Building a wall in the sea, often a seawall or retaining wall, requires careful planning, adherence to regulations, and specialized construction techniques. Here's a breakdown of the process:
1. Planning and Preparation
- Research Local Rules and Regulations: Before commencing any construction, thoroughly investigate and obtain the necessary permits and comply with local environmental regulations. This is paramount.
- Site Assessment: Conduct a comprehensive site assessment. This includes:
- Bathymetric survey: Mapping the seabed's topography.
- Geotechnical investigation: Analyzing the soil composition and stability. This is crucial for determining the foundation requirements.
- Wave and current analysis: Understanding the forces the wall will need to withstand.
- Design: Based on the site assessment, design a wall that is structurally sound and environmentally appropriate. Consider factors like:
- Wall type: Options include sheet pile walls, concrete walls, rock revetments, and gabion walls. The choice depends on the site conditions, budget, and desired aesthetics.
- Height and thickness: Determined by wave action, water depth, and soil properties.
- Materials: Selecting durable, marine-grade materials that can withstand saltwater corrosion and erosion.
2. Construction Methods
The specific construction method depends largely on the type of wall being built. Here are some common approaches:
2.1 Sheet Pile Walls
- Installation: Sheet piles are interlocking steel or concrete sections driven vertically into the seabed.
- Drive Guide: Create a system to guide the sheets during the driving process. This ensures proper alignment.
- Driving: Using a pile driver (impact hammer, vibratory hammer), drive the sheet piles to the designed depth.
- Waler Boards: Secure waler boards (horizontal beams) to the sheet piles to provide additional support and stability.
- Anchoring (if needed): Install tie-back anchors to provide extra resistance against lateral pressure, especially in deeper water or areas with strong currents.
2.2 Concrete Walls
- Foundation Preparation: Prepare a stable foundation, which may involve dredging, leveling, and installing a layer of gravel or other suitable material.
- Formwork: Construct formwork to define the shape of the concrete wall.
- Reinforcement: Install steel reinforcing bars (rebar) within the formwork to provide tensile strength to the concrete.
- Pouring Concrete: Pour marine-grade concrete into the formwork. Ensure proper compaction to eliminate air pockets.
- Curing: Allow the concrete to cure properly, typically by keeping it moist for a specified period.
2.3 Rock Revetments
- Base Preparation: Prepare a stable base by grading and compacting the existing soil.
- Filter Layer: Place a filter layer (geotextile fabric or graded gravel) to prevent soil erosion through the revetment.
- Rock Placement: Carefully place large rocks or boulders on the filter layer. Interlock the rocks to create a stable and durable barrier.
2.4 Gabion Walls
- Gabion Basket Assembly: Assemble wire mesh baskets.
- Placement: Position the empty gabion baskets in the desired location.
- Filling: Fill the baskets with rocks, ensuring proper compaction.
- Securing: Secure the baskets together with wire ties to create a continuous wall.
3. Considerations for Underwater Construction
- Dewatering: If possible, dewater the construction area using cofferdams or other techniques to create a dry working environment. This can significantly simplify construction.
- Diving Operations: Employ commercial divers for underwater tasks such as foundation preparation, formwork installation, and inspection.
- Equipment: Use specialized marine equipment such as barges, cranes, and excavators to transport materials and perform construction tasks.
- Weather: Closely monitor weather conditions and avoid construction during periods of high winds, waves, or storms.
4. Environmental Impact
- Minimize Disturbance: Take steps to minimize disturbance to the marine environment during construction.
- Sediment Control: Implement sediment control measures to prevent siltation and turbidity.
- Habitat Restoration: Consider habitat restoration measures to compensate for any habitat loss caused by the wall.
5. Post-Construction
- Inspection: Regularly inspect the wall for signs of damage or deterioration.
- Maintenance: Perform necessary maintenance, such as repairing cracks, replacing damaged rocks, or removing debris.
