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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~