The importance of geodetic control in the construction process
The purpose of construction guidance observations Geodetic observations are quite often required during the construction of the engineering structures. The purpose of these observations is either the geometric guidance of the construction or the control of the geometrical quality of the built structures. In both cases surveyors are required to compare the ’as built’ status of the structure with the geometric position and dimensions of the structures on the plans. Construction guidance observations are a part of the construction activities. These observations are carried out continuously during the construction process. The purpose of these observations is to quantify the discrepancies between the built structures and the planned positions and dimensions, thus these discrepancies can be corrected for by applying ’construction/assembly corrections’. Continuous guidance observations are required for example in the case of the construction of reinforced concrete structures using sliding formworks. Let’s imagine that the chimney is being constructed, that has a circular cross‐section.
The purpose of the construction guidance observations is to determine the radius and the center of the cross‐section of the built part. The discrepancies between the ’constructed’ cross‐ sections and the planned cross‐sections provide the geometrical correction for the placement of the sliding formwork. These corrections show not only the correction of the center line, but also the correction of the radius of the structure. Thus both the position of the sliding formwork and the dimension of the formwork can be corrected based on the construction guidance observations. In many cases the purpose of construction guidance observations is to adjust the position and the orientation of structural elements (for example: pillars). These adjustments are done with an iterative procedure. Firstly the structural element is placed approximately to the correct place and it is oriented approximately in the correct direction. Afterwards surveyors measure the exact position and orientation of the element and compute the required corrections to meet the planned position and orientation values. Based on these corrections, construction workers can adjust the placement and the orientation of the structural element. Unfortunately – mainly due to the large size and weight of these structures, this adjustment can not be done in a single step. Thus the new discrepancies in position and orientation must be quantified by the surveyor again, and new correction values are computed. Usually the correction values have a decreasing trend, thus the position and the orientation of the structure should converge to its planned position and orientation. This iterative process is done until the computed correction values are below the given tolerance. Construction control observations are done after the construction process is finished. In this case the purpose of the observations is the determination of the geometric error of the construction. The final approval of the construction usually depends on the results of these construction control observations. In some cases, when the construction process is separated into different individual steps, the results of the construction control observations are taken into consideration during the planning of the next construction step. Such an example can be the construction of the nuclear power plant in Paks, when the small geometric error of the construction of the buildings was taken into consideration in the planning of the technological facilities. In some cases the documentation of geometric construction error helps to determine the cause of malfunctioning structures. It is also important to mention that the positioning observations of the entire structures are usually distinguished from the positioning and dimensional observations of the structural elements. This is because of the fact that – in most cases – a lower accuracy is required for the positioning and the orientation of the entire structure compared to the positioning and the orientation of the structural elements. Imagine that a large hall can be misplaced by several centimeters without any serious consequences, but the misplacement of pillars inside the building by the same amount would lead to the failure of the structure.