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.