Digging Below Sea Level: Considerations and Safety in Deep Excavations Near Shoring Walls

[MikePhua]

Digging below sea level, especially at depths like 6 feet below, comes with several challenges that require careful planning, safety considerations, and specific techniques. When digging close to shoring walls, the risks increase due to the potential for wall instability, shifting soil conditions, and the possibility of flooding or water ingress. This article will provide insights into the complexities of excavations below sea level, the dangers of working near shoring walls, and strategies to mitigate those risks.
Understanding Excavation Below Sea Level
Excavation below sea level refers to digging in areas where the ground surface is lower than the average sea level. These projects often occur in coastal regions, around bodies of water, or areas subject to subsidence. The primary concern when excavating below sea level is water management—since the groundwater table may rise during digging or due to environmental factors like tides or rainfall.
Key Considerations for Excavating Below Sea Level:

1. Groundwater Control: As you dig below the water table, water ingress becomes a critical issue. Pumps, dewatering systems, or temporary barriers may be required to prevent flooding and ensure the excavation remains dry.
   
2. Soil Stability: Soils below sea level, especially in coastal areas, are often loose, sandy, or highly porous. These soils can behave unpredictably under excavation and may require the use of soil stabilization techniques, such as chemical grouting or geotextile fabrics.
   
3. Shoring and Wall Stability: Excavating near shoring walls can introduce stress on the wall, especially when working at significant depths. If the shoring is not properly designed to support the weight of the surrounding soil, the risk of collapse increases. It’s essential to use proper wall anchors, braces, or even temporary shoring solutions to mitigate movement.
   
4. Pressure from Surrounding Earth: As the excavation depth increases, the weight of the surrounding earth creates significant lateral pressures on the walls. These pressures can cause the walls to bow, crack, or collapse if not properly supported.
   

The Role of Shoring Walls in Excavations
Shoring walls are temporary structures designed to prevent the collapse of an excavation site. They are especially useful in deep excavations where soil stability can be compromised, and workers are at risk of being trapped by falling debris. Shoring walls come in various types:

• Soldier Piles and Lagging: Common in urban environments, these walls use steel piles driven into the ground, with wooden or steel lagging placed between the piles to prevent soil from shifting.
  
• Sheet Piling: Sheet piles are driven into the ground to create a continuous barrier that resists water and soil pressure. This type of shoring wall is particularly useful for excavation near water bodies.
  
• Hydraulic Shoring: Hydraulic shoring uses adjustable hydraulic cylinders to apply pressure to the soil, providing a tight seal and preventing movement.
  

Each type of wall is designed to resist the forces applied by surrounding soil, but their stability must be regularly checked, particularly when excavating below sea level.
Potential Hazards of Excavating Near Shoring Walls

1. Wall Failure: The biggest risk when working near shoring walls is the potential for wall failure. If the soil around the shoring becomes too loose or waterlogged, the pressure can exceed the structural integrity of the wall, causing it to collapse.
   
2. Shifting Soil and Ground Movement: Excavation near a shoring wall can cause the surrounding soil to shift, putting additional strain on the wall and potentially triggering ground movement. As the excavation progresses, it’s important to monitor the shifting of surrounding soil using sensors or manual checks.
   
3. Flooding Risk: When working below sea level, the water table can be high, increasing the risk of flooding. A breach in the shoring system or failure of dewatering equipment can lead to water inundation of the worksite, complicating the excavation process.
   
4. Equipment Instability: Excavation at significant depths, especially near shoring walls, can cause equipment to become unstable. The shifting of ground or soil can make it harder for heavy machinery to maintain stable footing, increasing the risk of accidents.
   

Precautionary Measures and Best Practices
To minimize risks and ensure the safe execution of the project, it’s essential to implement specific safety protocols and techniques when digging near shoring walls, particularly below sea level.

1. Thorough Site Assessment: Prior to excavation, conduct a thorough site assessment to determine soil composition, water table levels, and the stability of the existing shoring walls. Consider the history of flooding or groundwater issues in the area.
   
2. Dewatering Systems: Employ robust dewatering solutions, such as sump pumps, well points, or deep-well dewatering systems, to manage groundwater and prevent flooding. Regularly check these systems to ensure they are functioning properly throughout the project.
   
3. Reinforced Shoring: If excavation depth exceeds 5 to 6 feet, additional reinforcement may be needed for shoring walls. This could involve the installation of braces, anchors, or hydraulic support systems to ensure the stability of the walls under high pressure.
   
4. Progressive Excavation Techniques: When working near shoring walls, excavate gradually to avoid shifting large volumes of soil at once. Progressive excavation allows you to monitor the effects on the wall and surrounding soil, ensuring that changes are made safely.
   
5. Soil Stabilization: Consider soil stabilization methods like chemical grouting or soil nailing to improve the stability of loose or porous soils. These techniques can prevent soil from collapsing into the excavation site, thereby preserving both the shoring walls and the excavation site.
   
6. Frequent Monitoring: Continuously monitor the condition of the shoring walls and surrounding soil using geotechnical sensors or manual checks. Track any movement, displacement, or pressure changes in the walls to address issues before they escalate.
   
7. Proper Training: Ensure that workers are properly trained in excavation safety, including recognizing signs of wall failure and knowing how to operate dewatering equipment. Emergency protocols should be in place in case of a collapse or flood.
   

Conclusion
Excavating below sea level near shoring walls presents several unique challenges. Water management, soil stability, and wall integrity are paramount in such projects. By employing effective dewatering systems, reinforcing shoring walls, and adhering to best excavation practices, these risks can be minimized. The key to safely completing such excavations lies in thorough planning, regular monitoring, and appropriate use of technology to handle changing ground conditions. Excavation projects in these conditions require expertise and precision to ensure the safety of workers and the stability of the structure being built.