Underground injection plays a pivotal role in enhancing the durability of tunnels and preventing water leakage. Polyurethane and acrylic injections serve as powerful tools that aid engineers in overcoming challenges encountered during tunnel construction. Therefore, the correct application of these techniques is of vital importance for the long-term resilience of tunnels.
Each type of application utilizes cement injection for different purposes during tunnel construction or maintenance, contributing significantly to the safety, durability, and longevity of the tunnel. Proper planning and implementation of these stages facilitate the successful completion of tunnel engineering projects.
Cement injection is commonly employed in tunnels for various reasons and is typically crucial for the safety and durability of tunnel construction. This application is usually carried out in five stages.
Primary Support Elements for Excavation Progression: Issues such as tunnel wall or ceiling collapse or cracking pose significant risks to the safety of tunnel construction. Cement injection is a method used to prevent such problems and ensure the continuation of tunnel excavation.
Surface Soil Improvements:
- a. Robit: Robit is a technique used to improve the upper part of the soil during tunnel excavation. It can be employed to enhance the stability of the ground during excavation and support tunnel walls. In this method, cement injection is applied into self-drilling pipes to increase strength.
- b. Umbrella Arch: It aims to improve the soil around the tunnel during excavation. Typically, it is applied using a series of holes where a cement mixture is injected. Cement injection is performed into pipes placed in the ground to enhance strength.
- c. Grouting: Grouting is a method that ensures the support of the ground during tunnel excavation. As the tunnel progresses, the ground is reinforced and stabilized using cement injection. Cement injection is carried out into pipes inserted into the ground to increase strength.
Mirror Improvement (Mirror Ground Anchoring or Mirror Bolting): This refers to the reinforcement of the ground inside or surrounding the tunnel using cement injection. Mirror ground anchoring or mirror bolting is employed to prevent the collapse or subsidence of the ground inside or around the tunnel. This ensures the safe construction and operation of the tunnel. It involves drilling into the mirror and describing cement injections to be made with or without the insertion of iron bars into the drill hole.
Post-Excavation Tunnel Stability: Following the completion of the tunnel, cement injection is utilized to address stability issues that may arise due to environmental conditions or factors originating from the tunnel itself. This ensures the tunnel remains safe and resilient.
Bolting (Anchoring): Bolting is a method typically carried out using steel rods or anchors supported by cement injection. Here’s how the process works:
- Drilling: The first step involves drilling holes to the desired depth in the ground. These holes are usually placed in areas prone to weakness or fragility, such as tunnel walls or ceilings.
- Anchor Placement: Next, steel rods or anchors are inserted into the drilled holes. These rods are typically long and durable, designed to reinforce the ground.
- Cement Injection: Once the rods or anchors are in place, the holes are filled with cement injection. This allows the rods or anchors to grip the ground tightly and provide additional stability.
- Curing: Finally, the injected cement cures and hardens over time. This ensures that the anchors or rods remain securely in place and firmly anchored to the ground.
- Grouting: Steel rods are compressed (Torqued – Bolt Torquing) to provide additional support to the walls or ceiling of the tunnel.
Contact Injection: Aimed at filling any voids that may occur between the tunnel walls or floor and the shotcrete after excavation. This helps prevent water leakage and other adverse effects.
Soil Improvement Injections (Consolidation): If post-excavation tunnel stability cannot be ensured or deformations persist, the soil is improved according to geological conditions determined by on-site geological engineers. These injections are used to increase soil strength and ensure tunnel safety.
Waterproofing and Voids Filling: After permanent lining, cement injection is used to fill any voids that may occur between the waterproofing and lining concrete. This is important to prevent water leakage and enhance tunnel impermeability.
Sodium Silicate and Cement Ground Improvement: Water Management and Quality Control
The combined use of sodium silicate injection and cement can yield effective results in ground improvement and water management operations. This method is particularly applied in situations where the cement-water mixture alone cannot control the groundwater pressure, especially in tunnels. The cement-water mixture may lose its density due to the influence of groundwater, and it can be washed away before setting when it comes into contact with water. In such cases, injections made with chemical additives such as bentonite and retarders, along with sodium silicate, can help determine the setting time of the process on-site and control the impact of groundwater.
Recommended Mixing Ratio (per 1 m³) for Mixtures Prepared for a 470 L Boiler
| Material | Kg/m³ | Kg | Lt | A | B | C |
|---|---|---|---|---|---|---|
| Water | 1000 | 780 | 780 | 260 | 260 | 260 |
| Cement CEM-I 42.5R | 3090 | 295 | 95 | 98.33 | 98.33 | 98.33 |
| EUCON RETARDER 130 (High-Performance Retarder-Stabilizer) | 1095 | 5 | 5 | 1.67 | 1.67 | 1.67 |
| Bentonite | 1700 | 40 | 24 | 13.33 | 0 | 3 |
| Sodium Silicate (Filtered 3 Module) (CreteShot NS 303) | 1375 | 132 | 96 | 44 | 44 | 44 |
| TOTAL | 1252 | 1000 | 417.33 | 404 | 407 |
Polyurethane Injection – Chemical Injection
Polyurethane injections are used to prevent water from seeping into the tunnel. In this process, a special reaction is created by mixing 20 kg of polyurethane with 2 kg of catalyst. Holes are drilled at a 45-degree angle approximately 20 cm away from where water enters, and packers are placed inside the holes. When the polyurethane liquid is injected into the packers, it expands into foam upon contact with water, filling void spaces. This helps prevent the spread of water within the tunnel. However, polyurethane alone may not be sufficient, and therefore, acrylic injection is used as well.
Acrylic Injection
Acrylic injections are used to further prevent water leakage inside the tunnel. In this process, 25 kg of acrylic is mixed with 1.25 kg of catalyst, while in a separate container, 25 kg of water is mixed with 0.625 kg of Init (salt). Similarly, holes are drilled at a 45-degree angle approximately 20 cm away from where water enters, and packers are placed inside the holes. The acrylic-catalyst mixture and the water-Init mixture are directed to the same area using separate hoses. This separation is necessary because acrylic and catalyst must be kept together in the same container, while water and Init must be kept together in another container. Otherwise, the materials quickly react and become unusable. The total of four components, located in two different containers, are conveyed to the packer through two separate transmission lines, ensuring the completion of the mixture just before reaching the packer. Once the mixture is combined, it is immediately injected into the application point. Injection continues as long as the inflow of water is observed, and the injection process continues until it is observed that the injection has reached the point where water is observed. Upon completion of injection at a particular point, the plumbing at the junction of the four mixtures is cleaned, and prepared for the next injection point until a new mixture is prepared.
Chemical injection processes play a critical role in enhancing the durability of tunnels and preventing water leakages. Polyurethane and acrylic injections are powerful tools that assist engineers in overcoming challenges encountered during tunnel construction. Therefore, the proper implementation of these techniques is crucial for the long-term strength of the tunnel.
Each type of application utilizes cement injection for different purposes during tunnel construction or maintenance, and it is essential for the safety, durability, and longevity of the tunnel. Proper planning and implementation of these stages aid in the successful completion of tunnel engineering projects
