The first phase of a sewage treatment plant covers an area of 15.48 hectares, with a daily treatment capacity of 100,000 tons; the newly built coarse screen room and inlet pump house is an irregular open caisson, shaped like the Chinese character "凸" (convex). The caisson wall thickness is 1 meter for the outer wall and 0.6 meters for the inner wall. The caisson is constructed to a height of 14 meters during the sinking stage. The cutting edge elevation is -3 meters at the time of construction, and the final sinking elevation is -16.5 meters, with a sinking depth of 13.5 meters. The caisson has a plan area of 340 m², a total soil excavation volume of 4590 m³, and a sinking weight of 3158 tons. Due to the "凸" shape, the center of gravity is obviously offset, and the sinking coefficient is large, making it very sensitive to uneven settlement of the foundation. Slight deficiencies in construction measures may cause distortion, deformation, tilting, and cracking of the caisson due to uneven stress (see figure below).

 

　　The shallow sedimentary soil at the site is mainly marine mucky soil, with high water content and high compressibility, and is widely distributed. Therefore, the geomorphological type belongs to the Quaternary coastal mucky plain. The site was originally a silt tidal flat, later artificially enclosed into aquaculture fish ponds, mainly for shrimp and fish farming. The northeast periphery is a newly built seawall in recent years, so the site is not affected by seawater tidal fluctuations.

 

　　According to the geotechnical investigation report, the geological layers from top to bottom are:

 

　　Surface layer: site backfill pond slag layer: the top elevation of the current pond slag layer is 2.5 meters, relative elevation -1.4 meters; layer 0;

 

　　Plain fill: locally formed by recent artificial backfill, composed of block stones, crushed stones, and cohesive soil, in a loose state, with fly ash below. The layer thickness is 0.5m-1.0m, and the bottom elevation is 0.86m; layer 1b: fly ash, layer thickness 0.42m, bottom elevation -0.44m; layer 2a mucky silty clay: layer thickness about 1.9m, bottom elevation -2.34m, flow-plastic, saturated, high compressibility, characteristic value of foundation bearing capacity 50Kpa; layer 2b mucky silty clay: layer thickness about 5.3m, bottom elevation -7.64m, flow-plastic, saturated, high compressibility, characteristic value of foundation bearing capacity 55Kpa; layer 2c mucky clay: layer thickness about 6.9m, bottom elevation -14.54m, flow-plastic, good clay plasticity, high compressibility, characteristic value of foundation bearing capacity 50Kpa; layer 3 silty sand: layer thickness 5m, bottom elevation -19.54m, medium dense, poor sand sorting, rapid shaking reaction, characteristic value of foundation bearing capacity 140Kpa; layer 4a mucky silty clay, flow-plastic, general clay plasticity, high compressibility.

 

　　Hydrological conditions: The groundwater at the site is of the pore phreatic water type, mainly occurring in the layer 2 mucky soil, recharged by atmospheric precipitation and seawater. Deep confined water mainly occurs in the layer 3 silty sand. According to geological data, the soil conditions for caisson sinking are extremely poor: the soil has high water content, high compressibility, and low bearing capacity. The bearing layer is the layer 2c mucky clay, which is 1.9m above the top of layer 3 silty sand.

 

 

　　The original design of the caisson consists of four sections. After excavating the surface fill, the ground elevation corresponds to the elevation of the third caisson section. Therefore, the fourth section is no longer considered part of the caisson structure; it will be constructed after the three sections are sunk in place and the bottom is sealed. To reduce the weight of the first concrete pour and avoid excessive or uneven settlement causing concrete cracking, the first section is the cutting edge (height 3.85m). After the concrete strength reaches more than 70% of the design strength, the second section of the side wall (height 5m) is poured, followed by the third section of the side wall (height 5.15m). After all three sections of caisson concrete are completed, the caisson is sunk in place in one operation.

 

　　4. Caisson Sinking Construction

 

　　4.1 Due to the poor geology of the project, the surface backfill and fly ash layer are excavated down to layer 2a. Before the first caisson section is constructed, the foundation at the cutting edge must be treated. At the same time, to reduce the caisson sinking depth and lower the construction work surface, the cutting edge elevation is excavated to -4 meters. The foundation at the cutting edge is treated by laying a 1-meter-thick sand cushion and a 0.25-meter-thick C25 concrete cushion.

 

　　4.2 According to the structural characteristics of the caisson, the drainage sinking method is adopted in the initial stage. The water level difference causes part of the groundwater to seep into the caisson. Based on conventional experience, the ratio of the disturbance range around the caisson to the sinking depth is 1:1. During construction, close attention should be paid to timely backfilling and compaction with local sand or clay to prevent surface water from entering the disturbed soil and causing larger-scale collapse, while also ensuring the safety of the surrounding ring road. During drainage sinking, to prevent disturbance of the soil outside the caisson, the soil excavation inside the caisson should be effectively controlled: excavate the middle first and then the sides, properly retain the soil under the large beams and partition walls, and form seven small pot bottoms to prevent sudden sinking caused by extremely poor soil bearing capacity. Because the coarse screen room caisson is "凸"-shaped, the center of gravity is obviously offset, making the large end easy to sink and the small end difficult to sink. Therefore, during sinking, the principle of correcting the small end first should be followed: sink the small end first by 20-30 cm, then sink the large end by 20-30 cm to level, and repeat until final sinking.

 

　　4.3 Final sinking of the caisson: when the cutting edge elevation is 0.5 meters from the design elevation, the final sinking stage begins. Soil excavation at the bottom of the caisson should not form a pot bottom; instead, soil should be excavated quantitatively along the cutting edge, not exceeding 0.10 meters, so that the mud surface at the bottom forms an inverted pot bottom. Because the caisson is filled with water and the buoyancy is large, the caisson will not sink by itself after quantitative soil excavation under the cutting edge tread. At this time, the water level inside the caisson can be appropriately lowered to reduce buoyancy and increase the sinking force, and the caisson will sink steadily. Final sinking sequence: quantitative soil excavation → dewatering inside the caisson → observation → water addition inside the caisson. Through three to four cycles, the caisson elevation reaches the design requirements. When the caisson is 0.10 meters short of the design elevation, stop construction and observe the sinking amount. By flushing mud and leveling the pot bottom, if the cumulative self-sinking amount within 8 hours is not more than 0.1 meters, the caisson can remain stable; after 12 hours, it is basically stable, and then riprap and bottom-sealing concrete construction can begin.

 

　　4.4 Construction effect: The total sinking time was 19 days. Throughout the construction process, operations were strictly controlled according to the construction plan, with the maximum sinking rate of the caisson at 3 cm/hour and the maximum cutting edge height difference controlled within 36 cm. During sinking and bottom sealing, there was no deformation, tilting, or cracking of the caisson; no flow lines in the caisson structure and no water seepage in the bottom slab, achieving good results. After the caisson was finally sunk in place, the longitudinal deviation of the cutting edge plan center was 3.5 cm, and the transverse deviation was 5.8 cm; the bottom elevation difference of the cutting edge between any two of the four corners was not more than 1.8 cm; the average elevation of the cutting edge was 1.3 cm, all meeting design and specification requirements.

 

　　5. Summary of Construction Experience

 

　　① During caisson sinking, measure and correct frequently, use measurement data to guide sinking construction, and strictly control the caisson elevation deviation within 10 cm to prevent large displacement of the caisson.

 

　　② In the later stage, during undrained sinking of the caisson, frequently observe the water level inside the caisson, replenish water in time, frequently measure the underwater mud surface, maintain smooth communication with divers working underwater, and provide timely information on the caisson pot bottom and deviations.

 

　　③ In the final sinking stage, slow down the sinking speed and adopt multiple water pumping and releasing correction methods to ensure that the caisson reaches the design elevation and that indicators such as inclination and displacement meet requirements.

 

　　④ During sinking, observe the measurement monitoring points set around the caisson daily, keep records, and take timely measures to handle any abnormalities.

 

　　⑤ Prepare concrete test blocks properly, cure companion test blocks according to specification requirements, and only pump water after the concrete test block strength reaches 70% of the design strength.