A caisson is a structure used as a deep foundation in water supply and drainage, bridge engineering, and industrial and civil construction. It is a type of underground structure made of reinforced concrete in the shape of a shaft on the ground to serve as the support for the foundation pit wall. Under the protection of the shaft wall, soil is excavated inside the shaft by machinery and manual labor, allowing it to sink into the soil under its own weight. It has advantages such as good structural rigidity, simple construction, safety and reliability, investment savings, and accelerated construction progress, and has been widely used in foundation engineering. Although caissons have these advantages, any slight negligence or poor quality control during construction will lead to a series of quality problems, which are extremely difficult and costly to rectify, greatly affecting the normal use of the project. Based on my construction experience, the following focuses on the problems commonly encountered during an important process of caisson construction (the excavation and sinking process), analyzes their causes, and formulates corresponding treatment measures.

 

    Excavation and sinking of caissons is a very important process in caisson construction and a link that must be taken seriously in construction. After the concrete shaft wall of the caisson is completed and its strength reaches 100% of the design strength, excavation and sinking can proceed. Before excavation, an excavation plan must be prepared in advance, and construction personnel and mechanical equipment must be reasonably allocated. In the initial sinking stage, the depth of the shaft into the soil is not large, the soil resistance encountered by the sinking shaft is relatively small, and because most of the caisson is still above ground, the lateral constraint of the soil is very small, and stability is poor, so the caisson is most prone to deviation and tilting. In this stage, the excavation procedure and depth should be strictly controlled, and uniform excavation should be noted. For large shaft structures, multiple working faces must be arranged as needed, and excavation should be symmetrical and uniform at the same time. In reality, it is impossible to sink vertically and uniformly during excavation; each sinking inevitably involves some tilting. When excavation continues, the amount of excavation can be increased on the side opposite to the tilt of the shaft. In the initial stage, the plan position of the caisson should be checked frequently, marks should be made in both longitudinal and transverse directions, and large tilts should be prevented at all times. In the intermediate stage, difficulty in sinking may occur, but after the upper shaft section is extended, sinking will become obvious again, and tilting accidents may still occur. When sinking reaches the later stage, because a large part of the caisson shaft has entered the soil and is constrained laterally by the soil, the friction resistance increases, and the caisson will experience difficulty in sinking, while the possibility of tilting becomes very small. Therefore, the main problems in caisson excavation and sinking are deviation and difficulty in sinking. The following measures can be taken to address these two problems.

 

 

    1.1 Cause Analysis. The causes of caisson deviation may include skewing during the fabrication of the caisson, uneven excavation, uneven hardness of the soil layers, improper temporary soil stacking outside the shaft, or obstacles encountered at part of the caisson bottom, all of which can cause deviation.

 

    1.2 Corrective Measures

 

    ① During the fabrication of the cutting edge, the foundation ground (sleepers) must be leveled with instruments, and the foundation ground (sleepers) under the cutting edge around the perimeter must be of equal thickness and quantity, uniform and consistent.

 

    ② Excavate more soil on the side that has sunk less, and excavate less or no soil on the side that has sunk more.

 

    ③ Adopt measures such as asymmetric loading, asymmetric water jetting, and applying lateral forces.

 

    ④ When small isolated rocks are encountered, the soil around the obstacle can be excavated and the rock removed; when layered large cobbles are encountered, the covering mud and sand can be cleared first, then loose or weak spots can be found and removed by digging and prying; for larger cobbles, in the case of no dewatering, a suction dredger with a diameter larger than the cobble can also be used to suck them out.

 

    ⑤ According to different soil conditions, adopt different excavation sequences and layered excavation to make excavation symmetrical and uniform, so that the cutting edge is uniformly stressed and the caisson sinks evenly and vertically and smoothly. For soft soil, first excavate the soil layer in the middle of the caisson (about 40–50 cm deep per layer), and retain a soil berm around the cutting edge so that the caisson sinks by squeezing the soil; for medium-dense soil, if the caisson still sinks very little after the soil berm at the cutting edge is removed, the soil berm can be uniformly thinned in layers from the middle toward the cutting edge to make the caisson sink smoothly; for soil layers with uneven hardness, first excavate the hard side, then the soft side; for quicksand layers, excavate only the middle and not the perimeter; for hard soil layers, the cutting edge can be hollowed out in sections according to the sequence of removing the support sleepers, and immediately backfilled with sand and gravel; after the last few sections (i.e., where the positioning support sleepers are located) are hollowed out and backfilled, the backfill material is gradually excavated in layers to allow uniform sinking. If the caisson tilt is affected by the direction of groundwater, first excavate the soil on the downstream side, then the upstream side.

 

    ⑥ The soil excavated from the caisson should preferably be transported away promptly; even if conditions do not permit, it should be moved to a location outside the influence range around the shaft wall and stacked uniformly around the perimeter.

 

    ⑦ Strengthen measurement and observation during sinking, with no fewer than 2 observations per shift; any tilt found (plumb bob deviation of 5 cm) should be corrected promptly.

 

 

    2.1 Cause Analysis

 

    ① The friction resistance of the shaft wall is too large, exceeding the self-weight of the caisson.

 

    ② No soil is excavated at the tip of the cutting edge or the depth of soil cutting in the direction of the cutting edge is insufficient, resulting in excessive frontal resistance.

 

    ③ Isolated rocks or large stones and other obstacles may be encountered at the bottom of the cutting edge, causing the caisson to be locally stuck, or the cutting edge may be compacted by sand and gravel.

 

    ④ In soil layers with high friction resistance, no friction-reducing measures have been taken, or the friction-reducing measures have been damaged, increasing the lateral friction resistance.

 

    ⑤ When sinking in soft cohesive soil layers, if sinking is stopped for too long for some reason, the lateral pressure increases, causing sinking to be too slow or to stop.

 

    2.2 Sinking Assistance Measures

 

    ① The formwork surface used for the caisson must be flat and smooth, with tight joints during fabrication, and the height difference between adjacent blocks must meet requirements.

 

    ② Use the weighting method, i.e., lay a platform on top of the caisson and place heavy objects such as soil, sandbags, or stones on it, so that the caisson has sufficient sinking self-weight.

 

    ③ In soft cohesive soil layers, for caissons with a small sinking coefficient, adopt continuous excavation and continuous sinking, and do not allow excessively long interruptions.

 

    ④ Use the water jetting method, i.e., symmetrically embed water jet pipes at different heights in the shaft wall cavity, with water jet nozzles opening upward on the outside of the shaft wall; when sinking is slow or stops, use high-pressure water jetting to reduce the friction resistance between the shaft wall and the soil layer.

 

    ⑤ Use the mud lubrication jacket method, i.e., fill the gaps around the shaft wall with highly thixotropic mud (20% bentonite, 5% caustic soda, 75% water) or yellow mud slurry to form a lubricating mud jacket, which can greatly reduce the friction resistance acting on the shaft wall; strengthen management to prevent mud loss.

 

    ⑥ For caissons sunk by the undrained method, the pumping method can be used to pump out part of the water from the shaft to reduce buoyancy and increase downward pressure to make the caisson sink.

 

    From the above cause analysis and treatment methods for the problems encountered, it can be seen that in the caisson construction process, not only must quality be well controlled in the fabrication of the shaft, but quality must be more strictly controlled in the excavation and sinking stage, and construction must be carried out strictly in accordance with design and specifications. Once negligence occurs in this stage, or problems are not effectively treated in a timely manner, greater waste and serious consequences will result. The problems covered in this article are those I have experienced in construction and can serve as a reference.