Single-shield and double-shield TBMs are two common types of tunnel boring machines, playing a vital role in tunnel construction. Both use a rotating cutterhead to remove rock or soil, and then use a propulsion system to propel the tunnel forward. Below, I will detail the characteristics, advantages, and disadvantages of each type of TBM.

  I. Single-shield TBM

   1. Characteristics of Single-shield TBMs:

   ① Suitable for conditions with favorable geological conditions, such as hard rock.

   ② Frequent support operations are required in unstable geological conditions.

   ③ The shield structure is simple and relatively easy to maintain.

   ④ Propulsion relies on the reaction force of the tunnel wall.

   2. Advantages of the Single Shield TBM:

   Using precast concrete segments for support, it offers rapid support speed and high construction efficiency in weak and fractured surrounding rock, making it a clear advantage in short, soft rock, and shallow tunnels.

  3. Disadvantages of the Single Shield TBM:

   Compared to the Double Shield TBM, tunneling and segment installation cannot proceed simultaneously, resulting in certain process delays and relatively low tunneling efficiency.

   II. Double Shield TBM

   1. Features of the Double Shield TBM:

   ① High adaptability, capable of operating in a variety of geological conditions.

   ② High tunneling efficiency in stable rock formations.

   ③ Lining ring installation can be performed promptly in unstable rock formations, improving safety.

   ④ Relatively complex structure, resulting in relatively high maintenance costs.

   2. Advantages of the Double Shield TBM:

   ① Dual propulsion systems enable simultaneous tunneling operations while segment assembly is in progress, minimizing process delays and increasing tunneling efficiency.

   ② Open, single-shield, and double-shield tunneling modes can be flexibly selected based on different geological conditions. For example, high tunneling speeds can be achieved in good hard or soft rock formations. When encountering soft rock that cannot withstand the compressive stress of the support plate, the shield tail propulsion hydraulic cylinder can be supported on prefabricated lining blocks or steel ring beams to propel the cutterhead forward. When encountering hard rock, the support plate can be used to hold the tunnel wall in place, similar to an open-type TBM, while the main propulsion hydraulic cylinder propels the cutterhead forward.

   ③ The high degree of mechanization makes tunneling safer, more reliable, and faster, with minimal interference with surrounding ground structures.

   3. Disadvantages of the Double-Shield TBM:

   ① The equipment structure is relatively complex and the cost is relatively high.

   ② Its adaptability may be limited in some very complex geological conditions.

   Single-shield and double-shield TBMs each have their advantages. The choice of which TBM to use depends on the specific geological conditions and project requirements. In actual applications, engineers will decide which type of tunnel boring machine to use based on the geological survey report and the specific requirements of the tunnel to ensure the smooth progress of tunnel construction.