Brief Presentation Of The Works Of The First Phase

As reported in the relevant study, (Mitiadou-Fezans et al. 2003b), the main objective of this first phase of structural restoration interventions was (i) to improve

Figure 25. Old mortars of first and second construction phase and cracked mosaics that have to be preserved in situ.
Figure 26. Typical examples of the first phase structural restoration interventions (Recent mortars removal, grouting preperation, stiching of cracks, local reconstruction).

in the best possible way the mechanical behaviour of the masonry elements by reinstating their continuity (lost due to the numerous cracks) and increasing their resistance (mainly to traction and shear), without side effects due to possible durability matters. This objective had to be achieved taking into account the existence of mural mosaics, frescoes and old pointing mortars, that had to be preserved and conserved in situ. In fact all uncolored joints of Figures 12 & 13 still preserve the old Byzantine pointing mortars.

This first phase of masonry repair interventions comprised mainly the following works (Fig. 26): i) very careful removal of plasters and deteriorated pointing mortars applied during previous interventions,

Figure 27. Typical examples of in situ mural mosaics conservation works (including installation of fine tubes for grouting).

without harming the old ones, adjacent to or underneath them, ii) removal of tiles and other covering and filling materials to reach the extrados of all vaulted structures, iii) stitching of the most severe cracks, using long stones, bricks, or thin titanium plates, iv) few local reconstructions necessary either for the repair of dislocated or collapsed parts or for the restoration of past morphological alterations, v) deep re-pointing where necessary and preparation of the masonry for injection grouting, vi) implementation of injection grouting, vii) removal of all injection tubes, (viii) in situ conservation of all deteriorated old mortars using frescoes techniques, ix) execution of all necessary works to ensure the protection of the extrados from rainwater.

In parallel with the works for the masonry repair, the competent Conservators have carried out all the necessary works for the in situ conservation (including grouting) of severely damaged mural mosaics, (Chryssopoulos et al. 2003). As anticipated, in most cases, the cracks on the masonry affected also the mosaics beard on its internal face (Fig. 27).

From the above brief presentation of the works undertaken, it is ensued that the application of adequate mortars and grout compositions, constitute a key parameter in order to ensure a successful intervention, both for the masonry and the mosaics.

Thus, extended laboratory and in situ pilot tests have been undertaken for the formulation of the adequate mortars compositions taking into account the characteristics of the existing materials, the requirements set by the architectural and structural study and the worksite conditions. For the majority of the re-pointing works adequate lime-pozzolan based mortars were used. For the local reconstructions, deep re-pointing of extremely damaged critical areas and stitching with thin titanium plates, hydraulic lime based mortars were applied. Detailed information concerning the properties of existing mortars is given in Papagianni 2002, while all the results concerning both the properties of existing materials and those designed and applied to the monument are reported in Anagnos-topoulou & Miltiadou-Fezans 2007. In order to ensure the protection and in situ conservation of old pointing mortars, on the external facades, the upper central area just below the dome, and the groin vaults of the sanctuary, the deep re-pointing works have been executed by experienced Conservators.

As anticipated special attention has been given to the design of the adequate grouts, not only because grouting constitutes the main of the works undertaken that can substantially improve the mechanical behaviour of the masonry, but also due to the fact that it is an invisible and irreversible intervention, which affects the masonry elements and all the mosaics, frescoes and old mortars beard on them. Furthermore, as the proper design of a grout composition cannot ensure on its own the successful execution of the grouting intervention, a specific application methodology has been developed and applied. The most important aspects of this methodology are reported in Miltiadou-Fezans et al. 2008. This application methodology permitted the implementation of injections to this important monument, bearing mosaics, frescoes and old mortars that had to be preserved in situ, in a more rational and fully controlled way. The monitoring results and those obtained by sonic tomographies summarized below, gave important information for the effect and the efficiency of the whole grouting intervention, and enhanced the validity of the methodology applied.

During the whole project, all interventions have been documented in a detailed way (adequate as built drawings have been elaborated), together with all the new findings concerning the materials, types of construction, past interventions and pathology of various invisible elements (i.e extrados of groin vaults). Thus the historical and recent pathology drawings have been updated and all necessary information has been collected to be taken into account in the study of the second phase interventions.

11 SEISMIC MONITORING SYSTEM AND APPLICATION OF NDT'S

As aforementioned, in order to increase our understanding of the seismic structural response and to decrease the uncertainty of the seismic action, the installation of an earthquake monitoring system was decided. The system was installed on the central core of the monument at 2003, after the implementation of the aforementioned emergency measures, and before the beginning of restoration works.

Thus, when the hydraulic lime grouting application started (Miltiadou et al. 2008), in June 2006, the system was functioning and had already recorded two weak earthquakes. The first took place before the beginning of masonry repair (26-9-2004) and the second during the preparation of masonry for grouting

Figure 28. Variation of the period of the first mode.

by stitching of cracks and local deep re-pointing (8/1/2006). After three and four months from the end of the first grouting period, during which grouting was implemented up to the springing level of groin vaults, two more weak earthquakes were recorded (5/11/2006 and 12/12/2006). The analyzed data, have shown that the monitoring system installed can give important information for the overall behaviour of the structure, before during and after the implementation of various interventions. Detailed information is given in Mouzakis et al. (2008).

As shown in Figure 28, during the earthquake of 08/01/2006 occurred after the urgent measures and while preparing the masonry for grouting, the natural period of the first mode was increased, and hence the stiffness of the structure was decreased. This is totally expected, as only local deep re-pointing and stitching of some cracks had been applied at that moment, while the emergency measures have not been designed to undertake seismic actions. Moreover an increase of damping was recorded. These results may indicate a slight deterioration of damages. On the contrary, as a result of the implementation of grouting injections up to the springing level of groin vaults, the period and the damping of the first mode were decreased, along the EW and the NS directions. This result was reached by analysing the data of both seismic events, happened three and four months after the first grouting period. Thus, it was proved that the effect of interventions and more specifically of grouting on the overall behaviour of the structure can be evaluated by a seismic monitoring system. This promising finding led to the decision to install in 2007 complementary instrumentation, with the aim to cover the whole structure.

The extended seismic monitoring system has recently recorded two more earthquakes (6/1/2008 and 30/1/2008), and the data are still under processing. These recent seismic events took place after nine and ten months from the end of masonry repair works (28/4/20070), during which grouting has been carried out to the entire structure below the level of the base of the dome's drum. Thus, it is expected that the seismic monitoring records will give further information for the effect and efficiency of all the interventions undertaken until now.

Figure 29. Manual-sonic and sub-surface radar maps of "Saint-Orestes". Damaged zones are in dark.

Figure 30. Tomographical reconstruction in a grouted and in a non grouted area.

Figure 29. Manual-sonic and sub-surface radar maps of "Saint-Orestes". Damaged zones are in dark.

12 APPLICATION OF NDT'S FOR MAPPING MOSAICS SUBSTRATA AND GROUTING MONITORING

Figure 30. Tomographical reconstruction in a grouted and in a non grouted area.

in progress. In Figure 30 two indicative tomographi-cal reconstructions of a grouted and in a non grouted area are presented. The comparison of these two cases permits to conclude that the grouting survey of such masonry structures may be achieved by the observation of the velocities of their inner part (Cote et al. 2008).

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