DEFORMATION MONITORING OF MOTORWAY UNDERPASSES USING LASER SCANNING DATA
Other literature type
(issn: 2194-9034, eissn: 2194-9034)
The motorway Ourense – Celanova will become the next years in one of the main roads of inland Galicia (northwest region of Spain)
that will connect quickly with the cities of Northern Portugal. This highway is projected as a public – private partnership between
the regional government of Xunta de Galicia and the construction companies Copasa SA and Extraco SA. There are currently under
construction the 19 km of this road and presents a number of structures as viaducts, overpasses and underpasses. The viaducts are
part of the main road, allowing passage of the vehicles at conventional speed. Overpasses are mainly used in the connection of the
highway with secondary roads. Moreover, the underpasses are better suited for the passage of wildlife animals, persons or
The underpass arch-shape structures used for this project consist of two reinforced concrete voussoirs placed on two small concrete
walls. For each set of voussoirs there are three joining points, two between the walls and the voussoirs and one between the both
voussoirs at the top of the structure. These underpasses suffer significant mechanical stress during construction, because during the
backfilling process asymmetric loads are applied to both sides. Thus, it is very important the monitoring of the structure using
geodetic techniques as total stations, levels or laser scanners
The underpass selected for this study is located at the kilometric point 4.9 of the highway, with a total length of 50.38 m, maximum
span of 13.30 m and rise of 7.23 m. Voussoirs has a thickness of 0.35 m and a length of 2.52 m. The small lateral walls exhibit a
height of 2.35 m and thickness of 0.85 m. The underpass presents a slope of approximately 4 % and the maximum height of the
backfill over the top of the structure is 3.80 m. The foundation consists of a concrete slab arch-shape (curvature opposite the main
arch) with a thickness of 0.7 m.
The geodetic technology used for the deformation monitoring is a Optech Lynx mobile LiDAR. This laser scanner is based on time
of flight technology and presents an accuracy of 6 mm in the determination of the geometrical coordinates. This accuracy can be
improved to around 1 mm using fitting post-processing techniques and makes this technology very useful for studies related with
deformation monitoring. The laser scanner, in comparison with other geodetic techniques as total stations, allows the control of all
the structure, including unexpected deformations. Reflective targets are permanently positioned over the small walls of the structure
to allow the 3D orientation of the different scans.
Two main scans are made for this study, before and after the backfilling process. Backfilling takes about 10 days for the construction
companies. The scans need a time of approximately 12 minutes. Construction works do not need to be interrupted during the scans.
Point clouds are then post-processed using QT Modeler Software. First, the point cloud is cleaned to use only the data directly
related with the structure under study. Then, using the target coordinates, both point clouds are moved to the same coordinate
system. Finally, the deformation of the underpass is studied using two algorithms specifically developed using Matlab software.
First algorithm fits a geometrical surface to the point cloud of the first scan and evaluates the residuals of both scans for this fitting
surface. Differences in the residuals give the deformation map of the structure. Second algorithm takes a portion of the point cloud
from the top of the structure, where it is located the joining point between the voussoirs. The joining between two voussoirs shows a
height step that in an ideal case must tend to zero. Deformations produced by the loading of the structure are measured as a
comparison between the steps before and after the backfilling process. The analysis of the results show as some deformation occurs
in the structure in the joining point of the voussoirs ranging between 1 mm and 5 mm. These deformations are under the tolerances
predicted by the structure and confirm the success in the construction works developed. The laser scanning and the post-processing
algorithms here developed appear as an easy methodology to make deformation monitoring of underpass structures and guarantee
the load capacity of the structure.