Underpinning is a critical construction process used to strengthen and stabilize existing foundations, typically when a building shows signs of settlement or subsidence. Given the potential risks involved, effective monitoring is essential to ensure structural integrity and safety throughout the project. This article explains why monitoring is important for underpinning works, the methods commonly used, and practical advice on setting up a reliable monitoring scheme.
Why Monitoring is Crucial for Underpinning Works
Underpinning involves excavating beneath an existing structure, which can significantly alter the way forces are transferred through the building and into the ground. Without careful monitoring, this process can lead to unexpected ground movement, foundation instability, or even structural failure.
Key Reasons to Monitor Underpinning Projects
- Early Detection of Movement: Identify small shifts before they become serious problems.
- Quality Assurance: Ensure that the underpinning process is stabilizing the foundation as planned.
- Health and Safety: Protect the safety of workers and nearby structures.
- Regulatory Compliance: Meet local building codes and engineering standards.
- Cost Control: Prevent costly remedial work by catching issues early.
Common Monitoring Methods for Underpinning Works
1. Crack Gauges and Tell-Tales (Most Common)
- How it Works: Simple devices that measure the width of cracks over time.
- When to Use: Effective for monitoring internal walls and masonry structures.
- Advantages: Low cost, easy installation.
- Limitations: Only measures surface cracks and provides limited data.
- Example: Install crack monitors on visible cracks within the structure being underpinned and nearby structures to detect any changes as the work progresses. Use baseline measurements for accurate tracking.
2. Total Station Surveys (Most Common)
- How it Works: Uses robotic total stations to measure the 3D position of monitoring points.
- When to Use: Suitable for high-precision work where lateral and vertical movements need to be tracked.
- Advantages: Real-time data, high accuracy, long measurement range.
- Limitations: Requires clear line-of-sight and regular calibration.
- Example: Set up retroreflective targets on the structure being underpinned and nearby buildings. Baseline readings should be taken before work begins, ideally 3 initial readings to account for daily variations.
3. Inclinometers and Tilt Sensors (Advanced)
- How it Works: Measures the angle of tilt within structural elements or retaining walls.
- When to Use: Useful for detecting early signs of ground movement or wall displacement.
- Advantages: Continuous monitoring, remote access to data.
- Limitations: Higher cost and more complex setup.
4. Ground Movement Sensors (Advanced)
- How it Works: Measures subsurface movement using probes or strain gauges.
- When to Use: Suitable for deep excavations or underpinning close to sensitive structures.
- Advantages: Detects ground movement before it reaches the surface.
- Limitations: Requires careful installation and calibration.
5. Vibration Monitoring (Advanced)
- How it Works: Uses accelerometers to measure vibrations in the structure.
- When to Use: Critical for sites where nearby structures could be affected by the works.
- Advantages: High precision, real-time alerts.
- Limitations: Can be expensive and requires regular calibration.
- Example: Install accelerometers before work begins, with baseline readings taken 1 week in advance.
6. Groundwater Monitoring (Advanced)
- How it Works: Uses piezometers to measure changes in groundwater levels.
- When to Use: Essential when dewatering is required or when working in water-sensitive areas.
- Advantages: Provides critical data on groundwater behavior.
- Limitations: Requires careful installation and maintenance.
7. Soil Testing (Advanced)
- How it Works: Regular particle size distribution testing within boreholes to assess soil stability and water flow.
- When to Use: Useful in complex geological formations or when underpinning in variable soil conditions.
- Advantages: Provides direct data on soil behavior.
- Limitations: Requires regular sampling and lab testing.
Practical Tips for Setting Up a Monitoring Scheme
- Choose the Right Methods: Select the appropriate combination of techniques based on site conditions and project requirements.
- Establish Baselines: Measure initial conditions before any excavation begins to provide a reference for future readings.
- Automate Where Possible: Use wireless data loggers and real-time systems for critical sites.
- Regular Inspections: Check that all sensors are functioning correctly throughout the project.
- Data Review: Regularly review the collected data to spot trends and make informed decisions.
Conclusion
An effective monitoring scheme is essential for the success of any underpinning project. It provides early warning of potential problems, helps maintain safety, and ensures the long-term stability of the structure. By selecting the right monitoring methods and implementing a well-planned scheme, contractors can reduce the risk of unexpected issues and keep their projects on track.