Critical Analysis of Interventions in Historical Rammed-Earth Walls—Military Buildings in the Ancient Kingdom of Seville

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 decades, 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 interventions 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. response, although further studies are needed Nevertheless, the current state of conservation in order to prove its efficiency in medium—and of both covering and wall implies that restora- long-term responses. Even so, there is no perma- tion work was largely ineffective mainly due to nent treatment; therefore periodical maintenance the low infiltration of the limewash that would is required, whose cycle and intensity depends on have required a regular maintenance on order to the level of exposure of the wall, among other be effective. factors. Another kind of inorganic compound in the form of ethil silicate has been used in several cases 2.3 Restitution/restoration of mass studies of restoration work, although it was not designed specifically for earthen construction. The restoration of mass occurs when the loss of Its usage is widely used for the enhancement of the original mass, due to erosion, is significant, stone cohesion, and is especially appropriate in and may even render the wall unstable, or when the case of sandstone and limestone (Zoghlami integration or reconstruction of certain sec- 2003). Furthermore, this compound is permeable tions is intended. This technique is much more to water vapour, is suitable to siliceous materials, destructive than consolidation since the carving and presents no chromatic change. Ethil silicate of original rammed earth is required. In addition, was used in the restoration of the Golden Tower new volumes of rammed earth are placed cover- in Seville (2004) and in the case of the restoration ing and hiding the wall, and may even lead to the of the Macarena sector of the City Wall of Seville wrong interpretation of constructive characteris- (2008). The performance in the Tower seems to tics when no criteria is borne in mind. Moreover, have yielded positive results so far, mainly since since this technique is permanent, International no damage has appeared because its rammed Restoration Charters advise against its indiscrim- earth is protected by a lime mortar. Regarding the inate use. Macarena sector of the City Wall of Seville, there Various techniques of the restitution of rammed is currently no serious damage where ethil silicate earth may be carried out according to the specific has been sprayed in consecutive layers. However, circumstances of the wall. All of these techniques due to the lack of maintenance, earth is beginning are based first on the placement and stabiliza- to appear at the base of the wall, as a consequence tion of a formwork and the later compaction of of erosion and loss of cohesion of its consolidated a certain mixture of earth. Both one-sided and surface. Some specialists (Aymat 2000) argue that two-sided restitutions have been registered cor- this compound is the direct cause of some losses responding to surface restitutions and those of of surface material in certain earthen walls. Hence, whole sections. it is always recommended to test the response of However, it remains essential to design the tech- rammed earth before any application since this is nique in accordance both with the physical and not a reversible technique. chemical parameters of the old wall and with the Organic compounds, such as asphaltic emul- constructive characteristics of each component of sions and acrilic epoxie resins, constitute another the rammed-earth wall. If these guidelines are fol- line of consolidant products. These have been lowed, the restoration will certainly be appropriate proved to be non-suitable products however, and coherent. mainly due to the low permeability of the layers There are several solutions for fixing the generated on the surface of wall, which drastically formworks, whether they be wooden or metal- changes the hygrometric behaviour of the wall and lic, from ancient techniques based on ropes and consequently weakens the structure of the rammed nails to modern systems. The first step is always earth due to water ponding. the placement of the needles, which will sup- Therefore, polymeric and resin-based consolid- port the entire formwork structure. To this end, ants have proved to be largely unreliable due to a small aperture is carved in the lower course, their major physical and chemical differences to allowing the formwork to enclose a few centime- earthen wall characteristics. The use of inorganic tres of the top of this lower course. Before set- compounds is currently more common in restora- ting the wooden needles in place, it is necessary tion work. Some authors argue that lime in the to completely soak them in water for one day

291 in order to prevent any unexpected increase in holes totally different to those of the original their volume. The dimension of wooden needles wall; at Castillo de las Guardas lime mortars may widely vary, from those square needles used were supposedly used to repair losses of mass, in the Alcazaba of Reina and the City Walls of but no constructive or even aesthetic criteria were Seville, to those of a rectangular cross-section of applied. For other examples, as in the Hacienda the Monteagudo Castle in Murcia (López 1999). de los Quintos, rough fired bricks were used all The typology of the needles should be carefully over the rammed-earth surfaces, thereby totally chosen in accordance with the characteristics enclosing and hiding the perception of the origi- of the original needles; a consideration largely nal wall. Finally, other restorations, such as that ignored in the past. of the Castle of Paderne (Cóias & Paulo 2004), Regarding the case studies, both formwork sys- chose to project earth into the hollows, hence tems have been used; in certain restorations in the generating an unsuitable texture to the original form of traditional wooden formworks made of rammed earth. 4 or 5 nailed planks (walls of Seville, Alcazaba of In general, there is a certain trend towards a Reina, Castle of Paderne, walls of Niebla), on other greater respect for original rammed-earth con- sites in the form of metallic boards for concrete structive characteristics, and a greater focus on res- (Castle of Toral de los Guzmanes). Despite this, titutions and consolidations of a more controlled it is clear that the key to accomplishing an ade- and limited nature. quate response is to guarantee the adherence of the material of the new restitution with that 2.4 Rendering/coating of the original. A number of techniques have been proposed and applied, based on protected When rammed earth is appropriately protected, or unprotected metallic connectors (Castle of it may be unnecessary to render the surface. Clas- Paderne), insertion of ceramic pieces, stones sic coverings may be applied, such as lime mortar or creosote-protected wooden beams and carving and limewash, after the wall is built, or even during of hollows or cavities in the existing wall (walls of the construction process as crusting layers of lime Niebla). However, prior cleaning and soaking of mortar (calicastrados), which are more commonly the surface by means of limewash or lime water found in military buildings. (Goreti 2005) is essential. Since rammed-earth Although earth rendering may be the most walls depend largely on vertical compression, suitable material for rammed earth, it is not it is crucial to assure a maximum of horizontal widely used for repairs in military rammed-earth surfaces to properly hold newly rammed earth. walls. Lime mortar rendering is the most frequent To this end, the original wall will be cut out, in technique under these circumstances, and should order to generate a stair-shaped cross-section. be spread in several layers; the first layers being It is also essential to protect the upper end of thicker and with an embedded mesh to reinforce the restitution, especially the interface, in order where necessary, and with less lime to adapt to to prevent infiltration of water, internal erosion, the original wall. External layers should be thin- and the consequent collapse of the restitution. ner and more resistant in order to obtain a longer- This is the reason why intervention of the walls lasting surface (Doat et al., 1991). In the same way of Niebla (1982–83) failed and currently entire as for restitutions, surfaces should be clean and sections of restituted one-sided rammed earth are free of loose chunks before any render is spread. falling apart. An adequate selection of soils and Lime mortars provide the correct protection since stabilizers and their compaction is also crucial. they guarantee adequate hygrometric perform- Restoration in both this last cases and in the City ance due to the ability of these mortars to tran- Walls of Seville (1987), failed to properly deal with spire and their similarity in rigidity to that of these factors. The latest experiences carried out rammed earth. in Niebla in 2010, Reina in 2009 and at the City In general, the responses in the case studies of Gate of Seville (Carmona) are currently showing lime mortars are satisfactory. For instance, the good technical responses thanks to constructive mortars of The Golden Tower, the Castle of Los solutions suitable for the original rammed-earth Molares and the Tower of Saint Antonio are cur- walls. Nevertheless, other restoration work, even rently preventing any new pathological processes. where technical solutions are working appropri- On the other hand, lime mortars used for the res- ately, have failed to observe and follow the con- toration of the City Walls of Niebla (2010) have structive patterns of the original wall, since, for recently shown new damage due to infiltrations example, the dimensions, measures and chro- of water from the back of the wall. In other cases, matism of its constructive elements are utterly such as in certain sections of the City Walls of distinct. Work undertaken on the City Walls of Écija, the incompatibility of the technical solution Sanlucar la Mayor show sequences of putlock is evident since extensive areas of the rendering

292 are coming off and revealing a rusty mesh. For the It is necessary to start repairs by analyzing same sample, the formal solution criteria of the new the state and evolution of the crack or fissure in covering do not match the constructive parameters order to determine whether it is due to a degen- of the original wall, since proposed courses, loca- erative damage process in order to assess not only tion and shapes of needles do not correspond to its scope and severity but also whether the crack the original wall. is going through the thickness of the wall. These Other rendering than lime mortar has been parameters will help to define the most suitable observed in certain analyzed interventions, and repair method, which may be classified as soft has resulted in a variety of responses. Brickwork and hard methods (Jaquin 2008). Soft techniques is a common alternative technique to mortars are appropriate when the crack is not under any when the protection of a military wall is needed, kind of tension, while hard techniques are more although Compressed Earth Blocks could have suitable if certain tension or movement is likely. been used instead. Rough fired brick, with typi- In any case, these methods are not applied until cal Islamic dimensions, 28 × 14 × 4 cm, inherited the wall is completely stabilized since they are from the Taifal models (Tabales 2000), is nor- not designed to properly withstand and transmit mally used. When brickwork is carried out with a the tension caused by unexpected movements prior cleaning of the original wall and breathable (Pearson 1977). mortars, a new suitable surface can be achieved, Soft methods consist of simply filling up the although the real constructive identity of the crack in order to prevent weathering agents from rammed-earth wall shifts into the background. attacking and progressively eroding the core In certain cases, as in the Tower of Saint Antonio, of the wall, and hence structural continuity of brickwork is limited to restricted areas where it the rammed earth is not completely achieved. was required as a structural reinforcement or sim- New filling material should have similar char- ply to rebuild the original shape whereby restored acteristics to those of rammed earth in order to areas are evident. However, there are other kinds prevent or minimize differential shrinkage. Before of repairs whose responses are controversial. any application, cleaning should be undertaken. This is the case of the restoration of the Southern Narrower fissures and cracks are easily repaired sector of the wall of San Juan de Aznalfarache, by successive injections of limewash (Goreti where both formal and constructive parameters 2005). When the opening is wider, a denser mate- from original rammed earth have been utterly rial is needed; to this end first a fluid lime mortar ignored, since a new covering made of dark stone might be used and afterwards the aperture could and cement mortar has generated a barely tran- be finished with a limewash. If the aperture is spirable skin, which, as a consequence, has led to significant, say of 5–10 cm, it will be necessary further damage in the rammed earth due to water to first apply lime mortar containing ceramic, gathering. brick or adobe rubble or even small chunks of Finally, a special rendering made of lime mortar rammed earth. is worth mentioning, which is reinforced by means In the restoration of the Castle of Alcalá de of an embedded steel mesh in both sides of the wall, Guadaría in April 2010, cracks were sealed up which are stitched together with steel bars through with a lime mortar leaving an inner void. Several the thickness of the wall. This rendering aims to plastic tubes were then placed at various heights increase the compression strength of rammed which entered the mortar inside the cracks, and earth, and it was put into practice in the Hacienda these were used to pour a fluid lime mortar into the de los Quintos in order to reinforce a load-bearing wall until the cavity was completely filled. Finally, wall faced with an increase of vertical loads. Other these tubes were cut off. The main inconvenience similar interventions in adobe or CEB walls have is the major lack of control in the application chosen nervometal (AIS 2009) and plastic meshes and the great loss of fluid mortar that this tech- (Vargas et al., 2007) in order to improve seismic nique involves. As an alternative, in the Macarena performance. Nevertheless, these experiments have sector of the City Walls of Seville, crushed chunks never been carried out for military rammed-earth from the original rammed earth together with walls, partly owing to the inconvenience of their lime mortar were used to fill the widest cracks. great thickness. Although a simulation of original textures and colours was intended, aesthetic results remain questionable. 2.5 Crack and fissure repairs Hard tying techniques are developed from soft Rammed earth tends to suffer from cracking due to techniques by means of embedding a connector the scarce tensile and shear strength. Thus, vertical that stitches the crack and improves the struc- cracks are more frequent since horizontal cracks tural continuity. Several authors suggest cutting a are linked to compression and buckling. horizontal chase into the face of the wall, across

293 the crack. Reinforced courses of adobe or CEB factor can be evaluated as a way to indicate how are then laid (Ashurst 1988; Pearson 1997; Hurd effective the corrective, preventive and mainte- 2006). When the hollow is narrow, mesh-reinforced nance actions are. layers of mortar may be used instead (Keefe 2005). Finally, it will be essential to analyze the suit- As an alternative, Guillaud (2004) suggests using ability of the material of the technical solutions to various types of wooden staple. However, none of the repair carried out in the particular case on mili- these techniques have been carried out in the case tary rammed-earth walls. In this way, the physical- studies. For instance, in both the restorations of the chemical compatibility of construction materials Castle of Lebrija (Torrecillas & Romero 2006) and can be assessed, as well as the technical viability of the walls of Sanlúcar la Mayor (Callejas & Martín the proposed solution. 2005), steel staples were set across the cracks, and sealed by an epoxy mortar, showing how techniques designed for other materials are indiscriminately ACKNOWLEDGEMENTS used in earthen building repairs. This work was carried out within the Research and Development Project (I + D Project) (2004–2008) 3 CONCLUSIONS BIA2004-01092, entitled Maintenance, evaluation and restoration proposals for rehabilitation of From the analysis and comparative evaluation buildings and urban infrastructures with histori- of the case studies, it may be concluded that cal rammed-earth walls in the Province of Seville there is a need to establish certain criteria that (Spain), headed by Dr. Graciani García, within reflect the different responses of each interven- the framework of the National Plan of Scientific tion, thereby allowing objective comparisons and Research. This work is currently being carried improving future technical strategies of rammed- out within the Research and Development Project earth walls. In order to accomplish a more pre- (I + D Project) (2011–2013) BIA2010-18921, enti- cise classification of interventions, the selected tled The Restoration of rammed-earth architec- criteria may be gathered into three categories ture in the Iberian Peninsula. Criteria, techniques, according to formal-constructive, pathological- results and perspectives, headed by Dr. Mileto, risk, and technical reliability responses. To this within the framework of the National Plan of end, characterization and classification of the Scientific Research. interventions is intended under the same perspec- tive, in order to obtain reliable and consistent conclusions. In reference to the formal-constructive response, REFERENCES it is concluded that evaluating formal and con- Ashurst, J. & Ashurst, N. 1988. Practical building constructive parameters of the intervention is essen- servation: English Heritage technical handbook. 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