In the construction project "REKO VTL Květnov - Švarec, Vatín I, PS-01 VTL gas pipeline" the designer proposed a crossing with the double-track electrified railway line Havlíčkův Brod - Brno. The crossing point of the gas pipeline with the railway was designed without excavation, using controlled overpressure. Geodetically, it was necessary to measure and determine possible height shifts and deformations of the rail strips.
Our measurements were also intended to provide information on the current condition of the railway line and to ensure the safety of traffic during and after the completion of construction works. Permanent points outside the overpressure area were used as reference points for precise leveling, namely the footings of the overhead line supports on which the measuring studs (railway point field, securing mark - ZZ) were placed.
Due to the local nature of the measurements, it was not necessary to know the real elevations of these points, but it was sufficient to create a local elevation system. We chose 100 m as a baseline for one of the fixed points on a nearby bridge structure. From it, we created a local elevation system sufficiently different from the actual elevation of the points. We created a second reference point, which served as a control, by a leveling program.
For the geodetic measurements (leveling), seven fixed points were identified on each rail strip to detect displacements (one at the centerline of the thrust line and others at distances of one, three, and five meters from the assumed centerline). For unambiguous identification, the points were numbered 101 to 107 on the first rail and 201 to 207 on the second rail.
For the actual assessment of potential subsidence, we have coordinate-marked the axis of the controlled pipeline flow directly on the track bed, for which we used a precise and fast GNSS method. To compare the coordinates of the points on the rails, it was necessary to geodetically level them before and after the controlled overpressure. At the same time, we minimized the influence of weather and temperature conditions in both measurement stages so that the measurements could be compared with each other.
The Groma program and a Python script were used for the leveling calculations.MGEO software was used for subsequent processing of the geodetic data.
Based on the accurate measurements, there was no displacement or reshaping (change) of the track axis or change in the condition of the operating rail line. All observed deviations were within the limits of measurement accuracy and were consistent with the different temperatures at the measurement times.
To confirm the results in the long term and given other ongoing construction works, a control measurement was proposed to be carried out over a longer time.
Thanks to the VYG branch, we have tried out new procedures that can be applied to other jobs.
The DN900 guard (see photo) serves as protection for the DN700 guard, with the intermediate space to be filled with C25/30 grouted cement mortar. The DN500 pipeline itself will be enclosed in a protective element (DN700 guard) with a rubber sleeve.
Author: Ing. Pavel Hladík