Critical Analysis of Interventions in Historical Rammed-Earth Walls—Military Buildings in the Ancient Kingdom of Seville
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Rammed Earth Conservation – Mileto, Vegas & Cristini (eds) © 2012 Taylor & Francis Group, London, ISBN 978-0-415-62125-0 Critical analysis of interventions in historical rammed-earth walls—military buildings in the ancient Kingdom of Seville J. Canivell & A. Graciani Seville University, Seville, Spain ABSTRACT: Despite the general acceptance regarding the interest of dealing with heritage interventions using specialized approaches, thanks to the Charter of Krakow, actions on historical rammed earth walls are to often undertaken without support from any preliminary critical assessment. Hence, in recent dec- ades, this has contributed towards the generation of a wide disparity of intervention criteria and technical solutions. This paper addresses a critical analysis of interventions carried out over a specific period of time on military buildings of the ancient Kingdom of Seville. Prior recording and cataloguing of interven- tions was carried out in order to propose criteria for their value, in terms of their adaptation to formal and technical interventions characteristics. 1 INTRODUCTION selected, it was intended to cover the maximum number of constructive techniques and states Andalusia possesses a long military heritage as a of conservation, so that conclusions would be result of its ancient territorial and political struc- stronger and could be extrapolated to include a ture, consisting of towers, castles and city walls, variety of geographic locations. which have been restored over the last 7 years Specifically, this study takes the following under the framework of the Andalusian Plan of building as case studies: the Castle of Alcala de Defensive Architecture (PADA), designed and Guadaira, Castle of the Guardas, Alcazar of the developed by the Dirección de Bienes Culturales City Gate to Seville (Carmona), Hacienda de de la Junta de Andalucía. los Quintos (Dos Hermanas), Ecija City Walls, These buildings, whose most common con- Castle of Lebrija, Castles of Los Molares, Niebla structive technique is rammed-earth, are espe- City Walls (Huelva), Tower of Saint Antonio cially vulnerable to external agents that, together (Olivares), Sanlucar la Mayor City Walls, San Juan with inappropriate conservation and maintenance de Aznalfarache City Walls, Seville City Walls, techniques, have frequently increased their state Alcazaba of Reina (Badajoz), Castle of Utrera, of risk. and the Tower of Alcantarilla (Utrera). Hence, it is appropriate to propose a critical revision of the latest interventions on military rammed-earth walls in order to establish those 2 INTERVENTION COMPARISON material factors and technical solutions that must be assessed, and as far as possible to estimate the The analysis presented in this paper of the main efficiency and durability of their responses over intervention criteria used in the case studies has time. This analysis will lead to the establishment been developed according to the most frequent of objective criteria which may allow a classifica- technical and formal solutions, as well as to their tion of interventions according to their responses results and responses. Furthermore, repair work to these criteria. due to water erosion, and in the form of consoli- The proposed critical analysis has been applied dations, restitution/restoration of mass, renderings to a group of military buildings of the ancient and crack repairs are pointed out for each study Kingdom of Seville that have been totally or par- case. tially built with military rammed-earth during the North-African (Almoravid and Almohad, 2.1 Repairs due to water XII–XIII centuries) and Mudejar periods (XIII–XIV centuries), and have been restored in The vulnerability to water of rammed-earth walls recent decades. In spite of the clear predominance and its weakening effect on the inner structure of monolithic rammed-earth walls in the cases are normally manifested at the base and top of 289 a wall. Several techniques may be considered in to choose an alternative solution such as the order to prevent the gathering of water through placing of coverings or layers of a sacrifice mate- capillary action and through filtration at the base. rial at the top of the wall, which would have to First, passive techniques limit the access of water be permeable to water vapour exchange in order but fail to remove it. In comparison to the pro- to prevent water ponding under this protection. posed technique of Macías & Espino (2001) to To this end, Oliver (2000), considering erosion as dry out interior walls of low ventilation, which an unavoidable process, suggests using adobe as a consists of the placing of a ventilation pipe with sacrifice material that should be replaced within the two ends at different heights, or similar solu- a regular maintenance program. A number of tions based on ventilated chambers (Ashurst 1988, technical solutions have been applied in the case Monjo 2001), other kinds of simple actions may studies, commonly in the form of sacrificing lay- help the proper hydrothermic performance of the ers made of lime or lime-cement mortars. Some wall. For instance, when rammed-earth walls are examples, such as restoration work at the Cas- confined between hard and impermeable surfaces tle of Lebrija and the City Walls of Sanlúcar la (such as the Seville City Walls in the Macarena Mayor, show this solution, in the latter case with sector, which are surrounded by concrete and the worst esthetical results. As an alternative, asphalt), the removal of these materials and the at the Castle of Alcalá de Guadaría, special pieces action of leaving 50 mm free from the surface of of steel-reinforced limecrete were produced on- the wall (Ashurt 1988) or the replacement of those site, designed with sufficient slope and overhang dense materials with softer materials (Walker et al., for the correct protection, and were simply laid on 2005) constitute a major step towards correct top of the walls. However, it is remarkable how, hydrothermic behaviour. Nevertheless, in the res- for such outstanding restorations as those carried toration of the Seville City walls, as in other case out on the Seville City Walls, no effective technical studies (Écija, Niebla, Córdoba, and Cáceres), solution has yet been accomplished. no action has been carried out to prevent this Although the protection of the base and the top situation, and the consequent erosion of the base of the wall would suffice, high exposure levels to of the wall due to water splashing. In contrast, weathering agents may demand greater attention in the Portuguese Alentejo, certain straightfor- to be paid to the surface of the wall by means of ward and inexpensive solutions have been accom- an adequate rendering. When dealing with retain- plished through the placement of a small slope ing walls, opening certain channels through the along the base of the wall in the form of a nar- wall could help the drainage of water from the row pavement, in order to prevent the water from terrain, so that infiltrations may be prevented for gathering (Guillaud 2004). the whole surface. This technique was efficiently Although the usage of drains, prudentially put into practice in Alcazaba of Reina, however placed away from the wall, is very effective in order for other case studies under similar circumstances to control the amount of water that reaches the (Castle of Lebrija and the City Walls of San Juan base of the wall, is has not been possible to check de Aznalfarache) no technical solutions were neither analyze this situation on the study cases. planned even where damp and consequent damage Other passive systems, such as certain kinds of is currently evident. waterproof barriers (Ashurst 1988; Keefe 2005; Walker et al., 2005; Easton 2007), may be applied 2.2 Consolidations in order to control capillary rising damp. Although it is a simple solution in case of new buildings, it Consolidation is one of the most frequent actions becomes seriously complicated when dealing with on rammed-earth walls due to their specific char- restoration work. Siloxane injection systems into acteristics. Through industrial or traditional prod- historical walls (Ashurst 1988, Ortega 1994) show ucts, it is intended to stabilize the deterioration certain difficulties for military rammed-earth process of the inner structure of the mass and walls, due to their great thickness and inner irregu- likewise to improve cohesion and adherence of lar void distribution. This technique is therefore its particles. The efficiency of the consolidation proved less feasible, and hence its responses have usually depends on the infiltration capacity of the not been assessed for this study. product. The tops of walls, principally in the lengths Inorganic consolidants fill up the accessible of city walls, tend to constitute the most eroded pore system thanks to their chemical precipita- area due to their high exposure to weathering tion in water, thereby cementing particles of the agents. Although the placement of a roof and inner structure of the earth. Limewash or lime overhangs might appear to present a straightfor- water, of 1:5, 1:7 lime/water proportions respec- ward solution, this is not the case when dealing tively, applied with brush or spray improve cohe- with military walls, since it would be necessary sion when the carbonation process occurs inside 290 the wall. To this end, consecutive layers are spread form of limewash, lime water, or fluid mortar leaving one day between each application in order may present the best option in consolidations to let each layer properly harden (Goreti 2005). whether they be rammed earth or any other kind For instance, surfaces of the Alcazaba of Reina of earthen walls (Goreti 2005). Lime provides (Rocha 2005) were brushed with limewash con- extraordinary compatibility, and induces no taining a small proportion of sand, not only to damage. On the other hand, certain compoundds harden the wall but also to smooth over the con- such as ethil silicate currently display a good trasts between restored and original surfaces.