In the step method construction, the advance small pipe grouting is an important pre-reinforcement measure for the weak and broken surrounding rock. Its construction points can be specifically explained from the aspects of pipe design, processing and installation, grouting process and quality control, as follows:

  I. Pipe design and layout

   1. Pipe parameters

   ① Pipe diameter and length: usually use hot-rolled seamless steel pipes with an outer diameter of Φ42~Φ50mm and a wall thickness of 3.5~5mm, with a length of 3~5m (adjusted according to the degree of surrounding rock crushing, generally not exceeding 6m), to ensure that the reinforcement range covers 1~3m in front of the excavation step.

   ② Arrangement range: arranged along the outer contour line of the tunnel arch or side wall excavation, with a circumferential spacing of 30~50cm and an external insertion angle of 5°~15° (a smaller angle is usually used for the upper step arch, and the side wall can be appropriately increased to 20°) to form a uniform reinforced arch shell.

   2. Pipe processing

   ① Front end: Processed into a pointed cone shape to facilitate driving into the surrounding rock; Φ6~Φ8mm grouting holes are opened within 1~1.5m from the front end, arranged in a plum blossom shape, with a spacing of 15~20cm, and 30~50cm is reserved at the tail as a stop grouting section without holes.

   ② Tail: Weld Φ6~Φ8mm steel hoop or flange to prevent deformation of the end of the pipe during driving, and facilitate welding and fixing with steel frames (such as grid steel frames or I-beams).

   II. Construction process and installation points

   1. Construction preparation

   ① Measurement and positioning: Mark the position of the pipe on the face according to the design requirements, with an error of ≤5cm, to ensure that the circumferential distribution meets the reinforcement range.

   ② Equipment and tools: Use rock drill, hydraulic hammer or drilling rig to make holes (select according to the hardness of the surrounding rock, hard rock needs to be drilled and then the conduit is inserted, soft rock can be directly driven in), equipped with grouting pump (such as double-liquid grouting pump), mixer and pressure gauge (accuracy ≥1.5 level).

  2. Conduit installation

   ① Drilling and insertion: The diameter of the drilled hole is 10~20mm larger than the outer diameter of the conduit, and the drilling depth is 10cm longer than the length of the conduit; after inserting the conduit, check the exposed length (the tail needs to be exposed to the steel frame 10~20cm), ensure that it is firmly welded with the steel frame to form an integral support structure.

   ② Sealing: The gap between the conduit and the orifice is sealed with quick-setting cement slurry or anchoring agent to prevent leakage during grouting, and a stop valve (such as a ball valve) is installed at the tail to control the grouting pressure.

  III. Grouting process control

   1. Grouting materials

   ① Single-liquid slurry: cement slurry (water-cement ratio 1:1~0.8:1), suitable for surrounding rocks with developed cracks and good permeability.

   ② Double-liquid slurry: cement-water glass double-liquid slurry (volume ratio 1:0.5~1:1, water glass concentration 35~40Be′), initial setting time 30 seconds~2 minutes, used in areas rich in water or requiring rapid consolidation.

   ③ Admixtures: early strength agent, water reducer, etc. can be added to improve the slurry performance, and the specific ratio needs to be determined through field tests.

   2. Grouting parameters

   ① Pressure: initial pressure 0.3~0.5MPa, final pressure 1.0~1.5MPa (adjusted according to the density of the surrounding rock to avoid overpressure causing surface uplift or surrounding rock splitting), grouting speed is controlled at 5~15L/min.

   ② Sequence: follow the principle of "from low to high, interval jump hole", first grout the side wall hole and then the arch hole, and end when the single hole grouting volume reaches the design value or the pressure rises to the final pressure and stabilizes for more than 10 minutes.

   ③ Termination standard: mainly based on grouting pressure and supplemented by grouting volume, stop when the single hole grouting volume reaches the design calculation volume (considering the porosity of the surrounding rock) or the pressure reaches the standard and there is no leakage.

  IV. Quality control and precautions

   1. Process inspection

   ① Closely monitor the settlement of the surface and surrounding buildings during grouting, and immediately suspend grouting if abnormalities are found, analyze the reasons and adjust parameters (such as reducing pressure and shortening grouting time).

   ② Check the angle and depth of the guide tube to avoid excessive angles that lead to insufficient reinforcement range, or too small angles that lead to intrusion into the tunnel excavation contour.

  2. Effect inspection

   ① After grouting, check the strength and uniformity of the reinforced area by drilling and coring, sonic detection and other methods. The bearing capacity of the reinforced surrounding rock is required to be ≥ the design value, and the permeability coefficient is ≤10⁻⁵cm/s (water-rich areas).

   ② During excavation, observe the stability of the surrounding rock of the face. If there is block drop, water seepage, etc., it is necessary to add a small pipe or adjust the grouting parameters.

   3. Safety measures

   ① The grouting pump pressure system needs to be equipped with a safety valve to avoid overpressure and pipe burst; operators need to wear protective equipment to prevent slurry from splashing into the eyes or skin.

   ② When the step method is used for construction, after the grouting of the small pipe ahead of the step is completed, it is necessary to wait until the slurry reaches the design strength (usually 4~6 hours, and the double liquid slurry can be shortened to 1~2 hours) before excavation to ensure the reinforcement effect.

   The core of advanced small-duct grouting is to form a temporary reinforcement circle outside the excavation contour through "closely spaced ducts + pressure grouting". The key points of its construction require strict control of the duct angle, grouting pressure and material ratio to ensure close coordination with the step excavation sequence (such as giving priority to excavation after grouting the upper step, and supplementing support for the lower step as appropriate), and ultimately achieve the safe construction goal of "support first, then excavate, and proceed steadily".