J. Clapes

GPR survey to confirm the location of ancient structures under the Valencian Cathedral (Spain)

This paper describes the ground-penetrating radar (GPR) survey performed inside the Cathedral of Valencia, Spain. It is part of historical studies performed in the Cathedral in order to add information to old maps and documents in the Cathedral Archives and also to analyze the extent and importance of potentially destructive moisture areas that were appearing on the floor. The construction of the Cathedral of Valencia occurred in three stages, all of which are well-documented in the Cathedral Archives with detailed drawings, maps, and charts. The radar data were successful in locating crypts, ossuaries, sepulchers, and graves, and the location of ancient walls that existed before the final Cathedral expansion. Three cultural layers corresponding to the three periods of construction were also identified corresponding to the Roman, Arabian and Middle Age Epochs. Measurements of relative sub-floor moisture were obtained by comparing dielectric permittivity changes and radar velocity differences between materials in humid and non-humid areas.

Assessment of Complex Masonry Structures with GPR Compared to Other Non-Destructive Testing Studies

Columns are one of the most usual supporting structures in a large number of cultural heritage buildings. However, it is difficult to obtain accurate information about their inner structure. Non-destructive testing (NDT) methodologies are usually applied, but results depend on the complexity of the column. Non-flat external surfaces and unknown and irregular internal materials complicate the interpretation of data. This work presents the study of one column by using ground-penetrating radar (GPR) combined with seismic tomography, under laboratory conditions, in order to obtain the maximum information about the structure. This column belongs to a “Modernista” building from Barcelona (Spain). These columns are built with irregular and fragmented clay bricks and mortar. The internal irregular and complex structure causes complicated 2D images, evidencing the existence of many different targets. However, 3D images provide valuable information about the presence and the state of an internal tube and show, in addition, that the column is made of uneven and broken bricks. GPR images present high correlation with seismic data and endoscopy observation carried out in situ. In conclusion, the final result of the study provides information and 3D images of damaged areas and inner structures. Comparing the different methods to the real structure of the column, the potential and limitations of GPR were evaluated.

Application of Particle Motion Technique to Structural Modal Identification of Heritage Buildings

Determining the behavior of a structure estimated by means of finite elements analysis requires not only an in-depth knowledge of its geometry and dynamic properties but also an experimental validation to corroborate the adequacy of the characteristics of the structure. Most of the current structural identification techniques are based on linear methods that call for many measurement points and/or a relative simple structure. Complex structures are somewhat still an unexplored field due to the difficulties with the finite element method and the experimental corroboration of its results. This study presents the use of particle motion computation applied to each structural vibration mode to improve the identification of its dynamic properties, and its application to the Gothic Cathedral of Palma de Majorca (Spain).

Nanozonation in Dense Cities: Testing a Combined Methodology in Barcelona City (Spain)

Microzonation is widely used in seismic risk evaluations to define the predominant period values, which are usually associated with extended areas of a few hundred meters. However, the representative values corresponding to these areas are obtained from few measurements in each area. Thereby, results are accurate only in the case of depth-dependent soils. However, not detected narrow and sharp lateral changes in soil are potentially the cause of imprecision and could be a source of specific errors. This article aims to present several tests conducted in order to emphasise the importance of accurate selection of points, to underscore the necessity of more precise and detailed evaluations, and to suggest a possible methodology to select the most appropriate data acquisition points. Results highlight the need to divide microzonation areas into smaller zones for a precise evaluation in locations where sudden changes in soil characteristics exist. Therefore, in such sites the requirement of nanozonation appears; defining zones with the same soil response. Distance between vibration measurements could be the main problem for nanozonation; data acquisition in areas with irregular geology can be time consuming when a precise analysis is required. In the most complicated environments or in dense cities, it could even be unfeasible. Consequently, it is necessary to establish a functional methodology to adequately distribute the measurement points throughout the area. On this occasion, three sites in Barcelona city were studied. This city is surrounded by mountains at NW, W, and S, and by the Mediterranean Sea at N and E. As a consequence, the shallow geology is characterized by many paleochannels and streams that are currently buried. These geological structures most likely affect the soil response. Several tests were carried out to determine this dependence. The tests were based on Ground Penetrating Radar (GPR) surveys to define the paleochannels position and on vibration measurements in order to define properly the soil response. The results from both methods were compared to the known geology to accurately define the effect of the shallow geological structures in the predominant period and in the GPR images. Areas with the same geological unit but different materials were identified in the GPR images, allowing the selection of the most appropriate distance between vibration measurements in each place. As a final result, predominant periods that were measured over the same geological unit but over different material showed changes higher than the 40% in short distances. This procedure could improve the soil response maps, including nanozonation.

GPR applications in dense cities: Detection of paleochannels and infilled torrents in Barcelona GPR applications in dense cities

Barcelona is placed in a basin delimited by the Mediterranean Sea (E), the Collcerola Mountains (W) and the rivers Besos (N) and Llobregat (S). The city was built on Quaternary alluvial deposits and on the Tertiary and Palaeozoic materials of the surrounding mountains. The Quaternary materials are preponderant in the plane of the city, presenting a high lateral variability due to the paleochannels and infilled torrents existing between the Sea and the Mountains. The city was built on these heterogeneous materials. In order to prepare a detailed vulnerability map of the Barcelona city, a GPR survey provides information about the position of these geological structures. Results were compared to H/V spectral ratio measurements and soundings.

Geophysical exploration of columns in historical heritage buildings

Columns are one of the most usual supporting structures in cultural heritage buildings. This work presents the study of different columns using ground-penetrating radar (GPR) combined with seismic tomography. These columns belong to a Gothic Cathedral and to a Modernista Building. In the first case, columns are constructed with regular ashlars. GPR provide images that could define the internal ashlar distribution and shape, and the possible existence of inner cracks. However, the images must be supported with additional information from a seismic survey that could provide evidences about the stone quality. In the second case, columns are built with irregular and fragmented clay bricks, and mortar. The internal irregular and complex structure causes complicate 2D images. However, 3D images provide valuable information about the condition of the internal tube and indicate the existence of irregular elements. Results present high correlation with seismic data. In conclusion, results provide information and 3D images defining damaged areas and inner structures. Also, comparing the different methods with the structural knowledge, it is possible to define potential and limits of non-destructive testing applied to these elements.

Study of wood beams in buildings with ground penetrating radar

A large number of buildings in Barcelona city were built during the XIX Century, using wooden beams and masonry. Nowadays, these structures, some of them declared architectural heritage, suffer damages and important deterioration. Geophysical surveys by means of ground-penetrating radar could provide valuable information on the most damaged areas of the beams, and could supply useful data to improve restoration policy and heritage protection. The knowledge of the structure is needed in many cases, but often there is not information about the internal structure and the constructive techniques. However, a large number of supporting structures use to be embedded in the floor and covered by mortar, obstructing the direct visual inspection. Therefore, the access to the structure can be one of the highest difficulties in those inspections because the direct access is not possible. Hence, GPR was applied to determine internal constructive wooden elements and its state of conservation. In this work, several buildings were studied to locate damaged beams and to identify internal structures. Differences in reflections due to damaged and healthy beams appear are clearly highlighted in GPR images in several cases, while in other, differences are not so apparent.

Geological structures evaluated by means of scattering noise in ground penetrating radar images

Coastal geology in Barcelona City is formed by Quaternary sediments overlying Tertiary strata. The Quaternary plane is crossed by paleochannels and streams, all of them under the city structures and constructions. Boreholes demonstrate differences in the grain size distribution of the materials from the different geological structures. In this work, Ground-penetrating radar was proved as a tool to obtain quick information about these changes. The results from more than 20 km of GPR profiles in Barcelona city, seems to indicate that, depending on the relationship between the wavelength and grain size, GPR signal scattering increases significantly. Therefore, the analysis of the noise in GPR signals supplies information to determine the possible existence of geological changes in the quaternary deposits. Several tests and measures in well-known emplacements, as well as models and simulations, were used to determine the ability of this method in the study of the Barcelona plain shallow geology. Correlation between paleochannels and infilled streams and higher scattering noise is observed in this study. The analysis of the scattering effect on the amplitude of the radar data could help to locate paleochannels and subterranean infilled streams, being a valuable tool to improve the knowledge of the city subsoil.

GPR building inspection: Examples of building structures assessed with Ground Penetrating Radar

The GPR assessment of buildings depends on the structure or part of the building studied. In this paper, three different case study are presented, all of them about old buildings that need to be restored or modified. The radar study objective was, in all cases, the detection of hidden targets or structural elements. The first case presents the study of some terraces in buildings in order to determine the structural elements and also to determine approximately the materials, previous to an important modification. The second case is based in the study of the ground underneath the building. Several structures that could affect to the further restoration were detected. The last case presents the study of floors in the different levels of a building, showing the changes in the structural elements that support the floors. All the examples demonstrate the ability of the method applied previous to the restoration projects.

Ground penetrating radar assessment of historical buildings: The study of the roofs, columns and ground of Santa Maria del Mar, in Barcelona

Santa Maria del Mar is a magnificent gothic church built between 1329 and 1383 in a neighbourhood outside the city walls, over the remains of a more ancient church. The inhabitants of this district (merchants, downloaders of the port, ship-owners artisans and craftsmen) contributed and took part in the construction of this building. Nowadays it is one of the most representative gothic buildings of Barcelona. The structure was completely studied with GPR in order to obtain useful information for a further restoration. Some of the most interesting results were obtained during the evaluation of the vaults and roofs of the building. Radar images demonstrate that the inner structure was designed in order to diminish the load on the arches and walls. Hollow elements were used to support partly the roof in some areas. Moreover, walls, floors and columns were also assessed, and also the ground outside the building. A large number of graves were located under the church floor, but also some of the radar images suggest the existence of large underground walls. The GPR study of the columns and walls was completed with a seismic survey that demonstrates the existence of zones of non-consolidated materials and defined the joints of the ashlars.