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Mine Law
Mining Law, also known as the traditional tunneling method, refers to the construction technique where tunnels are excavated by creating underground passages. The fundamental principle of this method involves blasting, which loosens the surrounding rock mass and makes it prone to collapse. Based on this concept of "loose load," the excavation is carried out in segments, with each section being dug and supported immediately to ensure safety. This approach requires extensive support structures and high wood consumption. However, with the development of spray-on concrete and anchor support techniques, the number of divisions has been reduced, leading to the evolution of the New Austrian Tunneling Method (NATM).
One common excavation method involves drilling and blasting to create sections for tunnel and underground work. It was named after the mining technique used for developing roadways. When using this method, the entire cross-section is excavated to the design contour, and lining is constructed simultaneously. In softer ground, a small excavator may be used, and if necessary, the sides are excavated and supported based on the stability of the surrounding rock. For branch tunnels, a guide pit is first excavated, then expanded to reach the final design shape.
The primary purpose of dividing the excavation is to minimize disturbance to the surrounding rock. The size and number of divisions depend on geological conditions, tunnel size, and support type. In stable and intact rock formations, full-section excavation is possible for medium and small tunnels. However, in loose or fractured areas, multiple divisions are required, with temporary supports provided during excavation to prevent collapse. The introduction of spray-anchored support has significantly reduced the number of divisions and led to the development of the New Austrian Method.
Based on the sequence of lining construction, the method can be divided into two main types: the "first arch, then wall" method and the "first wall, then arch" method. The latter includes several sub-methods such as the funnel scaffold method, step method, full-section method, and the upper-lower guide pit first wall then arch method. In soft ground or large-span tunnels, a special method called the "half-arch first wall then arch" is used, also known as the "side wall pipe first wall then arch" method. Additionally, a hybrid method called the mushroom-shaped method combines elements of the first arch and funnel scaffold methods.
The "first arch, then wall" method is also known as the top arch method. In unstable soft rock formations, the arch section is excavated first, and the roof arch is built to support the surrounding rock before excavating the lower section and side walls. Before excavating the side walls, the top arch must be properly supported, hence the name. When digging the side walls (commonly referred to as "digging the horse’s mouth"), the work should be staggered to avoid the top arch from becoming suspended and sinking. The construction sequence is illustrated in Figure 1, where Arabic numerals indicate the excavation order and Roman numerals show the lining sequence.
During construction, both upper and lower guide pits are excavated, and a large number of concrete sills are placed at the upper section. These sills can be transported through a series of holes between the upper and lower guide pits, improving exit efficiency and reducing construction interference. If the tunnel is short and the rock is dry, only guide pits may be used. In such cases, the first half of the section can be fully constructed before starting the second half to avoid conflicts between transportation and construction. This method is suitable for loose rock formations but requires sufficient compressive strength at the arch seats. It can also be applied to large-span or high caverns in hard rock formations, simplifying the arching and concrete construction process. This method is sometimes referred to as the "foreign law" in some languages.
The funnel scaffold method, also known as the "lower wall first" method, is suitable for harder and more stable rock formations. During construction, the lower guide pit is excavated first, followed by upward expansion from the lower part to reach the vault. Then, the sides are excavated from top to bottom until the side walls are completed. After full excavation, the lining is built from the side walls up to the arch. The construction sequence is shown in Figure 2. A temporary wooden scaffold is set up in the lower guide pit, with lightweight steel rails laid on the crossbeam, leaving a longitudinal gap above the transportation line. The crossbeam is spaced with a funnel-shaped opening for debris to fall through. As excavation progresses, the scaffold serves as a working platform. The broken stone from positions 2 to 5 falls onto the scaffold and is discharged through the funnel into the bucket car. This system reduces labor intensity. The width of the lower guide pit is usually determined by two-lane transport, and a middle column may be added for temporary reinforcement under the crossbeam. The length of the scaffold section is established, and the extension of each enlarged excavation is determined by adding a certain margin. This method is widely used in railway tunnel construction due to its significant advantages.
The step method is divided into the positive step and reverse step methods. In poor stability rock formations, the positive step method divides the entire tunnel section into several layers, excavating from top to bottom. Each layer's excavation surface is close to the next, forming multiple steps. The upper step drilling and lower step excavation can be done in parallel, increasing efficiency. Once the entire section is excavated, the lining is built from the side walls up to the arch. The first layer excavated at the top is a curved guide pit, requiring more blastholes. The guide pit's lead distance is very short, allowing the blasted material to be directly thrown out, reducing workload and increasing tunneling speed. If the top rock is loose, temporary support using bolts or steel arches is needed in the guide pit to prevent collapse.
The reverse step method is used in stable rock formations. The entire tunnel section is divided into several layers, with the bottom excavated first using a large downward guide pit, then expanded from bottom to top. When drilling the upper layers, work platforms or funnel scaffolds must be set up for mounting.
The full-section method involves excavating the entire section at once, suitable for medium and small tunnels in good rock conditions. This method allows the use of large-scale machinery such as rock drills, large mounting machines, trough trains, shuttle carts, formwork trolleys, and concrete filling equipment for comprehensive mechanized construction. The emergence of the New Austrian Method has expanded the application scope of the full-section and step methods.
The method of excavating the front wall from upper and lower guide pits is also known as full-section excavation. In the past, in soft and unstable rock formations, this method was used to improve lining quality, excavating the entire section and constructing the lining in the order of the front wall and rear arch. However, this method required a lot of wooden support and frequent replacement, making it difficult and unsafe, so it is no longer used in China. It is also known as the Austrian national law or old Austrian law in foreign languages.
The mushroom-shaped method combines the features of the first arch back wall method and the funnel scaffold method, forming a hybrid approach. After excavating parts 1-4, the section resembles a mushroom shape, hence the name. A funnel scaffold is set up in the lower guide pit for upward expansion. If the arch conditions are poor, the arch can be built first for safety. This method offers flexibility, allowing easy transition to other methods. If the rock is poor, it can switch to the simple arch back wall method; if the rock improves, it can use the funnel scaffold method. In China, it was first used for railway tunnels with stable rock and later adapted for large-section tunnels, reducing moldwork requirements and accelerating construction progress.
The sidewall guide pit first wall behind arch method, also known as the core support method, is used for large-span tunnels in very soft and unstable formations. To ensure safety, the perimeter is first excavated, and the side walls are lined up to the arch to prevent strata collapse. During excavation, temporary support and arch support are provided on a large central core that remains unexcavated. Under the lining protection, this core is finally excavated, and an inverted arch may be built. The width of the side guide pits is large, providing space for excavation trucks, workers, and masonry operations, allowing simultaneous excavation and lining. Maintaining a sufficient width for the core ensures stability, and the larger the width, the more material is left for final excavation, reducing costs. This method is commonly used in large-span tunnels with high surrounding rock pressure and unstable formations, such as double or multi-line railway tunnels, road tunnels, and canal tunnels. It is also frequently used in hard rock formations, where the core serves as a foundation for supporting the arch and side wall formwork, greatly reducing temporary support needs. This method is still referred to as German law in some foreign languages.
In addition, when constructing large-section caverns, hybrid schemes combining the first arch back wall method with the core support method, or the first arch back wall method with the positive step method, are also employed.
Blasting in tunnel excavation differs from general stone blasting. To avoid damaging nearby temporary supports or permanent linings, the rock mass is crushed but not thrown far, so loose blasting is typically used. A blasting cycle includes drilling, loading, sealing, detonation, ventilation, temporary support, and excavation. Drilling and boring take most of the time and should be mechanized using rock drills, mining traction locomotives, etc.
To improve blasting results and avoid over-excavation or under-digging, the tunnel contour must meet design specifications. Factors like the number, diameter, depth, and charge of blastholes, as well as their layout, are crucial. The layout depends on the rock formation and tunnel size.
To create a new free face during blasting, a central trench is usually placed on the excavation surface, especially in pit excavation and full-face excavation. Auxiliary blastholes around the trench help expand the blast range. The three types of blastholes—contour control, peripheral, and inner—are detonated in the order of the circumference. Squeaky blastholes are generally arranged straight and flat, with axes perpendicular to the excavation face, often in grid, quincunx, or spiral patterns. Squint blastholes have oblique axes and are arranged in wedge, cone, or fan shapes depending on the geological structure.
Most blasting materials use less powerful and cheaper explosives. Water is used when available. Previously, sparks were used for detonation, but now electric or millisecond detonators are preferred. Recently, non-electric detonation with a detonating tube has become popular.
During blasting, to ensure accurate and smooth excavation contours and control surrounding rock vibration, modern techniques like smooth blasting, presplit blasting, and millisecond blasting have been developed and applied to achieve desired blasting effects.
Common equipment used in the mining method includes Nortex spray nozzles, concrete trolleys, aerial platforms, ** trolleys, armored trolleys, and drilling rigs from SADVIK and ATLAS.