Constructing a new foundation near an old structure requires careful planning and execution to ensure the stability of the existing structure is not compromised. The vibrations from excavation, as well as potential shifts in the ground, can cause damage to nearby buildings if proper precautions are not taken. Here are some important considerations and precautions to follow when planning and constructing a new foundation near old structures.
1. Ensure Adequate Distance Between Foundations
- The horizontal distance between the new foundation and the adjacent face of the old foundation should be at least equal to the width of the larger foundation.
- This buffer space reduces the chances of undermining or destabilizing the old structure by distributing the load more evenly across the soil and limiting vibration effects.
2. Control the Depth of New Foundations
- If the new foundation is to be constructed adjacent to the old one, ensure that it does not extend deeper than the old foundation.
- However, if a greater depth is required for the new foundation, provide lateral support to the old foundation. This can include methods such as underpinning or retaining walls to protect the old structure from the effects of excavation.
3. Observe Safe Excavation Angles
- To maintain soil stability around both structures, draw an inclined line from the outer bottom edge of the old foundation.
- This angle should ideally be 30° for soft soils and 45° for average soils. The excavation depth for the new foundation should not exceed the point where this line intersects the soil. By respecting this angle, you avoid cutting into the structural support of the older building.
4. Provide Lateral Support
- Where needed, provide lateral support to the old foundation through temporary retaining walls, bracing, or sheet piling.
- For older buildings, the soil beneath their foundation may have already undergone settlement over time. Lateral support helps maintain this soil’s stability, preventing subsidence and maintaining structural integrity.
5. Limit Vibration During Construction
- Use low-impact excavation methods to reduce vibrations. Techniques such as manual digging, controlled blasting, and mini-piling can minimize the risk of shifting or damaging adjacent structures.
- Avoid heavy machinery and large equipment near the old structure. Excessive vibrations can lead to cracks, weakening, and, in extreme cases, partial collapse of older buildings.
6. Monitor Groundwater Levels
- Excavation can alter groundwater levels, which might affect the stability of nearby structures. Dewatering processes, if not managed correctly, can lead to soil compaction or settlement around the old foundation.
- Install monitoring equipment to keep track of groundwater levels and make necessary adjustments to ensure the old structure remains unaffected.
7. Inspect and Reinforce the Old Structure
- Before starting any work, conduct a thorough inspection of the old structure. This will help identify any existing weaknesses, such as cracks or settlements, which could worsen during construction.
- Reinforce the old structure as necessary. Methods include masonry repointing, injecting resin or grout into cracks, and even strengthening walls with steel or carbon fiber to improve their resilience to vibrations.
8. Choose the Right Construction Techniques
- Use trenchless techniques or slurry walls to limit disruption to the surrounding soil. These methods support the soil during excavation, minimizing the risk of settling or shifting near the old foundation.
- Consider micropiling techniques that reduce the extent of excavation while providing adequate foundation support. Micropiles can often be drilled around or below existing structures, limiting the impact on nearby buildings.
9. Install Settlement and Vibration Monitors
- Install vibration monitoring devices to track the level of ground movement during construction. In many cases, it is required by law to measure and manage vibrations to keep them within safe limits.
- Settlement monitors on the old structure can help detect any early signs of movement, allowing for timely corrective measures to prevent further damage.
10. Comply with Building Codes and Standards
- In India, refer to IS codes for guidance on foundation construction and soil stabilization. The Indian Standards (IS) provide specific criteria for construction near existing structures, helping ensure safety and stability.
- For instance, IS 1904:1986 provides general design requirements for foundation stability, while IS 1080:1985 outlines guidelines for shoring, underpinning, and scaffolding. Following these standards not only ensures the structural integrity of both new and old foundations but also ensures compliance with legal regulations.
IS Codes Related to Foundation Construction Near Existing Structures:
- IS 1904:1986 – Code of Practice for Design and Construction of Foundations in Soils: General Requirements.
- IS 1080:1985 – Code of Practice for Shoring and Underpinning.
- IS 2974 (Part 1 to 5) – Code of Practice for Design and Construction of Machine Foundations.
- IS 6403:1981 – Code of Practice for Determination of Bearing Capacity of Shallow Foundations.
- IS 8009 (Part 1 and 2) – Code of Practice for Calculation of Settlements of Foundations.
Conclusion🎯
Constructing new foundations near existing structures requires meticulous planning and adherence to standard construction practices. By following these precautions, such as ensuring adequate separation, controlling excavation depths, and using appropriate reinforcement methods, you can minimize the risks associated with construction near old structures. Additionally, referring to the IS codes for foundation and soil management will provide further guidance to ensure the safety of both new and existing buildings.