Will going underground and climate change cause more sinkholes?

Key Concepts

Sinkholes, natural vs. man-made causes, underground construction, void formation, soil erosion, water leakage, heavy rainfall, water table, monitoring protocols, instrumentation (settlement markers, inclinometers, piezometers), ground improvement, reclaimed land, underground master plan, impact assessment, influence zone, soil investigation, geological complexities.

How Sinkholes are Formed

• Natural vs. Man-Made: Sinkholes are primarily a natural phenomenon caused by water dissolving underground rock or sediments. However, human activities like construction can exacerbate the problem, turning a natural process into an engineering issue.
• The Process: Water erodes underground space, creating voids. The size of the sinkhole depends on the size of the underground space being eroded and the weight (overburden) on top.
• Engineering Problem: Construction activities, especially underground work, can lead to void formation. Soil and water leakage into deep excavation sites or overcutting during tunneling can create these voids.

Engineering Challenges and Prevention

• Void Formation: Underground construction can lead to voids, which may propagate and cause cave-ins.
• Prevention:
  1. Prevent void formation by installing robust retaining walls to prevent soil and water ingress.
  2. Implement precautionary measures during tunneling to avoid over-excavation.
  3. Monitor Key Performance Indicators (KPIs) during tunneling to ensure no over-excavation occurs.
• Challenges: Unexpected ground conditions, such as loose sand filled with water, can lead to uncontrolled inflow into the tunnel boring machine, creating voids.

Impact of Rainfall and Climate Change

• Oversaturated Soils: Heavy rainfall and ice melt can lead to oversaturated soils, weakening the ground.
• Water Table: A shallow water table in Singapore can fluctuate with rainfall and sea-level rise, affecting underground stability.
• Intense Rainfall: Forecasts predict more intense rainfalls for longer periods, increasing the risk of soil saturation and weakening.

Monitoring Protocols and Adaptation

• Detailed Monitoring Programs: Comprehensive instrumentation is used for underground construction, including:
  • Settlement Markers: Monitor ground settlement.
  • Inclinometers: Monitor lateral soil movement.
  • Piezometers (Dissometers): Monitor groundwater pressure changes.
• Early Warning Signs: Changes in groundwater pressure or unusual ground movement trigger investigations.
• Holistic Instrumentation: A comprehensive approach to instrumentation monitoring is essential for detecting early signs of movement.

Geological Considerations

• Varied Geology: Singapore has a juxtaposition of different geological units, with varying sturdiness.
  • Example: The area around Bond (where the podcast was recorded) has tough, folded sandstone from the Triassic age (200-300 million years old).
  • East Side: Unconsolidated sands, muds, and silts, which are less sturdy.
• Lack of Dissolvable Rock: Singapore lacks easily dissolvable rock types like gypsum, chalk, salt, or limestone, which reduces the risk of natural sinkholes.

Reclaimed Land

• Engineered Land: 25% of Singapore's land is reclaimed, but it is engineered, not just dumped.
• Ground Improvement Techniques:
  • Consolidation of clay by placing sand layers.
  • Vibration (flotation) to densify sand.
  • Prefabricated vertical drains to accelerate clay consolidation.
• Not Riskier: Reclaimed land is not necessarily riskier for sinkhole formation due to these engineering efforts.

Underground Master Plan

• Key Consideration: When moving rail lines, utilities, and storage underground, it's crucial to avoid adversely affecting the surrounding environment.
• Impact Assessment: Detailed impact assessments are needed to evaluate the effects on nearby structures.
• Computer Modeling: Computer analysis models the work and construction sequence to estimate potential movement of nearby structures.
• Tight Supervision: Strict on-site supervision ensures adherence to approved designs.

Risk Perception and Influence Zone

• Influence Zone: The area affected by a sinkhole is typically a radius of three times the depth of the crater.
  • Example: A 10-meter deep crater has a 30-meter radius influence zone.
• Soil Movement: Within the influence zone, soil movement towards the void is expected.
• Safety Measures: Authorities close off the influence zone for investigation using GPR scanning and probing.

Responsibility and Coordination

• Multiple Parties: Sinkhole incidents often involve multiple parties (LTA, PUB, BCA) due to the complex urban environment.
• Coordination: Agencies coordinate during project approval and execution.

Areas for Improvement

• Extensive Soil Investigations: More detailed soil investigations are needed to understand underground complexities.
• Mindful of Sedimentary Units: Be more aware of unpredictable sedimentary units.
• Density of Boreholes: Increase the density of boreholes during soil investigations to better picture the underground environment.

Conclusion

Sinkholes in Singapore are complex issues influenced by both natural processes and human activities, particularly underground construction. While natural sinkholes are rare due to the absence of highly dissolvable rocks, construction-related activities can create voids leading to sinkholes. Mitigation strategies involve careful planning, robust engineering solutions, comprehensive monitoring, and coordinated efforts among various agencies. Ongoing research and improved soil investigations are crucial for understanding and managing the risks associated with sinkholes in Singapore's urban environment.