Monitoring of Hydraulic Structures Using Monitron Automated Hydrostatic Leveling System (on the example of monitoring the Moscow Canal Lock # 9)

The aim of deformation monitoring (deformation survey) of navigable hydraulic structures (NHS) is to ensure their safety [1-3]. Deformation monitoring is used to detect unfavorable changes in the technical condition of an object as early as possible so that there is enough time to prevent a potential accident. Therefore, it is important to increase the speed of the monitoring process, which is achieved by automation: from telemetric (remote) data acquisition from deformation monitoring instruments to the real-time analysis of the collected data by the intelligent data analysis software.

The Monitron automated hydrostatic leveling system is a state-of-the-art monitoring system that has been developed in Russia since 2012 using the latest industrial and information technologies. The Monitron system measures vertical displacements (settlement) in real-time, which is new for the deformation monitoring of navigable hydraulic structures since measurements were usually carried out manually by optical leveling (and the approximate regularity of these measurements is once every three months). Leveling automation expands the monitoring capabilities, in particular, lays the foundation for digital twins of hydraulic structures, as will be discussed below.

Hydrostatic leveling (HL) operates with the system of communicating vessels, in which the surface of the liquid remains always at the same level horizontally (see Figure 1). The HL is one of the classic leveling methods. This method has been used in various sectors of construction and geophysics to measure settlement with any required accuracy at any distance between sensors [4–8].

The technical characteristics of the hydrostatic leveling systems (HLS) accepted in international construction practice were fully observed during the development of the Monitron system. The crucial innovation is an optoelectronic system for determining the liquid level inside the measuring vessel [9]. Not only did it provide the same leveling accuracy and reliability, but it also significantly reduced the cost of equipment (up to 10 times) in comparison with foreign counterparts, which use extremely expensive pressure transducers.

The Monitron system has the following key advantages over geometric (levels) and trigonometric (total stations) leveling methods:

1. The elevation of a point is continuously transmitted to all HL sensors according to the principle of communicating vessels. Thus, at each scheduled measurement, the labor and time costs for carrying out a leveling traverse and setting up instruments are excluded.

2. As it follows, all HL sensors take measurements simultaneously in real-time (regularly once per minute).

3. The measurements are uninfluenced by temperature and humidity, weather and climate conditions, hazards, or disruptive factors of various kinds; there is no need for a clear line of sight between the sensors. Such reliability and continuity cannot be achieved by manual or robotic optical levels.

The main part of the Monitron system is measuring vessels, namely the digital hydrostatic sensors (HLD-21) with an accuracy of ± 0.05 mm and a measurement range of 100 mm. The sensors operate in a temperature range from minus 65 to plus 50 °C. Our product has a service life of 15 years and provides a high protection degree of IP66 (dust-tight, protected against powerful water jets).

The sensors have a contemporary design and can be disguised as lighting fixtures or architectural lighting (see Figure 2). On the surface of earth structures, the sensors can be mounted on ground marks.

To assess the differential settlement, tipping and bending of structures, a group of two (2D rotation) or more (3D rotation) HLD-21 sensors is sufficient. It is also possible to use only one HLD-22 sensor that can measure the angles of rotation, which is currently being used experimentally and tested for registration in the National Register of Measuring Equipment (Russia).

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.

The Monitron system was used at the Karamyshevsky canal lock # 9 during the excavation of twin tunnels with a diameter of 6.0 m each between the Narodnoe Opolchenie and Mnevniki stations of the Bolshaya Koltsevaya line. The tunnel route crosses the lock below sections 8-9 at an angle of 70° (see Figures 3 and 4). The clear distance between the tunnels and the bottom of the lock chamber was about 15 m, with a total length of the chamber of 300 m and a cross-sectional area of 30×12.5 m2. The width of the chamber along the bottom is 40.4 m.

Considering the high importance of NHS and a certain geotechnical risk when tunneling under an operated facility, two duplicate deformation monitoring methods were used, providing measurements at the same points:

• optical leveling with an electronic total station once every 6 hours;
• hydrostatic leveling by the Monitron system based on HLD-19 with measurements performed every minute.

The Herrenknecht S-791 earth pressure balance machine (EPBM) was used for the excavation. The annular gap was grouted through holes in the tail of the shield. The tunnel has an outer and inner diameter of 6.0 m and 5.4 m, respectively. The segmental concrete lining has a thickness of 300 mm. Each ring is 1.4 m in width.

The calculated settlement of the lock sections above the twin tunnels is 4.9 mm. The predicted settlement area (1 mm or more) has a width of 61 m, within which sections 7-10 are located.

The deformation monitoring was carried out from November 2018 to April 2019. The first tunnel was excavated in December 2018, the second tunnel in February 2019.

The excavation rate averaged 10.3 m/day, i.e., 2.6 m for a 6-hour total station measurement cycle (which is carried out sequentially, from point to point, therefore there are some leveling errors due to timing). Comparison of the measurements of optical and hydrostatic leveling showed that the residual did not exceed 0.3 mm (see Figure 5).

Excavation-induced impact on the canal lock lasted about eight days for each tunnel. Settlement telemetry allows promptly adjusting the technological parameters of the excavation that cause ground movement (earth pressure, grouting parameters). As a result, the lock settlement decreased to 2.1 mm, i.e., by 60% compared to the calculated settlement (obtained for regular excavation without prompt corrections according to telemetry monitoring data).

The Monitron system can be connected and integrated to other measuring systems or operate with our own software.

The Monitron real-time measuring system includes all necessary tools for the analysis of settlement, graphing, and trend research. The system is a cloud service and is accessible on https://monitron.ru from any computer without installing any additional software. This provides online access to the leveling data to all responsible people. Should threshold value be reached, the system will generate and send e-mails and SMS to the users in a timely matter.

Moreover, the Monitron real-time measuring system allows you to access calculated stress-strain state and flow results directly from the Internet browser without installing specialized engineering software (ZSoil, Plaxis, and Midas file formats are currently supported).

Digital twin (DT) in the Monitron measuring system uses a mathematical model (usually FEM) that calculates the SSS at any given point in time considering the loads and actions active at this step (for example, water level, ambient temperature, etc.).

The physical and mechanical characteristics of structures and soils are subject to natural variability (variations), their actual values differ from the design ones. This requires the calibration of the mathematical model to determine the actual values of the parameters. For this, the parameters are varied many times in the tolerance range, and the calculations are compared with field data. Once the calculated and field data match, the model is verified against an additional field data and validated during the trial period.

A calibrated, verified, and validated DT effectively controls the stress-strain state and flow, evaluates the agreement with the designed settlement and stress state, predicts exceeding the limit values (for example, limit state of cracking), and optimizes funds for repairs. If the field data deviates from the data of the DT, it will be possible to identify destructive processes at the initial stage and respond to them in advance.

Due to the regularity of measurements, the Monitron system allows accumulating data for the calibration and verification of the DT over a six-month period (between extreme seasonal temperatures), while the traditional measurements 4 times a year requires data for several years.

Thus, the DT reflects the unique state of the structure, which occurs every day due to a new combination of loads and actions to which the structure will be subjected (and the way they change in time). This is especially evident in the effect of temperature, which is unique every year.

The agreement between the field data and the data of the DT indicates that the technical condition of the structure corresponds to the project.

The analysis of a condition of a structure using the DT can be carried out parallelly with the use of safety criteria for hydraulic structures [10-11].

The digital twin of the Moscow Canal Lock # 9 (see figure 6) makes it possible to assign to each Monitron sensor an assumed relation between the settlement and the position of the tunnel face in a form of the settlement corridor (see figure 7). The advantage is that if the behavior of the ''lock-tunnels-soil'' system deviated from the expectations in an unfavorable direction, it would be recorded even with a minor settlement when there is still the possibility of adjusting technological and structural solutions. This avoids situations where threshold values are reached in the middle of the construction process and opportunities to reduce settlement have already been lost.