A Complex Eco-friendly Geotextile A Complex Eco-friendly Geotextile Container Used in the Dredging and Container Used in the Dredging and Disposal of Contaminated Materials Disposal of Contaminated Materials

at Victoria Harborat Victoria Harbor

Felix Tseng1 Clark Chu1 Amy Tang1

1ACE Geosynthetics, No.33, Jing 3 Rd., C.E.P.Z. Wuchi, Taichung City, Taiwan, R.O.C.

Outline

• Case Introduction• Design Concept• Result• Conclusion

Case Introduction - Trial location

• Limited capacity for disposing dredged sediment in Hong Kong

• Unpolluted sediment was dumped in the disposal site of high seas: South Cheung Chau, East of Ninepin, North Lantau, East Tung Lung Chau, Tai Mo To and South Tsing Yi

• Polluted sediment was stored in confined disposal site: East Sha Chau

Case Introduction - Sediment type• It’s estimated that 50,000

cubic meter among the total dredged sediment, around 1.15 million cubic meter, was classified as Category H (>10xLCEL) which required special treatment of Type 3 contaminated dredged sediment

Note:Marine sediment is classified as Category L, Category M, Category H by polluted level based on ETWB TCW No. 34/2002LCEL – Lower Chemical Exceedance Level

Case Introduction - Monitoring work• A complex geotextile

container prefabricated with geotextile retains Type 3 sediment to avoid spread of suspended solid during disposal operation.

• In this trial test, the geotextile container was designed for the dredging and disposal work, and the water quality monitoring was performed during the operation.

Case Introduction - Geotextile Container• Big geotextile container of

volume of 100 to 1000 m3

for filling sand or slurry can be transported and dumped by split barge.

• Geotextile container of 28m in circumference and 12m in length was designed for this project. Compared with previous design, the new one saved 18% usage amount of fabric, moreover, the filling volume reaches 300 m3.

Design Concept - Mathematical Calculation• With following input, the

required tension of geotextile container is shown below:

– Dumping depth, h=20m– Width of hopper gate, b0=2.0m– Length of each chamber, L=11m– Depth of hopper, hb=4.92m– Unit weight of sediment, γ=12kN/m3

• Tension while geotextile container goes through hopper T1=83.08kN/m

• Tension while geotextile container impacts the seabed T2=86.38 kN/m

• ACETex® GT200-II PP, 200x200kN/m and seam strength of 140kN/m can meet required design tensile strength.Description of ACETex® GT200-II

Test Standar

d

Material Polypropylene

(PP)

Nominal tensile strength - MD kN/m

≧ 200

ASTM D4595

Nominal tensile strength - CD kN/m

≧ 200

ASTM D4595

Elongation at nominal strength - MD

% ≦ 23 ASTM D4595

Factory seam strength kN/m

≧ 140

ASTM D4884

Design Concept - Hydraulic Trial

• Hydraulic trials were conducted to a scale of 1:20 for imitating the dumping process:– Dry dumping– Dumping in water with 1m

depth (Actual dumping depth is about 20m)

　Prototyp

e Model

Length of split barge (m) 11 0.55

Depth of water (m) 20 1

Tensile strength (kN/m) 200 10

Factory seam strength (kN/m) 140 8.67

In-situ seam strength (kN/m) 82.5 5.04

Design Concept - Dry Dumping Trial• Main Purpose

– Measuring the dimension of geotextile container

– Checking dumping damage

• Trial Condition– Metal case was made as

split barge model– Same filling volume and

materials (silica sand)– Different opening speed

Design Concept - Dry Dumping Trial• Result

– Filling volume: 0.0275m3– Filling rate: 66%– Results is shown as below

table:

Item D1 D2

Hopper open time (s) 32 24

Hopper gate width (cm) 17 20

ACEContainer Avg. height (cm)

11.3

8

ACEContainer Avg. width (cm)

58 63.7

Filling rate (%) 66 66

Design Concept - Dumping in Water Trial• Main Purpose

– Observing dumping behavior

– Checking dumping damage

• Trial Condition– Metal model case in water

tank with 1m depth water– Excluding the in-situ

influence of current and wave

Design Concept - Dumping in Water Trial• Result

– Filling rate: 75~78%– Observing if there any leak in

the dumping procedure– Result is shown as below

table: Item W1 W2 W3 W4

Hopper open time (s) 18 25 33 34

Hopper gate width (cm) 17.5 22 21 20

ACEContainer Avg. height (cm)

7 7.1 6.75 6.9

ACEContainer Avg. width (cm)

59 57.7 61 62

Filling rate (%) 75 75 78 77

Design Concept - Water Quality Monitoring• Seven monitoring stations

located around 200~600m from disposal location were selected, two at the upstream and five at the downstream.

• Monitoring was undertook at around 30-minutes interval for 2 hours after dumping.

Result - Dissolved Oxygen Monitoring

• Disposal Zone Monitoring Stations:

Dissolved Oxygen (Surface & Middle) at Ebb Tide

2

4

6

8

10

14:24 14:52 15:21 15:50 16:19 16:48 17:16 17:45 18:14

Concentr

atio

n, m

g/L

M6 M7

`

Baseline

PostImpact

Turbidity (Depth-averaged) at Ebb Tide

0

5

10

15

20

25

14:24 14:52 15:21 15:50 16:19 16:48 17:16 17:45 18:14

Concentr

atio

n, N

TU

M6 M7

`

Baseline

PostImpact

• Upstream & Downstream Monitoring Stations:

Result - Turbidity Monitoring

• Disposal Zone Monitoring Stations:

• Upstream & Downstream Monitoring Stations:

Turbidity (Depth-averaged) at Ebb Tide

0

5

10

15

20

25

14:24 14:52 15:21 15:50 16:19 16:48 17:16 17:45 18:14

Concentr

atio

n, N

TU

M6 M7

`

Baseline

PostImpact

Turbidity (Depth-averaged) at Ebb Tide

0

5

10

15

20

25

14:24 14:52 15:21 15:50 16:19 16:48 17:16 17:45 18:14

Co

nce

ntr

atio

n, N

TU

M1 M2 M3 M4 M5 M6

Baseline PostImpact

Result• Water quality monitoring was continued for 5 days after

disposal operation at East Sha Chau• Baseline conditions was took 2 hours prior to disposal

operation• No pollution was found at upstream and downstream

monitoring stations• Exceeded result of DO in the monitoring record was due to

natural wave fluctuation

Conclusion• Mathematical computation and hydraulic trial can be

reference info. for designing geotextile container for different conditions.

• High tensile strength, excellent permeability and fitness to configuration of hopper barge makes ACEContainer a effective solution for this project.

• Actual disposal operation proved ACEContainer is applicable to disposal of dredging material: a speedy solution without influencing navigation operation and avoiding possible pollution.

Thanks for your attention~