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

Contaminated Materials at 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.

Type B. In-situ Seam WorkType A. Factory Seam Work

Type B. In-situ Seam WorkType A. Factory Seam Work

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-IITest

StandardMaterial 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)

Prototype 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

Con

cent

ratio

n, m

g/L

M6 M7

`

Baseline

PostImpact

Turbidity (Depth-averaged) at Ebb Tide

0510152025

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

Con

cent

ratio

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

0510152025

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

Con

cent

ratio

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

Con

cent

ratio

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~