Geotextile tubes for cofferdams in land reclamation

Geotextile tubes are typically made of high-tenacity polypropylene and are generally black in color. While material specifications vary, a common weight is 400 g/m². Length is unlimited, and widths can be extended by stitching multiple panels together; however, the required material strength increases with the width. The filling height is generally set at two-thirds of the tube's diameter. Customization is available based on the diameter of the site's grouting equipment, and chemical additives can be incorporated to accelerate solidification and the separation of solids from water.

Comparison of geotextile tubes with other dredging methods:

1. Traditional natural dewatering in sediment disposal areas: This method for treating river and reservoir sediment is technically simple but requires a large land footprint; it is a common approach in domestic sediment treatment projects (including the cleanup and treatment of Taihu Lake sediment). However, this dewatering method is highly inefficient, and the use of disposal yards for contaminated sediment poses certain environmental and safety risks. Because the sediment in the yard remains exposed and in direct contact with the surrounding environment, failure to properly treat residual water can lead to secondary pollution of nearby water bodies.

2. Mechanical dewatering: While effective for treating sediment contaminated with organic matter and heavy metals, this method has drawbacks: high initial equipment investment, energy consumption, the need for dedicated processing facilities, and processing capacities that often fail to meet the tight schedules required for on-site river and reservoir sediment remediation.

3. Geotextile tube dredging: Convenience: Tube diameters and lengths can be adjusted as needed; they are highly adaptable, stackable, and easy to transport. Environmental friendliness: Operations are fully enclosed, generating minimal noise and posing little risk of secondary pollution. High efficiency: Capable of handling required wastewater and sludge volumes; the number of tubes used can be scaled up or down based on pump flow rates. Effective volume reduction: Sludge volume can be reduced by over 90% within a month, simplifying subsequent removal and disposal. Wide applicability: Suitable for wastewater treatment and sludge dewatering projects of various scales. Construction Method for Geotextile Tubes (Cofferdam Tubes):

Step 1: Filling. High-strength, permeable geotextile fabric is fabricated into the required tube shape, which is then filled with slurry.

Step 2: Dewatering. The geotextile fabric features fine pores that retain solid particles while allowing water to drain out. This effectively reduces the volume of the contained material, enabling repeated filling cycles until the maximum permissible height of the tube material is reached; the discharged water can be recycled.

Step 3: Consolidation. Following multiple filling and dewatering cycles, the fine particles remaining within the tube gradually consolidate as they dry.

Technical Advantages of Geotextile Tubes:

(1) Tube volume is highly adjustable, allowing for flexible handling capacities;

(2) No investment in mechanical dewatering equipment or power supply is required, and the system does not need to be moved;

(3) Low investment cost, minimal labor requirements, and simple operation;

(4) Tubes can be placed at any suitable location along riverbanks or lakeshores, and the clear effluent can be discharged directly back into the water body;

(5) Tubes can be positioned at multiple locations to match dredging vessel operations, making them ideal for in-situ sludge dewatering;

(6) The dewatering process is a closed operation, preventing secondary pollution;

(7) Sludge moisture content can be controlled by adjusting filling frequency, pressure, and resting time;

(8) Uses specialized filter fabric offering excellent filtration performance, high strength, and resistance to aging;

(9) Reagent formulas can be adjusted based on sludge characteristics to ensure environmentally safe treatment;

(10) Dewatered sludge can be utilized for river/lake bank protection, agriculture, or landscaping;

(11) Low system investment and operating costs, combined with a short construction period. Laying and Anchoring of Cofferdam Geotextile Tubes:

The cofferdam geotextile tubes are laid manually during low tide when the mudflat is exposed; prior to laying, any riprap, debris, or other obstructions that could compromise quality must be removed from the area. The prefabricated tubes are transported to the site and positioned according to the layout plan. To prevent rolling, sliding, or displacement during filling and to ensure accurate placement, the tubes must be anchored after laying. The bottom-layer tubes are anchored by driving steel anchor pipes into the ground alongside the tube and connecting them via anchor rings attached to the tube. Once the bottom-layer tubes are filled, the upper-layer tubes are anchored by tying them to the filled bottom tubes using geotextile straps connected to the lifting/anchoring loops on both sides.

Filling of Cofferdam Geotextile Tubes:

High-tenacity polypropylene cofferdam tubes are filled using a 22 kW jet-suction slurry pump unit mounted on a flat-deck barge. After extracting soil underwater via jet-suction, the slurry discharge pipe is directed into the filling sleeve of the laid tube to pump in the sand-slurry mixture.

Filling begins at the sleeve located at one end of the tube, while water drains from the other end; this facilitates sediment deposition and improves efficiency. During the process, the accumulation of sand and silt at the discharge point is monitored, and the position of the discharge nozzle and filling sleeve is continuously adjusted. Workers walk back and forth across the top of the tube to rearrange soil particles for greater density, ensuring the sand fill is uniform and compact, the surface is level, and the rate of drainage and consolidation is accelerated. Once the tube reaches the "slurry-blocking" stage (indicating it is full), the filling operation is reduced or halted to prevent the fabric from bursting, allowing time for consolidation and dewatering. During the filling process, survey personnel control the top elevation of each sandbag layer based on the cross-sectional drawings of the sandbag-core dike. If the desired height is not achieved in a single pass, filling is resumed—once or multiple times—after the sandbags have undergone some initial consolidation, continuing until the target degree of fullness is reached.