How Are Underground Tunnels Supported? Underground tunnels are critical infrastructures for transportation, utilities, and urban development. Their structural stability relies on sophisticated engineering techniques tailored to geological conditions and project requirements. This article explores the key methods and technologies used to support underground tunnels, drawing insights from real-world practices and academic research.
1. Immediate (Temporary) Support:
As the tunnel is excavated, temporary supports—such as steel sets, timber, or specialized shields—are installed to prevent collapse. In many modern methods, tunnel boring machines (TBMs) not only excavate but also install precast concrete segments or spray shotcrete (a type of fast-setting concrete) immediately behind the face. This rapid support minimizes deformation and stabilizes the cavity as work progresses.
2. Permanent Lining and Reinforcement:
Once the tunnel is advanced, a permanent lining is constructed. In rock tunnels, the lining might consist of concrete, steel ribs, or a combination thereof—often reinforced with rock bolts that anchor the lining to the surrounding rock mass. This creates a composite structure where the tunnel lining and the natural rock work together, forming an arch that distributes the overburden loads safely.
3. Traditional Support Methods
a) Steel Arch Supports
Steel arch supports are widely used in tunnels with unstable surrounding rock or soil. These prefabricated steel frames are installed during excavation to provide immediate structural reinforcement. For example, in China’s Luojiang Tunnel, steel arch supports were combined with shotcrete to stabilize the surrounding rock.
b) Rock Bolting and Shotcrete
Rock bolting involves drilling anchors into the tunnel walls to bind loose rock layers, while shotcrete (sprayed concrete) forms a protective shell. This method, known as the New Austrian Tunneling Method (NATM), was applied in the Xuefeng Mountain Tunnel to address complex geological conditions.
c) Concrete Lining
Permanent concrete linings are cast after excavation to withstand long-term loads and groundwater pressure. In Tongluochuan Tunnel, precast concrete segments were used to reinforce the tunnel structure.
Tunnel support systems are a synergy of geology, engineering innovation, and continuous monitoring. From traditional rock bolting to AI-enhanced TBMs, these methods ensure the safety and longevity of underground structures, enabling modern cities to thrive beneath the surface.