Caissons (also known as caissons) are widely used in various specialties in urban construction due to their minimal impact on the surrounding environment and relatively small site requirements. For example, during urban pipeline construction, caissons can be used as working pits or inspection wells to avoid excessive land occupation caused by direct excavation that may affect road traffic or inability to excavate due to surrounding buildings. However, caissons are more complex than general excavation projects, and the problems encountered are more difficult to handle. How to avoid these problems or address them in the early stages is key to the success of caisson construction.

 

 

　　A caisson foundation is a shaft-like structure that is sunk to the design elevation by excavating soil from within and relying on its own weight to overcome the friction resistance of the shaft wall, then sealed with concrete at the bottom and filled in the shaft to become the foundation of a building. Caisson foundations can have a large embedment depth, strong integrity, good stability, and a large bearing area, capable of withstanding large vertical and horizontal loads; the caisson serves both as a foundation and as a cofferdam structure for retaining soil and water during construction. The construction process is not complicated. The caisson construction sequence includes pre-construction preparation—three connections and one leveling (ground treatment), excavation of the foundation pit, caisson fabrication, caisson sinking, caisson bottom sealing, and capping, among other main construction procedures. The following will detail the caisson construction technology:

 

　　2 Caisson Fabrication

 

　　2.1 Preparation Work

 

　　First, the caisson construction site must be leveled, with the leveling range extending 1m to 3m beyond the outer side of the caisson. After leveling, setting out and positioning should be carried out accurately, and formal construction can only begin after acceptance.

 

　　2.2 Laying Timber Mats, Erecting Formwork for the Bottom Section of the Caisson, and Tying Reinforcement

 

　　Caissons are usually fabricated in situ, and the following different methods can be used:

 

　　2.2.1 Timber Mat Method

 

　　First, spread a sand cushion layer of about 0.5m thickness on the site (to facilitate the extraction of timber mats). Symmetrically lay appropriate timber mats at the cutting edge on the sand cushion, fill sand between the mats, then erect formwork according to design dimensions and pour the first bottom section of the caisson.

 

　　2.2.2 No-Mat Method

 

　　On uniform soil layers, the no-mat method can be used. Pour a layer of concrete of the same thickness as the caisson wall to replace the timber mats and sand cushion. The poured concrete is located below the cutting edge to ensure the caisson remains vertical during fabrication and initial sinking.

 

　　2.2.3 Earth Mold Method

 

　　If the foundation is uniform cohesive soil, the earth mold method can be used to fabricate the caisson. There are two types: filled earth inner mold: first, use clay or silty clay to fill and compact in layers according to the shape and dimensions of the cutting edge and partition walls, and trim the surface to match the design. Excavated earth inner mold: when the groundwater level is low and the soil quality is good, the excavated earth inner mold can be used. At the cutting edge, excavate the foundation trench according to design dimensions, using the foundation soil as a natural mold.

 

　　2.3 Pouring Concrete

 

　　When pouring caisson concrete, it should be done symmetrically and uniformly to avoid uneven settlement of the ground causing cracks in the caisson; it is best to complete the pouring continuously in one go.

 

　　2.4 Removing Formwork and Timber Mats

 

　　When the concrete strength reaches 25% or more of the design strength, the inner and outer side formwork can be removed; when it reaches 75% of the design strength, the formwork at the bottom of the partition walls and the inclined surface of the cutting edge can be removed. When the concrete reaches the design strength, the timber mats can be removed. The last four timber mats to be removed are called positioning mats and are usually marked with red paint. The sequence for removing timber mats is: first the inner wall, then the outer wall; first the short sides, then the long sides. For the mats under the long sides, remove every other one, centered on the positioning mats, symmetrically from far to near. Before removing the mats, the foundation soil under the mats can be loosened; after each removal, backfill and compact with sand at the cutting edge.

 

　　3 Caisson Sinking

 

　　Caisson sinking involves uniformly removing soil inside the caisson by mechanical or manual methods to eliminate or reduce the frontal resistance of the soil under the cutting edge; sometimes methods to reduce the friction between the shaft wall and the surrounding soil are also used, so that the caisson sinks gradually from the ground into the underground by its own weight. The sinking construction methods include: drainage sinking, undrained sinking, or central island sinking, etc.

 

　　For caissons on land, drainage sinking is predominant, and methods such as mechanical grabbing with manual cooperation, or manual excavation with mechanical vertical transport in open excavation with drainage, as well as drainage sinking using hydraulic machinery instead of manual excavation, can be used. Undrained sinking with air-lift dredging and diver cooperation or mechanical grabbing and diver cooperation can also be used. For caissons on artificial islands in rivers, floating caissons, or caissons in silty sand layers at waterside beaches, undrained sinking with underwater soil removal should be the main method. It can be seen that whether to adopt drainage or undrained sinking depends on the location of the caisson and the soil layers it passes through.

 

　　For caissons with greater depth and larger plan dimensions, in the later stages of sinking, the friction resistance of the outer wall will increase significantly, or for caissons with thinner walls, lighter self-weight, and smaller sinking coefficients, after sinking to a certain depth, due to increased friction resistance of the outer wall, sinking becomes difficult and may even stop. Before design and fabrication, consideration should be given to installing mud lubrication sleeves on the outer side of the shaft wall, or embedding pipes in the shaft wall (outer side) to reduce the friction resistance of the outer wall by high-pressure water jetting or compressed air injection, as well as measures such as adding counterweights beside the shaft wall or lowering the groundwater level inside the shaft wall during sinking to assist the caisson in sinking smoothly to the required position.

 

 

　　When the caisson sinks to within the required range of the design elevation, and after settlement observation, the settlement rate (cumulative self-sinking of the caisson within 8 hours not exceeding 10mm) is within the allowable range, bottom sealing can be carried out. Bottom sealing is divided into dry sealing and underwater sealing. The following will detail the dry bottom sealing construction technology:

 

　　4.1 Preparation Work

 

　　(1) Before sealing the caisson bottom, the reserved reinforcement at the bottom groove of the shaft wall and the partition walls need to be roughened, and the sludge in the groove at the cutting edge should be cleaned out.

 

　　(2) For caissons constructed under wellpoint dewatering conditions, before bottom sealing, large stones or other blocks should be used to pad the inclined surface of the cutting edge, and the wellpoint dewatering equipment should be checked to ensure it is in good condition and can maintain continuous pumping. For caissons sunk with thixotropic mud slurry wall protection, when sunk to the design elevation, immediately prepare for the replacement of thixotropic mud slurry.

 

　　(3) Prepare various materials and equipment (including concrete) required for bottom sealing, and formulate a bottom sealing plan. Before the caisson sinks to the design elevation (with a certain allowance for over-sinking), bottom sealing should be carried out promptly. Delays may lead to changes in conditions, deviations, or large amounts of soil rushing into the shaft, causing great difficulties for dry bottom sealing. Therefore, preparation for bottom sealing should be done promptly.

 

　　(4) Before bottom sealing, a sump well should be constructed inside the caisson to be pre-placed at a certain position (in each compartment) at the bottom for pumping out groundwater after sealing. Its depth and size should meet the water pump suction requirements.

 

　　4.2 Dry Bottom Sealing Construction

 

　　Before bottom sealing, according to design specifications, generally a layer of crushed stone (or gravel sand) is first spread. After leveling, plain concrete is poured for bottom sealing (also called cushion layer). Concrete pouring should proceed from the surrounding shaft wall towards the center. The plain concrete bottom sealing should be poured in one go, in compartments, in batches, in sections, and symmetrically, without interruption, to avoid construction joints that may cause leakage. When pouring the bottom sealing concrete, the sump well must not be blocked, ensuring that drainage inside the caisson can continue, so that the concrete is not immersed in water before final setting. The surface of the plain concrete bottom sealing should be flat. Only when the strength reaches 25% or more of the design strength can the bottom slab reinforcement be tied on top. The reinforcement must be tied properly.

 

　　5 Conclusion

 

　　(1) During caisson sinking, construction observation should be strengthened; when deviations occur, causes should be analyzed and adjustments made promptly.

 

　　(2) During caisson construction, to ensure project quality, preventive measures for common quality issues should be formulated before construction, and follow-up inspections should be conducted during construction to eliminate various quality hazards in the bud.

 

　　(3) In caisson design, sinking verification and cutting edge calculation are key, and rigorous calculations should be based on site geological conditions and structural characteristics.

 

　　(4) When there are fine sand or silty sand layers below the groundwater level, if drainage and soil extraction are used, quicksand phenomena will occur, and a large amount of sand will rush into the shaft, causing the caisson to tilt and creating great difficulties for construction.