FOUNDATION CHARACTERIZATION SYSTEM FOR PROTECTED BUILDINGS

Abstract

A system for characterizing the foundations of protected buildings, characterized in that it comprises: a continuous ERT profiling system (1) along the external and internal perimeter of the walls of a tower (T) to obtain a real 3D representation of the electrical values of the foundation, which are processed by a first computer program; a passive nonlinear L-shaped MASW profiling system (2) adapted to the configuration of the tower (T), where these profiles coincide within the studied surface with the ERT system; and where the data are processed in a second computer program; and a ground-penetrating radar profiling system (3) arranged on the entire accessible interior surface and along the external perimeter of the tower (T); and where the data are processed in a third computer program; a fourth computer program for correlating the results obtained in the previous programs; and at least one computer module where the computer programs are executed.

Description

BUILDING FOUNDATION CHARACTERIZATION SYSTEM PROTECTED Technical field The present invention relates to a new 3D characterization system of the existing foundation under historical buildings, specifically in towers declared as Cultural Heritage Assets (BIC), the objective of the invention being to determine the geometry and type of unknown foundations in the subsoil of this specific type of protected constructions and buildings. The present invention falls within the sector related to soil characterization. State of the art As is well known, the number of properties registered as Cultural Heritage Assets (BIC) at the national level in Spain has been steadily increasing. This also applies globally. As an example, in Spain, the latest statistical record from 2021 shows a total of 16,292 properties declared as Assets of Cultural Interest (BIC), of which 1,491 are towers in the monument category. This represents a significant number of unique properties that must be protected and preserved. This leads to a growing need for intervention, maintenance, and preventive conservation, which in turn implies, as a primary premise, rigorous research and diagnostic knowledge to establish appropriate measures for these BIC buildings. Specifically, properties can be declared BIC in the categories of Monuments, Gardens, Historic Sites and Complexes, as well as Archaeological Zones. Turning now to the subject of the invention, historic tower-type buildings declared as BIC are usually classified as Monuments, which is the category with the largest number of properties. In this sense, each building protected as a Site of Cultural Interest (BIC) possesses its own historical, constructive, architectural, structural, aesthetic, and symbolic values, which must be understood to allow for appropriate intervention on the property, as its value is unique. Therefore, knowledge of the foundations is fundamental. Currently, at both the Spanish and European Union levels, regulations stipulate that no interior or exterior work affecting listed Buildings of Cultural Interest (BIC) or their surroundings may be carried out without the express authorization of the competent authorities. Furthermore, any construction that alters the character of the building or modifies its appearance is prohibited, thus establishing specific conservation and enhancement measures. For this reason, intervention criteria for historical heritage are complex today. Therefore, a rigorous, typically multidisciplinary, investigation of the property must begin before any work is undertaken, with the aim of achieving a thorough understanding of the building that allows for an accurate diagnosis and the development of appropriate intervention proposals. The general procedure currently used to study the subsoil beneath historical buildings involves the use of various non-destructive geophysical techniques, specifically electrical resistivity tomography (ERT). ERT profiles are arranged linearly in 2D in the vicinity of these types of buildings to reach adequate depths of investigation, given the limited interior space available. Therefore, they are not located directly on the tower's foundation, characterizing the surrounding area but not the foundation itself. Consequently, in many cases, soil samples are taken by drilling boreholes with continuous core sampling to determine the characteristics of the foundation and subsoil, causing damage to the protected environment with these destructive techniques. Along these lines, there are known publications that seek to obtain 3D models for soil characterization based on two types of methodologies. On the one hand, there is the parallel arrangement of several 2D linear profiles, which are subsequently joined using software to form the 3D model; and on the other hand, using a continuous profile with a non-linear distribution, obtaining a 3D model. For example, there is the information published in the document "Integrated Ground Penetrating Radar, Electrical Resistivity Tomography and Multichannel Analysis of Surface Waves for detecting near-surface caverns at Duqm area, Sultanate of Oman" by AME Mohamed, I, et al (07-2019), where the obtaining of 3D models from the making of longitudinal and parallel profiles is described, where the equipment used measures only between the electrodes of each 2D profile, and a 3D block is obtained from interpolations between profiles, not with real measurements between them. This has an essential problem in the case of foundations in BIC tower buildings, which is that with this methodology it is not possible to obtain reliable data of the area existing under the load-bearing walls of the BIC building. Therefore, it is understood that a solution must be sought to obtain a characterization of the foundation that corresponds to historical tower-type buildings with the highest degree of protection, whose foundation is unknown, and where these foundations generally have a particular geometry, in which it is necessary to know the characteristics of the foundation in order to establish appropriate measures of intervention, maintenance and conservation. In these constructions, given the limited interior space to place 2D linear profiles that can reach adequate investigation depths, and knowing that this type of measurement would also not be able to know the characteristics of the subsoil under the walls of the tower, the solution proposed to the problem described is the arrangement of a continuous non-linear electrical resistivity tomography profile along the exterior and interior perimeter of the building, and the use of techniques such as multichannel analysis of surface waves (MASW) and ground-penetrating radar to compare results. It is reiterated that known methodologies are based on the distribution of parallel 2D longitudinal profiles to obtain a 3D block. Therefore, if a 2D profile is placed on the exterior of the wall and another parallel to it on the interior, the interpolation performed between them will not take into account the bottom of the wall, and consequently, its foundation. To obtain a 3D model from 2D profiles, the spacing between the 2D profiles must be twice the distance between the electrodes of the 2D profile. This requires creating a number of longitudinal 2D profiles depending on the tower's dimensions, which necessitates passing through the tower walls. On the other hand, sufficient space is needed to accommodate the minimum length of the surface profile, as the depth of study achieved is estimated to be around 20% of the length of the linear 2D surface profile. This is unfeasible in listed buildings located in urban centers or protected areas, in addition to the building's geometry itself, which results in limited interior space. Also known is the information published in the document "Non-destructive DC resistivity surveying using flat-base electrodes" or the document "Enhancing Electrical Contact with a Commercial Polymer for Electrical Resistivity Tomography on Archaeological sites: A Case Study, by MD Vascónez-Maza et al (07-2020)," which describes a system for performing electrical resistivity tomography. This system can be used in archaeological excavations, but this information neither suggests nor explains that this technology allows for the characterization of the existing foundation under a building, much less that it can be used for listed buildings with the problems and limitations previously indicated. In any case, this technology could only be implemented, along with the previously mentioned background, to provide a partial solution to the problem that is the subject of the present invention.In fact, what the document justifies is the improvement of data recording thanks to the provision of a conductive gel. Taking these aspects into account, the present invention establishes a system for characterizing the foundations of towers listed as BIC, which are known to have a unique structural foundation and the information available is usually non-existent or comes from unconfirmed historical documents, and therefore, a precise knowledge of the foundation is required to establish the corresponding protection and / or intervention measures that are necessary, something that with known technologies is unfeasible and not possible to be carried out. To this end, the invention proposes, on the one hand, the arrangement of a continuous, nonlinear electrical resistivity tomography profile along the exterior and interior perimeter of tower-type buildings to obtain a 3D model of the foundation, the use of ground-penetrating radar and multichannel surface wave analysis, and the corroboration of the actual 3D model of the foundation obtained with the electrical resistivity tomography technique. This system, due to the arrangement of the continuous, nonlinear profile along the entire perimeter of the building (interior and exterior), allows for obtaining an actual 3D model of the entire foundation block of the building, including the area under its load-bearing walls, and also allows this model to be corroborated with additional data.Obtaining the data and interpreting these results allows for the reliable determination of the dimensions and geometry of the foundation, as well as the possible existence of cavities, thus promoting knowledge of the subsoil and the foundation in this type of construction. Given the technologies known in the state of the art and the problems indicated above that are not solved or are solved in an unsatisfactory way, the present invention presents a system that, unlike any other known in the state of the art, allows the 3D characterization of the existing foundation under historical buildings, specifically in towers declared as Cultural Heritage Assets (BIC). Explanation of the invention The present invention consists of a system for determining the foundation of historic tower-type buildings declared as BIC (Bien de Interés Cultural, or Asset of Cultural Interest), which are known to be unknown, both in their geometry and structure, in order to establish future studies of intervention, maintenance and conservation. This will be carried out by implementing a real-world 3D model using electrical resistivity tomography (ERT), performed directly on the building. Due to space limitations, parallel profiles cannot be created to achieve adequate depth of investigation. Aluminum plates will be used to avoid damaging the protected building. This increases the area and depth of the study, reduces application time, and eliminates the need for destructive techniques. Subsequently, ground-penetrating radar (GPR) and multichannel surface wave analysis will corroborate the 3D model of the foundation obtained with the ERT. The system that is the subject of the present invention has the following operation: A continuous electrical resistivity tomography profile is performed along the external and internal perimeter of the tower, with the aim of obtaining a real 3D electrical values ​​of the subsoil of the building, with depths varying according to the configuration used and the distance between the electrodes. In this regard, a system is required for the implementation of the ERT (Energy Reduction Test), which comprises equipment with plates, springs, multiple electrodes, and gel applied to the pavement. To avoid damaging the protected historic building, the electrodes will be placed on specially designed and manufactured aluminum plates measuring 100x100x10 mm, with a steel spring securing the electrode to the plate. The spring is attached to the plate at its ends by two sleeves welded to opposite sides of the plate. These sleeves are 20 mm in diameter and 1.50 mm thick, also made of aluminum. Furthermore, to improve electrical contact between the building floor and the electrode, a conductive gel will be applied in the area between the building floor and the plate, and between the plate and the electrode.This carbomer-based conductive gel, commonly used in laser hair removal and ultrasound analysis, does not leave stains and is easily removed, making it suitable for use on protected pavements, which is equipment known in the state of the art, and specifically in the previously mentioned document by Vásconez-Maza et al., (2020). The electrodes located on the outer and inner perimeter of the tower will have a variable separation of approximately 0.5 to 2 meters, depending on the building's dimensions and the equipment cable. This will cover the largest possible area to detect the foundation type and its geometry. A dipole-dipole configuration will be used to ensure good resolution and data coverage at the surface, while a pole-dipole configuration will be used to reach greater depths. The measuring equipment will consist of a resistivity meter with multi-conductor cables connected to the stainless steel electrodes. The continuous profile must be distributed along the outer and inner perimeter of the building to obtain a true 3D model of the subsurface's electrical resistivity values, enabling the detection and estimation of the unknown foundation type and its geometry. It should be emphasized that, in the scenario where the outer perimeter of the tower is not paved, the stainless steel electrodes will be connected to stainless steel metal rods driven into the ground to reduce electrical contact resistance and properly transmit the electrical current to the subsoil. The recorded data is processed, including filtering, topographic correction, and removal of erroneous data, resulting in a 3D model of electrical resistivity values ​​for the tower's subsoil at appropriate investigation depths. This process is then carried out using computer software. Furthermore, a preliminary estimate of the foundation's dimensions is obtained by locating changes in the material's electrical resistivity that may define it, as well as characterizing any potential cavities. These ERT results will be compared with MASW and ground-penetrating radar measurements. Passive nonlinear L-shaped Multichannel Analysis of Surface Waves (MASW) profiles, adapted to the tower's configuration and located within the area studied by the ERT (Earth Reduction Tracking System), are performed. The profiles will be executed on the building's inner and outer perimeters (2), specifically, each L-shaped profile will be placed at each corner of the tower, both inside and out. The equipment for performing the passive nonlinear L-shaped MASW profiles comprises a plurality of geophones with variable spacing between them and a seismograph. The objective is to obtain shear velocity (Vs) values ​​at each of the outer and inner corners and thus corroborate the data obtained from the ERT measurements. Ground-penetrating radar profiles are carried out across the entire accessible interior surface and along the exterior perimeter, covering an area of ​​2 m from the building facade (3), coinciding with the surface studied with the ERT and the MASW. High and low frequency antennas will be used, with the aim of reaching different depths of investigation. There is a processing, by means of computer programs, of the MASW and ground point radar measurements, obtaining the results of each of the profiles made. A computer program establishes a correlation between the results obtained with each geophysical technique, allowing the 3D model of the ERT (Environmental Risk Assessment) to be double-checked for accurate interpretation. This enables the characterization of the foundation, identifying its type and geometry, and improving the understanding of the building's subsoil for the proper execution of future interventions. Furthermore, it allows for the identification of cavities, for example, characterized by very high electrical resistivity values, always verified with the support of MASW (Movement Structures and Ground Penetrating Radar) results. It can therefore be stated that the system comprises: - a team performing a continuous electrical resistivity tomography profile along the external and internal perimeter of the tower, - a computer program executed on a computer module, which processes the data obtained by the equipment executing a continuous electrical resistivity tomography profile, carrying out the filtering, topographic correction and elimination of erroneous data and obtaining a 3D model of electrical resistivity values ​​for the subsoil; - a team for executing non-linear passive L-shaped Multichannel Surface Wave Analysis profiles adapted to the tower configuration, and these profiles being located within the surface studied with the ERT equipment; - a team to carry out ground-penetrating radar profiling across the entire accessible interior surface and along the exterior perimeter; - computer programs executed on a computer module, which processes the data obtained by the Multichannel Surface Wave Analysis profiling equipment and the ground-penetrating radar profiling equipment, obtaining the results of each of the profiles carried out; - a computer program executed on a computer module that establishes the correlation of the results obtained with each of the geophysical techniques, allowing the real 3D of the ERT to be doubly verified by the MASW and ground-penetrating radar profiles. In a possible embodiment of the invention, the computer programs are processed in a single computer module, meaning that this single computer module is common for processing the data with each of the techniques. The applicant is unaware of a system as effective as the one previously described. It should be noted that, throughout the description and claims, the term "comprises" and its variants are not intended to exclude other technical features or additional elements. Brief description of the invention's features Figure 1 shows a schematic representation of the arrangement of the different equipment of the invention, where the arrangement is shown of the execution team of a continuous ERT profile (1) along the external and internal perimeter of the walls of a "typical" tower (T) with the objective of achieving a real 3D of the electrical values ​​of the foundation by processing the data recorded by means of a first computer program of the MASW profiling equipment (2) passive nonlinear L-shaped profiles adapted to the tower configuration (T), carrying out these profiles within the studied area with the ERT equipment; and the arrangement of the ground-penetrating radar profiling equipment (3) on the entire accessible interior surface and along the outer perimeter of the tower (T); and where the data from both pieces of equipment are processed in their respective computer programs obtaining the results of each of the profiles carried out; and where there is a fourth computer program that establishes the correlation of the results obtained with each of the geophysical techniques, allowing the real 3D of the ERT to be doubly verified by the MASW and ground-penetrating radar profiles. Figure 2 shows a perspective view of an electrode of the continuous ERT profile execution equipment, where a plate with a steel spring for clamping an electrode onto the plate can be seen, where the clamping of the spring to the plate is carried out at the ends of the spring through two sheaths welded to two opposite side faces of the plate, and where the arrangement of the conductive gel is established between the pavement and the plate, and between the plate and the electrode. Figure 3 shows an aerial view consistent with Fig. 2. Figure 4 shows a front elevation view according to Fig. 2 Figure 5 shows a side elevation view according to Fig. 2 Figure 6 shows the correlation of values ​​in an example of (A) values ​​obtained with the ERT equipment using a real 3D model; (B) Vs values ​​obtained with the MASW equipment; radargram obtained with the ground-penetrating radar equipment, specifically a radargram obtained with a 250 MHz antenna (C1) and a radargram obtained with a 500 MHz antenna (C2). Description of a method of carrying out the invention. In a practical example, the selected historical tower building, listed as a Site of Cultural Interest (BIC), is the tower of Murcia Cathedral. As is known, this tower is located in the city's urban and historical center, surrounded by other buildings and with limited space for implementing known techniques. This invention allows for the optimization of the placement of continuous ERT profiles (1) with reference to accessible areas and the condition of the pavement. A measurement recorder is available using ERT. This is achieved by a plurality of electrodes (8) interconnected by multi-conductor cables (10). Figure 2-5 shows a preferred embodiment of one of these electrodes, where the electrode holder for ERT measurement consists of a square aluminum plate (4) measuring 100 x 100 x 10 mm, with a steel spring (5) for securing the electrode to the plate. The spring is secured to the plate at its ends by two aluminum sheaths (6) welded to opposite sides of the plate. These sheaths are 20 mm in diameter and 1.50 mm thick. To improve electrical conductivity, a carbomer-based conductive gel (7) is placed in the area between the plate and the electrode (8), and between the building floor (9) and the plate.In this regard, a continuous nonlinear electrical resistivity tomography (1) profile was performed along the internal and external perimeter of the tower, with a total length of 88 m. For the first phase, a DC multi-electrode resistivity system was used. This system consists of a high-precision resistivity meter, a central electrode control unit, eight cables with seven passive electrodes each, a current generator, cable connectors, and a tablet controller. The eight cables allowed for the installation of a total of 56 passive electrodes along the external and internal perimeter of the tower, following a continuous arrangement. The electrode spacing was 2 m, resulting in a total area of ​​484 m² (a 22 x 22 m square). It should be noted that this multi-electrode resistivity system requires the cable terminations to be connected to the equipment near electrodes 28 and 29. The entire 3D array of an electrode control unit was connected to the resistivity meter. This central, computer-controlled unit provided electrical measurements of the subsurface, allowing the generation of a 3D model based on the electrical resistivity of the tower's subsurface. The instrument was used in the resistivity model for current injection at 1.2 s intervals, with a range of 0.1 to 450.52 mA between each point, and for voltage measurement with a range of 0.01 to 945.35 mV between each point. A total of 56 electrodes were used in a 3D design with dipole-dipole and pole-dipole configurations. For the dipole-dipole configuration, the instrument calculated and stored a total of 924 apparent resistivity values ​​with an interval range of 1 to 9760 Ω·m. For the pole-dipole configuration, it calculated and stored a total of 640 apparent resistivity values ​​with an interval range of 1 to 9800 Ω·m. The quality of the resistivity data from the 3D model is crucial for inferring the subsurface structure of the building. As mentioned previously, in this study, the solution adopted for carrying out the ERT measurements was the use of two different configurations: dipole-dipole and pole-dipole. Therefore, for these electrical tomography profiles, multi-conductor cables connected to stainless steel electrodes are used, placed on the invention of aluminum plates with steel springs on the conductive gel arranged on the accessible surface of the building floor; where the electrodes can be arranged on aluminum plates with steel springs for clamping the electrode to the plate so as not to damage the protected element, on the other hand, if the outer perimeter is not paved, the stainless steel electrodes will be connected to stainless steel metal rods driven into the ground;where the use of the aluminum plate with a steel spring for securing the electrode to the plate can be implemented, and two sheaths welded to two opposite side faces of the plate for securing the ends of the spring to it, allowing data to be recorded directly on the property without damaging the protected historic building, since it is not necessary to make perforations to carry out the measurements; where the use of a conductive gel based on carbomer material does not leave a stain and is easily removed, placed in the area located between the plate and the electrode, and between the floor of the building and the plate, with the aim of improving the electrical contact between the floor of the protected property and the electrode;It should be taken into account that the continuous profile must be distributed along the exterior and interior perimeter of the building, in order to obtain a real 3D representation of the electrical resistivity values ​​of the subsoil, and to obtain a first approximation of the dimensions of the foundation, locating the changes in the electrical resistivity of the material that can define it. The data is processed, including filtering, topographic correction, removal of erroneous data, and inversion, resulting in a 3D model with a surface area of ​​22 x 22 m2 and a depth of 17.7 m. This provides a first approximation of the foundation's condition, particularly useful for towers without existing subsurface documentation that require immediate intervention. Once the real 3D model was created with ERT and the results obtained, MASW and ground-penetrating radar profiles were carried out on the surface studied with the ERT. For the MASW measurements, eight passive nonlinear L-shaped 1D profiles were created, adapted to the tower's unique features. These profiles combined the use of 4.5 Hz and 1 Hz geophones, spaced 1 m apart on the exterior and 0.5 m apart on the interior. The equipment used was a seismograph with 24 geophones: 17 at 4.5 Hz and 7 at 1 Hz. A total of 35 ground-penetrating radar (GPR) profiles were carried out using multiple 250 and 500 MHz antennas to reach the established investigation depths. Measurements were taken across the entire accessible interior surface of the building with both antennas and subsequently along the exterior perimeter, up to a distance of approximately 2 meters from the facade. The MASW and ground-penetrating radar measurements are processed using computer programs, obtaining the results for each of the profiles carried out. Using a computer program, the correlation of the results obtained with each of the geophysical techniques is established, allowing the real 3D of the ERT to be doubly verified for a correct interpretation, making it possible to characterize the foundation, and where it is possible to identify the type of foundation and its geometry, improving the knowledge of the subsoil of the building for the proper execution of future interventions, and allowing, for example, the identification of the presence of cavities, characterized by very high values ​​of electrical resistivity, and always verified with the support of the results of MASW and ground-penetrating radar. The results obtained from the practical example of the tower of Murcia Cathedral are shown in Fig. 6. With the application of the system of the present invention, two notably differentiated zones are determined, one zone located in the first 5-6 m of depth with electrical resistivity values ​​above 100 Ohm.m, and another located from 5-6 m of depth with low electrical resistivity values ​​(< 100 Ohm.m), marking the limit between the foundation and the substrate. To verify the ERT results obtained, Vs values ​​and radargrams were acquired. The results show significant increases in Vs values ​​between 5 and 6 m depth, and significant changes in reflections were also observed in the radargrams at these depths, indicating changes in the material's characteristics. This can all be related to the presence of a foundation slab in the first 5–6 m of depth. The results are shown in Figure 6. Furthermore, highly resistive zones with values ​​exceeding 5,000 Ohm·m were found, coinciding with a significant decrease in Vs values ​​along a length of approximately 1 m. Significant reflections consistent with cavities were observed in these same zones on the radargrams. Thus, data obtained with the 500 MHz antenna at a shallower depth but with higher resolution helped confirm the existence of hyperbolas consistent with cavities that could be approximately 0.80 m high. All of this can be related to the presence of cavities in the subsoil of this building, as shown in the aforementioned Figure. The results obtained show that the described system allows for the rapid and versatile characterization of the foundations and their geometry in listed buildings. This invention represents a significant advance in determining the type and shape of the foundations beneath these protected buildings using non-invasive techniques. It allows for reaching adequate depths of investigation in confined spaces, as well as identifying potential cavities. Therefore, uncertainty regarding the foundations is minimized, as reliable information is obtained about the state of the subsoil in the area under study. This information enables the scientifically sound definition of future maintenance and conservation measures for the protected heritage, saving both time and costs.

Claims

1. System for characterizing foundations of protected buildings, characterized in that it comprises: a continuous ERT profile execution device (1) along the external and internal perimeter of the walls of a tower (T) to obtain a real 3D representation of the electrical values ​​of the foundation, which are processed by a first computer program; a passive nonlinear L-shaped MASW profile execution device (2) adapted to the configuration of the tower (T),where these profiles coincide within the studied surface with the ERT equipment; and where the data are processed in a second computer program and a ground-penetrating radar profiling system (3) arranged across the entire accessible interior surface and along the outer perimeter of the tower (T); and where the data are processed in a third computer program and a fourth computer program for correlating the results obtained in the previous programs; and at least one computer module where the computer programs are executed.

2. A system according to claim 1, wherein the continuous ERT profiling system (1) comprises a plurality of electrodes (8) connected via multi-conductor cables (10), located on an aluminum plate (4), with a steel spring (5) for securing the electrode to the plate,wherein the spring is attached to the plate at its ends by means of two sheaths (6) welded to two opposite lateral faces of the plate.

3. A system according to claim 2, wherein a conductive gel (7) is disposed in the area between the plate (4) and the electrode (8).

4. A system according to claim 2, wherein a conductive gel (7) is disposed in the area between the building floor (9) and the plate (4).

5. A system according to claims 3 and 4, wherein the conductive gel (7) is based on carbomer material.

6. A system according to claim 2, wherein the central electrode control unit is connected to a resistivity meter.

7. A system according to claim 2, wherein the central electrode control unit is connected to a current generator.

8. A system according to claim 1, wherein the computer programs are processed in a single computer module.

9. A system according to claim 1,wherein the MASW profiling equipment comprises at least one seismograph with a plurality of geophones between 4.5 Hz and 1 Hz.

10. A system according to claim 1, wherein the ground-penetrating radar profiling equipment (3) comprises a plurality of high- and low-frequency antennas.