Structural Analysis of Historical Constructions - Modena, Lourenço & Roca (eds) © 2005 Taylor & Francis Group, London, ISBN 04 1536 379 9

Structural studies developed on "The Miracles Roman Aqueduct" in Merida (Spain)

A. Garay Palacio Labein Foundation, Bilbao, Spain

ABSTRACT: In roman times Emerita Augusta (Mérida) was an important city in the eastem part of the Iberian Peninsula, with an important roman aqueduct that started from Proserpina dam and carried water to the city crossing the Albarregas river. The Mirac1es Roman Aqueduct of Merida exhibits a series of deformations, fissures and leanings which have motivated the undertaking of a series of previous studies for the design of an execution project for the strengthening of the structure and/or restoration works. In 2002 , an important team of professionals leaded by LABEIN studied several aspects of the construction of this roman aqueduct to the Ministry ofCulture ofthe Junta de Extremadura. This paper deals with the structural and foundation performance founded during the development ofthe studies aforementioned .

INTRODUCTION

The structural study that will be analyzed in the present document is part of lhe works of Technical Assess­ment and Redaction of the Project of consolidation and Illumination of The Miracles Roman Aqueduct (Mérida). The Technological Center Labein, institu­tion in charge of this work, has studied the monument in coordination with other organizations, due to the multi-disciplinary character required. The participants involved have been:

- LABEIN Technological Center - rNTROMAC Technological Institute - Laboratory of Architectonic. Photogrammetry.

University ofValladolid - Arqueocheck - INASMET Technological Center - Fernando Hernandez Mancha, Architect - Manuel Martín Castizo, Engineer

The global project has consisted of 5 parts:

- Part I : Planimetry survey. The mapping, carried out with the techniques of c1assic topography and pho­togrammetry, has the following characteristics, suit­able metric precision on the edition scale, intensity in the representation of the detail and codification ofthe result.

- Part 2: Historical-archaeological analysis. By means of explorations, excavations and readings of faces a complete historical-archaeological study of the monument has been made and a wide

data base composed of texts and images has been created.

- Part 3: Structural and constructive studies. Within this phase, an analysis of the present structural pathologies in the monument has been carried out, as well as a study of the laying of the foundations and the development ofseveral models ofthe struc­tural behavior of the monument by the method of the finite elements.

- Part 4: Project of consolidation and restoration. Based on the knowledge adquired on previous phases, is inspired and presided over by the inten­tion to preserve ali and every single constituent elements ofthe good and its surroundings, avoiding aggressive interventions.

- Part 5: Project of artistic illumination of the aque­duct. The projected illumination aims at empha­sizing the characteristic interesting differentials, discarding any electrical infrastructure on the same one and correctly balancing the light in the differ­ent planes, giving with a form ofthe possible most natural illumination.

2 DESCRIPTION OF THE STRUCTURE AND THE MATERlALS

The Aqueduct of the Mirac1es comprises of a complete project ofhydraulic engineering that is bom in the dam of Proserpina and transports water to the city over the depression ofthe Albárregas River.

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Figure I. Faetory of ashlar masonry in abutments and eoat­ing of eolumns. Briek faetory in eross-seetional lines of the eolumns and lhe ares. Conerete in the infill ings of the eolumns.

2.1 Elements and materiais that form the structure

The aqueduct is constitued by a succession ofcolumns jointed to each other by arcs at different leveis. The degradation undergone by the structure has caused the elimination of an important number of columns and arcs between them.

The column, consist of a central square nucleus and two buttress, one to each side, in perpendicular direction to the plane that forms the structure.

This central nucleus of 3 x 3 meters is made up of an equipment formed by 5 lines of granitic ashlar work and next to 5 brick lines. The ash lar work is approx­imately of 35 cm of thickness, being the central zone ofthe column infilled by roman concrete.

The brick lines occupy ali the section of the bat­tery, and in the zone corresponding to these lines inner filling does not exist. Between the granitic lines the existence of a mortar has been detected, even in the superior zones of the columns.

The concrete f illing ofthe interior ofthe columns, are formed by sands of great grain size combined with others of so large minor and ali assembled with a mortar. In general this fi lling displays an excellent adhesion to the granitic ashlar work.

From the base to an approximate height of 15 meters, the buttress are completely formed by granitic ashlar work without any infilling (verify by endoscop­ics explorations). From the 15 meters of height, the buttress only let be of granite and happen to have the same constructive form that inside the columns.

The arcs between lhe columns are of makes of brick of average point. In the inferior lines the arc this trimmed by the lateral faces ofthe columns.

2.2 Geometric relations

The following geometric relationships between the different elements that compose the monument,

have been obtained. This relations characterize the structure and fac ilitate the comprehension of its behavior:

- Width of columnldistance between columns (light of the arc), this data provides information on the influence that horizontal thrusts ofthe arcs can exert on the global stability of each column. Consider­ing the following average values: 3 meters width of column and 4.5 meters of distance between the columns, or light of the arc, its ratio has a value of 0,66 for the analyzed structure. That value sug­gests that the horizontal thrusts of the arcs do not constitute structurally, resistant problem in the pillars.

- Thickness ofthe arc/light ofthe arc. Taking a value ofO.75 meters as average thickness ofthe arcs, and an average light of 4.5 meters, the calculated value is 1/6. This is a rather elevated value, considering the usual values in Roman works; 1/10 for small lights and 1/20 for great dimensions. Considering the strength, the greater the relation, the greater the weight the arc has and therefore the greater the efforts that it transmits.

2.3 Mechanical characteristics ofthe materiais

Several tests of mechanical characterization of the materiaIs have been carried out and the fo llowing val­ues have been obtained. Just a few values have been inferred from the norms PIET 70.

Granite physical properties: Density = 0.00259 kg/cm3, Unconfined compres­

sive strength of the stone=547.75kg/cm2, Uncon­fined compressive strength of the masonry = 40 kg/ cm2, Module of elasticity (e) = 46,612.25 kglcm2.

Brick masonry physical properties: Density = 0.00228 kg/cm3 , Unconfined compres­

sive strength ofthe brick = 223.5 kglcm2 , Unconfined compressive strength of the masonry = 28 kg/cm2,

Module of elasticity (e) = 44,800 kg/cm2.

The concrete of the inte rior of the nucleus of the columns that form the aqueduct, is made up of sand and mortar. The characteristics of sand and mortar have been analyzed separately, obtaining the following values:

Density of morta r = 0.00231 kg/cm3 , Density of the sand = 0,00284 kg/cm3 , Unconfined compressive strength of the sand = 1.218,56 kg/cm2 , Module of elasticity of the sand = 74.939 kg/cm2

.

By means of a digital treatment ofthe images, it has been possible to calculate the approximated percent­age of sand and mortar that constitute the concrete. Finally, the properties values adopted are:

Density = 0.002628 kg/cm3 , Unconfined compres­sive strength = 25 kg/cm2, Module of elasticity (e) = 50,000 kg/cm2.

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3 ASSESSMENT OF DEFORMATIONS

A detailed analysis of deformalions of the structure has been made with the data provided by the group of architectonic photogrammetry of the University of Valladolid.

The following deformations have been detected: Overhangs of the colunms, overhangs of ashlar masonry ofsome bodies ofthe colunms, deformations ofthe arcs. In the aqueduct, two types of deformations can be observed, in the longitudinal axis and in the transverse axis.

The deformations detected in the longitudinal axis, in the form of slight collapses, oscillate between 5 and 7 cm., from the base to the higher parI. That supposes between a 0.25% to a 0.35%.

The aqueduct exhibits deformations in the trans­verse axis in several colunms that preserve lhe totality or most of the buttress. The higher colunms, which have lost their buttress, are lhe ones lhat display greater deformations.

The arcs in its three leveis, are of average point, except those located between the columns number 9 and 14. ls possible, according to the data, that they have been recovered at some time. Some of the columns show losses of section in the inferior part of the same one but that does not seem to indicate, analyzing the data provided by the planimelry, thal the deformations ofthe columns are due to that reason.

4 ASSESSMENT OF FOUNDATIONS

The following data have been obtained. The foundation of the aqueduct, was studied by

means ofthe accomplishment ofa campaign of archae­ological trial pits and mechanical drilling, on the granodioritic rocky substrate.

Considering the observed data we intuited that more different types of foundations had been used.

For the time being, the materiais of the founda­tions, maintain their geotechnical parameters with a sufficient safety factor.

The most important pathology that can generate some degree of uncertainty, is the loss of volume in the bases of the columns. In order lO limit this phenomenon one specific study would be would be necessary.

5 STRUCTURAL CALCULATIONS

Ali the structural calculations were made with the intention of calibrate the behavior of the structure in the present conditions, by means of the obtaining the tensionallevel and cracking of the different structural elements. We obtained the safety facto r comparing the

tensional leveis with the ultimate or limit tension of the materiais ofthe structure. The knowledge ofthese data constitutes an objective criteria, and aided by the visual inspection, allowed the accomplishment of an approximated diagnosis of the structure, one of the ma in objectives of the works. Previous to the accom­plishment of the mathematical model that reproduces the behavior ofthe structure from the methodology of the finite elements, simple calculations of the differ­ent elements from the structure have been made, with the intention of obtaining approximate values of the unitary tensions in the masonry.

5.1 Modelling oflhe slruclure

We used the commercial program ANSYS(v6.0) to analyze the structure by the Method of the finite elements.

Two material models were considered:

- Linear elastic Analysis. Used like auxiliary tool , having in account its limitations and without the pretension to obtain exact results. This type of anal­ysis constitutes a very useful approach to the studied problem. Nonlinear Analysis. Realized to represent the main characteristics ofthe masonry, practically null resis­tance to traction and essentially fragile character. By means of a model of elastic-fragile material to traction and elastic-plastic to compression, the type of element makes specific of ANSYS (SOLlD65) allows, with measure models sophistication, to reach sufficiently realistic conclusions.

In a begilming, we introduced in the models the dif­ferent properties ofthe materiais, being created models of great complexity. This way, we considered an only material in the analyses, with the average values ofthe mechanical properties.

With the intention of simplifying the entrance of data in the program, and to maintain coherent units in each parameter of calculation, the data have been trans­formed to adapt to the international system, obtaining this way the results of displacement in meters and the tensions in Pascals (10- 5 kg/cm2).

The considered loads are the following ones:

- Own weight ofthe elements - Punctual load of 10.000 Nw, the weight of a stork

nest. - Pressure of the wind of 1.000 Nw/m2 , equivalent

to a terminal velocity of 144 km/h. This maximum value of the speed has been determined from the data provided by the installed automatic weather station in Mérida.

In order to modelling the foundations with the suit­able conditions of contour, a first rough estimate was

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Figure 2. One example ofmade analysis .

made, being observed that it hard ly did not exist differ­ence, between the considerations of embedded support and articulated support. It was decided therefore on an embedded support preventing this way turns in the base. The considered average length ofthe columns is of 27 meters. From ali the exposed data, a total of 11 structural analyses have been made.

The behavior ofthe columns was studied with great detail, contemplating the following cases:

Complete column, with both complete buttress in all his length Column without buttress Column with only one buttress.

In each one of these variables the following cases of load have been considered:

Own weight Own weight + wind pressure in one of its faces Own weight + wind pressure in one of its faces + punctual load of stork nests.

AIso, a model has been made in two dimensions of the original longitudinal section of the aqueduct submissive its own weight in which it has been tried to reproduce the original behavior of the structure. In order to be able to compare this model, with the present situation, a model with the present geometry of the structure has been developed, also in two dimensions and with the same situation of load.

Finally, a model in three dimensions with the geom­etry that displays at the present time the aqueduct, without considering the action ofthe buttress has been developed.

6 CONCLUSIONS

The fo llowing conclusions have been obtained: The considered maximum tensionallevels, with the

limitations that the finite element method has in the app!ication to this type of structures, are considered low, not surpassing the values of 14.7 kg/cm2 in com­pression and 1.2 kg/cm2 in traction. This low levei of tensions is considered essential for the structural integrity of the set.

Several columns ofthe structure could see affected its stability by the diminution of section in its inferior zones, if they are not replaced of an effective way.

Any indication has not been detected that associates the 10ss of abutments with structural fa ilures of such.

The use of the brick factory combined with the ash­lar masonry and the Roman concrete in the columns, characterizes the work, not as much deeply aesthet­ically, that without a doubt it does, like from the structural point of view. The brick lines appear only in those zones of the columns that have filling con­crete (nucleus of the columns and superior zones of the buttress).

Considering this fact, the following functions, all of them of great importance score, that in our opinion these lines fulfill:

Increase ofthe cohesion ofthe structure, by means of the connection, at regular intervals, of the four faces of the columns, in certain way bracing them. This way it has been avoided the separation offaces and the collapse ofthe structure by the propagation of small f issures.

- Also, the !ines divide the mass of filling, avoid­ing that the retraction of the mortar generate great hollow spaces in the nucleus. Improvement of the transmission of loads of the structure, by means of the distribution of the loads on both material (granite and concrete) that forms the mixed section. This fact, along with the great adhesion which they display stones of granite to each other and the concrete of the interior of the columns, has been able to lessen the negative effects of the disappearance of stones in the low zones.

REFERENCES

Carlos Casado Fernandez, 1972, Roman Aqueducts in Spain Antonio Jose Mas-Guindal Lafa rga, 1992, The computer

science methods in the diagnosis of old buildings: the Aqueduct of Segovia, Ministry of Culture

Auguste Choisy, 1999, The art to construct in Rome October 2000, Days on bridges of bovedas of factory,

Ministry ofPublic Works and the Economy Gerónimo Lozano Apolo, 1995, Course of Techniques of

intervention in the arquitectonico patrimony. Volume 2: reconstruction of buildings of factory wall s

PIET 70, 197 1, Institute Eduardo Torroja 1998, Notes of the great course bovedas Hispanic, Ministry of Public Works and the Economy

Carlos Casado Fernandez, History of the bridge in Spain Roman Bridges

Jean-PierreAdamm, 1996, The Roman construction, material and technical

Aurel io Striking Ramirez, 1992, Survival ofa hidraulic work. The Aqueduct of Segovia

Heyman, 1999, Checks the stone skeleton , Ministry ofPublic Works and the Economy

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