#Article Launch Challenge# Before reading this article, we sincerely invite you to click the "Follow" button, so that you can continue to push such articles in the future, and at the same time, it is also convenient for you to discuss and share, your support is the driving force for us to keep writing
A 15,000-tonne steel caisson suffered a violent tilt on the turbulent waters of the Yangtze River, with a vertical drop of more than 13 metres from its highest and lowest points, and less than 4 metres from its lowest end. Faced with such a huge steel caisson, how should the mainland straighten it?
What is a steel caisson?
In fact, this steel caisson was used as part of the bridge pier foundation. There are usually two technical approaches to the construction of the main tower foundation, namely the piling method and the caisson method. The caisson method was chosen to construct the Hutong Bridge mainly based on the geological structure on which the bridge is located, and the area is dominated by sandy soil layers. If piling is used, the pillars must be drilled deep into the rock to ensure stability, but the sand layer is still a few hundred meters below the pier and the solid rock layer cannot be reached, which can cause the entire construction effort to be wasted.
Therefore, the caisson rule has become the most suitable choice. In contrast to piling, caisson creates a massive, downward extending vessel in a soft layer of soil, which is then filled with concrete to form a solid base. During the construction of the Hutong Bridge, this caisson was a steel structure that was carefully designed to withstand tremendous pressure and was able to maintain its structural integrity during the settlement process.
The construction of caissons is a slow and difficult process, which requires a gradual sinking in the water to a predetermined depth. First, the engineering team placed a steel caisson on the surface of the riverbed. Then, the caisson is added to its weight by filling the inside with sand and concrete, allowing it to sink gently. As the caisson sinks, divers and robots constantly remove obstacles underneath the caisson to ensure that it sinks evenly and does not tilt.
After the caisson reaches a predetermined depth, the next step is to remove the water and sediment from the inside of the caisson, and then lay a hard concrete layer at the bottom as a base, which eventually forms the pier foundation. In this way, the caisson method provides rock-solid support for the main tower of the Shanghai-Tong Bridge. Since the caisson requires long-term exposure to seawater, the team selected steel with excellent corrosion resistance and special treatment of reinforced concrete to improve its salt and alkali resistance and water resistance.
Why does the steel caisson deviate when it is so heavy?
Due to the rapid flow of the Yangtze River, the construction site was located in the tidal zone of the lower reaches of the Yangtze River, where the flow rate can be as high as 2.5 meters per second, and the river water constantly washes the sediment from the upper reaches of the riverbed, which eventually causes the caisson to gradually tilt. If the caisson continues to tilt, the stability of the entire structure will be seriously threatened, and may even lead to the failure of the entire project. They had to find a solution as soon as possible to ensure that the caisson could be safely and stably embedded in the riverbed.
What is the way to straighten the inclined steel caisson?
There are many ways to straighten the inclined steel caisson, and the common one is to adopt the correction strategy of combining excavation adjustment and lateral support technology, which is an effective means to correct the deflection foundation. It involves soil excavation on the higher side of the caisson and placement of cushion blocks on the lower blade foot side for support. In this way, as the caisson progresses deeper into the ground, the originally higher sides gradually descend to achieve equilibrium. When implementing excavation adjustments, the professional team uses advanced monitoring equipment, such as inclinometers and GPS, to track the angle and position of the caisson in real time.
There are also off-well bias, crest bias or casing methods, which combine deflection excavation and the application of lateral pressure or horizontal force to enhance the correction effect achieved by simply excavating the soil on one side. Due to the large volume of soil when excavating the outer groove, the depth of the excavation work is generally limited to about 1.5 to 2 meters, so this technology is particularly suitable for scenarios that require correction operations in deeper soils. Of course, there are also the off-well support pad method, the off-well water injection method, etc.
Continental's engineers initially opted for a bias technique that adjusted the load by pumping water on the lower side and filling the other side, with the aim of correcting the skew. Subsequently, they used the method of throwing stones and sandbags around the caisson, especially in the lower parts of the shaft, to delay the erosion of the sediment. After these steps, they moved on to solving the problem of vertical correction of the caisson.
This process involves performing sludge pumping at the bottom of the inclined side of the caisson and subsequently injecting concrete into the inner wall of the caisson to depress the angle of inclination of the caisson. As a result, previously placed gravel slides naturally to the bottom of the well, helping to stabilize and support the offset caisson.
As the caisson gradually regains verticality, a heavy concrete foundation is laid at the bottom of the caisson to ensure that the caisson has solid support when it reaches a predetermined depth. This layer of concrete not only provides physical support, but also acts as a barrier to prevent further groundwater infiltration and damage to the caisson structure.
In order to ensure an even distribution of the concrete layer, the technician must carefully calculate the flow and curing time of the concrete to ensure that the laying work is completed without affecting the caisson correction process. After countless monitoring and adjustments, the caisson was finally positioned in its intended position.