Soft ground tunnelling refers to the construction of tunnels in soil or other weak, unconsolidated materials. This type of tunnelling presents unique challenges due to the lack of natural support and stability of the surrounding ground. Various techniques are employed to ensure safe and efficient tunnelling, such as using tunnel-boring machines (TBMs) equipped with specialised cutters, grouting to stabilise the soil, and installing temporary support systems like steel ribs or shotcrete. Proper geo-technical investigations and monitoring are essential to assess ground conditions and adapt tunnelling methods accordingly, ensuring the successful completion of soft ground tunnelling projects.
Soft ground tunnelling refers to the construction of tunnels in geologically unstable and weak soil conditions, where the ground lacks sufficient strength to support the tunnel without additional stabilization measures. Soft ground typically includes soils such as clay, silt, peat, and loose sand, which can pose significant challenges during tunnelling. Specialized tunnelling methods and ground improvement techniques are employed to ensure the safety and stability of the tunnel in such conditions. Here’s an overview of soft ground tunnelling:
- Challenges in Soft Ground Tunnelling: Soft ground tunnelling presents several challenges, including:
- Ground Settlement: Soft soils can undergo settlement or compression due to the weight of the tunnel, leading to surface subsidence and potential damage to buildings and infrastructure above.
- Water Ingress: Soft ground is often more permeable, leading to increased water ingress into the tunnel, which can cause flooding and reduce the stability of the tunnel face.
- Tunnel Face Instabilities: The lack of ground support can result in tunnel face collapses and the potential for accidents during construction.
- Excavation Support: Adequate support systems are required to prevent tunnel collapse during excavation.
- Tunnelling Methods for Soft Ground: Different tunnelling methods are used to address the challenges posed by soft ground conditions, including:
- Shield Tunnelling: Shield tunnelling methods, such as Earth Pressure Balance (EPB) and Slurry Shield, are commonly used in soft ground conditions. The tunnel boring machine (TBM) is equipped with a shield that provides support to the tunnel face while the excavated soil is removed.
- Sprayed Concrete Lining (SCL): SCL, also known as the New Austrian Tunnelling Method (NATM), involves the application of shotcrete or sprayed concrete to stabilize the tunnel face and provide immediate support.
- Segmental Linings: In this method, precast concrete segments are installed to form a lining that provides structural support to the tunnel.
- Ground Freezing: In certain cases, ground freezing is used to stabilize the soil before excavation by injecting a refrigerant into the ground to create a frozen soil wall.
- Jet Grouting: Jet grouting involves injecting a high-pressure grout into the ground to mix with the soft soil and create a stabilizing column.
- Ground Improvement Techniques: Various ground improvement techniques are used to strengthen soft ground before tunneling, including:
- Soil Stabilization: Adding stabilizers like cement or lime to the soil to increase its strength and reduce settlement.
- Grouting: Injection of grout into the ground to fill voids and improve soil cohesion.
- Soil Mixing: Mixing the soft soil with cement or other binders to create a more stable soil-cement mix.
- Monitoring and Safety: Soft ground tunnelling requires careful monitoring of ground movements, settlement, and water ingress during construction. Instrumentation and monitoring systems are installed to ensure the safety of the tunnelling process and to provide early warnings of potential issues.
Soft ground tunnelling demands a thorough understanding of the soil conditions and geology to implement appropriate tunnelling methods and ground improvement techniques. Each soft ground project is unique, and engineers must tailor their approach to the specific conditions and challenges at hand to ensure the successful and safe completion of the tunnel.