Abstract: Based on a project case, this paper elaborates on the construction methods, key points, and difficulties of using a steel open caisson as a foundation pit support under soft geological conditions, providing a technical basis for further deepening and perfecting steel open caisson construction technology.
Open caisson construction methods are increasingly applied in projects with soft soil foundations. Typically, an open caisson, as a type of underground structure and deep foundation, is first manufactured on the surface as a shaft-like structure (open caisson), and then, under the enclosure of the shaft wall, soil is continuously excavated from inside, causing the caisson to sink gradually under its own weight and additional counterweight. After reaching the predetermined design elevation, the bottom is sealed and the internal structure is constructed. The steel open caisson foundation pit support construction method introduced in this paper refers to the principle of open caisson construction and uses steel plates. However, the steel open caisson is not part of the overall structure but only serves as a support structure for the pit wall.
Keywords: foundation pit; steel open caisson; construction

1 Project Overview
This project is the foundation engineering for an electron beam cold hearth furnace (EB furnace). According to the engineering geological and hydrological conditions and the EB furnace foundation construction drawings, the deepest foundation is -12.3007 m, and the groundwater level is about -7 m. Therefore, excavation below -7 m is not feasible (if wellpoint dewatering is adopted, since the municipal pipeline network is not yet connected, a large amount of water in the plant area has nowhere to be discharged). According to the investigation report, the foundation below -6 m is a silty sand layer, which may cause flowing sand. To ensure safety and schedule, open caisson construction is adopted below -7 m. If a reinforced concrete open caisson were used, the construction period would be longer, the cost higher, and stiffening ribs would be necessary to meet requirements, making it impossible to construct the concrete side walls of the foundation. Using a steel open caisson not only shortens the construction period, but the internal steel supports do not affect the construction of the foundation side walls; the outer wall of the steel open caisson can be used as external formwork for the side walls, and the project cost is similar to that of a reinforced concrete open caisson. Therefore, after comprehensive consideration of various factors, steel open caisson construction was decided.
2 Fabrication of Steel Open Caisson
2.1 Fabrication Plan of Steel Open Caisson
1) The main shaft wall is proposed to use 18 mm thick steel plates, with shaped steel and steel pipe supports inside, as shown in Figure 1. 2) The fabrication of steel ladders and counterweight baskets is shown in Figures 2 and 3.
2.2 Quality Control of Open Caisson Fabrication
1) Before fabrication, lay springboards and wooden battens at the shaft wall position.
2) During fabrication, control welding deformation and take corrective measures if necessary.
3) Weld water-stop rings at the center of the concrete side walls on the support steel pipes, so that after the equipment foundation concrete formwork is removed, the pipes can be filled with expansive concrete.
4) Steel plates and profiles are fed according to processing dimensions, with allowance for loss.
5) Plate welding adopts double-sided bevel continuous welds; profile welds adopt continuous welds; rigid beams are intermittently welded to steel plates. 3 Construction of Steel Open Caisson
3.1 Construction Sequence
Caisson positioning survey → erect scaffolding inside shaft wall → lay springboards and wooden battens outside shaft wall → fabricate and install caisson, counterweight baskets, and ladders → sinking (with deviation correction simultaneously) → bottom sealing → concrete construction inside caisson.
3.2 Construction Preparation
1) Level the site
Remove all earth around the EB furnace and level the site until construction conditions are met.
2) Set up survey control network
Set up a survey control network and benchmark points as required by the design drawings, conduct positioning and setting out, and determine the central axis of the caisson and the outline of the foundation pit.
3) Determine the plane position of the steel open caisson
The deepest elevation of the steel open caisson is -12.3007 m, and the bottom elevation of the support piles is -17.500 m.
4) Excavate soil above groundwater level and support the pit using a long-arm excavator down to the groundwater level, and perform anchor rod anchoring. After the anchor rod anchoring is completed, immediately install the steel open caisson. At the bottom of the foundation, construct drainage ditches and interception ditches, leaving sufficient working space. The ditch width is 300 mm, depth 400 mm, and the distance from the ditch edge to the slope bottom is 400 mm, and to the caisson edge 500 mm (see Figure 4).
3.3 Excavation and Sinking
1) To ensure smooth sinking of the caisson, before construction, calculate the sinking coefficient in stages based on the caisson shape, weight, sinking depth, and hydrogeological conditions, as a basis for determining the sinking construction method and technical measures. The sinking coefficient is calculated as: k0=(G-B)/Tf
Where: G is the self-weight of the caisson; B is the buoyancy of groundwater during sinking; Tf is the total friction resistance of the shaft wall; k0 is the sinking coefficient, preferably 1.05–1.25. For caissons in mucky soil, take the smaller value; in other soils, take the larger value. The shaft wall friction resistance is shown in Table 1. 2) Extract the bearing timbers in zones, sequentially, symmetrically, and synchronously. After each extraction, immediately fill the space under the cutting edge with sand or gravel. The timbers at the positioning supports should be extracted last and simultaneously.
3) During excavation and sinking, proceed in layers, uniformly and symmetrically, to ensure uniform vertical sinking without excessive inclination. Generally, excavate from under the cutting edge first, allowing the steel caisson to sink first, then excavate the middle part, thus preventing flowing sand during pumping. If the sinking coefficient is found to be small, stop excavation immediately, analyze the cause, identify problems (e.g., ① uneven settlement causing lateral deviation; ② cutting edge stuck on support piles), and take other measures (e.g., add counterweight frames, cut off the cutting edge with gas welding, break pile heads) to keep the caisson sinking continuously without long pauses, which may cause flowing sand and other hazards. Also, do not excavate the bottom too deep.
4) After excavation, before concrete pouring, clean the soil above the anchor cables to prevent soil from falling into the concrete during pouring and affecting concrete strength.
5) During sinking, in addition to preventing uneven sinking and sudden sinking, special emphasis should be placed on preventing torsional deformation of the caisson. When excavating inside, strictly control the excavation thickness, proceed from periphery to center, uniformly and symmetrically, and leave soil berms as needed, cutting layer by layer to ensure uniform sinking.
6) When sinking caissons in silty and fine sand layers, flowing sand is often encountered. If no measures are taken in advance, severe inclination may occur during sinking. During construction, closely observe the foundation pit. If water flow is too large or too rapid, which may cause flowing sand, stop construction immediately and adopt dewatering measures.
4 Earthwork Excavation and Transport
On site, six soil buckets cooperate with one 40 t crane to alternately transport soil outward, and a loader pushes the soil to an open area 10 m away from the EB furnace construction site to ensure no surcharge around the foundation pit.
5 Prevention and Correction of Sinking Inclination, Displacement, and Torsion
Set settlement observation points around the caisson, strengthen observation during sinking, measure at least twice per shift, and inspect after each sinking. If inclination or torsion is found, correct it promptly. To prevent sudden sinking, control uniform excavation.
6 Counterweight Installation
1) The installation sequence of counterweight baskets is: first hoist counterweight basket 1 to the middle of the caisson, observe whether the caisson sinks; if not, add a second counterweight basket 1, then gradually separate the baskets to both sides to avoid inclination due to eccentric load. The installation sequence of counterweight basket 2 is the same as that of basket 1.
After installing the counterweight baskets, if the caisson has no deviation, symmetrically add counterweight blocks to make the caisson sink uniformly. If deviation occurs during sinking, adjust the deviation by asymmetrically installing counterweight blocks. The counterweight blocks are precast concrete blocks.
2) Fill the gap between the cutting edge and the side wall soil. After completing the first section of the caisson (about 2 m), fill once with medium sand, filling while inserting a ?20 mm water pipe to compact. After completing other sections, first flush sand into the lower layer with a water pipe, then fill the upper layer, and repeat until the final elevation.
7 Bottom Sealing
1) When the caisson sinks to within 0.100 m of the design elevation, stop excavation and pumping, allowing it to sink to or near the design elevation under its own weight. When the caisson reaches the design elevation, conduct settlement observation. Bottom sealing can proceed only when the settlement within 8 hours is ≤10 mm. Drain the water inside the caisson and remove floating mud before sealing.
2) Use crushed stone as a reverse filter layer at the bottom of the caisson, with a total thickness of 400–700 mm. The thickness of the reverse filter layer and the shape of the bottom can be adjusted according to construction needs to ensure stability.
3) Pour plain concrete cushion (C40P8R impermeable concrete with early strength agent) on the reverse filter layer, and fill and vibrate compactly under the cutting edge. After the cushion concrete reaches 50% of the design strength, tie the bottom slab reinforcement and pour the bottom slab concrete.
4) From the start of pouring the bottom sealing concrete (C40 plain concrete) until the reinforced concrete bottom slab is poured and reaches 30% of the design strength, pump out the water in the filter drums. After the bottom slab reaches 100% of the design strength, seal the filter drums. Use C40 concrete for sealing, preferably with appropriate early strength agent and expansive agent. Before sealing, pump out the water in the filter drums.
5) Dry bottom sealing shall comply with the following: ① The soil surface at the caisson base shall be excavated entirely to the design elevation; ② Water inside the caisson shall be drained as much as possible; ③ During pouring, prevent uneven sinking of the caisson; in soft soil layers, bottom sealing should be carried out in symmetrical sections; ④ Before the bottom sealing and bottom slab concrete reach design strength, continuously pump water from the sump below the bottom sealing.
8 Safety and Environmental Protection Precautions and Measures
1) During caisson sinking, regularly observe settlement of existing buildings and take appropriate safety measures if necessary.
2) When arranging cranes, pipelines, and other heavy equipment around the caisson, consider possible ground subsidence and take corresponding technical measures.
3) During the initial sinking operation, continuously supply compressed air to the caisson working chamber through two or more air pipes to ensure safety and normal construction.
4) Excavate in layers to prevent the bottom from being dug too deep or the cutting edge from being excavated too fast, which may cause sudden sinking and injury. When excavating, no one is allowed to work or pass under the cutting edge or supports to avoid injury from rapid soil cutting at the cutting edge.
9 Conclusion
The construction technology of steel open caissons is increasingly applied in projects with poor geological conditions and high water levels. With the continuous development of construction technology, the support area is gradually increasing, the operation technology is becoming more mature, and the application fields are becoming wider. Compared with steel sheet pile construction and concrete open caissons, it has many advantages such as short construction period and low cost, and is worth promoting.