During the sinking of a caisson, people always hope to maintain an ideal vertical sinking state, but a large number of caisson sinking practices have proven that this is unrealistic. This is mainly due to the following reasons:

 

　　1. Due to structural reasons, the weight on each side of the caisson in plan is unequal.

 

　　2. During sinking, it is impossible to excavate soil absolutely evenly on all sides inside the caisson.

 

　　3. Within the sinking depth range, the foundation soil layers have uneven thickness and different strengths.

 

　　4. On the ground surface around the caisson, additional loads may be distributed unevenly (such as formwork, scaffolding materials, reinforcing steel, and lifting equipment), causing differences in the lateral pressure acting on the outer wall of the caisson.

 

　　Therefore, during sinking, a caisson always deviates: if it does not lean to the left, it leans to the right; if it does not tilt forward, it tilts backward. This is an objectively existing and inevitable phenomenon.

 

　　First, let us clarify the relationship between sinking and rectification. To illustrate the issue:

 

　　When the caisson sinks and, due to unbalanced soil excavation inside the caisson and other effects, deviates to the left, the cutting edge A moves to position A’ and cutting edge B moves to position B’. Although the caisson has sunk a certain depth, deviation has occurred, and at this time cutting edge B’ is higher than cutting edge A’. To rectify the deviation in time, more soil is excavated inside the caisson at cutting edge B’, reducing the supporting reaction of the soil, causing cutting edge B’ to sink excessively to position B’’. At this time, cutting edge A’ also sinks, but by a smaller amount, to position A’’, and the caisson then deviates to the right.

 

　　From the above rectification process, the following conclusion can be drawn——the rectification process is the sinking process.

 

　　Since the rectification process is the sinking process, during the sinking of a caisson, rectification should be treated as the primary task and sinking as the secondary task, with rectification as the main focus and sinking as auxiliary. Large deviations must be corrected, and small deviations must also be corrected. Through continuous rectification, the caisson will sink smoothly to the design elevation.

 

　　Next, let us analyze the relationships among the various forces during the rectification process. To illustrate the issue, the three-dimensional force problem of the sinking process is simplified into a plane problem:

 

　　When the caisson deviates to the left (solid-line position in the figure), the left wall presses against the soil on the left, putting the soil into a passive stress limit state, and the left wall is subjected to passive earth pressure (EP in the figure). At the same time, the right wall tilts to the left, putting the soil on the right into an active stress limit state, and the right wall is subjected to active earth pressure Ea. When rectifying to the right (dashed-line position in the figure), the right wall instead is subjected to passive earth pressure EP’, and at the same time the left wall is also subjected to active earth pressure Ea’.

 

　　Since the caisson is also sinking during the rectification process (solid-line position → dashed-line position in the figure), the outside of the right wall is subjected to soil friction resistance FP’, and at the same time the outside of the left wall is subjected to soil friction resistance Fa’.

 

　　In the figure, W is the self-weight of the caisson during sinking, and RB is the supporting reaction of the soil under the right cutting edge. Here,

 

　　The rectification moment is:

 

　　The moment resisting rectification is:

 

　　To make the caisson rectify to the right, the following formula must be satisfied:

 

　　This is the main relationship among the various forces during the rectification process.

 

　　Once the main relationship among the various forces during the rectification process is understood, corresponding rectification measures can be adopted.

 

　　1. Methods to increase the rectification moment (referring to the left side of the inequality)

 

　　(1) Add a certain amount of surcharge on the ground on the left side of the caisson to increase Ea.

 

　　(2) Add surcharge on top of the caisson, especially on the left half, to increase W.

 

　　(3) Sling a wire rope over the top of the caisson and use a winch arranged on the right side of the caisson to pull the caisson to the right.

 

　　(4) When the caisson is sinking underwater, pumping water out of the caisson reduces buoyancy and increases the sinking force. Used in combination with the above methods, this is also very effective. (See Figure 3)

 

　　2. Methods to reduce the moment resisting rectification

 

　　(1) Excavate soil at the right cutting edge to reduce the soil supporting reaction RB.

 

　　(2) Jet water to scour the outside of the right wall to reduce the soil friction resistance FP’. (See Figure 4)

 

　　(3) Dig trenches outside the caisson wall to reduce the passive earth pressure EP’.

 

　　(4) When the caisson has bottom beams, excavate the soil under the bottom beams to reduce the soil supporting reaction.

 

　　(5) When the caisson is sunk by the air curtain method, start the air curtain on the right side to reduce FP’.

 

　　The most commonly used rectification method is to excavate the soil at the cutting edge and under the bottom beams.

 

　　Let us give an example, a plan view of the cutting edge and bottom beams:

 

　　At this time, the measured elevations of the four corners A, B, C, and D are A=-10.43 m, B=-10.31 m, C=-10.18 m, and D=-10.06 m. Comparing corner A (the lowest) with the other three corners, corner B is 12 cm higher than corner A, corner C is 25 cm higher than corner A, and corner D is 37 cm higher than corner A (see the numbers at the corners in the figure), with corner A as 0. It can be seen that side AB is lower and side CD is higher. During rectification, the line EF on the left side of diagonal BC should be used as the boundary, and the soil under the bottom beams within the shaded area and at the cutting edges of CD and BD should be excavated. First excavate the soil on the inclined surface of the cutting edge (from D→C→F and from D→B→E), and measure whether the elevation difference among the four corners has decreased. If there is slight improvement but it is still not ideal, then excavate the soil under the bottom beams within the shaded area, and again measure the changes in the elevation differences among the four corners. If the rectification effect is satisfactory, switch to excavating soil and sinking; if it still does not reach the expected value, continue excavating the soil under the cutting edges on both sides CD and BD. Generally speaking, for caissons with balanced structural self-weight and when the foundation soil layers do not vary much, this excavation rectification method can achieve relatively satisfactory results.

 

　　When the centerline of the caisson in plan is displaced (mostly caused by failure to correct the elevation difference of the cutting edge in time), the method of excavating soil unevenly inside the caisson can also be used to correct the displacement.

 

　　For example, when the north-south centerline of the caisson is displaced eastward, we can first excavate the soil at the east cutting edge inside the caisson to straighten the caisson again. After straightening, measure whether the north-south centerline still shows eastward displacement. If some correction has been achieved but it is still not ideal, repeat the above uneven excavation method 1 to 2 more times, and the centerline displacement will generally be corrected.

 

　　In short, as long as the relationships among the various forces during the rectification process are mastered and fully utilized, more and better methods will be devised to achieve the purpose of rectification.