1. Overview

 

　　The source storage well of an irradiation center is used to “store” radioactive sources, and its depth is determined by the source loading amount and the height of the source rack. The source storage well is located in the irradiation room and is a fully underground reinforced concrete structure. Different construction methods can be adopted according to different geological conditions, such as open caisson or direct excavation. Generally, the direct excavation construction method is suitable for rock sites, while the open caisson construction method is suitable for cohesive soil sites. Below, the author gives a brief introduction to the open caisson construction method in combination with an actual project.

 

　　2. Open Caisson Construction Method

 

　　An open caisson is a deep foundation sunk into the ground by cast-in-place construction and soil excavation and sinking. It is generally divided into circular and rectangular types according to cross-section. As part of the source storage well, the open caisson generally has a rectangular cross-section. It acts like the “outer shell” of the source storage well, bears the external water and earth pressure, and serves as the outer formwork for the secondary casting of the source well.

 

　　The open caisson construction method has a small excavation volume and causes little disturbance to the surrounding soil, making it suitable for operation on narrow construction surfaces. The source storage well of an irradiation center in Shanghai adopted the open caisson construction method.

 

　　The calculation of an open caisson is relatively complex and requires sinking calculation, anti-floating stability checking, well wall reinforcement calculation, cutting curb calculation, and bottom-sealing concrete thickness calculation.

 

　　2.1 Sinking Calculation

 

　　Sinking should be designed based on self-weight sinking. The open caisson relies on its own weight to sink and must overcome the friction between the well wall and the soil and the buoyancy of water.

 

　　Sinking coefficient: k0=(Gk-Fg)/Tf

 

　　Where GK——standard value of the well body self-weight (KN);

 

　　Fk——standard value of groundwater buoyancy during sinking (KN);

 

　　Tf——standard value of total friction resistance of the well wall (KN);

 

　　k0——sinking coefficient;

 

　　The Shanghai Code for Design of Foundation Engineering DGJ08-11 stipulates that k0 should be taken as 1.05~1.25. After calculation, the sinking coefficient k0 of the open caisson of this irradiation center is 1.12, which meets the construction requirements.

 

　　2.2 Anti-floating Stability Checking

 

　　The anti-floating stability checking includes checking at two stages: sinking and bottom sealing, and service.

 

　　DGJ08-11 requires that, without considering the friction resistance of the well wall, the anti-floating stability coefficient may be taken as 1.0~1.05.

 

　　Anti-floating stability coefficient: kf=(Gk+Tf)/Fk

 

　　2.2.1 Sinking and Bottom Sealing Stage

 

　　After the open caisson is sunk to the design elevation and the bottom-sealing concrete is poured, the design conservatively does not consider the friction resistance of the well wall. Gk is taken as the standard value of the self-weight of the open caisson and the bottom-sealing concrete, and the groundwater level is taken as the initial sinking elevation of the open caisson, -2.200 m. After calculation, Kf is 1.13, which meets the requirements.

 

　　2.2.2 Service Stage

 

　　In the design, the friction resistance of the well wall is not considered. Gk is taken as the standard value of the self-weight of the open caisson, the secondary cast source well, and the bottom-sealing concrete, and the groundwater level is taken as ±0.000 m. After calculation, Kf is 1.14, which meets the requirements.

 

　　2.3 Calculation of Open Caisson Well Wall

 

　　2.3.1 Checking of Vertical Reinforcement in Well Wall

 

　　The most unfavorable condition for tension in the well wall occurs when the open caisson is sunk to the design elevation. Because the upper well wall is “clamped” by soil and the soil under the cutting curb has been excavated, a relatively large tensile force may occur in the well wall. To calculate the maximum vertical tensile force generated in the open caisson, it is assumed that the friction resistance of the well wall is distributed in an inverted triangle, so the most dangerous section is at half the height of the open caisson. The design value of the total tensile force Nmax is 1/4G, where G is the design value of the total weight during sinking of the open caisson (KN), and the partial factor for self-weight is taken as 1.2. After checking, the actually provided vertical reinforcement meets the requirements.

 

　　2.3.2 Calculation of Horizontal Reinforcement in Well Wall

 

　　It is divided into two stages: sinking and bottom sealing, and service. However, in the service stage, the secondary cast source well and the open caisson act together, so the sinking and bottom sealing stage should control the reinforcement design, and the internal force calculation for this stage is also used as the basis for horizontal reinforcement in the design.

 

　　Because the water and earth pressure acting on the well wall gradually increases along the depth of the open caisson, the open caisson is divided into three sections along its height for reinforcement in the design. DGJ08-11 recommends using active earth pressure. Considering that in the service stage, the secondary cast source well and the open caisson jointly bear the external water and earth pressure, active earth pressure is adopted in the design, but the cohesion of cohesive soil is ignored, which is also an amplification of the active earth pressure. The groundwater level is taken as -2.200 m, and a ground surcharge of 20 KN/m2 is considered.

 

　　The open caisson of this irradiation center is rectangular, with an outer plan dimension of 5x5.8 m. The internal force is calculated as a “planar frame”.

 

　　2.4 Calculation of Cutting Curb

 

　　The cutting curb is the “sharp corner” part at the lowest end of the open caisson well wall. Its function is to cut into the soil when the open caisson sinks. The cutting curb is the part where the force is most concentrated in the open caisson and must have sufficient strength to avoid deflection or damage.

 

　　2.4.1 Calculation of Vertical Internal Force of Cutting Curb

 

　　On the one hand, the cutting curb can be regarded as a cantilever beam fixed at the root of the cutting curb, with the beam length equal to the height of the inclined surface of the cutting curb; on the other hand, the cutting curb can also be regarded as a closed horizontal frame. When calculating the vertical internal force, only the action of the cantilever beam is conservatively considered, that is, it is calculated as a cantilever beam, and two unfavorable conditions are considered: inward deflection and outward deflection of the cutting curb.

 

　　Condition 1: Outward deflection of the cutting curb (arrange inner vertical reinforcement):

 

　　When the open caisson sinks and the cutting curb has been embedded in the soil, the main forces on the cutting curb are shown in Figure 1 and can be calculated by the following formulas:

 

　　V1=G-V2

 

　　V2=[(b/2)/(a+b/2)]G

 

　　H=V2tg(α-δ)

 

　　Where: V1——design value of reaction on the cutting curb tread per unit perimeter (KN/m);

 

　　V2——design value of reaction on the inclined surface of the cutting curb per unit perimeter (KN/m);

 

　　H——design value of self-weight of the open caisson per unit perimeter during sinking (KN/m), with the partial factor for self-weight taken as 1.2;

 

　　α——angle between the inclined surface of the cutting curb and the horizontal plane (°);

 

　　δ——friction angle between soil and concrete cutting curb (°), which may be taken as the standard value of the internal friction angle of the soil.

 

　　According to the external forces V1, V2, and H, the axial force F, horizontal shear force V, and moment M acting at point O at the root of the cutting curb can be obtained. The vertical reinforcement required on the inner side of the cutting curb is calculated from F and M; the shear reinforcement arranged along the full height of the cutting curb is calculated from V.

 

　　Condition 2: Inward deflection of the cutting curb (arrange outer vertical reinforcement):

 

　　When the open caisson is sunk to the design elevation and the soil under the cutting curb has been completely excavated, the self-weight of the open caisson is entirely borne by the friction resistance of the well wall. At this time, the water and earth pressure (PW, PE) on the outer side of the well wall causes the cutting curb to deflect inward. The cutting curb is calculated as a cantilever beam bearing the external earth and water pressure.
 

　　(PE1, PE2——earth pressure; PW1, PW2——water pressure)

 

　　According to the water and earth pressure, V and M at point O are obtained. The vertical reinforcement required on the outer side of the cutting curb is calculated from M; the shear reinforcement of the cutting curb is checked from V.

 

　　2.4.2 Calculation of Horizontal Internal Force of Cutting Curb

 

　　As a closed horizontal frame, the external force acting on the cutting curb should be multiplied by the distribution coefficient β, and β is calculated by the following empirical formula:

 

　　β=h4/(h4+0.05l4)

 

　　Where: h——height of the inclined surface part of the cutting curb;

 

　　l——calculated span of the short side of the open caisson;

 

　　After calculation, the β of the cutting curb of the open caisson of this irradiation center is 0.02, which proves that the external force distributed to the horizontal frame is very small. The actual horizontal reinforcement is the same as the horizontal reinforcement of the upper part of the cutting curb.

 

　　There are also some structural requirements for the cutting curb. The bottom plane of the cutting curb is called the tread, and the tread width b is generally 150—300 mm, with the inclined surface angle α≤60°. When the soil layer is soft, b is taken larger and α smaller; when the soil layer is hard, b is taken smaller and α larger. The groove on the cutting curb is designed to provide a better connection between the bottom-sealing concrete and the open caisson wall, and the groove depth is generally 150—250 mm.

 

　　2.5 Calculation of Bottom-Sealing Concrete Thickness

 

　　The bottom-sealing concrete can be considered as a two-way slab simply supported on four sides under uniformly distributed load. According to DGJ08-11, the thickness of plain concrete for bottom sealing can be calculated as ht=1384 mm, and the actual thickness adopted is 1600 mm.

 

　　The design of the open caisson construction method should fully understand the actual site environment and consider unfavorable factors such as groundwater level, stratum soil stability, and quicksand. The design and calculation should be comprehensive and accurate, and close cooperation with the construction party is required to ensure the safety and reliability of the open caisson construction.