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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.
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Outline
• Case Introduction• Design Concept• Result• Conclusion
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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.
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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:
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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
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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
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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.
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Thanks for your attention~