The caisson construction method is one of the important construction methods used for building underground works and deep-buried foundation works. In water supply and drainage engineering, it is commonly used for water intake structures, drainage pump stations, large collection wells, shield and pipe-jacking working shafts, and other projects. The plan layout is mostly circular, rectangular, elliptical, prismatic, or irregular in shape. The interior of the well may be single-cell or multi-cell (i.e., with partition walls), depending on the requirements of use. Caisson construction includes several main parts such as caisson fabrication, caisson sinking, and caisson bottom sealing. Depending on different situations and conditions, either one-time fabrication and one-time sinking may be adopted, or fabrication and sinking may be carried out alternately. The caisson shaft is generally fabricated on the ground; soil is excavated inside the shaft so that the shaft, relying on its own weight to overcome the friction between its outer wall and the soil, gradually sinks to the design elevation. Then the soil layer inside the shaft is leveled, a concrete cushion and concrete base slab are cast, and the bottom-sealing work of the caisson is completed. Therefore, under conditions such as high groundwater level, large seepage volume, or the presence of confined groundwater, running sand, soft soil layers, narrow site areas, and nearby existing underground pipelines or above-ground structures, its construction advantages are even more evident. Below, based on the construction practice of the coarse screen caisson of the collection well at the wastewater treatment plant in the Taicang Port Development Zone, Jiangsu, the author briefly discusses the application performance and understanding of the open-well caisson construction method with a planar opening.

 

　　Jiangsu Taicang Port Development Zone Wastewater Treatment Plant Co., Ltd. is located in the central area of the Taicang Port Development Zone, Jiangsu, and was invested in, constructed, and operated by Zhejiang Fuchun Ziguang Environmental Protection Co., Ltd. in BOT form. The first-phase wastewater treatment capacity is 10,000 tons/day, and the long-term wastewater treatment capacity is 20,000 tons/day. The first-phase site area is more than 52 mu. The wastewater treatment portion consists of: the collection well primary pump house, grit removal and fine screen, equalization tank, aeration tank, secondary sedimentation tank, effluent stabilization tank, effluent pump house, etc. The auxiliary production portion consists of: blower room, sludge thickening tank, sludge dewatering room, power distribution room, maintenance workshop, garage, gatehouse, etc. The management and living facilities portion (mainly concentrated in the comprehensive building) includes offices, meeting rooms, activity rooms, canteen, bathrooms, chemical analysis laboratory, staff shift dormitories, etc.

 

 

　　The primary pump house is a cylindrical concrete structure with a diameter of 9 m and a depth of 11 m. The coarse screen of the collection well is 8 m deep and connected to the primary pump house, forming a convex shape in plan. The original design was for open-cut excavation (excavation depth below 12 m). Due to soil conditions, the soil below 3 m is silty mucky soil, which is highly prone to liquefaction once disturbed. During the open-cut excavation, serious collapse occurred. The site is very close to the grit removal and fine screen and the comprehensive building, and construction could not proceed. Under these circumstances, a caisson scheme was proposed. Considering the stability of the caisson during sinking, the design institute modified the primary pump house to caisson construction. After the caisson bottom sealing was stable, the coarse screen of the collection well would then be constructed by open-cut excavation.

 

　　After the bottom sealing of the primary pump house caisson was stable, during construction of the coarse screen of the collection well it was found that, because the earlier open-cut excavation had caused liquefaction of the lower soil mass, it was impossible to excavate by open cut to the design bottom elevation of the coarse screen of the collection well. Therefore, the site considered and compared multiple construction schemes to solve this construction difficulty:

 

　　Scheme 1: Driving steel sheet piles

 

　　Drive steel sheet piles as enclosure, then carry out open-cut excavation. However, the effect was not significant. Because the lower silty mucky soil had already been disturbed, driving the steel sheet piles caused further disturbance, and the steel sheet piles were stressed in a cantilever manner, with the fixed end in the lower soil mass. Liquefied soil could not serve as a fixed end, construction safety could not be guaranteed, and the project cost would also increase considerably, so this was not advisable.

 

　　Scheme 2: High-pressure grouting

 

　　Improve and reinforce the soil mass by high-pressure grouting so that the liquefied soil mass is modified and consolidated, and then carry out open-cut excavation. However, the construction period is long and the cost is high. The entire periphery of the construction face, including the open-cut slope, would need to be reinforced, and a curing period would also be required after grouting.

 

　　Scheme 3: Caisson construction scheme

 

　　The caisson construction scheme should be the best scheme: low cost, short construction period, and safe. Ultimately the caisson construction scheme was selected. However, the plan shape of the coarse screen collection well is U-shaped with one side open, so making it a caisson seemed impossible, and the geological conditions were extremely poor, making it extremely difficult.

 

　　1. Because the lower silty mucky soil had been disturbed and liquefied, possessing flowability, the friction resistance decreased and it could not bear the weight of the caisson. The elevation would be very difficult to control, and excessive sinking was highly likely. The following measures were taken.

 

　　a. Reduce the dead weight of the caisson: construct half of the caisson first, excavate 4 m first, construct the caisson below 4 m, and after bottom sealing was stable, cast the upper 4 m in place, excavate and backfill, and lower the initial sinking elevation. According to the geological conditions at the caisson location, in order to ensure balanced sinking during the initial sinking stage of the caisson, the artificial fill layer was removed. This created two favorable conditions: first, because the initial sinking stratum was mud, its water content and bearing capacity were uniform, facilitating stable initial sinking; second, the total sinking amount of the caisson was reduced by 4 m, and the upper part of the caisson did not need to be constructed as a caisson but could be cast after the caisson bottom sealing, thus reducing the dead weight of the caisson (while still meeting the sinking weight requirement) and shortening the sinking depth, speeding up construction progress, achieving two benefits at once.

 

　　Sinking coefficient formula:

 

　　K = Q/(f·h·L) > 1? (1)

 

　　Where Q — dead weight of the caisson

 

　　f — friction coefficient; for soft soil, take 9.8–11.76? kN/m2

 

　　h — maximum sinking depth

 

　　L — perimeter of the outer wall of the caisson

 

　　The friction coefficient is taken as the maximum value for soft soil. Generally, the dead-weight sinking coefficient of a structural caisson can still reach 3.0, let alone in mud, so there is absolutely no problem of delayed sinking. The existing problem is that when the sinking depth reaches the requirement, it will still continue to sink, so control measures must be taken.

 

　　b. In order to further reduce the dead weight of the caisson, first construct the outer perimeter as the caisson, and cast the internal partition plates and other parts for the second time after bottom sealing is stable, while also leaving a construction working face.

 

　　c. Local soil replacement: first excavate part of the surface mucky soil and backfill with sand and gravel to ensure stability during caisson casting, so as to avoid uneven settlement during casting that would cause structural damage, and at the same time increase friction resistance during caisson sinking.

 

　　d. In order to solve soil heave and the inability to seal the caisson bottom, the cutting edge elevation was deepened by 500 mm. When sinking a caisson in flow-plastic mud to the elevation, soil heave is very likely to occur. Once it occurs, the heaved soil often rises above the base slab height, and this part of the soil cannot be removed; once removed, the stress balance of the caisson would be broken, and the caisson would continue to sink to reach a new stress balance. Deepening the cutting edge elevation was intended to pre-set a certain amount of heave soil, and at the same time the cutting edge increased the friction area and could also bear some upward resistance. When sinking a caisson in flow-plastic mud, sudden sinking, tilting, sliding, water and mud inrush into the well, and uncontrolled excessive sinking are very likely to occur. The speed and direction of caisson sinking are extremely difficult to control, and this is a construction difficulty that has not yet been well solved.

 

　　The plan shape of the coarse screen collection well is U-shaped with one side open. The shape is complex, the forces during sinking are complex, and it must also be connected to the primary pump house, so making it a caisson seemed impossible. In order to complete the construction task safely and effectively, a comprehensive analysis of the stress conditions of the caisson was carried out, and various difficulties were solved one by one. 

 

　　a. In order to reduce the lateral earth pressure during caisson sinking, construct half of the caisson first, excavate 4 m first, construct the caisson below 4 m, and after bottom sealing is stable and the lower caisson is completed, cast the upper 4 m in place. The maximum lateral earth pressure at the bottom will be reduced by half, simplifying structural reinforcement treatment, reducing the designed wall thickness of the reinforced concrete caisson and the amount of horizontal reinforcement, and lowering project cost.

 

　　b. In order for the U-shaped plan of the coarse screen collection well to serve as a caisson and sink independently, it must first be separated from the primary pump house. A 1 m reserved zone was designed as a secondary casting area, to be cast for the second time after the caisson bottom sealing is stable. Reserved dowel bars were provided at both ends to facilitate connection of the secondary casting reinforcement and ensure integrity.

 

　　c. The plan shape of the coarse screen collection well is U-shaped. Lateral earth pressure causes excessive bending moment in the middle of the arc, leading to fracture. In order to overcome the unfavorable stress condition, one reinforced concrete support beam was added at the upper and lower parts of the opening respectively, changing the stress condition from the original cantilever slab to a simply supported slab. The lower beam can also provide certain resistance during caisson sinking, making up for the defect of no friction resistance at the opening. After construction is completed and before equipment installation, whether to chisel it out depends on the situation; if it hinders equipment installation, it shall be chiseled out.

 

　　d. Because the plan shape of the coarse screen collection well is U-shaped and serves as a caisson that can sink independently, it must first be separated from the primary pump house. A 1 m reserved zone was designed as a secondary casting area. During sinking, the caisson is subjected to lateral earth pressure N, and the entire caisson will shift toward the primary pump house, causing incorrect plan dimensions. In order to resist lateral pressure, four pulley support arms were specially added, two upper and two lower, subjected to horizontal thrusts N1 and N2 from the primary pump house, so that during caisson sinking the stress balance is always maintained and sinking remains vertical. The lower pulley support arms bear a larger force N1 and should be stronger. In order to allow the caisson to sink smoothly and reduce resistance, two No. 10 channel steels were added along the pulley path as guide rails, to be cut off before secondary casting.

 

　　e. Earth retaining at the secondary casting area: a secondary casting area was reserved, and during caisson sinking soil would intrude into it. To solve this difficulty, 5 mm thick steel plates were used as earth-retaining plates. Embedded iron plates were placed during caisson casting, and before caisson sinking the earth-retaining steel plates were fixed and welded firmly to the embedded iron plates, with angle steel stiffeners and triangular braces provided, so that they had sufficient strength to retain soil during caisson sinking without deformation. Fig. 5

 

　　Through the above detailed analysis, the feasibility of caisson construction was determined, and construction was carried out according to the above construction steps, processes, and sequence requirements, and the construction task was successfully completed.

 

　　From the foregoing construction process, it can be seen that in caisson design and construction, the bearing capacity can be adjusted through many means such as changing the cutting edge area, diameter, strength, and structural form. The methods are diverse, the process is simple, and the cost is low; it is a successful construction technique.? By improving the structure and improving the stress form, processes that were originally impossible to construct as caissons became possible. The design of the caisson must also fully take into account the actual on-site construction conditions, and must be continuously adjusted as the actual on-site construction situation evolves, by adding temporary beams and other measures to change the stress conditions, so that processes that were originally impossible to construct can be implemented.