PSI - Issue 32

V. Epin et al. / Procedia Structural Integrity 32 (2021) 64–70 V.Epin/ Structural Integrity Procedia 00 (2021) 000–000

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Fig. 2. A typical photo obtained from a video camera of a hydro-leveling sensor; the green lines indicate the search area for the liquid column, the red line indicates the liquid level.

The principle of measuring the settlement using the hydrostatic leveling method is as follows. The sensor system is rigidly fixed at the points where the relative vertical displacement should be measured during monitoring. The system of communicating tubes is filled with liquid. The position of the liquid column in each sensor is determined by the markers located on a linear scale. These measurement results determine the zero count of the vertical displacements and determine the horizontal position of the liquid level in the hydro-leveling system. From the entire set of sensors, one is selected, which is the basic one, relative to which the differences in the readings of the liquid level position are calculated.

0 i i U A B   0 (

0

)

(1)

0 A – indication of the liquid level on the base sensor in zero count, 0

i B – indication of the liquid level on the

sensor with the number i in the zero count, 0 i U – the value of the difference between the readings of the sensor with the number i and the base sensor in the zero count. All subsequent measurements of the liquid level readings n A , n i B allow you to determine the settlement values at the sensor mounting points relative to the zero reference. The values of these settlements are determined by the ratios:

0

k i i U U U    k i

(2)

k

k

k

(

)

Where i i U A B   – the difference between the readings of the base and arbitrary sensors at the current time. The choice of a liquid that does not freeze at subzero temperatures is important for the measurements by the hydrostatic leveling under direct environmental conditions. For this purpose, an ethylene glycol solution was chosen as such a liquid, which retains its operability up to -40℃. It should be borne in mind that the greatest influence on the error of the measuring system is the change in the density of the liquid caused by various factors, including temperature and possible delamination of the liquid. To reduce the temperature error (the negative effect of temperature on its density), a scheme with a minimum height of the liquid column (about 8-9 cm) was chosen. For periodic equalization of the liquid density, a mixing scheme is provided, including a mixing tank and a pump. Measuring modules are installed in the required number at various points of the object and are connected by tubes to a single hydraulic alignment loop. These modules allow you to track the settling of the building in the area where each sensor is installed relative to other sensors of this loop. Installation of sensors of the hydraulic leveling system was carried out directly on the supporting vertical metal columns located in the axes A, B, C (Fig. 3). The system consists of two closed loops of polymer tubes filled with the water solution of the ethylene glycol.