1. Steel support construction process

1. Installation and removal of steel support

(1) According to the steel support plan, before excavating the foundation pit, mark the center line position of the steel support plane around the foundation pit and make a good point. Use two theodolites to control the support axis during construction.

(2) After the working surface is excavated, weld the triangular support and bracket from the elevation control line marked with ink lines on the steel concrete ring beams on the three sides of the foundation pit, and install the steel support;

(3) According to the actual size of the foundation pit on site, equip the required support and pad materials, assemble the steel support to the designed length, and wait for the working surface to be excavated before installation.

(4) A 50-ton crawler crane is used for support installation (installed in the foundation pit), and the support is pre-assembled on the ground (depending on the construction site); during construction, ensure that the roads around the foundation pit are unobstructed and the road width is not less than 6 meters;

(5) The center points of the two ends of the support are determined on the steel concrete ring beam to ensure that the support axis is stressed, and the actual length of the two contact points is measured. Cutting and splicing are carried out according to the measured length.

(6) Since the steel support is long, columns are set in the pit, so column brackets are required. When the support trench is excavated, the trench for the steel bracket is also excavated. Generally, the trench is excavated to about 1.3 meters below the support. The steel bracket is installed to facilitate the segmented installation of the steel support.

(7) The steel support is pre-assembled to the designed length and the steel support is installed in segments according to the excavation sequence. Each segment of the steel support is installed in the east and west directions first and then in the south and north directions. Use a 50-ton crawler crane (or crawler excavator) to first lift one section, place one end on the edge of the steel concrete ring beam, and the other end on the steel bracket of the column. After it is firmly fixed, lift the other section and bolt the two sections together until the entire steel support is assembled. Then connect the rods.

(8) After the steel support is hoisted into place and assembled, first apply stress in the east and west directions, and then apply stress in the south and north directions. Four 100-ton hydraulic jacks were placed at the end. To facilitate construction and ensure uniform jacking force, the four jacks were secured together using a dedicated bracket. This bracket was then placed on the flexible head. Once the oil line was connected, the pump was turned on to apply prestress. Once prestress was applied, a spacer was wedged into the flexible head and secured with electric welding. The oil was then drained, the jacks were released, and the lifting wire ropes were untied, completing the installation of the support. While the jacks were applying prestress, the prestress value was recorded for future reference. The prestress was approximately 400 kN per support, and prestressing was applied in two stages.

First: Each support was uniformly stressed, requiring a preload of approximately 100-150 kN, ensuring that both ends of the support were aligned closely with the embedded steel plate section of the steel-concrete ring beam.

Second: Any gaps between the ends of the support and the embedded steel plate section of the ring beam were sealed with reinforced grouting and sealed with rebar heads or steel plate sheets. After the strength of the reinforced grouting material reaches C30 strength, a second pre-compression axial force of 300~400KN is applied. The lower soil can only be excavated after the support installation is accepted. The inspection unit must monitor the change of the support axial force for 24 hours. Generally, an axial force device or stress plate is installed.

(9) All parts that require electric welding should ensure equal strength connection.

(10) According to the design requirements, excavation should follow the principle of supporting first and then excavating. After the strength of the concrete ring beam reaches the design strength, the first steel support is installed. After the support system is formed and prestressed, the next process can be excavated.

(11) After the steel support is installed, excavation is carried out to 200mm above the bottom of the foundation pit, leaving 200mm of soil for manual removal, and at the same time, the concrete cushion layer is constructed.

(12) Coordination between excavation and steel support installation: When the support installation is completed, the roadbed box is first laid (crossed) on the steel support during excavation to ensure that the roadbed box is 40 cm higher than the support surface after being raised. During the excavation process, the roadbed box is raised again at any time according to the sinking of the roadbed box to ensure that the roadbed box does not touch the support and does not cause stress. Excavation and support installation are closely related. Regular inspections and observations should be made to check and observe the loosening of the support, displacement of the foundation pit retaining dam, etc. If any problems are found, timely measures should be taken to ensure safety and foolproofness. 2. Removal of steel supports ① According to the requirements of the design drawings: A. Remove the steel support after the deep pit foundation bottom plate reaches the design strength. B. Remove the steel support and cast the main structure wall and top plate. C. Before casting the force transmission belt between the bottom plate and the retaining pile, the gap between the main structure and the retaining wall should be filled with sand to the corresponding position before casting. ② First, erect steel pipe scaffolding at the support disassembly points. This is done at the ends of each support, at the column piles, and at the steel support joints.

③ Remove support clamps and other metal parts using gas cutting.

④ To de-stress the support, four 40-ton hydraulic jacks are placed at the end. To facilitate construction and ensure consistent jacking force, a dedicated bracket is fabricated to secure the four jacks together and place them on the end. Once the oil pipes are connected, the pump can be turned on to apply prestress. Once prestressing is in place, the steel wedges are promptly removed to prevent the steel support from falling and injuring construction workers during transportation.

⑤ Once the support stress relief is complete, the connection at the end of the steel support is cut, leaving one end resting on the scaffolding while the other end is removed by crane. After the flange screws are removed, the support is lifted out of the foundation pit. Due to its length, the steel support needs to be disassembled in sections. Depending on the construction site and other conditions, the support removal machinery is coordinated by the general contractor’s tower crane, with a separate 16-ton truck crane deployed outside the foundation pit.