I. Project Overview

The external retaining wall of the excavation pit in this project uses 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 barrier. The mixed piles and advance steel pipes were constructed prior to earthwork excavation. During the earthwork excavation, steel mesh pipes and sprayed concrete mesh were inserted as needed 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 layer is sprayed. When coordinating with the steel mesh pipe construction during earthwork excavation, each step is excavated to 0.5m below the corresponding steel mesh pipe, then the steel mesh pipe is inserted, followed by the installation of the steel mesh and the spraying of the concrete layer. The lower earthwork excavation must wait until the upper support structure reaches 80% of the design strength.

II. Construction Techniques

Hammer drilling for holes - 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 Spray Concrete Construction

Construction Technology

1) Locate and lay out steel pipe lines according to the design drawings, with each pipe's position accurately marked by surveyors.

2) Steel Flower Pipe Manufacturing and Installation Steel Flower Pipe Manufacturing: The end of the steel flower pipe entering the ground is processed into a pile point shape; a grouting hole with a diameter of 8mm is 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 barbs that are fully helical and set at a spacing of 500mm. Steel Flower Pipe Installation: Use an impact hammer or soil nail machine to align the steel flower pipe at the designed angle and position, and hammer it into the ground to the designed 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 position for reinforcement.

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

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

5) Shotcrete - The shotcrete has a strength of C20 and a designed thickness of 100mm; it is required to excavate the earthwork in layers and sections. The layer height for the shotcrete anchoring section should not exceed 1.5m, and the section length should not exceed 20m. The next layer's shotcrete anchoring work can only be carried out once the anchoring strength reaches 70% of the design strength.