What specific chemical properties of grouts are crucial for long-term tunnel stability?

Several chemical properties of grouts are crucial for ensuring long-term tunnel stability. Durability and resistance to degradation are paramount. The grout must be resistant to chemical attack from groundwater and other environmental factors. This includes resistance to sulfates, chlorides, and acids. Permeability is critical. The grout should have low permeability to prevent water inflow into the tunnel. Water inflow can lead to erosion of the surrounding soil or rock and corrosion of the tunnel lining. Chemical compatibility with the surrounding ground is essential. The grout must not react negatively with the soil or rock. For example, some grouts can react with certain types of clay, causing swelling or shrinkage. Shrinkage upon curing should be minimal. Excessive shrinkage can create voids between the grout and the surrounding ground, reducing its effectiveness. Long-term stability of strength is key. The grout must maintain its strength over time. Some grouts can lose strength due to creep or other forms of degradation. Viscosity and gel time are important for placement. These properties determine how easily the grout can be injected into the ground and how quickly it sets. Toxicity should be low. The grout should be non-toxic to minimize the risk of environmental contamination and health hazards to workers. The term 'grout' refers to a pumpable material injected into the ground to improve its properties. 'Tunnel stability' refers to the ability of the tunnel to maintain its structural integrity over time. 'Permeability' is a measure of a material's ability to transmit fluids.