In step construction, advance support is an important pre-reinforcement measure for weak surrounding rock, shallow buried tunnels or areas with complex geological conditions. Its core role is to improve the stability of the formation by reinforcing or supporting the surrounding rock in front of the face in advance, creating safe conditions for subsequent excavation and support. The following are the specific roles of advance support:

   I. Active reinforcement of the surrounding rock in front of the face

   1. Preventing surrounding rock collapse

   ① In weak and broken surrounding rock (such as IV and V grade surrounding rock), the face is prone to collapse due to poor self-stabilization ability. Advance support (such as advance small guide tubes and pipe sheds) cements the loose rock into a whole by driving steel pipes and anchors into the surrounding rock and grouting, thereby improving the strength and bonding force of the surrounding rock, forming an "advance bearing arch", and bearing the formation load in advance to avoid instability of the face during excavation.

   ② For example: Pre-grouting with small pipes can fill the cracks in the surrounding rock, reduce water seepage and soil loss, and prevent the risk of collapse caused by water softening the surrounding rock.

   2. Control the deformation of the surrounding rock

   As the "leading structure" of the initial support, the pre-support can constrain the early deformation of the surrounding rock (such as vault settlement and peripheral convergence). Especially in shallow buried tunnels, it can effectively reduce surface settlement and avoid affecting ground buildings or pipelines.

  II. Auxiliary excavation and support construction

   1. Form a temporary support structure

   ① Pre-support (such as large-diameter pipe shed) can be used as a "temporary beam" during excavation, spanning the broken zone or karst cave, and providing a safe working space for the upper step excavation. For example, in the water-rich sand layer, the pipe shed combined with grouting can form a water-stop curtain to prevent sand and water from gushing during excavation.

   ② For the upper step operation of the step method, the pre-support can reduce the risk of the air face during excavation and avoid the collapse of the top surrounding rock due to the large height of the upper step.

   2. Reduce over-excavation and under-excavation and support difficulty

   The reinforced surrounding rock has better integrity, the excavation contour is easier to control, and the probability of over-excavation (increase backfill volume) and under-excavation (secondary treatment required) is reduced. At the same time, the unevenness of the initial support shotcrete is reduced, and the support efficiency is improved.

   III. Enhance the integrity of the support system

   1. Connect the advance support and the initial support

   ① The tail of the advance support (such as a small guide tube) is usually welded with a steel frame (steel or grid steel frame) to form a three-dimensional support system of "advance support + steel frame + anchor rod + shotcrete". This connection enables the load of the advance support to be effectively transferred to the subsequent initial support structure, improving the overall bearing capacity.

   ② For example: the lock-foot anchor rod (tube) cooperates with the advance small guide tube to enhance the stability of the arch foot and prevent the arch foot from settling or displacement due to the excavation of the lower step.

  2. Respond to sudden geological changes

   When excavation reveals unexpected weak interlayers or cracks, advance support (such as temporarily added advance anchors) can quickly reinforce local weak areas to avoid the expansion of risks, reflecting the principle of "support first, then excavation, and pre-reinforcement first".

   IV. Ensure construction safety and efficiency

   1. Reduce construction risks

   ① In high-risk areas such as shallow buried bias, fault fracture zones, etc., advance support is a key measure to prevent collapse and roof fall, which can greatly reduce the probability of safety accidents (such as landslides and burials) and ensure the safety of workers and equipment.

   ② For the construction of urban tunnels or adjacent buildings, advance support can effectively control surface subsidence and avoid secondary disasters caused by damage to the surrounding environment.

   2. Optimize the connection of construction processes

   The pre-reinforcement effect of advance support reduces the need for emergency support after excavation, making the "excavation → support → lower step construction" process of the step method smoother, avoiding shutdowns due to handling collapse or large deformation, and improving construction efficiency.

   In the step method construction, advance support is the embodiment of the concept of "prevention first". By "reinforcement first, then excavation", the surrounding rock is controlled in a stable state, creating safe conditions for step excavation and step support. Its core role can be summarized as: strengthening surrounding rock, controlling deformation, assisting excavation, connecting support, and ensuring safety. Especially in soft strata, it is a key technical measure to avoid collapse accidents and ensure the safety of tunnel construction.