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Mine Law
The **Mine Law** refers to a traditional construction method used for excavating tunnels by creating underground passageways. This technique involves breaking the rock mass through blasting, which causes it to become loose and unstable, potentially leading to collapse. To ensure safety, the excavation is typically carried out in sections, with each part being supported as it is dug. This process requires extensive support structures and significant use of timber, making it labor-intensive and costly. However, the introduction of **spray-anchored support** has significantly reduced the number of divisions and led to the development of the **New Austrian Tunneling Method (NATM)**.
One common approach is the **full-section excavation**, where the entire tunnel cross-section is excavated at once, followed by immediate lining. This method is suitable for smaller tunnels in stable rock formations. When the surrounding rock is weak or unstable, the excavation is done in smaller segments, with temporary supports provided to prevent collapse. In such cases, a **guide pit** is first excavated, then expanded to reach the full design contour.
Another technique is the **arch-first method**, also known as the **top arch method**. It is particularly useful in soft or unstable rock conditions. The upper section of the tunnel is excavated first, and the roof arch is constructed to provide initial support before proceeding with the lower parts. This ensures that the upper structure remains stable during the excavation of the side walls. The sequence of this method is critical to avoid instability, especially when digging the "horse mouth" areas on either side of the tunnel.
The **funnel scaffold method** is another variation, often used in harder and more stable rock. It involves excavating a lower guide pit first, then working upward to enlarge the tunnel. A temporary scaffold is set up in the lower pit to support the excavation process, allowing for efficient removal of debris through a funnel-like structure. This method was widely used in railway tunnel construction due to its efficiency and ease of operation.
The **step method** divides the tunnel into multiple layers, with excavation progressing from top to bottom. It can be either a **positive step** or **reverse step**, depending on the stability of the rock. In poor conditions, the positive step method is preferred, while in stable environments, the reverse step allows for more efficient excavation. This method helps manage the load on the surrounding rock and improves safety.
The **mushroom-shaped method** combines elements of the arch-first and funnel scaffold techniques. After excavating several sections, the shape resembles a mushroom, hence the name. This method offers flexibility, allowing for adjustments based on geological conditions. It was initially used in stable rock but later adapted for larger tunnels, reducing the need for extensive formwork and improving construction speed.
For large-span tunnels, the **core support method** is employed. This involves excavating the side walls first, leaving a central core that provides temporary support until the entire structure is completed. This method is ideal for unstable ground, as it minimizes the risk of collapse and reduces the need for temporary supports.
In addition to these methods, **smooth blasting**, **pre-split blasting**, and **millisecond blasting** are modern techniques used to control the shape of the tunnel and minimize damage to the surrounding rock. These methods help achieve precise excavation and reduce vibration, ensuring structural integrity.
Common equipment used in these methods includes **spray nozzles**, **concrete trolleys**, **aerial platforms**, **drilling rigs**, and **armored trolleys**. Each tool plays a vital role in ensuring the efficiency and safety of the excavation process.
Overall, the evolution of tunneling methods—from traditional mining techniques to advanced mechanized systems—has significantly improved the safety, efficiency, and adaptability of underground construction. Whether in hard rock or soft soil, the choice of method depends on the specific geological conditions and project requirements.