How are highway tunnels constructed? Tunnel engineering is a complex and systematic project, comparable to "performing surgery underground on mountains or underwater." Its core lies in safely and precisely carving out a stable space within underground rock and soil, and constructing a permanent support structure. Common construction methods for highway tunnels include drill-and-blast, shield tunneling, and cut-and-cover. Among these, drill-and-blast and shield tunneling are the mainstream choices for mountain tunnels and urban tunnels. Below is a detailed introduction.

   I. How are highway tunnels constructed?

   The construction of highway tunnels typically includes the following stages:

   1. Survey and route selection: Investigating geological conditions, hydrological conditions, rock strata stability, etc., and designing the tunnel route, cross-sectional dimensions, and construction methods.

   2. Portal construction: Treating mountain slopes, cutting away debris, providing support, and constructing infrastructure such as pilot tunnels, access roads, and drainage systems.

   3. Main Excavation and Support: Excavation is carried out section by section using machinery or blasting. The surrounding rock is reinforced according to the principle of "rapid excavation, strong support, and early closure" to ensure the safety and stability of the tunnel structure.

  4. Secondary Lining Construction: After the initial support has stabilized, concrete lining is poured to form a long-term structure, improving waterproofing, fire resistance, and load-bearing capacity.

   II. Commonly Used Mainstream Construction Methods and Characteristics of Highway Tunnels

   1. Drill and Blasting Method (Preferred for Mountain Tunnels)

   ① Core Logic: Rock is broken through drilling, explosives, and blasting, and excavation is gradually carried out to form the tunnel shape, followed by support and lining.

   ② Applicable Scenarios: Mountainous areas, rock strata, and relatively hard and intact geological conditions.

   ③ Key Steps: Drilling → Blasting → Muck Removal → Initial Support (Shotcrete, Steel Arch) → Secondary Lining → Drainage and Waterproofing.

   2. Shield Tunneling (Mainstream for Urban Tunnels)

   ① Core Logic: A tunnel boring machine (TBM) advances underground, simultaneously completing excavation, muck removal, and lining processes; the entire process is automated.

   ② Applicable Scenarios: Suitable for soft soil strata in urban areas (such as subway tunnels and river-crossing tunnels), where high requirements for ground settlement control are necessary.

   ③ Key Advantages: Safe and efficient construction, minimal disruption to the surrounding environment, suitable for long-distance, large-diameter tunnels.

   3. Open-Cut Method (Applicable to Shallow Tunnels)

   ① Core Logic: First, the surface is excavated to form a foundation pit. The main tunnel construction is completed within the pit, and finally, the soil is backfilled to restore the surface.

   ② Applicable Scenarios: Suitable for shallow burial depths (usually less than 10 meters) and open areas, such as suburban tunnels and tunnel entrance/exit sections.

   ③ Key Characteristics: Simple and intuitive construction, lower cost, but significant impact on surface traffic and surrounding buildings.

   4. New Austrian Tunneling Method (NATM) – The most common method for mountain tunnels

   ① Core Logic: Utilizing the bearing capacity of the surrounding rock, a self-stabilizing structure is formed through reasonable support.

   ② Applicable Scenarios: Mountain tunnels, tunnels with relatively intact or moderate surrounding rock conditions.

   ③ Key Features: Sectional excavation (full-face, bench method, CD method, CRD method, etc.), initial support (shotcrete, anchor bolts, steel arches), multiple measurements and monitoring of the surrounding rock, and secondary lining after stabilization.

   5. Immersed Tunnel Method: Used for underwater tunnels. Tunnel sections are prefabricated on shore, floated to a designated location, immersed and connected, and then backfilled and fixed, such as the Hong Kong-Zhuhai-Macau Bridge undersea tunnel.

   Modern tunnel engineering often combines multiple construction methods. For example, a long mountain tunnel might mostly use the New Austrian Tunneling Method (NATM), but switch to a more conservative support method when encountering extremely fractured zones; while a tunnel crossing a city or river might use cut-and-cover, shield tunneling, and immersed tube tunneling methods simultaneously. Engineers will choose the safest, most economical, and most reasonable solution based on the principle that "geology is God, design is the leader, and construction is the key."