The study of interventions was approached differently in both projects. The RES-TAPIA project studied monumental architecture, that is, buildings with heritage protection, catalogued and appreciated as monuments. The actions and interventions analysed received public funding and extensive documentation was obtained from different archives.

Therefore over 200 interventions carried out on rammed earth buildings in the last 30 years were cross-referenced and analysed, based on crossed parameters. It was concluded that in the early 1980s, interventions were often based mostly on reconstructions, which were more or less general and aimed to restore the monument to its original aesthetic condition. At times this was more restrained, with partial reconstructions, while at others it was more extensive with complete reconstructions, aiming to restore the building to a specific point in its history which was considered of greater architectural splendour and interest. These criteria can be linked to a more historicist line of thinking forgotten in much of Europe but still widespread in the Iberian Peninsula until the late 1980s. In the late 1980s-early 90s criteria more in line with conservation became more prevalent, so that a greater number of interventions based on partial reconstructions of walls and reintegration of missing elements were carried out, while extensive reconstructions were much more infrequent at that point. Over time, the preference for conservation increased and in the 2000s actions for reintegration and the partial reconstruction of volumes were the main proposals favoured.

In terms of the materials used in the interventions it was observed that despite the interest of well-meaning conservation architects to use the original constructive techniques, these techniques were possibly unknown or barely explored in the 1980s so that contemporary materials such as cement were used in the mix to improve the strength of the walls proposed for the intervention.. The use of cement makes these walls less permeable and there is a considerable reduction in material compatibility between the original wall and that of the intervention, usually leading to a series of associated degradation problems (Fig. 4).

Since the late 1980s the cases analysed have shown an increasing use of traditional materials (earth, sand, gravel, masonry, lime) in the new rammed earth mixes, while cement content has generally been reduced although it can still be found in numerous interventions dating from the 1990s. The passing of time has shown the hits and misses of these interventions, and this in turn has influenced the more recent intervention proposals, which have sought to improve technical solutions and respect for pre-existing elements. Therefore, interventions from the 2000s showed an increased use of traditional constructive techniques as the most appropriate options, and other projects appeared in which the aim was to use the original materials, with no additions. This change is the result of increased study and knowledge of the rammed earth technique in recent decades, allowing professionals to use these techniques more precisely, in the knowledge that the use of cement in the mix was in many cases the cause of major degradation in the wall. This caused the appearance of salt efflorescence, which in an advanced state has even caused major material damage, so that the use of cement was reduced (García Soriano 2015).

In contrast, the SOS-TIERRA Project focused on the study of earthen vernacular architecture which was neither catalogued nor protected. In fact this architecture is barely appreciated as built heritage and has therefore undergone spontaneous interventions with no design, generally interventions attempting to respond to the functional needs of owners and paid for with much more limited private funds. These actions are also linked to the lack of specific knowledge on traditional construction, as the current generations missed the transfer of knowledge from previous ones.

The lack of specific regulatory legislation for interventions in traditional buildings, specifically earthen buildings, in most population nuclei has resulted in a wide range of heterogeneous solutions mainly dependent on the needs or the will of owners. The new needs and changes of use, generally made by both the permanent and temporary residents, and the need to repair existing damage generally lead to most interventions. Interventions in monumental architecture are based on projects which follow specific intervention criteria, and techniques are chosen based on these. In most cases the interventions in vernacular architecture lack a deep previous project or specific design analysis and are constrained by necessity and budget.

Therefore, the analysis of interventions in vernacular architecture, rather than analysing the criteria underlying the intervention aims to understand the interventions being carried out and the reasons for these, while identifying the dynamics of transformation of this architecture due to the interventions carried out. Therefore, the information recorded in the study record was analysed to extract numerical and percentage data regarding the type of general intervention and the different parts of the building. These data were cross-referenced with the information from other parameters found in the study record, including type and frequency of use in order to establish the relationship which these factors may have with the number or type of interventions executed in the different buildings. The sample consists of 274 case studies, 234 of which (85%) have undergone interventions and therefore make up the general sample used in this phase of the work. Among the 234 cases which make up the sample, distinctions were made depending on the constructive technique. Rammed earth accounts for the highest number of cases (107) while adobe and half-timber make up the remaining cases (60 and 67 cases respectively).

The study of interventions and specific intervention criteria as practised in monumental architecture is an arduous task in the case of traditional architecture. The interventions carried out are born in most cases from the need to repair the building to prevent further damage, from their change of use or mere aesthetic transformations (usually with no established criteria), and are greatly conditioned by speedy execution and obtaining materials used. Furthermore, it should be highlighted that in the case of interventions in vernacular architecture the prior condition of most buildings is not known so that prior lesions can only be identified when occasional interventions have been carried out on them or when the initial degradation mechanisms persist despite the interventions. Therefore, intervention criteria had to be simplified and linked to intentionality or prior reflection, and they have been defined as spontaneous intervention or planned intervention.

In general, interventions are not based on a series of conservation or restoration principles and most actions usually only attempt to eliminate problems or lesions as soon as possible, without seeking solutions that are reversible, compatible or distinguishable (Fig. 5). Therefore, the criterion for analysis has focused on studying the type of intervention in relation to the technique and type of material used. The complexity of the sample enabled partial studies to be carried out around different groups of cases with comparable characteristics such as the technique used initially in the construction of the building (rammed earth, adobe or half-timber). These partial analyses allowed to identify and compare the particular aspects of each group. Furthermore, comparative partial analyses were also carried out which were linked to the general level of intervention of the different cases, on the one hand isolating cases with only maintenance and repair intervention, and on the other, cases with a greater level of intervention: restoration, partial rehabilitation, full rehabilitation, expansion and demolition. As update interventions are mostly aesthetic they appear in connection with other types of intervention. Finally, it should be noted that in all cases a general analysis of the type of intervention was carried out for the building, and subsequently the partial interventions were studied in each of the parts of the building specified on the data collection record: walls, base, rendering, openings, flooring and roof.

Fig. 5
figure5

Examples of interventions and criteria concerning historic earthen architecture. Related to the main 8 actions stressed in the projects (whole rehabilitation, extension, demolition, conservation, repair, maintenance, updating, partial rehabilitation (Source: RES-TAPIA & SOS-TIERRA Projects)

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