repeated. When completed, an opening remains inthe middle of the package. In Figure 3,square section 8 is the filling opening.

The ratio between length of the Geotube®and the thickness of the folded packagedepends on the circumference of theGeotube®, the thickness of the geotextileand the dimensions of the filling opening.With commonly used dimensions ofGeotubes®, the theoretical ratio rangesbetween 100 and 250. For a 5 metrecircumference Geotube® of 200 kN/mgeotextile with a thickness of 3.3 mm,

Figure 1. First concept of the continuous Geotube®.

Figure 2. Geotube® on the bottom section of the slide according to the computer model.

Feasibility Study of the Continuous Geotube® 21

the theoretical ratio is 1:113. A package of1 metre contains a tube with 113 metrelength. When a 100 kN/m geotextile ischosen, the thickness is 1.6 mm and theratio is 1:234

The method was tested with a 3.5 metrecircumference and 5.2 metre lengthGeotube® (Figure 4). The size of theopening is 0.3 by 0.3 metre and thethickness of the geotextile 1.2 mm. The theoretical ratio is 240, so thecalculated height of the package is 22 mm.The measured height was also 22 mm.

CLAMP SYSTEM AND SCALE TESTS

A clamp system is needed to secure theGeotube® to the pontoon and to lower the Geotube® at the same time. A clampsystem with the clamp completely outsidethe Geotube® has the most advantages.The forces acting on the clamp can easilybe transferred to the pontoon and theclamp is always accessible for maintenance.The Geotube® is clamped on two sidesnext to the filling pipe. The clamp will befitted with a tensioner system to get acontinuous process.

MARCO VAN ZIJL

Marco van Zijl graduated in 2004 from

the University of Technology, Delft,

the Netherlands (TU Delft). For his master’s

study he worked at the Rotterdam Public

Works, investigating the problems of

constructing a continuous Geotube®.

Presently he is working for the dredging

and marine contracting company Van Oord.

WILLEM J. VLASBLOM

Wim Vlasblom received an MSc in Civil

Engineering, specialising in hydraulics,

from TU Delft in 1968. From 1968-92 he

was in the research departments of three

major dredging contractors. From 1992-94

he was Head, Planning and Production

Department at the Airport Platform

Contractors Marine Works JV for Chek Lap

Kok Airport, Hong Kong. Since 1994 he is

Professor of the Chair of Dredging

Technology at TU Delft.

JARIT G. DE GIJT

Jarit de Gijt graduated in Civil Engineering

from the TU Delft in 1976 and worked for

Fugro from 1975 to1987. In1987 he moved

to Rotterdam (Public Works) in the

engineering department and has been

involved in designing, constructing and

contracting for port infrastructures projects.

He has been a guest lecturer since 1999

and part time lecturer since 2004 at

TU Delft in the hydraulic engineering section.

ERIK BROOS

Erik Broos received his MSc in Civil

Engineering from the TU Delft in 1999.

He then went to work for the Rotterdam

Public Works Engineering Consultants.

He is currently working for the harbour

engineering department as a consultant for

various hydraulic engineering projects and

as project leader for the dredging works

on the existing (first) Maasvlakte.

JAN DE BOER

Jan de Boer graduated from TU Delft in

1967 with an MSc in Civil Engineering.

Initially working as an Engineer with the

Dutch Association of Contractors in Shore

and Bank Protection, he joined Zinkcon

International BV later in 1968. From 1984-

88 he was with DEMAS (Dredging,

Engineering and Management Studies)

and thereafter to the present became

Director of Geopex Products (Europe) BV.

Figure 3. Diagram of two views of the folding method.

Figure 4. Two views of the folding test

with a Geotube®.

22 Terra et Aqua | Number 102 | March 2006

Scale tests were performed to determinethe maximum axial force that the clampcan transfer and to understand the processof placing the Geotube®.

A model of the clamp was made on a1:5 scale. The clamp was not fitted withtensioners but with clamping plates forsimplicity (Figure 5).

For the strength test, a 1 metrecircumference and 25 kN/m strengthGeotube® was fixed in the clamp. The other end of the Geotube® wasclamped circular (see Figure 6). In the firsttest the Geotube® tore at the circularclamped end at a load of 12.4 kN (Figure 7).In the following test the maximum load on

the geotextile was 19 kN; at that momentthe geotextile was about to tear again atthe circular clamped end of the Geotube®. The tests showed that a clamp systemfixing the Geotube® on only 40 percent ofthe circumference can transfer up to75 percent of the total axial strength of theGeotube®.

The filling tests were carried out in a4 metre deep basin with a 1 metrecircumference Geotube®. The tube is placedwithout a slide for simplicity (Figure 8).

The filling height was kept at 1 metreabove the bottom. The axial force on theclamp was measured and was equal to thecalculated static force with the model. The calculated height of the Geotube® is0.29 metre while the measured height ofthe Geotube® varies between 0.25 and0.29 metre.

This variation in height is caused by thesimplification of the scale model. Becauseof the clamping plates, the Geotube® hasto be lowered after every metre of placed

Figure 5. Scale model of the clamp.

Figure 6. Two views of the strength test.

Figure 7. Result of strength test with 11.9 kN load (left) and 12.4 kN load (right).

Feasibility Study of the Continuous Geotube® 23

tube to loosen the clamp plates and re-clamp the tube a metre higher. Hereafterthe tube is lifted again, but the decrease inheight cannot be undone. Because theGeotube® was placed without a slide, thecurve of the tube between the vertical tohorizontal position is not circular aspresumed in the model. The shape of thecurve of the Geotube® somewhat differsfrom the shape of the slide but thedifference is not significant.

Filling of the Geotube® can be donehydraulically or mechanically. The settlingvelocity of the sand particles is, partlydependent on the influence of processwater, too low to fill the Geotube®.

When the tube is filled mechanically, as inthe scale tests, the sand will act as onemass and will achieve a much highersettling velocity than an individual sandparticle. During the tests this influence wasnoticed in an increase of the axial forceacting on the clamp (Figure 9).

Another result of the test is that a slide isnot necessary to place a Geotube®. The Geotube® will curve in a manner sothat an equal tension distribution will beachieved. The place of the seam hasinfluence in the tension distribution in thegeotextile. The tension distribution will bebetter when the seam is orientated on topof the Geotube® after it is placed.

From the standpoint of process control, aslide is not even desirable. Without a slidethe filling height can be determined by theaxial force. With a slide an unknown partof the forces are transferred through theslide, so there is no relation between axialforce and the filling height.

Figure 8. Filling test.

Figure 9. Filling test results.

STRUCTURE OF THE INSTALLATION

The structure of the installation isdetermined mainly by the presence orabsence of swell compensation. When thepontoon is positioned and the clamp islocated in the centre of mass of thepontoon, rotation around horizontal axes of the pontoon does not lead to verticaltranslation of the clamp. Vertical translationof the pontoon can be seen as an increasein filling height. An increase in filling heightleads to higher axial and tangential forceswhich can be taken up by the geotextile,i.e. the geotextile acts as a swellcompensator.

As long as the rotations around thehorizontal axes of the pontoon are smallcompared to the water depth, the clamp

can be fixed to the pontoon and swellcompensation does not seems to benecessary. Gripping the package and themanner of changing the packages alsoinfluences the structure of the installation.

There are several options to fulfil thesetasks (Figures 10, 11 and 12).

To obtain the desired final product,complete control of the total process offilling and placing of the Geotube® isrequired. To keep variation in the finalheight of the Geotube® small, the fillingheight must be kept as constant aspossible. The influence on the filling heightof the descending speed can measureddirectly from the axial force, while theinfluence of the filling speed can only bemeasured with a delay. Therefore the

24 Terra et Aqua | Number 102 | March 2006

descending speed is made slave to thefilling speed to control the process.Between the folded Geotube® and theclamp is a small buffer, so the supply ofGeotube® from the package may differfrom the descending speed.

CONCLUSIONS

The main conclusions from this study are:- By folding the Geotube®, the problem of

integration of sewing the geotextile andfilling the Geotube® with sand is avoided.

- A clamp system that clamps theGeotube® on 40 percent of thecircumference can transfer up to75 percent of the total strength of theGeotube®.

- The mathematical model can be used topredict the axial force on the Geotube®and the shape of the Geotube® on thesea bottom with sufficient accuracy.

With the information obtained by the scaletests and the solution for the integrationproblem, a prototype test is now necessary.For the execution of a prototype test thefollowing recommendations are made:- The clamp should be placed rigidly and

without swell compensation on thepontoon.

- Prior to the prototype test, the manner ofgripping the package Geotube® and thesupply of Geotube® to the clamp has tobe tested.

Figure 10. Three-

dimensional model

of the installation.

Figure 11. Schematic drawing

of the installation structure.

Figure 12. Three-dimensional

overview of placing

the continuous Geotube®.