Excavation projects, particularly those in urban settings or near sensitive structures, come with their own set of challenges. One of the most significant concerns is ground settlement. When soil shifts during excavation, it can lead to subsidence, which has the potential to damage nearby structures, infrastructure, and utilities. This is where diaphragm walls come into play. Known for their robustness and effectiveness, they have emerged as an essential solution for minimizing ground settlement and ensuring stability during excavation.
What are Diaphragm Walls?
Diaphragm walls are vertical, reinforced concrete walls that are constructed below ground level. These walls are designed to resist the lateral forces exerted by surrounding soil and groundwater, making them ideal for deep excavations and supporting structures in unstable soils. They are typically constructed using the technique of slurry trenching, where a trench is excavated, and a special slurry is used to support the trench walls before concrete is poured in to form the diaphragm wall.
The Role of Diaphragm Walls in Reducing Ground Settlement
Ground settlement occurs when soil undergoes compression or displacement during the excavation process, which can cause a decrease in the ground’s surface level. This could affect not just the excavation site but also the surrounding area. The use of diaphragm walls plays a vital role in mitigating these risks by providing physical barriers that prevent excessive soil movement during the excavation phase.
Here are some key ways in which diaphragm walls contribute to reducing ground settlement:
1. Providing Lateral Support to the Soil
When excavation is carried out, the surrounding soil experiences lateral stress that can lead to deformation or displacement. It act as robust vertical supports that counterbalance these lateral forces. By providing resistance to the soil, these walls ensure that the surrounding soil structure remains intact, preventing unwanted shifts and settling.
2. Minimizing Horizontal Displacement
One of the most critical functions of diaphragm walls is to reduce horizontal soil displacement. Without proper support, excavation can cause the ground to shift inwards, which can result in subsidence of adjacent areas. The strength and rigidity of diaphragm walls help in reducing the horizontal displacement of soil, thereby stabilizing the surrounding terrain.
3. Control of Groundwater Pressure
Groundwater pressure plays a significant role in ground settlement during excavation, particularly in areas with a high water table. Diaphragm walls, with their impermeability and strength, help to control groundwater flow. By preventing water from infiltrating the excavation site, these walls reduce the risk of soil erosion and hydraulic pressure, which are major contributors to ground settlement.
4. Prevention of Soil Surcharge
In areas with dense structures or sensitive infrastructure nearby, the impact of excavation-induced settlement can be severe. It serve as a physical barrier that absorbs and distributes the load of the surrounding soil, reducing the risk of surcharge (excessive weight) on the underlying layers. This minimizes the chances of settlement in the adjacent structures.
5. Enhanced Excavation Safety
Excavation often involves cutting into soil layers that are prone to collapse. They provide structural support during the excavation process, preventing the excavation area from caving in. This not only reduces the potential for ground settlement but also enhances the safety of the construction site, making it safer for workers.
Advantages of Using Diaphragm Walls in Excavation
The use of diaphragm walls in reducing ground settlement is backed by several distinct advantages that make them a preferred option for many construction projects:
1. Adaptability to Different Soil Conditions
One of the standout features of diaphragm walls is their adaptability to various soil conditions. Whether the soil is soft, loose, or contains a high water table, diaphragm walls can be designed and constructed to offer the required support. This versatility makes them highly effective in urban and dense construction sites where soil conditions may vary significantly.
2. Permanent Support Solution
Unlike temporary excavation supports such as sheet piles or bracing systems, diaphragm walls provide permanent solutions to ground settlement. Once constructed, they can continue to serve as a barrier, offering long-term stability and safety for the surrounding infrastructure. This reduces the need for additional reinforcement or support systems during or after excavation.
3. Minimized Disturbance to Adjacent Structures
Because diaphragm walls help to maintain soil integrity around the excavation site, they reduce the risk of damage to adjacent buildings and underground utilities. The process of excavation is less likely to cause settlement-related issues, such as cracks in foundations, tilting of buildings, or damage to underground pipes and cables. This makes diaphragm walls an ideal choice for construction in areas with a high density of existing structures.
4. Cost-Effective for Deep Excavations
While the initial costs of diaphragm walls can be higher than some temporary support options, they prove to be cost-effective in the long term, particularly for deep excavations. Since they offer permanent support, they reduce the need for ongoing maintenance or reinforcement systems, and their ability to prevent settlement-related damage can significantly lower the overall costs associated with repairs and delays.
Diaphragm Walls in Complex Excavation Projects
The use of diaphragm walls becomes particularly beneficial in complex excavation projects, such as those involving underground parking, metro tunnels, or high-rise buildings with deep foundations. These projects typically require a high level of stability, as they often extend far below ground level.
In these cases, diaphragm walls act as retaining structures that provide the necessary support to stabilize the ground and prevent it from shifting. For example, during the construction of underground transportation networks like metro stations or subway tunnels, diaphragm walls are used to isolate the excavation site from the surrounding soil, minimizing the impact on the surrounding urban environment.
Additionally, diaphragm walls are crucial in projects where excavation extends below the groundwater table, such as in the construction of underground tanks or basements. Here, the walls help prevent water ingress and provide structural stability, reducing the risk of ground settlement caused by changes in water pressure.
Conclusion
The use of diaphragm walls is a crucial technique in reducing ground settlement during excavation. Their ability to provide lateral soil support, control groundwater pressure, and minimize horizontal displacement makes them an invaluable tool in maintaining excavation stability, especially in urban environments. The benefits of diaphragm walls extend beyond reducing ground settlement; they also offer long-term cost savings, enhance safety, and minimize the risk of damage to surrounding infrastructure. As excavation projects continue to grow in complexity and scale, diaphragm walls will remain a key solution for safe and efficient excavation operations.
