Method of intervention for transforming and via …€¦ · Method of intervention for transforming...

Method of intervention for transforming and

modernising old town centres: the case of the

vaulted masonry pipeline laid underground along

via Garibaldi in Venice

G. Riva,* P. Valle^

a Department of Construction Processes in Architecture, Venice

University Institute of Architecture, Tolentini, Venice, Italy

b Valle Architecture Office, S. Polo 1739, Venice.

Abstract

If on one hand the rationalization of sub-services in the old centres of Italian towns is anessential stage in their revitalization, on the other hand it involves complex methodologicaland planning aspects that are sometimes underestimated and which concern thecompatibility of the systems of the new plant with the existing masonry infrastructures and theproblems of keeping them in an efficient state of repair and recovering their functionality.The case considered is located in the old centre of Venice and concerns an important operationof transforming the urban structure at the beginning of the nineteenth century. This consistedof laying a masonry pipeline with a length of about 380 metres to convey the waters of a canalbranch and to catch rainwater; the pipeline is still in operation. The constructive technology ofthe vaulted masonry system is analysed by means of a survey of the elements of which it iscomposed and the structure in its static consistency and affidabiliry.is evaluated.Finally the procedures and criteria adopted for the correct preservation of these, structure areexamined with the objective of achieving equilibrium between the constructive signs of aperiod and the present-day functions of the contemporary urban system.

1. Introduction

During its execution, the project for the recovery of Via Garibaldi, previouslyStrada Eugenia, in Venice revealed a series of methodological implicationsconcerning operation on old structures in the historic town centre.The different contexts - natural, historic, narrative and scientific - were ofconsiderable importance.Strada Eugenia was created in the nineteenth century as part of a precise formaland symbolic programme with a European connotation, with the aim ofmodernizing the old town by means of a town planning project which identifiesa number of strategic areas with a view to enhancing the functionality of thetown as a whole.

Transactions on the Built Environment vol 26, © 1997 WIT Press, www.witpress.com, ISSN 1743-3509

644 Structural Studies, Repairs and Maintenance of Historical Buildings

Research in the archives enabled us to trace important iconographic documentsconcerning the formation and transformation of this part of the town.The monumental entrance to the new gardens laid out in the nineteenth centurywas obtained by covering the Rio di S. Anna, building an undergroundmasonry vault. The logic behind this springs from the very origins of the town,the creation of which always involved the reclaiming and excavation of naturalchannels and their urbanization, for civil purposes.The dimensions of this underground structure and of the street are unusual forVenice: about 380 metres long by 17.5 metres wide.The architect Giannantonio Selva, known for other work including theconstruction of the theatre, La Fenice, describes and graphically represents(figs. 1 and 2) the project for the creation of Strada Eugenia "tramite lacreazione di un acquedotto coperto a volto, per cut Vacquapossa ognorafluiree rifluire e possano in esso sfogare le fosse dei conterminanti stabilF ("bycreating a vaulted aqueduct, through which the water can flow, receiving influxfrom the drains of the neighbouring buildings"). [1]The gradual functional and economic abandoning of the peripheral areas withrespect to the flow of tourists in Venice, and especially the Castello district, ledto the physical and ecological decline of these sites, which received very littleattention as regards maintenance and repairs.

2. Characteristics of the recovery project

The recovery project by Patrizia Valle, (fig. 3) enabled the particular planningand technical features of this collective urban space, with its unusualdimensions, to be turned to account.The creation of paving with the materials historically used for pavements inVenice, such as 1 stria stone and trachyte, made it possible to exploit a space ofaround 6000 square metres, a kind of elongated piazza, for various collectiveuses.The work carried out at the same time underground made it possible to performthe necessary maintenance operations and cleaning of the principalunderground structure and of the side tunnels (fig. 4), built of masonry in thenineteenth century and now in a good state of repair.The operating criteria were fundamentally based on the maintenance andpreservation of the masonry by repairing the interruptions. In places where amore radical intervention was not required, the superficial restoration of themasonry was performed exclusively by filling in the spaces between the bricksremoving the brickwork and laying a new bed of mortar.In other cases large spaces had to be filled and parts had to be repaired whichhad been involved in falls or deep cracks, largely due to the laying ofexcessively invasive collectors, used in the Fifties and Sixties for sub-services.The maintenance and repair work also involved the side trunk lines runningparallel to the underground vault, for collecting rainwater and water from the


Structural Studies, Repairs and Maintenance of Historical Buildings 645

1 G.Selva, Public gardens and walk inCastello. General plan.

2 G.Selva, details of pavement infront of Lagoon

3 Section of underground arc andnew pavement4 Sections of on side collectors

adjoining buildings, by means of themake and mend technique usingbricks and mortar with the similarcharacteristics as the structure,reusing the old bricks whereverpossible.The possibility of doing a radical job,replacing all the existing collectorsand ducts (fig. 5) used for light,water, gas and communications, withothers that were less intrusive, laid insuitable paths, allowed high levels oftelematic and technologicalmodernization to be achieved in a partof the old town, of which ViaGaribaldi forms the backbone. Theseeffects will gradually become moreapparent also in the improvement ofthe state of repair of the facades ofhistoric buildings, which may be ridof the enormous amount of externalcables that at present spoil theappearance of Gothic doorways andwindows of Istria stone.The pavement, made of large pavingslabs and blocks of gray Euganeantrachyte, follows the path of the streetthat lined the old canal (fig. 6),physically showing the process oftransformation of the area andrepresenting, in the procedurefollowed, a precise desire to prefer thetechniques traditionally used inVenice.These techniques, both in the hand-chiseling of the stone and in thejointed laying in a herring-bone orcontinuous pattern, have proved to bethe best, despite recent attempts tointroduce the use of other materialssuch as asphalt and slabs or cubeshaped of porphyry, or the sametrachyte slabs laid interspaced withcement mortar, all of which, as wellas being less decorous, are lacking in



5 View of installation ofsubservices.

6 Project plan towards the balustradeover Rio di S. Anna.7 P.Patte. Section of a street withcentral trunk line, Paris 1796

resistance and durability. The use oftrachyte blocks coincides rationallywith the general need for organizationof the subsoil. Furthermore constant,non-destructive maintenance can becarried out regularly, both on thesophisticated technological aspects ofthe subservices and on the masonrystructures and the underground vault,which require periodic cleaning andchecking, by means of the seals whichmay be opened for inspection; theseare made of the same stone materialand form an integral part of the designof the street paving.

3. The scientific historic context

The creation of this hand-manufactured belongs to a precisehistoric and scientific context.Between the seventeenth and the lateeighteenth century, hydraulic sciencein the Veneto - among the oldest andbest known in Europe - was markedby macrohydraulic operations,diverting rivers and makingtransformations in the lagoon, and bythe conception of structures used inVenice and the territory of the Venetowith the aim of improving the qualityof life in town and country.These were real hydraulic machinessuch as transverse road drains,inverted siphons and sluices, at theservice of farming or of thesettlement, designed and calculatedaccording to the flow of runningwater and of tides. Even today thesestructures offer ample guarantee oflong life and efficiency, in a naturaland historic context such as that ofVenice which is particularly subject tophenomena of corrosion and physicalaggression, caused by living forces.



Hydraulic science in the Veneto has been mainly inspired by needs of a practicalnature and put to civil use. [3]Many of the technicians engaged in studies and structural work in the final yearsof the Venetian Republic were mathematicians and architects, like Zendrini [4]and Temanza, who also contributed to the training of others who worked in thenineteenth century. It is possible to see a number of common characteristicsboth in the Venetian school and in the Ecole des Fonts et Chaussees in Paris [5],especially as regards the aim of the projects, with respect to the town and theterritory, in the first phase of modernization.In the nineteenth century, with the design of the Strada Eugenia, GiannantonioSelva seems to inherit and almost want to summarise different forms ofknowledge, developed for joint purposes, which are at heart the necessities ofmanagement of the new town that has emerged from the ancien regime.Some principles linked with regulating the flow of waters are applied in theproject for the new access street to the gardens, which is part of a larger projectinvolving the whole town with the new Town-planning Scheme of 1807.The construction of the new street, with the building of the undergroundmasonry vault, was begun in 1808 and continued until 1812, when the trachytepaving was completed.The designs of Selva and Romano are reminiscent of the tables published in thetexts of Patte and Navier (figs. 7 and 8), which show detailed sections of thesubsoil of the new town roads, conceived in such a way as to optimize thedistribution and disposal of water, through trunk lines specially planned toimprove the old town.Instead of filling in the canal, as habitually occurred even in Venice with the so-called "/•// terci" - such as the closing of the rio Batario or of the rio degliAssassini - Selva opted for a modern solution which, by allowing the waters toflow freely in the specially built underground channel, guaranteed thehealthiness of the area by allowing the flow and ebb of the tide.

4. Constructive characteristics and consistency of the masonry vault

The construction is a masonry structure of solid bricks and lime mortarcomposed of a vault with a slightly flattened arch (chord/height ratio 2.3), withoffset on the extrados with variable sections, from three heads (about 36,0 cm)at the key section to five heads (about 63.0 cm) at the springers, which areraised on two sturdy foundations with a square base section 1.30 m wide (fig.9). The foundations rest on a floor of wooden boards which connects the headsof a pile, also made of wood, driven into the soil beforehand to consolidate it.The vault, which has a span of about 4.00 m and a total length of 380.00 m, hasdimensions that are exceptional when compared with those of the undergroundmasonry ducts constructed in Venice in the eighteenth and nineteenth centuriesduring the phases of the urban transformation.The intended function of the main trunk line of the sewage system in a vastpopulated area, with numerous ducts from the side streets and the discharges of



the latrines in the houses facing onto them, together with its extraordinarylength, justifies the adoption of a pipeline width that was adequate to ensure themovement of water to carry away the materials deposited there and the flow ofthe current between the two opposite entrances of the channel, preventingrepeated silting up. It was probably with this in mind that Selva first planned avault with a width of 9.40 m, equal to that of the canal, and more than twice thewidth of the one later adopted. However, that decision would have requiredlowering of the vault, giving a value of the chord/height ratio 5.2 times higherthan that found in the flattened arch structures of bridges in Venice, as thelateral thrusts could scarcely have been absorbed by the foundation structurestraditionally used.The first hypothesis was apparently abandoned after the failure of the attempt toput it into practice, in favour of a solution which combined a drastic reductionof the chord from 9.40 m to 4.00 m and a change in the geometric configurationof the section, from a flat arch to a round arch, with the greatest care in thepreliminary consolidation of the soil and the construction of the foundations.The result is a vaulted structure that is still exceptional, at least double the widthof those normally used for sewage pipelines, more similar to a bridge than to anunderground tunnel. As the vault is under the ground, the designers also had toconsider another factor that does not occur in the case of a bridge: that is thepossible reduction to zero of the internal angle of friction of the embanked earthused to support it, due to the temporary or permanent presence of water in thesubsoil, causing a possible, quite significant increase of the overall thrustingaction on the structure.When the site was opened, the masonry structure appeared to be in goodcondition (fig. 10), without any sign of cracking in its component materials,mortar and bricks, or decay of the mortar, thus justifying the decision to reclaimthe structure from the functional point of view. Its durability was certainly aidedby the deviation of the mouth of the pipeline into the adjacent rio della Tanainstead of directly into the basin of San Marco, during maintenance works onthe bank overlooking the basin, thus protecting the structure from the effects ofsurge. In the town a phenomenon is encountered which gives rise to concern,generally along the canals with major traffic, involving the disintegration of thebottom structures and the erosion of the foundation of many buildings; insewage pipelines this is caused by the negative surge ("restia") due to theprolonged lowering of the water surface when large ships with a deep draughtare passing.The masonry technique adopted reproposes the traditional structural featurestypical of the Venice area. The structure is well equipped with joints in themortar bed which never exceed the thickness of one centimeter, speciallydesigned to offset the cyclical washing action of moving water due to the riseand fall of the tide, thanks to two devices aimed at limiting its effects: theadoption of very thin joints and the application of a protective jacket on theextrados of the vault. The first device, which helps provide the masonry withgood resistance, was adopted mainly with the purpose of limiting the



infiltration of the embanked earthwith water under pressure from theintrados of the pipeline as the tiderises.The practice was well known and wasalso reinforced with the constructionof vertical strips of masonry facingthe canals exposed to the action ofmoving water, tn the case of masonrywith stone quoins, this went as far asto eliminate the connections, whichwere sometimes replaced with thinlayers of lead to absorb theimperfections of the contact surfacesand thus favour uniform settling ofthe masonry under the effect of thebuilding loads. The second device,that is the waterproofing of theextrados surface of the value with ajacket of lime mortar 1.5 to 2.0 cmthick, aimed at preventing theembanking material from beingwashed away with the water flowingout with the ebb tide, thus avoidingthe negative consequences both of thedeposit of silt on the bottom of thepipeline and of the gradual reductionof the stabilizing effect of theembanking material on theequilibrium of the vault.The mortar used both for the liningand for the joints in the masonry ismade with the traditional lime binder,taking care to apply it only afterquenching of the quicklime, andadding sand (the so-called "maltabroada").so as to obtain goodadhesion between mortar and bricks .The bricks, made at the time of theconstruction of the vault and onlypartly integrated with other bricksreclaimed from the demolition ofprevious old buildings in the area,reflect the characteristics of lightweight, good resistance to

8 Ing. A. Romano, Plan and crosssection of a stretch road ....

9 Survey of the underground vault.10 Via Garibaldi, Venice. Inspectionsof the underground vault.



Load condition Minimum principalstress (compression)on the intrados of the

build section(N/mm*)

Deformation at thekeystone

(mm)Permanent load (dead weight,weight of the filling and of thepaving in trachyte slabs 10 cmthick). 0,22 0,30

Permanent andaccidental loads (50KN/nf) on the entiresurface. 0,25 0,44

Permanent andaccidental loads(50KN/nf) limited to half thespan. 0,28 036

11 Table L Summary of the investigation of the distribution of stresses andstrains in three different load condition on the vaulted structure examined.

compression and durability, as found in brick products made in the pre-modemperiod with discontinuous fire baking procedures in the Venetian area.A sample of three or four bricks was taken from six different areas of theextrados of the masonry, brought to light after having freed them from theembanking earth; from these samples 4.0 cm cubes were taken and subjected tocompression test in the laboratory. The values obtained in the tests werebetween 12.3 and 39.8 N/mm^ which, grouped together us significant means,give values varying between 15,6 and 23,2 N/mm^ for the brick. From acomparison of the values obtained with the forecasts for the mechanicalbehaviour of brick masonry in Venice, assessed from the resistanceencountered on removing the brick, found in the technical literature [8] [9],hypothetical resistance values between 3.1 and 4,6 N/mm^ may be obtainedand an expected mean elastic modulus of around 2000 N/mm̂ . Thesecircumstances confirm the indication of a bearing masonry structure withsufficient mechanical qualities, such as emerged on a visual inspection of theextrados, but penalized in the resistance and with a great deformability for the


Structural Studies, Repairs and Maintenance of Historical Buildings 65 1

poor contribution of the mortar.The theoretical investigations of the finished elements, carried out with the aidof numerical models in the elastic field using two-dimensional elements such asPLANE2D with 3 or 4 nodes, were based on the following properties of thematerials:Masonry: Young module 2000 N/mm^, Poisson coefficient v - 0.2 specificweight = 18KN/m3;Filling: Young module = 1 N/mm^, Poisson coefficient v - 0.2 specific weight =

For the vaulted structure examined, these investigations give a stress due tocompression only, for all the following examined conditions: 1. Permanent load(dead weight, weight of the filling and of the paving in trachyte slabs 1 0 cmthick), 2. Permanent and accidental loads (50KN/nf) on the entire surface. 3.Permanent and accidental loads(50 KN/nf) limited to half the spanThese stresses reach on the intrados of the build sections the maximum value of

0.28 N/mm^ for the condition ( (permanent and accidental load on the entiresurface)). From the comparison between the modelling results and thoseobtained from the interpretation of the laboratory tests on bricks removed fromthe site, we can obtain an indicative certainly acceptable value of the safetycoefficient which is between 1 1 and 17.Tests of deformability also give low yield values which reach a maximum valueof 0.44 mm at the keystone for the condition of permanent and accidental loadon the entire surface.Table 1 (fig. 11) shows the results obtained in modelling.

5. Conclusions

The studies and tests carried out during work performed in the historic town onthe lagoon highlighted a number of general principles and research themes thatdeserve investigation for the recovery of structures in areas subject to highsedimentation.The town may be considered as a palimpsest in which it is possible to read thegenerating principles of urban form and its morphogenesis.The relationships formed with the history of the town involve different contexts,some of which have been referred to in this work, which may be criticallyassumed in a project of operation.The figurative and technical contexts were given particular attention,considering the scenario that was created in Venice with the advent ofnineteenth, century classicism which aimed to rationalize civil architecture.Wherever possible, the intervention tried to respect the materials and thetraditional techniques originally used, considering also that their preservation isoften more easily guaranteed by a regular maintenance process that constantlyapplies the same methods, rather than by intrusive and non-reversibleinterventions.



Acknowledgments

We thank the engineers Carlo Bettio and Mauro Ferrarese (of the 2FB Office inPadua) for their collaboration in processing the finished elements of thecalculation model.

References

1. Selva, G. Giardinipubblici a Castello, Report, AMY, Venice, 1807.2.Valle, P. Percorsi. Architetture nella citta, pp 58-63, II Car do

Editore,Venice,1996S.Valle, P. Tommaso Temanza e I'architettura civile. Venezia e il settecento:diffusione efunzionalizzazione dell'architettura, Officina Edizioni, Rome,1989

4.Zendrini, B.Leggi efenomeni delle acque corr£/7//.Venice, 1741.5.Belidor, B.f.De Architecture Hidraulique, Paris, 1735-1770, 2**. Ed. withnotes by Navier, Paris, 1810-1819, Italian translation of the 2™* ed., Mantua,1835.

6.Cavalieri di S.Bertolo, N. Istituzioni di architettura, statica e idraulica, F.lliNegretti, Mantua, 1831.

7.Salvadori, E. Preventivo di spesa per interventi in acquedotti sotterranei,Report, AMV, Venice, 1831.

8.Zago, F. Riva, G. Proprietafisico-meccaniche del mattoni e comportamentodella muratura del centra storico di Venezia. Parte prima: il mattone, Actsof the Institute of Building Science, Istituto Universitario di Architettura ofVenice, Scuola Tipografica Emiliana Artigianelli, Venice, 1981.

9. Zago, F. Riva, G. Proprietafisico-meccaniche dei mattoni e comportamentodella muratura del centra storico di Venezia. Parte seconda: la muratura,Acts of the Institute of Building Science, Istituto Universitario di Architetturaof Venice, Scuola Tipografica Emiliana Artigianelli, Venice, 1982.


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