I. Project Overview

The construction of this project utilizes a "6m long Φ600@400 cement-soil mixed pile + 6m long Φ60@800 advance steel pipe + Φ60×3.0 steel mesh pipe" as the upper support structure and water-shedding screen. The mixed piles and advance steel pipes were constructed prior to earthwork excavation. During the earthwork excavation, steel mesh pipes and sprayed concrete with mesh were installed according to the progress of the excavation. The spacing of the steel mesh pipes is 1000mm; the surface of the mixed piles is reinforced with a steel mesh, and a 100mm thick C20 concrete is sprayed. When coordinating with the steel mesh pipe installation during earthwork excavation, each step is excavated to 0.5m below the corresponding steel mesh pipe, followed by the insertion of the steel mesh pipes and the installation of the steel mesh and concrete spraying layer; the lower earthwork excavation must wait until the upper support structure reaches 80% of the designed strength.

II. Construction Techniques

Percussive hole drilling - Slope repair - Hole positioning - Air hammer anchoring pipe to design depth - Hole plugging with grout - Grout topping - Tying and fixing steel mesh - Reinforcement - End welding - Shotcrete surface layer - Concrete surface curing - Repeat for next layer of steel flower pipe construction

Section 3: Steel Flower Pipe and Mesh Sprayed Concrete Construction

Construction Technique

1) Locate and Mark Out: The surveyor accurately marks the position of each steel pipe according to the design drawings.

2) Steel Flower Pipe Fabrication and Installation: The soil-end of the steel flower pipe is processed into a pile point shape; drilling holes with a diameter of 8mm are set throughout the full length of the steel flower pipe, arranged in opposite directions with a spacing of 500mm; L25×3 angle steel is used to make inverted spikes that are spiral throughout the full length, with a spacing of 500mm. Steel Flower Pipe Installation: Use an impact hammer or soil nail machine to align the steel flower pipe according to the designed angle and position, and hammer it into the soil to the desired length. When using steel flower pipes longer than 6m, the pipe sections are aligned with a centerline and fully welded together, with additional welding of 3Φ16 rebars at the overlap for reinforcement.

3) Grouting - The grouting material uses pure cement slurry with a water-cement ratio of 0.4 to 0.5, with a grouting pressure of not less than 0.6 MPa. The grouting body strength is 20 MPa, the cement grade is 32.5R composite silicate cement, and the requirement is that the cement content per meter should not be less than 25 kg.

4) Install a single-layer Φ6 steel wire mesh with a 200×200 grid. At the exposed end of the steel flower pipe where it contacts the steel wire mesh, add two Φ14 reinforcing bars for enhanced connection and weld them to the steel flower pipe.

5) Shotcrete - The shotcrete strength is C20, with a design thickness of 100mm; the earthwork must be excavated in layers and sections, with the layer height in the shotcrete and anchoring section not exceeding 1.5m, and the section length not exceeding 20m; the shotcrete and anchoring strength must reach 70% of the design strength before the next layer excavation and shotcrete anchoring operation can proceed.