The article discusses the issue of monitoring hydraulic structures with the high damage potential. Taking this into account, monitoring should ensure the reliability and operability of a hydraulic structure during the entire period of its operation. Particular attention should be paid to the control of vertical displacements of the monitored structure since the settlement is one of the major causes of accidents. It may lead to serviceability failure and, in some cases, to a collapse. Various systems used for deformation monitoring are compared. Recently, automated hydrostatic leveling systems are widely used all over the world since they offer high performance and acquire data in real-time. By way of an example, the experience of using Monitron automated hydrostatic leveling system is considered. The system has shown efficiency and reliability in the deformation monitoring of various engineering structures.
KEYWORDS. Automated monitoring, hydrostatic leveling.
Similar to other major infrastructure projects, hydraulic structures require regular monitoring during the entire period of their operation [1-3]. At the same time, it is necessary to consider an increase in risks of emergencies and accidents as hydraulic structures are subject to wear and tear. Considering the life span of a hydraulic structure and in view of the high damage potential, the efficiency and reliability of monitoring, both during construction and operation, must be ensured and meet the highest requirements.
Monitoring efficiency refers to an ability to timely detect anomalies in the operation of hydraulic structures, since they may pose a higher risk of emergencies and accidents; to develop and implement measures to prevent them in advance.
Reliability is concerned with the continuous operation of a monitoring system regardless of technical, climatic and organizational factors.
Instrumental measurements can be manual or automatic. Manual downloading and analyses of the collected data are time-consuming, making the continuous monitoring of diagnostic indicators highly complicated. Thus, there is a need to develop monitoring measuring systems based on the fully automated acquisition and interpretation of data to control the operability and safety of hydraulic structures in real time. That is achieved by developing a measuring system consisting of a system that automatically acquires data from sensors and data analysis software.
At present, the monitoring systems used in hydraulic structures in Russia usually do not meet the above requirements since some of the subsystems are not yet automated, and instrumentation is still manual.
One of the critical diagnostic indicators by which the safety of hydraulic structures is assessed are displacements caused by ground-induced movements (see Figure 1). Statistically speaking, most failures of various types of concrete hydraulic structures, for example, the dams of St. Francis (USA) and Malpass (France), Zagorskaya PSP-2 (Russia), occurred due to excessive settlement caused by the failure of dam foundation [4-5]. Foundation material failure usually occurs due to unanticipated seepage leading to uncontrolled internal erosion. Studies show that these processes develop gradually until the volume of removed material reaches a critical value. The subsequent development of processes that adversely affect dam safety is difficult to predict. To prevent such erosion-related failures, continuous (real-time) deformation monitoring is required.
During the operation of hydraulic structures, it is also necessary to monitor the movements of high concrete dams when filling and emptying the reservoir, and that creates alternating loads on the foundation material and negatively affects its stress-strain state.
Nowadays, in Russia, the deformation monitoring of hydraulic structures is carried out manually by means of optical leveling with the regularity of once a month or less. Outside the structure, in open areas, atmospheric conditions often make optical leveling difficult, and in some cases, impossible.
An alternative method for measuring settlement of various types, which has become widespread, is hydrostatic leveling (HL). It operates with the system of communicating vessels, in which the surface of the liquid remains always at the same level horizontally (see Figure 2). Practice shows exceptional versatility of this method, efficient for measurements with any required accuracy for any distance between sensors [6–8].
The development of HL began in the middle of the last century. For example, in the 60s, an extended mercury-filled system was developed to place magnets of the proton synchrotron. This reference system was said to be accurate within ±0.025 mm across the 1-mile diameter machine .
The HL has been worked out both theoretically and practically. Measurement errors and methods of their compensation are well studied. This makes it possible to design such systems of any required scale with the required leveling accuracy.
The complexity and costs of the hydraulic leveling system are largely determined by the system for fixing the liquid level in the measuring vessel. Most common sensors designed for deformation monitoring of hydraulic and other engineering structures usually use a high-precision pressure transducer. Such transducers measure the liquid level with an accuracy of ±0.1 mm, and antifreeze is commonly used as a liquid.
Despite the obvious advantages of the HL, such monitoring systems have not been widely implemented in Russia before. However, recently, they are becoming more widespread.
As an example, the Monitron hydrostatic leveling system was used at the Moscow Canal Lock # 9 during the excavation of twin tunnels with a diameter of 6.0 m each between the Khoroshevskaya and Mozhaiskaya stations of the Moscow Metro. The clear distance between the tunnels and the bottom of the lock chamber is about 15 m, with a total length of the chamber of 300 m and a cross-sectional area of 30×12.5 m2. To ensure the safety of the lock, the chamber was monitored by one portable electronic total station, capable of measuring angles to 0.5'', and 24 HLD-18 hydrostatic levels combined into a single system with a measurement accuracy of ±0.1 mm mounted on the lock walls (see Figure 3).
A comparison of tacheometry and hydrostatic leveling showed that the maximum difference between measurements is 0.3 mm (see Figure 4). The data from the hydrostatic levels were collected every minute (1440 times a day), while the tacheometric survey was carried out four times a day with an interval of every 6 hours.
Of particular interest is the application of the Monitron system for lifting and leveling the Zagorskaya PSP-2 by means of controlled compensation. This project provides for the automated measurement of vertical movements inside and outside the powerhouse during restoration works. A full-scale model of the PSP concrete slab was equipped with HLD-17 hydrostatic levels during its lifting at experimental site # 3 (see Figure 5).
The hydrostatic levels used at the experimental site transmitted data every minute to cloud data analysis software on https://monitron.ru. That made it possible to remotely access the field data collected by the monitoring system and their analysis with simulation results. As an example, a graph of vertical displacements is shown (see Figure 6), compiled according to the data of hydrostatic levels for the period from 2017-06-20 to 2017-06-27 (results of 7 632 measurements were used.) For comparison, the graph for one of the measurement points (D-07) shows a dashed curve based on the data of numerical simulation of controlled compensation grouting during lifting of the full-scale model. Since the displacements were measured in real-time, the comparison of the calculated and observed lifting was carried out for all measurement points at each injection stage. Based on this, we identified grout pipes through which additional volumes of grout mixture had to be injected [9–10].
The technical characteristics of hydrostatic leveling systems accepted in international construction practice were fully observed during the development of the Monitron system. A brand-new optoelectronic system for data acquisition has been created. This technology comes up with similar accuracy while significantly reducing (up to 10 times) the total cost of measuring equipment in comparison with foreign counterparts.
The main part of the Monitron system is the measuring devices, namely the digital hydrostatic levels HLD-21 (see Figure 7), with the following characteristics:
The Monitron system has been used at dozens of facilities in Russia and abroad, in particular, during the construction of the Moscow Metro, as well as for deformation monitoring of wind turbines.
An advantage of hydrostatic levels is that they can be easily combined into a single system, regardless of the number and location, indoors or outdoors, which is significant given the size of hydraulic structures. For comparison, it is worth noting that automated optical leveling using robotic total stations in similar conditions is very expensive since it requires plenty of them when combined into a system.
We have developed a real-time measuring system with a built-in module for sending alerts, which allows us to deploy the system and commence deformation monitoring within a short period.
As mentioned above, the number of hydrostatic levels in one system is almost unlimited. With a very large number of the levels, for operational reasons, it is advisable to divide them into subsystems. Thus, a hydraulic structure of any size can be monitored.
Due to their compactness, the hydrostatic levels can be fitted in service areas, galleries, turbine halls of hydroelectric power plants, on structural elements, both inside and outside the structure, as well as on surfaces exposed to the environment. The housings of the hydrostatic levels can be combined with lighting fixtures, located at a height that is safe from accidental physical impact. On the surface of earth structures, the sensors can be mounted on ground marks (see Figure 8).
A key feature of the Monitron system is the continuity of measurements, which allows, thanks to automated notifications, to detect and forecast accidents in advance. This makes it possible, considering the simplicity and robustness of the system, to easily combine it with the currently used and developed digital automated systems for monitoring and analyzing the stress-strain state of hydraulic structures [11–12].
The conclusions are as follows:
1. Hydrostatic leveling can significantly improve the quality of deformation monitoring of hydraulic structures.
2. The experience of using the Monitron hydrostatic leveling system proves the robustness of the equipment and the accuracy of real-time measurements.
3. Simple integration of hydrostatic levels into a single system, regardless of the number and location of measuring equipment, allows you to deploy it and start monitoring within a short period, if necessary, combining it with currently used digital automated monitoring systems.
4. With a large number of points for leveling, the system can be divided into subsystems, which, in turn, makes it possible to use it on hydraulic structures of any side.
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