Resolving Subsidence in a Car Park at Bupa Nairn House Care Home, Enfield
Background
Bupa Nairn House Care Home, located in Enfield, experienced significant subsidence in a section of its car park. This issue, primarily identified near the retaining wall and drainage chambers, had led to surface deformations, including cracks and void formations. The subsidence risked further degradation of the car park and adjacent structures if not addressed promptly.
Investigation and Findings
A detailed ground investigation was conducted by Structa LLP to ascertain the causes of subsidence. The investigation revealed:
- Ground Conditions: The car park was underlain by a significant thickness of Made Ground, consisting of loosely compacted materials, including construction debris. These extended up to 5.5m below ground level (bgl) in some locations.
- Standard Penetration Test (SPT) Results: SPT ‘N’ values ranged between 0 and 3 in areas near the drainage chambers and retaining wall, indicating very loose soil conditions.
- Geotechnical Assessment: Settlement was attributed to poorly compacted Made Ground, likely backfill from historical gravel pit infilling, and potential water ingress from nearby drainage systems.
Alternative Solutions
Several alternative methods were considered to address the subsidence:
- Excavation and Replacement: This approach would involve removing the affected Made Ground and replacing it with properly compacted engineered fill. However, this method would require extensive excavation and disruption to the care home residents.
- Compaction Grouting: Another option considered was injecting cement-based grouts to increase the density of the ground. While effective, this method is slower and less adaptable to the site’s specific constraints compared to geopolymer injection. As an active car park at a care home, this was not a viable option.
Solution: Geobear’s Ground Improvement Method
To address the subsidence, Geobear was approached to design an innovative geopolymer injection solution. A Geobear solution uses a multi-component expansive geopolymer that is injected at various depths across the site. The geopolymer is injected via steel tubes that are positioned through holes drilled across the site. The process features:
- Injection Design: Targeted injections were planned at three depths—0.8m, 1.8m, and 2.8m below the surface—within a 17m x 4m affected area.
- Verification of Ground Improvement: Dynamic Cone Penetration (DCP) tests were performed after injections to measure the improvement in ground strength. These tests confirmed the enhanced compaction and density of the treated soils.
- Monitoring During Injection: Laser levels were employed throughout the injection process to monitor the car park surface in real-time. This ensured that the injection did not inadvertently lift or disturb the surface, maintaining the integrity of surrounding structures.
Key Benefits of Geobear’s Solution
- Minimal Disruption: The geopolymer injection process was non-intrusive and required no excavation. It allowed the care home’s operations to continue without significant interruptions.
- Speed of Implementation: The project was completed in just three shifts, reducing downtime compared to traditional methods.
- Environmental Considerations: The geopolymer material is lightweight and has a lower carbon footprint compared to cement-based solutions.
- Precision and Adaptability: Real-time monitoring ensured precise delivery of the geopolymer, tailored to the specific ground conditions.
Time Savings
One of the key advantages of Geobear’s geopolymer injection solution was the significant reduction in project duration:
- Traditional Excavation & Replacement Method: Typically, this method would require 2-3 weeks of work, including excavation, removal of spoil, backfilling, compaction, and resurfacing.
- Geobear Geopolymer Injection Method: The entire treatment was completed in just 3 shifts, ensuring that the car park remained operational with minimal disruption.
- Overall Time Savings: The geopolymer solution delivered a time saving of approximately 75%, allowing for quicker restoration of the affected area while reducing inconvenience to residents and staff.
Carbon Savings
By choosing Geobear’s geopolymer injection method over traditional excavation and replacement, significant carbon savings were achieved:
- Traditional Excavation & Replacement Method: Estimated emissions of ~8,160 kg CO₂e from excavation, transport, disposal, and new material import.
- Geobear Geopolymer Injection Method: Estimated emissions of only ~175 kg CO₂e.
- Total Carbon Savings: ~7,985 kg CO₂e, equivalent to removing ~30,000 miles of car travel (based on ~0.26 kg CO₂e per mile).
These savings are based on comparable, verified reports produced by Carbon Footprint Ltd and highlights the substantial environmental benefits of using Geobear’s solution.
DCP Test Results and Ground Strength Improvement
Post-injection Dynamic Cone Penetration (DCP) testing confirmed significant improvement in soil strength:
- Maximum improvement in Test 1: A 220% increase in ground strength was recorded at 1.0m depth.
- Maximum improvement in Test 2: A 250% increase in ground strength was recorded at 1.3m depth.
- Overall Improvement: The average strength increase across all tested depths was substantial, ensuring long-term stability of the treated area.
- Key Depths Strengthened: The upper layers (0.4m – 1.5m) showed the most significant improvements, confirming effective soil strengthening where required.
These results validate the effectiveness of the geopolymer injection in enhancing the compaction and stability of the treated ground.
Process and Execution
The works were executed as follows:
- Site Preparation: The work area was cleared, and utilities were marked to ensure safety.
- Drilling and Injection: 12mm holes were drilled at designated grid points. Geopolymer was then injected through tubes to the required depths.
- Safety Measures: Laser monitoring and strict adherence to the risk assessment and method statement (RAMS) ensured minimal impact on the surrounding structures, including the retaining wall and drainage systems.
- Post-Injection Testing: DCP tests were conducted to validate the improvement in ground conditions and ensure the treated areas met required standards.
Outcome
The Geobear solution successfully improved the soil strength, significantly reducing the risk of further settlement. Post-treatment testing confirmed the desired improvements, with enhanced compaction and stability in the affected zones. The non-disruptive method ensured minimal inconvenience to the care home’s operations.
Lessons Learned
This project demonstrated the importance of thorough ground investigations in identifying the root causes of subsidence. It also highlighted the effectiveness of Geobear’s geopolymer technology in providing a sustainable, low-disruption solution for sensitive sites.