Diaphragm walls are a critical component in modern construction, particularly in urban environments where space is limited and excavation near existing structures is unavoidable. These walls serve as underground retaining systems, providing lateral support to prevent soil movement and control groundwater. However, building diaphragm walls near existing structures presents unique challenges that require meticulous planning, precise execution, and advanced engineering solutions.
This blog explores the key challenges associated with constructing diaphragm walls near existing structures and how they can be addressed to ensure project success.
Understanding Diaphragm Walls
Diaphragm walls are reinforced concrete walls constructed in situ, typically used for deep excavations. These walls are preferred for projects such as basements, tunnels, underground parking, and metro stations, where lateral earth pressure and water ingress need to be controlled. Diaphragm walls are built using a sequence of excavation, slurry stabilization, reinforcement placement, and concrete pouring. Their construction, however, becomes increasingly complex when existing buildings or infrastructure are in close proximity.
Key Challenges in Building Diaphragm Walls Near Existing Structures
1. Ground Movement and Settlement
When excavating for diaphragm walls, the removal of soil can cause ground settlement, which may affect the stability of nearby structures. Even minor soil displacement can result in foundation cracking, misalignment, or structural damage to adjacent buildings.
Solution:
To mitigate ground settlement, techniques such as pre-loading, soil improvement, or using low-permeability slurry are employed. Continuous monitoring with geotechnical instruments like settlement plates, inclinometers, and extensometers helps detect early signs of ground movement, enabling corrective measures.
2. Vibration and Noise Impact
Excavation and construction equipment generate vibrations and noise, which can disrupt the integrity of nearby buildings and inconvenience occupants. Vibrations can lead to micro-cracks in foundations and walls, particularly in older structures.
Solution:
Low-vibration equipment, such as hydraulic grabs and trench cutters, can reduce the impact on adjacent structures. Additionally, noise barriers and working during designated hours minimize disruption to residents and businesses.
3. Control of Groundwater
Groundwater management is a significant challenge during diaphragm wall construction. High water tables can lead to seepage, instability, and reduced effectiveness of the retaining wall. Water ingress can also affect the foundations of nearby structures.
Solution:
Techniques such as dewatering, installation of cutoff walls, or using bentonite slurry during excavation can effectively manage groundwater. Waterproofing the diaphragm wall with additives or coatings further ensures its functionality.
4. Limited Space for Construction
Urban environments often provide limited working space, especially near existing buildings or infrastructure like roads, pipelines, or railways. This restricts the use of large equipment and storage of construction materials.
Solution:
Using compact and versatile equipment can help in space-constrained areas. Modular construction methods, precise planning of material logistics, and just-in-time delivery are strategies to manage space limitations efficiently.
5. Interference with Underground Utilities
Building diaphragm walls often involves navigating a maze of underground utilities, such as water pipes, electrical cables, and sewer lines. Any disruption to these utilities can lead to delays, additional costs, and inconvenience to the public.
Solution:
Conducting thorough utility surveys using ground-penetrating radar (GPR) or other mapping technologies is essential before construction begins. Coordination with utility providers and adopting trenchless techniques for utility relocation can prevent disruptions.
6. Structural Stability of Adjacent Buildings
Existing buildings near excavation sites may not be designed to withstand additional stresses caused by diaphragm wall construction. Soil displacement and vibrations can compromise their structural stability.
Solution:
Pre-construction surveys to assess the structural integrity of nearby buildings are crucial. Reinforcing the foundations of vulnerable structures using underpinning or jet grouting provides additional support during construction.
7. Challenges in Alignment and Precision
Diaphragm walls must be constructed with extreme precision to ensure their effectiveness. Misalignment during excavation can lead to gaps in the wall, compromising its ability to provide lateral support and resist water ingress.
Solution:
Advanced technologies such as GPS-based equipment, laser-guided systems, and real-time monitoring ensure accurate alignment during construction. Regular quality checks during excavation and concreting minimize errors.
8. Environmental Concerns
Construction activities near existing structures in urban areas often face scrutiny due to environmental concerns such as dust generation, noise pollution, and potential damage to heritage buildings.
Solution:
Implementing dust control measures, such as using water sprays and covering construction materials, helps reduce air pollution. Strict adherence to environmental regulations and obtaining necessary permits ensures compliance and minimizes environmental impact.
9. Coordination with Stakeholders
Building diaphragm walls near existing structures often requires close coordination with multiple stakeholders, including property owners, utility providers, government authorities, and the public.
Solution:
Clear communication, regular updates, and involving stakeholders in the planning process help build trust and avoid conflicts. Having a dedicated project liaison can ensure smooth coordination and quick resolution of issues.
Advanced Techniques for Overcoming Challenges
Modern engineering has introduced innovative methods to address the challenges of constructing diaphragm walls near existing structures. Some of these techniques include:
1. Top-Down Construction
In this method, the diaphragm wall acts as a load-bearing structure for the above-ground construction while excavation continues below. This approach minimizes disruption to adjacent buildings and accelerates project timelines.
2. Ground Freezing
Temporary ground freezing stabilizes soil and prevents water ingress during excavation. This technique is particularly useful in areas with high groundwater levels or loose soil conditions.
3. Slurry Walls
Using bentonite or polymer-based slurry provides temporary support to excavation walls, reducing the risk of collapse and minimizing soil movement.
4. Digital Twin Technology
Digital twins create a virtual model of the construction site, allowing engineers to simulate and plan diaphragm wall construction with high precision. This helps identify potential issues and optimize workflows.
Conclusion
Constructing diaphragm walls near existing structures presents a host of challenges, ranging from ground settlement and vibration to groundwater management and space constraints. However, with meticulous planning, advanced construction techniques, and effective stakeholder coordination, these challenges can be successfully addressed.
Diaphragm walls play a vital role in urban construction, enabling deep excavations while safeguarding adjacent buildings and infrastructure. By leveraging modern engineering solutions and maintaining a proactive approach, construction teams can ensure the successful implementation of diaphragm walls, even in the most complex urban environments.
