Structural assessment of a roman aqueduct “Pont del Diable” in Tarragona by F.E.M.

Jaume FABREGAT1, Anna ROYO1, Agustí COSTA 2, Gerard FORTUNY3, Josep LLUÍS4. (1)Architecture Student, ETSAR, University Rovira i Virgili, Reus, Spain. [email protected], [email protected]. (2) Pd.D., Student, ETSAR, University Rovira i Virgili, Reus, Spain. (3) Pd.D., Informatics Engineering and Mathematics Department, ETSAR, University Rovira i Virgili, Reus, Spain. (4) Pd.D., Construction Department, ETSAR, University Rovira i Virgili, Reus, Spain.

Abstract The object of study is a roman aqueduct which construction was ordered by Emperor Augustus the first century b.C. in Tarraco, a city in the north of Spain, today known as Tarragona. The city was declared Heritage of Humanity by UNESCO in 2000, some restoration works were planed since then and those concerned the aqueduct as well. The restoration work consisted of a landscape performance and a physical and mechanical review of the aqueduct state in order to reinforce it if it was required. The landscape performance had the responsibility to rehabilitate a green area. Its direct relation with the River Francolí makes it a performance of great interest for the city, since it is its most important green corridor. The study analyzes the entire structure of the bridge considering several load cases. The purpose is to obtain data about the structure physical and mechanical behavior. With this information we will reach several conclusions concerning deformation and stress parameters. Hypothesis of load cases will take into account weathering effects since these would have changed stone properties through the years. Furthermore the aqueduct will also be analyzed when affected by horizontal loads. By superimposing the entire cases we will highlight the weak points of the aqueduct. This could help taking relevant decisions in order to reinforce the zones detected as well as streamline the restoring process.

Keywords: Structural Assessment, F.E.M., Aqueduct, Weathering, Heritage.

1. Introduction Aqueducts are a sample of the Roman Empire engineering resources. Water engineering had been widely developed in times of the Roman Empire. They had conquered most of the land surrounding the Mediterranean Sea, and at their step they funded the most important cities which began as a military campus and have remained until today. These were located thoroughly in strategic and favorable places. Several parameters were considered for their set up. Topography, vision range and resources like freshwater were taken into account, parameters such as these formed the basis for a new urbanism which would help developing a new civilization over the rest. Tarragona is one of these cities which origin goes back to these times. Even though there is awareness of the presence of previous cultures in this site (Phoenician, Iberian,...), Romans were the ones who decided the nowadays city location. Moreover, they are responsible of its urban planning structure which considered as well the incoming canalizations of the aqueducts in order to provide the city with freshwater.

The water supply was effectuated by 2 aqueducts; they brought water from the River Francolí and from the River Gaià respectively. From these two hydrological structures just remain a few traces and stones. However, as these water webs were studied from a topographical point of view, sometimes,

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romans were forced to build big structures to cross valleys they couldn’t save in any other way. These great constructions had survived through the years because of their size and consistency. It is a structure of this nature in which we have focused on our analysis.

2. Aqueduct physical and mechanical description

2.1 Physical description The aqueduct was built in order to save a distance of 217m and a depth of 27m of a valley named “Vall de les Ferreres”(Fig.1). It is said that the name of the Valley rises from the idea of the shape of the bows recalling yellow bright horseshoes. The equilibrium of the structure is based on download arches. The bridge was built up with ashlars, superposing stones without any adding material. On the other hand, the bases of some of its pillars are widened to the contact with the ground. And there, in the basis they may be provided with a matrix of concrete which function is to fill the structure and promote its monolithic behavior. We can say that the structure of the bridge has survived until today due to stress compression. The more weight the aqueduct had, the more stable it would be. Ashlars had variable sizes. The bigger ones were placed at the bases whereas the smallest were placed at the top. Furthermore as can be imagined, there were other singular pieces specially sliced, and modeled. These are the belonging to the arches and the capitals. As can be seen in Fig. 1 the bridge is composed of 11 arches in the base and 25 in the first level.

2.2 Material Parameters Most of the stone used in the construction of public buildings and spaces of the city was taken from the Mèdol Quarry, located 6 km far from the aqueduct. However, recent studies have reached new evidence that the aqueduct was built with a stone coming from a quarry that used to be immediately under it. This seems to be logic since the pillars of the bridge rises on visible rocky foundations (Fig.2). IGC (Institut Geològic de Catalunya) studies have detected that the area where the bridge stands corresponds to a kind of sandstone “Calcarenita escullosa”(Fig.3).

Fig. 1. Avobe Fig. 3. Left

Fig.2. Up.

Fig. 1: 2D Plans for the aqueduct model. Fig. 2: Image of the base of a pillar in contact with the ground. Fig. 3: Image taken from de IGC data base. NMe - Local Stone.

This is a sedimentary rock. It may contain fossils and be quite soft but on the other hand may be well cemented and resistant. Its color changes as the weathering processes advance. In its origin, the color may have been pink and white but it can turn to grey or yellow depending on its composition. The porosity degree is high; this is a reason why it might be a rock which can be easily sliced. In consequence rock density is neither high. It is 24 kN/ m3 approximately. This affects directly to the results of the tests required to get the value of the resistance compression of the rock. So as to prove this resistance to compression we proceed to make a stress analysis. Its purpose is to compare the resistance of the stone of the bridge and the resistance of the rocks found in its surroundings. This way we could see how much the weathering processes have affected the stone of the structure. Data was obtained in the same place. The device used to help the analysis being developed was a Schmidt Hammer (Fig. 4). It had to be found a reliable stone in the surroundings which could be used

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as a basis. The same had to be done with the bridge, looking for a surface not too damaged to carry out the test.

Fig 4. Fig 5. Fig 6.

Fig 4: Schmidt Hammer while making the stress analysis to the local stone. Fig 5: Local Limestone Stone. Fig 6: Stone of the bridge under weathering process.

Table 1: A 30 31 32 36 36 37 38 38 40 41 42 42 43 43 44 45 45 46 47 50 40,2 B 0 0 14 14 14 14 14 18 18 18 18 19 21 22 22 23 24 25 26 26 22,6 B* 0 0 0 1 1 10 10 10 10 11 11 11 11 12 12 12 14 14 16 16 12,9 A: Local Stone. Found in the surrounding of the bridge. B: Weathered stone of the bridge. B*: Weathered stones of the bridge. It fades with the touch of the hand.

Table 1: Data obtained with the Schmidt Hammer.

Table 2: Table 3: SH Direction Density Resistance to value Test (kN/m3) Compression (MPa) A 40,2 24 kN/m3 70 MPa B 22,6 21 kN/m3 27 MPa B* 12,9 16 kN/m3 16 MPa A: Local Stone. Found in the surrounding of the bridge. B: Weathered stone of the bridge. B*: Weathered stones of the bridge. It fades with the touch of the hand.

Fig 7.

Table 2: Compression Stress Resistance. Table for Schmidt Hammer. Table 3: Resistance to compression values. Fig 7: Weathering adverse effects on the stones of the bridge.

Once a reliable surface is detected, we proceed to do several tests in order to get an average result. The results of the tests gathered with the ones obtained by the Schmidt Hammer, are shown in Table 1. These values correspond to the value for the rebound of the hammer when applied perpendicularly to a surface. The lower 50% of the results are not taken into account, and the average is obtained with

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the other 50% of the higher values. The Table 2 is used in order to determine the uniaxial resistance to compression of the stone. This Table relates the value gotten from the Schmidt Hammer with the density (kN/m3) of the rock so as to get the uniaxial resistance to compression (MPa). Results for tests uniaxial resistance to compression are shown in Table 3. It is shown that local stone presents higher results in comparison with the weathered one. These results could be associated to the starting properties of the stones of the bridge just being cut. The values in the second and third cases are not higher, as they belong to weathered stone tests. We can see that stone resistance tends to 0 as the weathering effects are more adverse. This kind of stone, calcarenita escullosa, turns to dust as it weathers as shown in Fig. 7. It is easy to imagine that a stone in this state of conservation will not offer any resistance to compression (Values of 0 in the Schmidt Hammer test).

3. The model The object of study was drawn and modeled in order to perform the structural assessment by the Finite Element Method. The calculation was carried out by the free software Salome Meca 6.3. An accurate drawing of aqueduct is made using a CAD application (Figure 1). These were the basis for the 3D model, done with the same software. The Model is drawn as a single solid and it could not contain any other elements such as lines, polylines or surfaces. These were basic impositions to enable the assessment. The modeling process is shown in Figure 8.

Figure 8: Modeling Process. From 3D CAD model to Mesh .med file in preparation for FE study.

These impositions do not allow modeling an exact replica of the aqueduct, so it is necessary to make a simplification of the geometry, which means the analysis will offer us a general view of the bridge behavior. When the model is prepared, stress assessment in Salome Meca 6.3 can start. We considered one single and homogeneous material which characteristic properties are introduced. The software will not consider the complexity of the components of the bridge, since the aim of the study is to approach us to a general acknowledge of the behavior of the entire body. Once the model 3D in .stp file is imported to Salome Meca 6.3, we turn it into a 3D Mesh (Hypothesis Netgen Simple Parameters). The mesh will be given nodes values. And the body is finally given the data gathered in the previous studies.

The calculation procedure will be an isotropic linear elastic study on the 3D Mesh. The first material properties required are the Young’s modulus and the Poisson’s ratio; these are shown in Table 4. Since the value of the density of the stone is known in each state we can deduce the Elasticity modulus by the checking Figure 9: In addition we will have to decide the boundaries conditions, such as adding imposed degreed of freedom on groups of the Mesh or adding pressure on them, depending on the case we will be working on.

3. Calculation Cases: The aim of the assessment is to get qualitative data about the structure behavior in order to detect the weak points of the structure, those which are more likely to fade. Highlighting these areas can help preventing future damage among the structure. First cases will look forward to get information about the structure natural compression working behavior. Horizontal loads will be studied as well in the last cases so as to figure out what are the lines and zones affected by this kind of load. The calculation cases look for the comparison between an ideal state of the rock and a weathered state of it, after 2000 years. This will show how much weathering affects the behavior of the structure in addition to the information related to the potential damaged areas provided by vertical and horizontal loads. This way the calculation cases are the following:

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Figure 9: Table 4: Elasticity Modulus Poisson’s ratio A 30 000 N/mm2 0.23 B* 7 000 N/mm2 0.23

Fig 9: Rock classification based on the relative modulus (E/σ) Table 4: Elasticity Modulus and Poisson’s ratio for Local sane rock and for weathered stones of the bridge.

Vertical Load Cases: a) Ideal Behavior of the bridge. Local Stone Data. a1) Under weathering processes. Bridge Weathered Stone Data. Horizontal Load Cases: b) Wind effect over Local Stone Data. b1) Wind effect over Bridge Weathered Stone Data.

Case A This first case looks forward to find the qualitative behavior of the initial state of the bridge. The model is provided with the values belonging to the local sane stone in order to approach the initial situation in which weathering agents had not affected the structure yet. So as to carry out the assessment an external load is needed, in this case as we want to know the behavior of the bridge against its own weight we divided the bridge in two so that we calculated the load of the top first 2 meters, and used it as a load. A scheme of this division is shown in Figure 10. The value for this ‘external load’ is 16 600 900 N.

Figure 10:

Figure 10: Scheme of added pressure on the bridge mesh.

After calculating the case, the information of the stress is offered as a Scalar Map (Figure 11). The degradation of the colors shows the magnitude of the tensions. Reddish points are the ones which stand more compression stress while cooler colors show which parts of the aqueduct do not suffer as much. This kind of representation makes it direct and simple to detect such critical areas. As can be seen weight goes directly to the ground through the vertical structure while the arches just help bracing the structure and download their own weight to the supporting pillars. It can be observed that capitals and other bulky items do not make any structural function as their coloring is the coolest. The deformation shape (Figure 12) shows which points are the ones to suffer more deformation.

Case A1 The second case is a replica of the first. Otherwise, we will use the data belonging to the weathered state of the stone (B*). We see that the degradation and distribution of the compression stress Scalar Map colors is the same since the scale in which stress is distributed does not change (Figure 13 and Figure 14). However stress and deformation values have increased. In this case the porosity increases and so the density drops, the resistance of the material is reduced and as a result deformations increase. So, logically the structure becomes much more vulnerable to the attack of weathering agents. In addition to the effect of the wind, water or salinization, there are others such as insolation which can

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make the stone surface suffer superficial movements of expansion and contraction which may weaken it. The collection of weathering agents can turn the stone into dust until it disappears.

Case A

Fig 11: Compression Stress Scalar Map. Case A. Fig 12: Deformed shape. Case A.

Case A1

Fig 13:Compression Stress Scalar Map. Case A 1. Fig 14: Deformed Shape. Case A 1.

It can be said that this case is a case of a state in the procedure of a stone in weathering, which means in the procedure of disappearing, unless any external agent acts in order to avoid it.

Case B The following two cases will make an assessment of the effect of the horizontal loads on the structure. To carry out the study we will take into account the year average load of the wind (0.52 kN/m2) in the location in addition to its own weight load. In this first case the properties of the stone will be the ones belonging to the sane rock. The load of the wind will be applied on the face facing the wind direction. Although the surface hit by the wind is lightened by arches still it offers resistance to the horizontal load. The Scalar Map and the Deformed Shape for the wind horizontal load first case show how much and in which zones the stress and the deformation are higher. Pillars are subjected to cut stress showing a kind of a circle stress area which center is in the middle of the top of the structure. This can be clearly seen in the deformation Scalar Map. So we can guess that in an extreme case, only the base of the pillars connected to the ground would remain, while the area described by the stress circle would collapse. Moreover, wind as a weathering agent, transports salinity from the sea, as well as grains of sand which hit the named surface. At the place we could check how a face of the aqueduct is more affected than the other because of the wind weathering effect. With this case we can add a stress line to the results gathered in the first two cases. Not just some isolated points and corners suffer stress, but the whole structure in case of horizontal loads. It can be seen in the Scalar Maps that the arches help the structure working as a web in its entirety.

Case B1 In this last case we will assess the structure of the bridge in a weathered state (B*) for horizontal wind load. Again, the Scalar Maps are similar to the previous. The values of deformation and stress increase since the stone is not as resistant as in Case B. It is proved that the structure suffering is higher if nothing stops the weathering effects.

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Figure 18 shows the effect of the wind on the edge of one of the first floor arches. The face hit by the wind tends to lose its original shape while the other remains. Lost volume of the edge of the arch is colored in red.

Case B

Fig 15: Compression Stress Scalar Map. Case B. Fig 16: Deformation Scalar Map. Case B.

Case B1

Fig 17:Deformation 3D Scalar Map. Case B1. Fig 18: Picture showing the bad state of the face of the arches facing the wind.

4. Results Structural assessments enable us to detect the points we have to take more care of in this type of structures. Cases A and A1, which carry on an assessment of vertical loads, show that in this case corners and the parts in which the volume is reduced such as singular points and edges are the ones that suffer more stress. In this case, some parts of the bridge held the function of bracing the structure and do not really support important loads. This would be the situation of the arches and in an ornamental way, of the capitals. After seeing the third case, we can see that there are other critical areas to take into account. We could add a new critic line to the scheme of points we have to beware of. It consists of an approximate semicircle drawn from the top center of the structure. This line marks the place that suffers most when the bridge supports horizontal wind load. As the Scalar Maps show the distribution of the stress and the scale of the deformation are the same in N and N1 cases, otherwise values change because of the loss of resistance. These values will always get worse due to the weathering effects, up until the extreme situation of making the stones disappear. Recently some restoration works have been done so as to protect the monument, among them to reinforce of weak points of the structure of the bridge. Some parts of it had been replaced in their entirety because of their bad state. In order to contrast the data gathered in this study with the current state of the aqueduct some pictures were collected to portray some damaged zones or parts of the bridge (Fig.19). Weak points detected in the analysis are highlighted with a red point in the Scalar Map elevation. These images corroborate the fact that the small parts of the structure such as corners and edges are the ones to suffer more and are harder to maintain trough time due to their fragile state. It can be seen that some of the parts have been already restored by the recent works due to the opening of the new green area surrounding the bridge (Fig 20).

5. Conclusion Structural assessment by Finite Element Method using software Salome Meca 6.3 enables easy and fast analysis of structures behavior. Although it is qualitative data what we get in the assessment, these procedures make the task of preventing deterioration of monuments more precise and comfortable. This kind of analysis helps deducing which parts of the object are likely to suffer problems. This is due to the collection of several weathering processes in addition to the own stress efforts. As seen the areas affected correspond in most of cases to bulky surfaces and parts in which

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the area of the stone is reduced. These are the cases in which weathering processes are more violent with the stone breaking it into small pieces or even turning it into dust so that the monument slowly reduces its shape and volume.

Fig.19

Fig 19: Pictures taken on 28th December, 2012.

Figure 20

Figure 20: Panoramic view of the bridge and its area.

Knowing the exact points that require more attention in the prevention of deterioration, it becomes necessary to imagine solutions so as to slow down the rhythm of the harming weathering processes. Sometimes there is no chance to act forcefully replacing damaged stones by new ones which weathering state is not as advanced. Other times, we may find ways to protect monuments in an indirect way. It should be necessary to develop a project focused on protecting the surface of the monument stones. These could consist on an aerodynamic project, for example proposing obstacles to the wind such as trees in order that it would not hit the surface of the aqueduct as hard. Water should also be considered proposing drains surrounding the aqueduct to reduce the capillarity of the stones. This kind of performances might be helpful to protect monuments while taking care of landscape. Nowadays, protecting Heritage of Humanity has become a worrying topic because of the dramatic fading of the entire built Heritage. It is a problem that deserves a special attention since it advances quietly. However, there are simple ways, such as this, to start preventing and taking into account those parts of the monument which require immediate attention.

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Bibliographical References [1] GONZÀLEZ de VALLEJO, L. Ingeniería Geológica. 1ª ed. Madrid: Pearson Ed., 2002. ISBN 84- 205-3104-9. [2] LAPUENTE MERCADAL, M.P; AUQUE SANZ L.F. Efficacy of several organic products for the treatment of sandstone at the Monastery of Sijena (Huesca). In AA.VV. Proceedings of. City: Geogazeta, 34 (2003), p. 75-78. [3] BENAVENTE,D., GARCIA DEL CURA, M.A., ORDOÑEZ,S(2003). Salt influence on evaporation from porus building rocks. Cnstr. Build. Mat, 17,113-122. [4]CHAROLA,A.E. (2004). Deterioration in historic buildings and monuments. 10th International Congress on Deterioration and Conservation of Stone, Stockhold (Sweden), Vol.1, 3-14.

[5] http://www.igc.cat

[6] http://www.icc.cat

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