I. Project Overview

The retaining wall of this project's foundation pit is lined with 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 before the earthwork excavation. During the earthwork excavation, steel mesh pipes and shotcrete with wire mesh are 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 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 designed strength.

2. Construction Techniques

Percussion drilling for hole creation – slope repair – hole positioning – pneumatically hammering anchor pipe to design depth – plug hole with grouting – top up grouting – tie and secure steel mesh – compress reinforcement – end welding – shotcrete surface layer – concrete surface layer curing – cycle to next layer of steel flower pipe construction

Section 3: Steel flower pipe and mesh concrete spraying construction

1. Construction Technology

1) Locate and Mark Out Lines: Accurately position each steel pipe according to the design drawings by the surveyor.

2) Steel Flower Pipe Manufacturing and Installation: The soil-end of the steel flower pipe 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 opposite to each other with a spacing of 500mm; L25×3 angle steel is used to make inverted spikes that are spiral-shaped 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 at the designed angle and position, and hammer it into the soil to the designed length. When using steel flower pipes over 6m in length, 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, the grouting pressure is not less than 0.6MPa, the grouting body strength is 20MPa, the cement grade is 32.5R composite silicate cement, and the requirement is that the cement content per meter should not be less than 25kg.

4) Install a single-layer Φ6 steel wire mesh with a spacing of 200×200mm. At the exposed end of the steel flower pipe where it contacts the steel wire mesh, add two Φ14 reinforcing bars for enhanced connection, which are then welded 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 shotcrete and anchoring layer height not exceeding 1.5m and the section length not exceeding 20m. The next layer of excavation and shotcrete anchoring can only be carried out once the shotcrete and anchoring strength reaches 70% of the design strength.