Comprehensive Technical Inspection of a Medieval Bridge (Ponte de Vilanova, in Allariz) Using Microtechnological Tools

Abstract

Ponte de Vilanova, a masonry bridge, was built in Allariz, Galicia in the 13th–14th centuries. It is still standing. The structure, generally well preserved, shows minor deformations and wear signs caused by environmental factors. To conduct a comprehensive assessment without impacting the bridge’s integrity, drones equipped with thermal and underwater imaging technology were employed. Aerial inspections revealed vegetation growth and minor efflorescence (salt deposits) in some areas, while aerial thermography detected temperature variations along the stone joints, indicating the presence of moisture. The granite blocks comprising the bridge showed consistent quality and preservation. The underwater inspection confirmed that the bridge’s piers are well set on the riverbed, with no major damage observed, ruling out the immediate need for repair. This approach allowed a thorough evaluation of submerged parts without requiring divers, enhancing safety and reducing costs.

1. Introduction

The Ponte de Vilanova (Vilanova Bridge) is located at the following coordinates:

• 42°11′33.0″ N.
• 7°48′10.0″ W.

It is located in the town of Allariz in the province of Orense, in Galicia, in the northwest of the Iberian Peninsula (Figure 1). Allariz is a historic town which was once of great economic and strategic importance [1].

The Vilanova Bridge is a medieval bridge, built in the 13th–14th centuries [2,3], which spans the River Arnoia, a tributary of the Miño. It is possible that this bridge replaced an earlier one built by the Romans in ancient times [4,5].

Several testimonies confirm that, throughout the 16th-18th centuries, the bridge was damaged and repaired several times [1,3] until it had the appearance it has today (Figure 2 and Figure 3).

The bridge has two arches that are very close to the semicircular point. The spans of these arches are almost equal: 11.25 m and 11.38 m (Figure 4). The intermediate pier, which is the only pier the bridge has, has a pointed starling and a cutwater. The starling is on the downstream side and does not reach the level of the roadway (Figure 2). This starling is topped with a short pyramidal cap. Curiously, the cutwater does reach the level of the road (Figure 3), forming a siding (refuge). This siding, protruding over the breakwater, has a trapezoidal shape when viewed in a plan format (Figure 4) and shows a pediment with an inscription and the coat of arms of Allariz [1,2]. The starling and the cutwater are not locked: they are attached to the bridge. This indicates that they were probably built during one of the repairs carried out on the bridge [1,3].

Various stonemason marks can be seen on the vaults, and all of them are alphabetical [1]. This detail confirms the medieval origin of the bridge [4,5].

The same goes for the profile of the bridge. It is a humpback bridge (or hump bridge): the deck follows the initial curve of each arch, rising from ramps on each side to the centre of the bridge, forming a hump-like arrangement (Figure 2, Figure 3 and Figure 4). It is therefore a humpback bridge with gentle slopes. Humpback bridges are characteristic of the Middle Ages.

The usable road width between parapets varies between 3.80 and 4.40 m (Figure 4).

The masonry of the arches is ashlar, with very uniform voussoir openings. The spandrel walls are also made of ashlars, which are of uniform size and arranged in horizontal courses. However, the masonry of the wing walls is of lower quality, as the ashlars are less uniform, and it is, in general, less well made.

The parapets are made up of two rows of horizontal ashlars with rounded ends. Their height varies between 0.50 and 1.05 m.

The bridge is in good condition. However, a slight deformation can be seen in its arches. This deformation is particularly noticeable in the ashlars of the central area of the upstream wall (Figure 3) and in the ashlars of the downstream cutwater.

Hence, the present investigation aimed to determine whether this recent deformation compromised the safety of the bridge. In order to minimize any impact on the medieval bridge, we decided to carry out this verification through non-contact auscultation techniques and mainly used drones.

Structural inspection is an essential operation in the conservation of any civil construction. In essence, it is based on checking, characterizing, and monitoring the construction as a whole; each of the different elements that make up the construction is also checked. This can be accompanied by tests that complement the diagnosis performed through visual inspection, depending on the type and scope of inspection undertaken [6].

In recent years, unmanned aerial vehicles (drones) have been used widely in various fields. These are aircraft that can be controlled remotely by an operator or programmed to be completely autonomous. The addition of certain accessories to these devices, such as recording cameras or high-resolution image capture cameras, and the development of increasingly precise and affordable microtechnology [7,8] have made it possible to use drones for this type of inspection.

Thus, in recent years, many advances have been made and a significant number of inspections have been carried out using drones [9]. The results have been very satisfactory, since in many cases drone inspections are more economical, faster, and safer than alternatives [10]. Therefore, we considered the possible application of this tool for the inspection of this bridge.

There are many and very diverse types of drones available today [11]. Therefore, it is important to know the most appropriate type of aircraft for each situation, and particularly for the operation analyzed here. Of all the classification criteria, the most relevant for us is the distinction between fixed-wing drones and rotary-wing drones [12]. There is no doubt that the fixed-wing drone has great advantages that make it suitable for a multitude of applications, but its inability to perform a vertical take-off and maintain a stable position in the air makes it unsuitable for the inspection of an old building. Therefore, the type of drone used for this work is usually a rotary-wing drone, and more specifically a multirotor drone (Figure 5). Multi-rotor drones are drones with multiple propellers (always in pairs) that take off vertically and also have the ability to rotate and hover. This makes them ideal for carrying out vertical work and maintaining a stable, fixed position in the air, thus allowing for precise analysis.

A purely visual inspection can often be insufficient, as it only analyses the visible elements that make up the buildings. This can be a significant limitation, as there may be damage phenomena that are invisible to the human eye and therefore escape the recognition and scope of a visual inspection.

Here, a relatively new technology emerges and gains importance: thermography. Thermography is an imaging technique based on the detection of radiation emitted by bodies, transforming the captured information into images that incorporate valuable data relating to their surface temperature. Such is its importance that it has not only become the subject of detailed study in the field of heritage and ancient buildings [13], but has now come to be considered as a fundamental and instrumental auscultation technique in the field of monument conservation [14].

A normal camera, such as the one built into any mobile phone today, detects the visible light emitted by objects and is able to capture it in a photograph. A thermal camera, on the other hand, detects infrared emissions: the frequency of infrared rays is lower than the frequency of visible light, which is why the human eye is unable to detect them. However, these infrared rays can be perceived organoleptically in the form of heat. The human eye is insensitive to the infrared radiation emitted by an object, but the skin is sensitive to this radiation. Therefore, a thermal camera can capture the surface temperature of objects and record it in a thermal image (or thermogram).

For this reason, and in order to obtain a more complete record of the functional strength and appearance of the Vilanova Bridge, it was decided to also carry out a thermographic analysis, complementary to the visual inspection. For this purpose, a thermographic camera attached to a mobile phone was used.

However, it can be difficult to position a thermal imaging camera correctly. For a proper analysis of any thermal anomaly, it is recommended that the thermal imaging camera be placed at a viewing angle of between 70° and 90°, measured with respect to the surface being analyzed [15]. The inaccessibility of certain areas of the bridge and the height of the photographer (in our case, 1.85 m) are important limitations: the most critical part of the bridge is the central area, which is over the riverbed.

Due to the small size and low weight of current thermal imaging cameras, any multirotor drone is now capable of carrying a thermal camera. As we have seen with visual inspections, drones, thanks to their versatility in terms of movement and positioning, can position a camera at difficult-to-access viewpoints.

This is how aerial thermography was born. The combination of thermography and the aerial piloting of the drone allows us to obtain specific information from a thermal camera from viewpoints that are very difficult to reach manually: a drone with a thermal camera is able to easily meet the condition of having the camera positioned with a viewing angle of between 70° and 90°, measured with respect to the surface being analyzed, thus overcoming the limitations of terrestrial thermography (Figure 6).

Thus, in aerial thermography, position is no longer a limiting factor. The primary limiting factor during measurement is often hydrometeorological conditions, as they can significantly impact measurement accuracy. It is recommended that survey campaigns be conducted at dawn or dusk, as this is when it is easier to detect heterogeneous areas that dissipate heat differently. At that time, cameras can locate potential construction defects in the materials making up the monument. Ideally, measurements should be taken when the ambient temperature is around 25 °C, with a solar intensity of 1000 W/m² and some cloud cover. Under these conditions, the contrast will be high enough to reveal any defects. Unfortunately, this is not always possible.

The damage that prompted this investigation, if confirmed, clearly corresponds to an issue with the bridge pier. Specifically, it involves the pier’s foundation. Part of the inspection cannot be conducted with the tools presented thus far due to the presence of the river. This water flow prevents visibility and access to certain elements of the bridge. However, analyzing these elements can be crucial for a complete diagnosis, given the frequency with which damage occurring in this type of structure originates in the foundation [16,17].

Any inspection of a civil structure begins with an analysis of the foundation and the base of the construction [18]. When these elements are visible, a visual inspection can adequately cover their assessment. However, in most cases, these elements are neither visible nor accessible. This limitation means that potential foundation failures can only be detected indirectly through visible external signs in other parts of the bridge, such as excessive movement, deformation, cracking, etc. [17,19,20]. The displacement observed between the ashlar stones in the upstream spandrel wall and in the downstream cutwater could indicate foundation issues in the pier.

Therefore, the geometric and pathological characteristics of the substructure and superstructure needed to be carefully monitored. To achieve this, we carried out visual and thermographic inspections. The first procedure that can (and should) be used to correctly detect problems related to a poor structural response in the foundations is the observation (and subsequent analysis) of the symptoms that may eventually appear in the superstructure as a consequence of the twists and absolute and differential settlements present in the foundations. For this task, the drone proved to be a key and very useful instrument.

However, there may be elements of the foundation that are in poor stability conditions with reduced levels of safety, but where these deficiencies are not manifested by obvious external symptoms at the time of the inspection. This is made even worse if one considers that there may be a risk of extreme situations or situations of unforeseen instability, as well as large-scale movements or even episodes of collapse, whether partial or even total. Therefore, we can say that it is not possible to carry out a complete inspection, which can provide guarantees of total certainty regarding the real state and stability of an ancient construction, when part of the monument is submerged under water.

The main drawback of localized damage to the foundations of buildings (not only bridges) is the difficulty of visual inspection. If the inspection is carried out during the summer period, the detection of possible problems relating to the degradation of the structural elements may be easier. Unfortunately, this is not always possible.

This detection can even be extended to problems arising from the deterioration of the soil–foundation complex. In the case of bridges, the foundations usually start at a certain depth. In addition, the foundations are usually located under water or are invisible for inspection purposes, as they are hidden by sediment. In these circumstances, a purely visual inspection would be an arduous task (if not overwhelming).

Direct damage to bridge foundations can essentially occur for two reasons [16,17,19,21]:

• Due to the degradation of the materials that make up the substructure of the building;
• Due to the poor performance of the soil–foundation complex in response to the different actions to which the complex itself is subjected.

Factors such as the action of water currents on submerged elements or the increase in the hydraulic speed of the current, due to a decrease in the section of the channel or a change in the longitudinal profile, can cause such deficiencies.

Therefore, given the huge number of catastrophes and bridge collapses linked to failures in their foundations, a detailed visual inspection of the foundation is not enough. As far as possible, it is important to estimate the type and dimensions of the foundation element, the type of terrain, the longitudinal profile of the river channel, and the cross section (upstream and downstream of the construction) and then determine the degree of cleaning of the channel and the layout of the drag elements in it. In all these operations, drones have great shortcomings and limitations.

Thus, it is often necessary to resort to the use of special inspection techniques, which can obtain sufficient information to provide complete knowledge of the real state of the base of the construction and the detailed bathymetry of the channel beneath it and in its closest surroundings, upstream and downstream of the construction. Thus, just as the use of drones was suggested some time ago for carrying out visual inspection, an equivalent tool has been sought to allow the recognition of the submerged part of the constructions under analysis: an aquatic drone (Figure 7 and Figure 8). Such a drone model has already been used for this purpose on an experimental basis [21] with notable success.

With this underwater drone, it is possible to inspect the submerged part of the Vilanova Bridge substructure. As with thermography, underwater inspection serves as a complement to visual inspections for which the common drone has proven effective.

2. Materials and Methods

In order to carry out a visual inspection using all the techniques described in the previous section, the Vilanova Bridge was chosen due to its historical and architectural value and its potential use by pedestrians or light vehicles. Going deeper, the reasons for the choice were multiple:

1.

The bridge must be preserved for its heritage value. As a medieval bridge, its original materials (stone and mortar) may have deteriorated due to the passage of time, humidity, and climate changes. A technical inspection is essential to evaluate its condition and ensure the preservation of this heritage. The early detection of collapses or erosion allows for planning interventions that respect the authenticity of the bridge.

2.

The structural safety of the bridge is essential. As the bridge is used by pedestrians, cyclists, or even light vehicles, it is crucial to ensure that its structure remains stable and safe.

The river’s flow may be eroding the bridge’s foundations (piers or abutments), which could compromise the structure. For this purpose, underwater inspection is necessary.

Other factors, such as vibrations from traffic or vegetation roots, may have affected its stability.

The separation between the ashlar blocks mentioned in the Introduction (Figure 3) contributed to making the bridge worthy of inspection.

3.

Sustainability factored into this decision. If any restoration intervention on the Bridge is necessary, it must align with principles of sustainable restoration, a practice which requires an appropriate technical diagnosis.

Furthermore, the bridge is one of the town’s biggest tourist attractions: it is essential to keep it in optimal condition to avoid accidents and ensure a safe experience. In addition, taking advantage of and optimizing a seven-hundred-year-old structure embodies sustainability.

Thus, in accordance with the above, the following microtechnological tools were used to carry out the planned complete inspection:

1

A simple Flir thermal imaging camera was used to capture photographs and thermograms. Terrestrial thermography operations were carried out with this tool. This camera was connected to an iPhone (iOS system). The characteristic parameters of this thermographic camera are as follows:

●

Thermal resolution: 80 × 60.

●

Object temperature range: −20 °C–120 °C.

●

Thermal sensitivity: 0.150 °C.

●

Accuracy: ±3 °C or ±5%. This figure shows the typical percentage of the difference between the ambient and scene temperature. This is applicable 60 s after start-up when the unit is within 15 °C–35 °C and the scene is within 5 °C–120 °C.

●

HFOV: 50° ± 1°.

2.

We used a drone Parrot Anafi model (Figure 5) that incorporated a thermographic camera that could oscillate vertically on its own axis, allowing greater adjustment for the arrangement of the viewing angle between 70° and 90° with respect to the surface to be analyzed. Visual inspection and aerial thermography operations were carried out with this tool. This drone was manufactured in Massachusetts, the United States. It weighs 320 g and is used in conjunction with a remote control linked to an iPad (iOS system) for better viewing. The technical characteristics of the thermal imaging camera attached to this drone are as follows:

●

Spectral range: longwave infrared, 8 μm to 14 μm.

●

Thermal resolution: 160 × 120.

●

Thermal sensitivity: 0.050 °C.

●

Object temperature range: −10 °C–140 °C.

●

HFOV: 57°.

3

A PowerVision PowerDolphin PDW10 (4K) underwater drone was used to carry out these inspections (Figure 7 and Figure 8). This drone was manufactured in Beijing, China. It weighs 2268 g and is used in conjunction with a remote control linked to an iPad (iOS system) for better viewing. This underwater drone has a 4K HD camera with multiple resolution settings and the ability to rotate up to 220° on its own axis, allowing it to see above and below the water’s surface. It also has a built-in GPS with bathymetric detection and “return home” functions. It has a two-hour battery life and can reach a maximum speed of 4.50 m/s (16.20 km/h). This drone model was chosen due to its simplicity in use and handling, as well as its accessibility. In addition, its low cost made it ideal for this initial research.

All operations were carried out with equipment that was calibrated before the start of the operations. We followed the instructions of each manufacturer, and always took into account the conditions of use prescribed for the equipment.

Cameras and drones were calibrated before the start of each inspection.

All operations were carried out at dawn on several days in August. Specifically, these operations were carried out at dawn on several summer days, with thermographic operations in mind. It should be remembered that thermal imaging cameras detect the different temperatures that appear on the walls of the bridge. At dawn, it is easier to observe the possible presence of non-homogeneous areas that dissipate the heat differently.

At dawn there are stable thermal conditions. Overnight, structures and their surroundings typically reach a uniform temperature due to the absence of direct solar radiation; at dawn, solar radiation is still minimal, reducing the risk of thermal interference from uneven heating.

This is very important because the Vilanova Bridge is near vegetation and water; these elements might influence the measurements due to their different heat retention and emission properties.

Thermography works best when there is a temperature contrast between different materials in the monument (e.g., stone, mortar, or metal elements) and the surrounding environment. In August, nights in Allariz tend to be cool, and if the previous day was warm, the internal temperature of the monument may differ from its external surface, creating the necessary contrast. Measurements were completed before the sun began to significantly heat the monument’s surfaces.

According to the State Meteorological Agency, temperatures ranged from 15 to 25 °C on the days of the operations at dawn. According to the Spain Meteorological Agency, temperatures on the days of dawn operations were 15 °C–25 °C. These temperatures are ideal for carrying out this work if there has been sufficient nighttime cooling.

3. Results

In the following sections, the results of each of the operations carried out are analyzed.

3.1. Principal Inspection: Visual Analysis

As already mentioned in the Introduction, the bridge is in good condition. In general terms, the granite that makes up the bridge is also in good condition. In the areas most exposed to the elements, such as the abutment (Figure 9), the drone allowed us to observe certain deteriorations that occurred as a result of the synergy of actions of a diverse nature, mainly chemical and biological. Thus, the formation of various black crusts can be observed, which is presumably linked to the action of polluting agents (particularly sulphur compounds). There is no doubt that the nearby circulation of vehicles, which still travel over the bridge, can influence the appearance of these black crusts [22].

Along with these crusts, there is an abundant presence of biocolonies (plants), which have grown by taking root in the joints between the ashlars, especially in the joints of angular points. These enter into feedback with the phenomena of humidity, efflorescence, and runoff water, as reflected in certain stains observed (Figure 9).

We decided to fly the drone inside both vaults to inspect their interiors (Figure 10). As can be seen, all the elements that make up this vault are made of granite, with dry-set ashlars and voussoirs. The rounding of the vertices, characteristic of the alteration of granite [22], is the most notable feature if we ignore the fracture observed in certain elements (Figure 11).

The inspection allowed us to verify the existence of small efflorescences, although these were found in a very small proportion of areas inspected: such lesions were observed on the intrados of the vaults (Figure 10) and on the elevation of the abutment (Figure 9). Efflorescence and crypto-efflorescence are usually concentrated around areas where there is a high concentration of humidity [22] (Figure 12). The Vilanova Bridge is a river bridge; therefore, it is evident that it is in a location where humidity is high.

On the one hand, efflorescence is a common phenomenon in construction materials, particularly on masonry surfaces. It occurs when soluble salts within these materials are transported to the surface by water and then crystallize as the moisture evaporates. It manifests as a white, powdery, or crystalline deposit on the material’s surface [23]. It results from water migration within the material, which dissolves soluble salts present in the material or its surroundings (such as soil, mortar, or additives): when the water reaches the surface and evaporates, the salts are left behind as crystals.

It is primarily a visual issue, affecting the appearance of the material, especially in structures with exposed finishes. Efflorescence itself rarely damages the structure directly. However, it may indicate moisture problems that, over time, could compromise the material’s durability [22].

The Vilanova Bridge had the right conditions for the appearance of efflorescence: we found the presence of water or moisture in the material and the presence of soluble salts in the material, its environment, and the surfaces exposed to air, where evaporation can occur [24]. It is primarily a visual issue, affecting the appearance of the material, especially in structures with exposed finishes. Efflorescence itself rarely damages the structure directly [22]. However, it may indicate moisture problems that, over time, could compromise the material’s durability. For this reason, it must be controlled.

On the other hand, crypto-efflorescence refers to salt growth within pores [24], which is more damaging than efflorescence [22,25]. This is a phenomenon similar to efflorescence, but it occurs inside the material rather than on its visible surface. It results from the crystallization of soluble salts within the pores of the construction material, and it can be more damaging than surface efflorescence. Unlike efflorescence, it is not visible on the surface but can cause structural damage: over time, it weakens the material’s structure, compromising its functionality and appearance [26,27].

The diagnosis of this issue is very complicated: since it occurs within the material, crypto-efflorescence may go unnoticed until significant damage becomes evident [28].

Both lesions are caused by salt crystallization. The phenomenon is triggered by the crystallization of soluble salts present in solution in the porous system of the masonry. If evaporation occurs on the surface, efflorescence is generated; if the drying surface is located on the inside, crypto-efflorescence is formed. On the one hand, the presence of efflorescence and crypto-efflorescence indicates that a chemical degradation process is taking place, which is generally not very dangerous; on the other hand, significant internal mechanical stresses may be generated due to the crystallization of the salt, depending on the porous system of the masonry (crypto-efflorescence) [22,24,28].

The drone also allowed us to appreciate in detail the separation between the exterior ashlars of the cutwater (Figure 13), downstream of the bridge, and at the spandrel wall (Figure 14), upstream of the bridge. Looking at this closely allowed us to see that our suspicions were well founded.

3.2. Thermographic Inspection

3.2.1. Terrestrial Thermography

Given the limitations already mentioned in relation to terrestrial thermography, the geometric characteristics of the bridge, the height of the inspector, and the inaccessibility of critical points of the structure, terrestrial thermography served above all to verify the homogeneity of the material used and the absence of anomalies in it.

Accordingly, the reconnaissance operation was carried out at dawn on a summer day. As a result, we were able to capture, among other images, a thermogram showing the downstream elevation of the bridge (Figure 15), and several details of the abutment walls (Figure 16 and Figure 17). These were accessible points at which images of the surface to be analyzed could be captured with a viewing angle of between 70° and 90° with respect to the structure.

The first conclusion is obvious: the presence of organic matter and vegetation constitutes the main anomaly observed in the thermograms obtained via terrestrial thermography. The points where the lowest temperatures are recorded correspond to the vegetation.

Neither the photograph nor the thermogram display any characteristic differences in terms of the constituent material of the different ashlars. This reflects uniformity in the granite of the ashlars. The only anomalies correspond to vegetation and cavities generated in the gaps between ashlars, which constitute critical points for future damaging phenomena [15].

3.2.2. Aerial Thermography

For the aerial thermographic auscultation, the observations made during the previous visual inspection were taken into account. Hardly any deficiencies were recorded during this inspection; the few deficiencies that were recorded were linked to the synergy of chemical and biological phenomena, which was aggravated by the humidity of the site. Fortunately, here, we have a structure made of large granite ashlars that is almost entirely lacking mortar in its joints.

The thermograms of the interior of the vaults revealed an observation (Figure 18): independently of the previous observations of possible efflorescence (Figure 9 and Figure 10), the maximum temperature values were recorded in the joints between the voussoirs or ashlars (especially in the larger cavities), while the minimum values were recorded on the most exposed external face of the vault. This is related to humidity.

As this is a bridge spanning a river with considerable flow, it is in a location with high humidity, which adds to the humidity characteristic of the site. Thermographic analysis of the intrados of the vaults allowed us to confirm this, although it was desirable to bear in mind the differential effect, where the upstream side is the west elevation (western side) and the downstream side is the east elevation (eastern side), in order to establish maximum and minimum temperature values.

The joints record the highest temperature values, with this value being especially high at the junctions between ashlars where, precisely due to the absence of mortar, cavities can be recorded (Figure 9, Figure 10, Figure 16a and Figure 17a).

The slight proliferation of efflorescence, which was diagnosed during the visual inspection carried out some months earlier on the intrados of the vaults (Figure 10), was also linked to humidity. Efflorescences are indeed usually seen around areas where there is a high concentration of humidity. However, it should be noted that the externalization of the efflorescences is not recorded on the thermogram (Figure 18).

This constitutes a risk in the face of phenomena such as the crystallization of salts or the freezing and thawing of interstitial water within the constituent material, which also demonstrates the great utility of exploring the monument with a thermographic camera.

The most interesting aspect of the aerial thermographic inspection was the detailed analysis of the areas of each elevation where the movements between ashlars were recorded (Figure 19 and Figure 20). The analysis led to the conclusion that these separations did not show any alterations in the thermograms, and therefore did not constitute any risk in terms of durability. Indeed, the temperature did not show any alteration in the cavities produced by the displacement of the ashlars.

3.3. Underwater Inspection

Many studies justify the decisions of engineers of antiquity, especially in the Roman era, to try to locate constructions (especially bridges) in sites with good foundation conditions [29]. Eugène Viollet-le-Duc assumed that, after the fall of the Roman Empire, technical innovation was practically non-existent until the Carolingian era [30]; at the end of the Middle Ages, the Renaissance recovered the Greco-Latin tradition in architecture and construction and it was imitated to unforeseen levels. During the following centuries, this imitation continued less intensely, but always with the premise of putting into operation what had proven effective, which can be seen particularly clearly in the foundations of masonry constructions. This bridge, in fact, is a clear example.

If there had been any problem with the foundation of the bridge, it would have occurred in the foundation of the intermediate pier. Therefore, the point of greatest interest for the aims of the inspection was, without a doubt, the surroundings of this construction element.

Based on this premise, the underwater drone used in the inspection was launched from an accessible point along the river, downstream of the bridge. Circulating along the riverbed against the current, it approached the Vilanova Bridge to perform inspection (Figure 7).

The drone made it possible to verify that the legs rest directly on the ground (Figure 21c), and it was possible to observe how the first ashlars rise from the sand of the river bed (Figure 21b,c).

Through the inspection of the submerged parts with the underwater drone, it was possible to verify the satisfactory state of the underwater elements: the absence of granite ashlar was not observed, and the granite constituting the submerged ashlars was in an acceptable state in terms of conservation (Figure 21b,c).

Likewise, it was possible to verify that the lower parts of all the submerged piers were in complete contact with the riverbed. This, together with the absence of deformations in the position of the ashlars, allowed us to confirm the absence of excavations under these piers.

In order to obtain a complete final diagnosis of the soil–pier interaction under the River Arnoia, a visual survey of the material that makes up the bed next to the submerged elements was carried out using the underwater drone (Figure 21a). We considered that this material, due to the erosive action of the water, can experience some displacement and that, as a consequence, the undermining of some of these partially submerged piers can occur. The analysis showed that there were hardly any irregularities along the bottom, and that the depth of the bed around all the partially submerged elements was practically identical. The river sand was interspersed with granite elements, with the bottom in this section remaining at a practically constant level.

The bed level was practically uniform, with no pits, deposits, or any other irregularities that could be caused by the action of water currents on the piers.

All of this allowed us to conclude that the submerged elements of the bridge were in good condition in terms of conservation and function, being able to serve their purpose in the short and medium term, thus guaranteeing the absence of undesirable phenomena linked to the foundation.

However, the drone here also made it possible to rule out the poor functioning of the foundation: it enabled us to rule out the existence of phenomena of degradation of the constituent material of the submerged elements and the presence of signs of the poor behaviour of the soil–pier complex due to the actions to which said complex is subjected.

4. Discussion

This work was based on the premise that it is not possible to carry out a complete inspection that can guarantee complete confidence regarding the real state and stability of an ancient construction when part of the monument is submerged under water. To achieve this, and to be able to obtain a complete diagnosis, an underwater drone was used as a complement to the airborne drone that could carry out visual inspections. This allowed the recognition of the submerged part of the substructure of a bridge that had allegedly suffered a settlement in its central pier. It also allowed the study of the bottom of the river basin in the vicinity of the bridge to verify its state in view of the verification of the soil–foundation complex.

During the development of these operations, it was possible to verify that the state of conservation of the submerged elements was notably good, with no significant deficiencies or damage related to the degradation of the immersed elements being observed in practically any of them: all the submerged constituent elements were in good condition. The underwater inspection carried out with the drone made it possible to verify that it is not easy to differentiate the structural elements of the foundation and the pier of a bridge, a distinction that is very common in old masonry bridges. In any case, the submersible drone made it possible to rule out deficiencies in the soil–foundation interaction and, therefore, any symptoms of settlement in its elements. If there was settlement, as evidenced by the ashlars of the breakwater and the spandrel wall, this settlement occurred some time ago, without having had the slightest subsequent effect on the pier that served as support and, therefore, on the rest of the bridge.

Obviously, the use of this tool can be extrapolated to other constructions of a similar nature [21]. The use of submersible drones to inspect the foundations of stone bridges is innovative because it offers significant improvements in safety, efficiency, and precision compared to traditional methods. The foundations of bridges, especially stone ones, are often located in areas that are challenging to inspect due to water currents, depth, and low-visibility conditions. Submersible drones can easily access these areas, eliminating risks to human divers, who might face dangers like entrapment, collisions, or adverse environments.

Submersible drones can inspect delicate structures without causing damage, which is essential for historic stone bridges requiring preservation.

Equipped with technologies such as sonar, underwater LIDAR, or depth sensors, drones create detailed 3D models of the riverbed and the foundation. This is critical for detecting erosion, cracks, or displacement in the base of stone bridges. Drones are equipped with high-resolution cameras and sensors that transmit data in real time. This allows engineers to directly observe the condition of underwater structures without long delays or extraction processes.

Compared to manual inspections or the use of specialized vessels, submersible drones significantly reduce operational costs and the time required to complete inspections. A very superficial economic assessment shows that, with an underwater inspection carried out with a drone (two inspections at most), compared to the cost of an underwater inspection with divers, the cost of the equipment used in the inspection of the Vilanova Bridge was amortized [21]. Drone deployment is quick and is less intrusive than traditional methods, such as building temporary structures or diverting water flow to enable physical inspections.

The bridge, despite the visible deformation, is perfectly safe. None of its elements show any significant damage. If the medieval bridge has survived so long, it is because it has been subjected to loads typical of the Middle Ages and, if these loads are not exceeded, it should have no problem surviving for many more years.

Once damage to the foundation has been ruled out, it is reasonable to assume that the separation of the ashlars is the result of the movement of the vault: if this is the case, the ashlars moved in a movement of compatibility produced in the spandrel wall, accompanying the movement in the vault. Such movement could have been the result of the formation of a triarticulation of the vault at a time when it settled. In any case, the damage is not recent and its effect on the foundation and on the bottom of the riverbed is null, as the underwater inspection was able to prove.

For its part, thermography (and especially aerial thermography) is a tool that can be used to obtain information regarding the temperature of a heritage element without the need for physical contact with it. Therefore, it can be said that it is a non-destructive technique that can complement other information (or be complemented by other information) obtained from other sources. This makes it possible to obtain real data on the state of the different constituent elements of a heritage construction and on the existence of possible damage to said elements. Thermography makes it possible to obtain information with which to predict future behaviour and thermal anomalies that may occur in some areas of the monument in order to make an adequate pathological diagnosis or, in the best of cases, to verify the good condition of the ancient construction.

As seen in the previous sections, a medieval stone bridge over a river might be exposed to erosion, moisture, and ageing. Aerial thermography can detect infiltrations or internal structural failures, monitor affected areas without the need for excavation or direct intervention, and generate thermal maps to guide precise conservation efforts.

Obviously, this experience can also be extrapolated to other constructions of a similar nature [13,15].

Thermal images can be integrated into 3D models of bridge to create detailed analyses. This helps to predict how environmental conditions or wear and tear might impact the bridge over time. Aerial thermography can complement other tools, such as photogrammetry or LIDAR scanners, to provide a comprehensive assessment of the bridge.

5. Conclusions

The results of the underwater inspection show that the use of appropriate micro-technology allows for a perfectly detailed and complete visual observation of all the submerged elements that make up a masonry bridge. This means that it is possible to avoid the use of divers for underwater inspections, which would have been necessary if this instrument had not been available. Therefore, in light of the experience collected here, the following can be concluded:

• The underwater drone allows the visual recognition of the state of conservation of the constituent material of the submerged elements.
• The underwater drone allows the recognition of the material that forms the bed next to the substructure and that, due to the erosive action of the water, can be displaced or altered, undermining piers or abutments.
• The underwater drone allows for the verification of the undermining conditions in each pier and abutment, and the estimation of the maximum depth of undermining.
• The underwater drone simplifies the planning work required for a traditional underwater inspection.
• The underwater drone simplifies the field work in terms of the identification and assessment of the deterioration of each of the constituent elements of the monument.
• The drone reduces all kinds of risks to the safety of the workers who collaborate in underwater inspections, which have inherent danger: with a drone, no worker has to, for example, expose themselves to the risk of drowning inherent to immersion.
• The six points outlined above demonstrate that drones bring considerable economic savings without compromising the quality of the work in any way.

Underwater inspection is very laborious and costly in terms of personnel, logistics and finances. Due to this cost, it is often resorted to when there is no other option. Inspection guides recommend carrying out underwater inspections every five years or even more frequently when the construction requires special monitoring, such as a construction where the foundations are particularly exposed to the action of water, where a rapid evolution of the hydraulic conditions of the river has been observed, or when there has been some anthropogenic intervention that may have harmful effects in the area of influence. The innovation of using drones, which makes underwater inspection much safer and more accessible, may be a stimulus in terms of these inspections being carried out more frequently, which will result in greater durability and better conservation of constructions with submerged elements. Aerial thermographic inspection is a complement that can help technicians in charge of heritage inspection and pathological diagnosis to obtain a new, highly relevant point of view given the angle and distance at which the photographs and measurements are taken. Similarly, when it comes to an inspection of significant dimensions, the time spent on it can be reduced, which will undoubtedly result in economic benefits for all the agents involved.

The use of aerial thermography to inspect stone bridges is innovative because it enables efficient, non-invasive, and precise detection of structural and material issues using advanced technology.

Aerial thermography utilizes infrared cameras mounted on drones to measure temperature variations on the bridge’s surface. These variations can reveal the following attributes:

• They can reveal internal cracks or microfractures that are not visible to the naked eye.
• They can reveal moisture zones or infiltrations within the stone structure, which often precede deterioration. Moisture trapped in the stone affects its thermal conductivity, which can be identified using thermography. This is particularly important for stone bridges, which are prone to damage from water infiltration and freeze–thaw cycles.
• They can reveal material degradation, such as weakened mortar or stone, which exhibit different thermal signatures.

A drone equipped with thermal cameras can inspect a bridge much faster than traditional methods, eliminating the need for scaffolding or traffic interruptions. This reduces costs and minimizes disruptions. Drone can inspect hard-to-reach parts of a bridge, such as arches, high vaults, or areas over rivers or cliffs, without putting technicians at risk.

For ancient or historic stone bridges, thermography is especially valuable for detecting early-stage deterioration without damaging the structure, allowing for targeted restoration efforts.

Therefore, aerial thermography introduces a new way to assess the condition of stone bridges by combining safety, speed, and precision. It is a key tool for preserving cultural heritage and ensuring the long-term functionality of these structures.

For historical stone bridges, such as those of Roman or medieval origin, these technologies help to identify structural issues and plan restoration interventions more accurately, ensuring the preservation of cultural heritage without compromising safety.