The vertical displacement of the dam, also known as subsidence, is the focus of current observations and is mainly carried out by precision leveling methods. Most of this measurement method follows:
The first and second level measurement is carried out, and the calibration of the level base point is mostly carried out according to the requirements of the first class level measurement. However, this uniform standard of observation accuracy may be too general to fully reflect the reality of different heights and types of dams.
Although precision leveling can achieve good results when carefully implemented and strictly followed by operating procedures, there are still some problems in practical application. Here are a few of the main issues and challenges:
Stability of the leveling base point: In order to avoid the influence of water storage in the reservoir area, the leveling base point usually needs to be located far away from the dam area. This requires periodic leveling transfer through the base points to check and measure the stability of the working base points in the dam area. However, leveling punctuation is often compromised, resulting in some projects not being able to calibrate the working base points shortly after the water is impounded.
Timing of dam subsidence observation: The subsidence of the dam foundation usually begins during the construction of the dam, from the micro-elasticity of the rock surface at the beginning of excavation to the gradual sinking with the elevation of the dam. Therefore, the observation of the vertical displacement of the dam foundation should start at the time of excavation in order to obtain the initial value. This requires close collaboration between design, construction and operation to ensure that the subsidence of the foundation can be observed throughout the process.
Subsidence measurement in the corridor: In some projects, the elevation cannot be transmitted into the corridor, resulting in the subsidence measurement value in the corridor can only reflect the relative rise and fall of the floor plate, but cannot obtain the absolute value after impoundment. In addition, the slope sections within the corridor are steep, making level transfer difficult. This needs to be taken into account at the time of design and, depending on the situation, the option is to transfer from the downstream lateral traffic corridor or vertically from the shaft with an indium steel tape ruler or displacement gauge.
Basic tilt observation accuracy: Some projects set punctuation points upstream and downstream in the transverse corridor to calculate the tilt value through the height difference. However, due to the limitation of observation accuracy, this method is often difficult to achieve the desired results. For example, for a 50-meter-long horizontal line, the accuracy of the height difference between the upstream and downstream endpoints needs to be greater than 025 mm to meet the requirements of 1 second for basic oblique observation.
Overall, although precision leveling plays an important role in the observation of vertical displacement of dams, there are still some challenges and problems. In order to solve these problems and improve the accuracy of observation, a series of strategies and technical measures need to be adopted, including improving measurement methods, improving equipment accuracy, and strengthening base point protection. At the same time, close coordination and cooperation in design, construction and operation are required to ensure the safe and stable operation of the dam.