Introduction

 

　　With the large-scale development of the national economy and urban construction, land use is becoming increasingly tense, inevitably leading to underground space development. Currently, open caissons are equipped with small-volume outlet wells. If excavation and support methods are used to fabricate outlet wells, there are problems such as large excavation depth, high support costs, and many safety hazards. This paper briefly discusses the construction technology of combining outlet well and open caisson fabrication and sinking, which solves the problems of high cost and high risk in outlet well fabrication.

 

 

　　1.1 Domestic Status Quo

 

　　For the construction of structures combining open caissons and outlet wells, the current domestic construction method is to first sink the open caisson into place, and then use excavation and support for the outlet well. This construction method has problems such as large excavation depth for the outlet well, narrow construction site, high support costs, and many safety hazards.

 

　　1.2 Introduction to the New Method

 

　　Based on the fabrication and sinking construction of open caissons, this paper explores a new construction technology: fabricating the open caisson and outlet well successively, and after their strength meets the sinking requirements, sinking the outlet well and open caisson simultaneously and then sealing the bottom.

 

 

　　Taking the coarse screen open caisson in the pretreatment area of the Phase II expansion of Shanghai Bailonggang Municipal Wastewater Treatment Plant as an example, this paper elaborates on the construction technology of combining outlet well and open caisson fabrication and sinking.

 

　　2.1 Project Overview

 

　　The Phase II expansion of Shanghai Bailonggang Wastewater Treatment Plant has a design service life of 50 years. The coarse screen open caisson has design dimensions of 28.4m×48.2m×22.2m. The wall thickness is 1300mm at elevations 1.4 to -2.3m, and 1600mm at -2.6 to -12.3m. The internal partition walls are 1.2m and 0.8m thick. The cutting edge of the caisson wall is 4m high, and the partition wall cutting edge is 2.5m high. The length-to-width ratio of the entire open caisson is nearly 2:1, and the height is relatively high. The outlet well has design dimensions of 14.0m×5.0m×10.7m. The wall thickness is 1000mm at elevations -6.0 to 4.7m. The cutting edge of the caisson wall is 1.0m high. The length-to-width ratio of the entire open caisson is nearly 3:1. The weight of the outlet well accounts for 1.02% of the open caisson weight, indicating it is an auxiliary counterweight structure of the open caisson.

 

　　According to the geological survey report, during the sinking of the open caisson, the cutting edge will pass through Layer ②3 gray sandy silt (prone to flowing sand, causing sudden sinking); pass through Layer ③ gray muddy silty clay in a fluid-plastic state with high compressibility, mainly containing mica, uneven soil, slightly smooth cut surface, no reaction to shaking, medium-high dry strength, and high toughness; Layer ④ gray muddy clay in a fluid-plastic state with high compressibility, mainly containing mica and organic matter, uneven soil, interbedded with thin layers of silty soil, slightly smooth cut surface, no reaction to shaking, high dry strength, and high toughness. The measured groundwater depth is 0.6 to 2.9m (elevation 4.03 to 2.54m). The height from the ground to the groundwater level below the bottom-sealing concrete of the open caisson is 17.1 to 18.4m, varying with tides, precipitation, seasons, and climate. The head of the slightly confined water in Layer ⑤32 is about 7.6m deep (elevation -2.1m).

 

　　2.2 Construction Technology

 

　　After the first sinking of the open caisson is completed, the next fabrication step is carried out. After the open caisson fabrication is completed and stable (the open caisson will continue to sink during fabrication), the outlet well is fabricated. When the open caisson concrete reaches 100% of the design strength and the outlet well concrete reaches 80% of the design strength, the sinking requirements are met. Since the weight of the outlet well accounts for only about 1% of the open caisson weight, it is required that the cutting edge of the outlet well always remains semi-suspended during the entire sinking process, ensuring that part of the outlet well's self-weight is transmitted to the open caisson through the connection.

 

　　2.3 Sinking and Stabilization

 

　　The sinking coefficient and stabilization coefficient are not specifically introduced here; refer to relevant specifications and requirements for open caisson design and construction.

 

　　A: When the open caisson strength reaches 100% and the outlet well strength reaches 85%, sinking can proceed. Calculated sinking coefficient: kc1=1.04>1. By removing part of the soil under the internal partition wall cutting edge, the open caisson will sink naturally, with the cutting edge cutting into the soil. The required sinking range is (1.05~1.25).

 

　　B: Sinking stabilization coefficient:

 

　　When the open caisson is less than 2.0m from the final position, adopt symmetrical compartmentalized soil flushing. Number the 8 compartments of the open caisson. Stop flushing soil in the outer 4 compartments. Flush the soil in the middle 4 compartments to 0.2m above the partition wall cutting edge. Monitor the sinking of the open caisson (at this time, the open caisson has not reached the final position, and the distance between the soil in the outer 4 compartments and the cutting edge increases). Maintain the relative height of soil in each compartment, and simultaneously flush soil in all 8 compartments to prepare for bottom sealing. When the open caisson is 0.15m from the design elevation, stop flushing soil. The wall cutting edge may cut into the soil to a depth of 1.5m. Calculate the sinking stabilization coefficient kc':

 

　　kc'=G/(F1+N)=0.863, meeting the sinking stabilization requirement.

 

　　The sinking stabilization coefficient range is 0.8~0.9. When the open caisson reaches the final position, it will stabilize at the design elevation. When the open caisson is about to reach the final position, adopt less excavation and frequent monitoring to ensure the sinking meets the design requirements.

 

　　2.4 Construction Key Points

 

　　After the secondary fabrication of the open caisson is completed and stable, fabricate the outlet well cushion (the cushion bearing capacity must meet the fabrication needs of the outlet well and ensure the sinking of the outlet well during fabrication is less than 15mm). After the strength meets the sinking requirements, carry out sinking construction. During sinking, ensure that the cutting edge of the outlet well always remains semi-suspended (if the soil under the outlet well cutting edge is compacted, the outlet well will bear part of the open caisson weight, possibly causing tension or shear failure at the connection between the outlet well and the open caisson; if the soil under the outlet well cutting edge is fully suspended, soil outside the outlet well cutting edge will flow into the outlet well).

 

　　2.5 Advantages of Simultaneous Sinking Construction

 

　　Compare the excavation and support scheme with the simultaneous fabrication and sinking scheme. The outlet well is only 5.0m from a 35,000-volt high-voltage cable to the north and only 6.0m from a road to the north. The bottom elevation of the outlet well base slab is -6.0m, the ground elevation is 4.7m, and the depth is 10.7m. The entire construction site is narrow.

 

　　Using the excavation and support scheme: the site is narrow. After the open caisson sinks, the soil around the outlet well collapses. The soil is loose with high water content and high groundwater level. First, use Larsen steel sheet pile support or cement mixing pile with H-beam support, layered excavation and layered horizontal support. The large excavation depth and loose soil result in high lateral pressure. The foundation pit operation plane无形间 increases the foundation pit excavation plane, and the support cost is high. The shallow groundwater depth requires wellpoint dewatering, and sumps are set in the pit (to prevent rainwater from soaking the pit bottom). The large pit depth makes the soil prone to collapse, posing high safety hazards, high risks, and high construction difficulty.

 

　　Using the simultaneous sinking of outlet well and open caisson scheme: no foundation pit excavation or support is required, reducing site requirements. During sinking, strictly control the outlet well cutting edge to remain semi-suspended, and carry out dry bottom sealing and base slab fabrication simultaneously with the open caisson.

 

　　Advantages compared with the new construction method: no excavation or support required, short construction period, low safety hazards, controllable construction process and quality, low investment cost, and significant benefits.

 

 

　　Compared with the open caisson, the outlet well has a small volume. The open caisson can carry the outlet well during sinking, always keeping the outlet well cutting edge semi-suspended, and the construction process and quality are controllable. Especially under poor working conditions, high risk sources, narrow sites, and difficult and costly excavation and support, the advantages of this construction technology are more evident. This method is of great significance for safe construction, cost saving, and quality control.