Construction of an offshore dike using slurry filled geotextile mats

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Geotextiles and Geomembranes 28 (2010) 422–433

Contents lists avai

Geotextiles and Geomembranes

journal homepage: www.elsevier .com/locate/geotexmem

Construction of an offshore dike using slurry filled geotextile mats

S.W. Yan a, J. Chu b,*

a Geotechnical Research Institute, Department of Hydraulic Engineering, Tianjin University, Chinab Centre for Infrastructure Systems, School of Civil and Environmental Engineering, Nanyang Technological University, Nanyang Ave., Blk N1, # 01A-10, Singapore 639798, Singapore

a r t i c l e i n f o

Article history:Received 21 July 2009Received in revised form25 October 2009Accepted 3 December 2009Available online 24 February 2010

Keywords:ClayCase historyConsolidationEmbankmentGeotextileSoil improvement

* Corresponding author. Tel.: þ65 67904563; fax: þE-mail addresses: [email protected] (S.W. Ya

0266-1144/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.geotexmem.2009.12.004

a b s t r a c t

A study on the use of clay slurry filled geotextile mats to construct dikes for land reclamation at TianjinPort, China, is presented in this paper. The dike so formed was covered by a thin layer of grouted geo-textile mattress for protection. Through laboratory tests, a type of low plasticity clay was chosen to be thefill for the mats. A simple method for estimating the required tensile strength for the geotextile mat andthe height of the mat was proposed. A preliminary design for the dike was made. Numerical analysis andcentrifuge model tests were conducted to verify the design and assess the stability of the dike beforeconstruction. A field trial was also carried out in which a 100 m long and 4.8 m high dike was constructedon soft seabed. The dike has been stable and the settlement has been within the expected limit since theconstruction was completed in September 2001.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Tianjin Port is 140 km away from Beijing and is one of the largestsea ports in China. Land reclamation has been carried out in thisarea since 1980s in order to cater for the rapid port development. Asgranular soil materials are scarce around Tianjin and the nearbycoastal region, clay slurry dredged from seabed has been used asthe main fill material for land reclamation. The vacuum preloadingmethod has often been adopted to consolidate the slurry (Chu et al.,2000). As the vacuum load causes an inward lateral moment in thesoil (Chu et al., 2000), a strong retention structure is usually notrequired for land reclamation. In the past, dikes have often beenconstructed using clay filled straw bags. However, this method isonly feasible in relatively shallow water.

To cater for further offshore land reclamation in relatively deepwater, the feasibility of constructing dikes using slurry filled geo-textile mats was studied. Similar methods such as the so-calledgeosynthetic tube have been used in China (Chi, 1991) and else-where in the world (Leshchinsky et al., 1996; Miki et al., 1996;Pilarczyk, 2000; Restall et al., 2002; Shin and Oh, 2003, 2007;Alvarez et al., 2007; Saathoff et al., 2007; Lawson, 2008). However,the geotextile mat method presented in this paper is different from

65 67910676.n), [email protected] (J. Chu).

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the geosynthetic tube method. The cross-section of the geotextilemat is not circular, but a flat mat with the horizontal dimensionmuch greater than the vertical one. For this reason, it will be calledthe geotextile mat or geomat in this paper. Despite a number ofstudies have been made on geosynthetic tubes (Kazimierowicz,1994; Leshchinsky et al., 1996; Miki et al., 1996; Plaut andSuherman, 1998; Plaut and Klusman, 1999; Cantre, 2002; Restallet al., 2002; Koerner and Koerner, 2006; Muthukumaran andIlamparuthi, 2006; Saathoff et al., 2007), a systematic constructionmethod for geomats has not been established. There are noguidelines on the selections of the fill materials and the geotextileto be used for the geomats. There are no established procedures onhow to design or construct such dikes either. Therefore, a researchproject was carried out to study the design and constructionprocedures for the use of geomats for dike construction. Anotherobjective of this research project was to seek an economical designfor the dike. As the dike under study was used as a retentionstructure for reclamation and the failure of the dike would notcause significant consequences, a design with a marginal factor ofsafety was targeted. Towards these purposes, the following taskswere performed for this research project:

(1) Find a suitable type of soil to be used as the fill material for thegeomats;

(2) Determine the suitable geometry of the geomat and the tensilestrength required for the geotextile;

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Table 1Physical properties of the soil used for the geomats.

Soiltype

Plasticlimit(%)

Liquidlimit(%)

Plasticityindex(%)

In-situ unitweight(kN/m3)

Finescontent(<75 mm)

Permeability(m/s)

SC-CL 11.5 20.4 8.9 20.0 55% 3.4� 10�7

0102030405060708090

100

0.0010.010.1110Particle size (mm)

% p

assi

ng

Fig. 1. Grain size distribution curve of the soil.

Table 2Change in water contents of the soils in the geomats.

Test number Water content Change in water content (%)

Before test (%) After test (%)

1 35.2 16.5 �18.72 32.2 17.6 �14.63 29.2 20.2 �9.04 26.2 17.8 �8.45 23.2 20.4 �2.8

hp

H

hpT

T

a

b

γ

Fig. 3. Calculation of the required tensile strength for the geotextile.

0

50

100

150

200

250

300

350

0 1 2 3 4 5

)m/

Nk(htgnerts

elisneT

Height (m)

Stacking

p0=100 kPa

p0=50 kPa

L_p0=50 kPa

L_p0=100 kPa

L_Stacking

Fig. 4. Calculated tensile stress in the geomats for a dike height H of 4.8 m.

S.W. Yan, J. Chu / Geotextiles and Geomembranes 28 (2010) 422–433 423

(3) Propose a preliminary design for the dike made of geomats;(4) Verify the preliminary design by conducting numerical analysis

and centrifuge model tests;(5) Conduct a field trial.

The results of the above study are presented in this paper.

2. Type of soil used

To construct offshore dikes using geomats, the soil to be filledinto the mats needs to be in a slurry form so that it can be pumped.

Fig. 2. The inflated geomat.

It would be ideal if the soil to be used for the mats can be sourcedlocally. Other requirements on the fill materials are (1) the soil hasto flow well in a slurry form; and (2) once the soil is pumped in themat, it should be able to consolidate relatively fast under its self-weight, the confining pressure asserted by the mat and wave forces.

Several silty clay soil samples were taken from or near thecoastal area where the dike was to be constructed and tested in thelaboratory. It was found out that soil with lower plasticity indexwould suit the purpose better. One type of soil was selected. Thephysical properties of the soil are given in Table 1. The grain sizedistribution of the soil is presented in Fig. 1. The fines content of thesoil was 55%. According to the Unified Soil Classification System(USCS), the soil is classified as a borderline case of SC-CL, that is,between clayey sand and low plasticity clay.

Table 3Comparison of the proposed method with that by Leshchinsky et al. (1996) for a dikeheight H¼ 4.8 m.

Pumppressure, p0

(kPa)

Tubeheight,h (m)

Tubecross-sectionwidth (m)

Tensile forceusing Eq. (6)(kN/m)

Tensile forceusing Leshchinsky’smethod (kN/m)

0 0.9 4.0 2.4 2.55 1.8 3.6 14.2 14.220 2.3 3.2 38.9 38.2100 2.7 3.0 156.9 154.8

Table 4Properties of the geotextile used.

Geotextile usage Thickness (mm) Mass per m2 (g/m2) AOS O95 (mm) Permeability (m/s) Tensile strength (kN/m) Max. elongation (%)

Longitudinal Transverse Longitudinal Transverse

Mat 0.52 131 0.145 3� 10�5 28 26 20.5 17.5Base layer 0.53 152 0.152 3� 10�5 33 27 22.0 16.0Geotextile mattress 0.61 284 0.088 4� 10�5 60 55 25.0 22.0

All made of woven polypropylene.

S.W. Yan, J. Chu / Geotextiles and Geomembranes 28 (2010) 422–433424

In order to study the consolidation behaviour of the soil underself-weight and wave forces, the soil was remolded into slurry andfilled in small size geotextile bags (40� 60 cm). The initial watercontents of the slurries varied from 1.1 to 1.7 times the liquid limit.The filled bags were firstly put into a wave generation trench for3 h. Waves were then generated in the trench for another 3 h. Thestill water depth was 0.53 m. The wave height was 0.22 m and theperiod was 1.54 s. The soil in the geotextile bag consolidated underthe wave force. The water contents of the soil measured before andafter consolidation are given in Table 2. It can be seen from Table 2that after consolidation, the water contents of all the soil samplesreduced to a value close to or slightly lower than the liquid limit ofthe soil regardless of the initial water content of the slurry. Thelarger the initial water content, the larger the reduction in watercontent. Therefore, for the soil used, the initial water content didnot affect much the final water content of the soil in the geotextilebag after it was consolidated under the wave force.

3. Design of geotextile mats

3.1. Required tensile strength

The geotextile used for the mats must have sufficient tensilestrength to sustain the pressure applied to the mat during pumpingand when the geotextile mats are stacked together. For designpurpose, the required tensile strength of geotextile needs to be

0

2

4

6

8

10

12

14

16

18

0 20 40 60 80

Water content (%), liquid limit (%), and plastic limit (%)

Dep

th (m

)

w /c

LL

PL

0

2

4

6

8

10

12

14

16

18

0 10

Bulk de

Dep

th (m

)

a b

Fig. 5. Soil property profiles (a) water content, liquid limit

calculated. The methods of analysis for the geosynthetic tubes havebeen proposed by several researchers (Kazimierowicz, 1994;Leshchinsky et al., 1996; Miki et al., 1996; Plaut and Suherman,1998; Plaut and Klusman, 1999; Cantre, 2002). For geomats witha flat cross-section, the calculation can be simpler. Based on themethod proposed by Kazimierowicz (1994), the following analysiswas proposed to calculate the required tensile strength.

A typical geomat after inflation is shown in Fig. 2. A simplifiedcross-section of the stacked geotextile mats is shown in Fig. 3a.Assuming that the height of the dike is H, the height of the bottomtube is h, and the unit weight of the slurry is g, the pressure insidethe bottom mat, p, can be calculated as

p ¼ gðH � hÞ (1)

Obviously, the pressure sustained by the bottom mat is thehighest among all the mats. Therefore, the tensile strength ofthe geotextile should be calculated based on the stress analysis forthe bottom mat. Considering the force equilibrium of the bottommat along the vertical section indicated by the vertical dash lineshown in Fig. 3a, the pressure distribution along this verticalsection is shown in Fig. 3b. T is the tensile stress in the geotextilemat. From force equilibrium in the horizontal direction, we have

2T ¼ phþ 12

gh2 (2)

Substitute Eq. (1) into Eq. (2), we have

20 30

nsity (kN/m3)

0

2

4

6

8

10

12

14

16

18

0 0.5 1 1.5 2

Void ratio

Dep

th (m

)

c

and plastic limit; (b) bulk density; and (c) void ratio.

0

5

10

15

20

25

0 25 50 75 100cu by field vane shear tests (kPa)

Dep

th (m

)

FVT1

FVT2

FVT3

FVT4

FVT5

FVT6

FVT7

FVT8

FVT9

Layer 1

Layer 2

Layer 3

Fig. 6. Field vane shear results.


T ¼ 12

gðH � hÞhþ 14

gh2 (3)

Given the design height of the dike, H, and the unit weight of theslurry, g, the required tensile strength of the geotextile and the limitheight of the bottom mat can be calculated using Eq. (3). It can bederived by letting dT/dh¼ 0 that T is the highest when h¼H. ForH¼ 4.8 m and g¼ 12 kN/m3, Eq. (3) becomes

T ¼ 6ð4:8� hÞhþ 3h2 (4)

If we rewrite Eq. (4) as

3h2 � 28:8hþ T ¼ 0 (4a)

For h to have real roots, the following condition must be met

Fig. 7. Key sketch of the designed d

28:82 � 12T � 0 (5)

that is, T� 69.12 kN/m for any h. Therefore, for the stacking casewith a dike height of 4.8 m, the maximum tensile strength requiredis less than 69 kN/m.

Eq. (2) can also be approximately used to estimate the tensilestrength required when the mat is filled under a given pumpingpressure, p0. By taking g¼ 12 kN/m3, then from Eq. (2), we have

T ¼ 0:5p0hþ 3h2 (6)

The T and h relations for p0 of 50 and 100 kPa and the stackingcase are plotted in Fig. 4. It can be seen that for p0 of 50 kPa, the Tand h relations are almost identical when h� 1.5 m. Usingp0¼ 50 kPa, T� 25 kN/m can be read from Fig. 4 for h� 1 m, that is,if a pumping pressure of 50 kPa is used and the high of the geomatis kept within 1 m, the tensile strength of the geotextile requiredwill not be more than 25 kN/m.

The above simplified method is compared with that of Lesh-chinsky et al.’s (1996). The tensile strength versus height relation-ships calculated using Leshchinsky et al.’s (1996) method are alsopresented in Fig. 4 using dash lines (labeled as L_p0¼ 50 kPa,L_p0¼100 kPa; and L_Stacking). Some numerical values are givenin Table 3. It can be seen that the curves and the values obtainedfrom the two methods agree with each other very well. Thecomparison indicates that the proposed method can also be usedapproximately for geotextile bags with circular cross-section.

3.2. Filtration requirements

In addition to having sufficient tensile strength, the geotextileused should also meet the filter criteria, i.e., its apparent poreopening size (AOS) has to be small enough to retain the soil insidethe mat and yet to provide sufficient permeability. Based on thegrain size distribution curve shown in Fig. 1, the AOS of the geo-textile should be smaller than 0.2 mm. This requirement can easilybe met by most geotextiles.

To meet all the above requirements, a woven polypropylenegeotextile with a mass per unit area of 131 g/m2 was chosen. It hada tensile strength of 28 kN/m in the longitudinal and 26 kN/m in thetransverse direction respectively. The AOS (O95) of the geotextilewas 0.145 mm. For the base layer, a stronger geotextile with a massper unit area of 152 g/m2 and a tensile strength of 33 kN/m and27 kN/m in the longitudinal and transverse direction respectivelywas used. Other properties of the geotextile are given in Table 4.

ike construed using geomats.

Fig. 8. (a) Finite element mesh; (b) the ultimate settlement contours at the middle cross-section of the dike.


4. Preliminary design

After the suitable soil and geotextile were selected, a prelimi-nary design for the dike built with geomats was made. The designrequirements set for the dike are:

(1) The soil in the geomats should be able to achieve a certaindegree of consolidation under the filling pressure and gravityduring the low tide period (about 5–6 h) and consolidatefurther under the wave force.

(2) The stress developed in the geomats should be less than thestrength of the geotextile.

(3) The settlement of the dike should be within a tolerate limit.(4) The stability of the dike should be maintained all the time.

The basic properties of the foundation soils below the dike areshown in Fig. 5. The results of 9 field vane shear tests are presentedin Fig. 6. As shown in Fig. 6, the soil profile can be roughly dividedinto 3 layers for analysis. The first layer was silty clay ranging from0 to about 5 m, where the water content of the soil was higher thanthe liquid limit (Fig. 5a) and the undrained shear strength wasgenerally less than 25 kPa (Fig. 6). The second layer was silt to siltyclay ranging approximately from 5 m to 15 m with the watercontent nearly the same as that of the liquid limit (Fig. 5a). Theaverage undrained shear strength increased with depth and variedapproximately from 20 to 40 kPa (Fig. 6). The coefficient ofpermeability for the first two layers of soil was in the order of10�9 m/s. Below this was a relatively stiff silty clay layer. The major

concerns in the design were whether the dike would be stable andwhether the settlement of the dike would be too large when it wasto be constructed on the soft ground.

Based on the past dike construction experiences and the resultsobtained from this study, a preliminary design of the dike usinggeomats was proposed. As shown in Fig. 7, the designed height ofthe dike was 4.8 m with the base and top elevations at 0.7 m and5.5 m respectively. The top width of the dike was 2.43 m. Thedesign water levels were at 4.7 m elevation during high tide and at0.7 m during low tide. The outer and inner slopes of the dike werechosen to be 2L:1H and 1.5L:1H, respectively. The length of thebottom mat was set to be 18 m. Nine layers of geomats were used.The bottom layer was filled with sand and the rest were with clayslurry dredged from the seabed of a nearby area. The clay slurry waspumped directly into the mats through injection holes. The heightof the geomat after consolidation was around 0.5 m. After thegeomats consolidated and the settlement of the dike stabilized,a grouted geotextile mattress of 25 cm was cast in-situ to forma cover to the surfaces of the slopes formed by the geomats. Thisgrouted geotextile mattress method is commonly used in Hollandand China (Pilarczyk, 2000; Chi, 1991; Chu et al., 2009). More detailon this geotextile mattress used in this project will be presented ata later section. As shown in Fig. 7, berms would also be used toenhance the stability of the dike and to protect the toes of theslopes. The berms were made of crushed stones of 50–80 kg. Theslopes of the berms were 2L:1H for the inner side and 3L:1H for theouter side. A 4 m thick of hydraulically filled slurry was to be placedbehind the dike after the dike was constructed.

-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

0.1

0 4 8 12 16 20

Distance from the centreline (m)

)m(tne

meltteS

1 day5151925303950701001501802503504505506507508501000

-0.4

-0.3

-0.2

-0.1

0

0.1

0 100 200 300 400 500 600 700 800 900 1000 1100

Duration (days)

Settl

emen

t (m

)

Centreline20 m away

a

b

Fig. 9. (a) Settlements with time measured at different locations below the dike; (b) settlements versus time measure at centerline of the dike and 20 m away from the dike.


5. Finite element analysis

Analysis of stability of embankment on soft soil is complicatedand finite element method is often adopted (Rowe and Li, 2005).

Fig. 10. Arrangement for

The overall stability of the dike at the end of construction wasanalysed using an elasto-plastic finite element package, PLAXISVersion 8.0. In the analysis, the soil was modeled as an elastoperfect-plastic material. The Mohr–Coulomb failure criterion was

the centrifuge tests.

Table 5Soil properties after consolidation in the centrifuge tests.

Test number Top layer silty clay Middle layer muddy clay Bottom layer

Water content (%) Density (kN/m3) cu (kPa) Water content (%) Density (kN/m3) cu (kPa) cu (kPa)

1 32.8 19.7 21 39.6 18.6 23 822 34.6 18.8 16 46.4 17.8 16 553 34.2 19.2 17 44.5 18.1 18 54


adopted. An undrained total stress analysis was carried out inwhich the undrained shear strength of the soil was selected basedon the field vane shear results shown in Fig. 6. The elastic moduluswas taken as 2.0, 3.5 and 4.5 MPa for the three layers respectively.The finite element mesh used is shown in Fig. 8a. The calculatedultimate settlement contours at the middle cross-section of thedike and foundation soil are shown in Fig. 8b. The calculatedground settlements versus distance from the centerline along thecross-section of the dike are plotted in Fig. 9a for different dura-tions. The settlements versus duration at the centerline and a point20 m away from the centerline are also shown in Fig. 9b. Themaximum ground settlement predicted was 42.5 cm. The analysisindicated that the dike would be stable.

6. Centrifuge test results

As the design was based on a thin safety margin, centrifugemodel tests were also performed to verify the design shown inFig. 7 and the study the failure mechanisms.

6.1. Material simulations

The centrifuge tests were carried out at the Nanjing HydraulicResearch Institute, China. The centrifuge machine used had

Fig. 11. Centrifuge tests. (a) Test 1: before test, (b) Test 1: after tested under 120 g

a capacity of 50 gt, with a maximum radius of 2.25 m andmaximum acceleration of 250 g. The dimensions of the model boxwere 685� 400� 200 mm. More detail on the centrifuge can befound in Liu and Zhu (1995). By considering the actual dimensionsof the dike and the size of the model box, a model ratio of n¼ 70was used. The model setup is shown in Fig. 10.

The properties of the foundation soils were the dominant factorfor controlling the stability and settlement of the dike. Soil samplesfrom all the three soil layers were obtained. To prepare the foun-dation soil, the soil samples were remolded into thick slurry andplaced in the model. The original soil profile was reproduced byplacing the soils in three layers. The soil was then consolidatedlayer by layer to achieve an undrained shear strength comparableto the in-situ values. The soil conditions used in the three centrifugetests and their undrained shear strength of the soil, cu, are given inTable 5. The cu values were measured using a pocket penetrometerwhich was calibrated against the unconsolidated undrained (UU)tests conducted on the same soil.

Two types of geotextile were used in the centrifuge tests.Type A was the same as that chosen for the geomats. As therewere uncertainties on how the geotextile should be modeled,a much weaker geotextile, Type B, was also used to form themats. The Type B geotextile was 0.4 mm thick and its unit weightwas 70 g/m2. The soil used to fill the mats was the same as that

, (c) Test 2: after tested under 120 g and (d) Test 3: after tested under 120 g.

0

2

4

6

8

10

12

0 20 40 60 80 100 12

Acceleration (g)

)m

m(tnemecalpsi

D

0

Test 1

Top of the dikeGround surface

Fig. 12. Settlements development with acceleration at the top of the dike and at the ground surface in centrifuge Test 1.


used in the project. The crushed stones used for the berms weresimulated using broken stones of 1–5 mm diameter. The slopesurface was covered with a thin layer of grouted geotextilemattress. The hydraulic fill was simulated with a slurry of a unitweight 14 kN/m3.

6.2. Testing procedure

The centrifuge tests were conducted in the following procedure.(1) The soils were placed in layers and consolidated to the requiredundrained shear strength; (2) the dike and berms were formedusing clay filled geotextile mats and broken stones respectively; (3)instruments were installed; (4) slurry was placed behind the dike;and (5) centrifugal load was applied by firstly increasing theacceleration to 70 g with 10 g for each loading step to check theperformance of the dike at the normal working condition, and thenincreasing the acceleration to 120 g to observe the development offailure.

Linear variable deformation transformers (LVDTs) were placedon the dike and the ground surface to monitor the settlement of thedike and the foundation soil (Fig. 10). Grid lines of 3� 3 cm weredrawn on the surface of the dike and the foundation soil (Fig. 11a) tofacilitate the deformation monitoring.

6.3. Test results

Three centrifuge tests were conducted. Tests 1 and 2 weresimilar. Type A geotextile was used in both tests. The differenceswere in the foundation soil properties. The soil in Test 1 had a lowerwater content and higher undrained shear strength as compared toTest 2 (see Table 4) to assess the effect of soil properties on theperformance of the dike. Test 3 was carried out using Type B geo-textile. The foundation soil properties in Test 3 were similar tothose in Test 2 (Table 4). The unit weights of the clay fill used for the

Table 6Prototype settlements measured along the centerline in the centrifuge tests at anacceleration of 70 g.

Test 1 2 3

Settlement measured at the ground (cm) 17.4 35.0 38.8Settlement measured at the top of the dike (cm) 57.2 74.3 81.9Vertical deformation of the geomats (cm) 39.8 39.3 43.1

mats and the slurry fill used behind the dike were 19.1 kN/m3 and14.7 kN/m3 respectively. The models before and after the three testsare shown in Fig. 11a–d respectively. It can be seen from the figuresthat the stability of the dike was maintained in all the three testseven after the tests had been conducted under 120 g. This could bedue to the ability of the mats to deform freely to redistribute load.

6.4. Displacements

The displacements measured for centrifuge Test 1 by LVDTsplaced on top of the dike and ground are plotted against acceler-ation in Fig. 12. It can be seen that the ground settlement increases

Fig. 13. Displacement vectors measured in normal working and failure. (a) Test 2under 70 g and (b) Test 2 under 120 g.

Fig. 14. A picture showing the fill process of the geomat.


with acceleration and the rate was getting smaller when theacceleration became higher. This was because consolidation of thesoil was mainly taken placed when the acceleration was picking upat the early stage. There were much more settlement at the top ofthe dike. This indicated that the soil in the geomats consolidatedwith the increase in vertical stress. The settlements at the groundsurface and at the top of the dike for the prototype at an acceler-ation of 70 g are given in Table 6 for all the three tests. The

Fig. 15. Dike constructed during field trial. (a) Initial stage, (b) intermediate stage, (c) use of cmattress, (d) placement of the grouted geotextile mattress for the slopes of the dike forme

differences of the two settlement values are the deformation of thedike, which are also given in Table 6. A comparison of the settle-ments observed in Tests 1 and 2 suggests that the properties of thefoundation soils affects considerably the ground settlement, butnot so much on the deformation of the geomats. A comparison ofthe settlements in Tests 2 and 3 indicates that the property of thegeotextile affects both the ground settlement and the deformationof the geomats.

6.5. Stability

Using the movements of the grid points, the displacementvectors at the grid points can be determined. The measureddisplacement vectors for Test 2 at g values of 70 g and 120 g arepresented in Fig. 13. The displacement vectors point toward thedownstream of the dike as anticipated. It can be seen from Fig. 13bthat a complete slip surface or zone was not formed even under120 g. The centrifuge test results indicated that the dike with thedesigned configuration was stable and the foundation soil appearedto be competent in providing sufficient bearing capacity andstability to the dike.

7. Field trial

After the preliminary study, a field trial was carried out. Asection of 100 m long dike with the base width of 22 m was con-structed using the design shown in Fig. 7. The clay slurry was

lay filled geotextile bags to form a smooth slope for the placement of grouted geotextiled by geomats and (e) final stage of the dike covered with grouted geotextile mattress.

- 100

- 50

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140 160

Duration (day)aPk(

daoL)

mm(tne

melttSe

)

- 1. 5m- 3. 0m- 6. 5mFill loadPredicted ground settlement

Fig. 16. Monitored settlements at different depths.


dredged using a cut suction dredger and pumped directly into themats. There was a pumping point on the mat for every 16 m2. Thepumping pressure used was 20–30 kPa. The maximum pumpingdistances was controlled within 500 m. A picture showing thepumping process is given in Fig. 14. Each mat was 20–33 m long.The thickness of the mat after it settled down was between 0.4 and0.5 m. The next layer was only placed when the settlement of thebottom layer became steady. Pictures that show the dike con-structed at different stages are given in Fig. 15.

In making the grouted geotextile mattress (see Fig. 15d), theslope of the dike was firstly smoothed by filling in the steps withcrushed stone filled geobags as shown in Fig. 15c. Moulds made ofgeotextile were then placed along the slope. The geotextile usedfor the grouted geotextile mattress had a thickness of 0.61, a massper unit area of 284 g/m2, AOS of 0.088 mm, and a longitudinaltensile strength of 60 kN/m. The other properties of the geotextileare given in Table 4. Cement grout was pumped into the geotextile

- 100

- 50

0

50

100

150

0 10 20 30

)aPk(

daoL)a

Pk(erusserp

eroP

- 1. 5m- 6. 5m- 12. 0mLoadPredicted -12m

Fig. 17. Monitored pore water pr

moulds using high pressure pumps. The cement grout used hada cubic strength of 25 MPa and the slump was 200–220 mm. Whenthe geotextile moulds were inflated, it exerted a pressure to thecement grout. Some water was then being squeezed out. Thecement grout began to harden in 30 min time. As shown inFig. 15d, the grouted geotextile mattress was designed withthickness varying at a certain pattern with a nominal thickness of25 cm. This design allowed the mattress to sustain limited defor-mation. Before the grouted geotextile in the mattress hardened,the grouted geotextile mattress was flexible and formed a coverthat fitted closely the profile of the dike. This had also made iteasier in accommodating limited settlement of the dike. For dikewith large settlement, a different design with the grouted geo-textile mattress blocked chained together (Wang et al., 2006) canbe used.

Instruments were also installed to monitor the settlement ofthe dike and settlements and excess pore water pressures in the

40 50 60 70

Duration (day)

- 3. 0m- 9. 0m- 15. 0mPredicted -1.5m

essures at different depths.

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5 6 7 8

Measuring points longitudinally along the dike

Settl

emen

t (m

)

Top of dikeGround

Fig. 18. Settlements measured at the points on top of the dike and ground selectedlongitudinally along the dike one year after constructions.


soil at different depths. The monitored settlement at differentdepths beneath the centerline of the dike and the surchargehistories are shown in Fig. 16. The excess pore water pressuresmeasured at different depths beneath the centerline of the dikeare also given in Fig. 17. The up and down in the pore pressures inFig. 16 was likely associated with the wave activities. It can be seenfrom Fig. 17 that the excess pore water pressure generations weregetting smaller with depth and the excess pore water pressure at15 m below the ground surface had became fairly small. Thisprobably defined the depth of stress influence. However, due toconstruction constraints, the settlements and pore water pressureswere only measured up to 140 days and 60 days respectively. Forcomparison, the predicted ground settlement (see Fig. 9b) is alsoplotted in Fig. 16. It can be seen that the predicted groundsettlement is smaller than monitored. This could be partiallybecause the deformation was assumed elastic as the elastoperfect-plastic model was adopted in the numerical analysis. Thepredicted excess pore water pressures at �1.5 m and �12 m arealso plotted in Fig. 17 for comparison. The predicted excess porewater pressures are also smaller.

Despite of the inability to monitor the settlements and porewater pressures in the soil in long term, the stabilized settlementsof the dike and the adjacent ground at different points along thelongitudinal axis of the dike were monitored. The data measuredone year after the construction of the dike are plotted in Fig. 18. Theaverage settlements of the dike and the ground are 84.1 cm and70.1 cm respectively. The average settlement of the dike was thus14.0 cm. This amount of settlement did not cause large cracks tooccur on the thin grouted geotextile mattress although small crackswere seen on the surface of the mattress. The average settlement ofthe dike is close to those observed from the second and thirdcentrifuge tests which were 74.3 cm and 81.9 cm respectively.Fig. 18 also indicates that the ground settlements along the longi-tudinal axis were quite even. This was expected as the length ofdike was much larger than its width. The trial has proven that thedesign is satisfactory and it is technically feasible to construct dikesusing slurry filled geomats. The dike was constructed in September2001 and has been perfectly stable since then.

8. Conclusions

A feasibility study on the use of clay slurry filled geotextile matsfor the construction of dikes was presented in this paper. Theselection of suitable soil to fill the geomats, the calculation of therequired tensile strength of the geotextile, and the design of

the dike using the geomats were discussed. Finite element analysis,centrifuge model tests and field trial were carried out to verify thedesign and establish the construction procedure. Based on thestudy presented, the following conclusions can be made:

(1) A low plasticity clay was proven suitable to be used as materialto fill the geomats in the slurry form;

(2) The required tensile strength for the geotextile used for thegeomats can be estimated using a simple analytical methodsuggested in this paper. The proposed method agrees well withthat by Leshchinsky et al. (1996);

(3) A preliminary design for the dike built using geomats on softseabed ground was proposed. The design was verified usingfinite element analysis, centrifuge model tests and a field trial.The centrifuge test results show that the dike built of geomatscan remain stable even under an acceleration of 120 g. Theground settlement is mainly controlled by the foundation soil;

(4) The field trial demonstrated that the dike built using theproposed method was stable. The use of grouted geotextilemattress as a cover for the dike after the geomats consolidatedwas suitable. The maximum ground settlement and thecompressibility of the dike measured in the field trial were84.1 cm and 14.0 cm respectively. These values agree reason-ably well with the centrifuge model tests. The smallcompressibility of the dike did not cause large cracks in thegrouted geotextile mattress although small crack occurred onthe surface of the mattress.

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Documents

Construction of an offshore dike using slurry filled geotextile mats