Structural Assessment of a Roman Aqueduct “Pont Del Diable” in Tarragona by F.E.M
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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, 676 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 677 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 678 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.