The Katholikon of Dafni Monastery, already inscribed in the world heritage list of UNESCO, is one of the major-byzantine (11th c.) monuments famous worldwide for its excellent mural mosaics (Millet. G. 1899, Bouras, CH. 1998, Delinikolas et al. 2003). The Katho-likon (main church of the monastery) belongs to the octagonal type and comprises the main church, the sanctuary, the narthex and four chapels, which complete its orthogonal plan.

In the western part, only the perimetric walls of an exonarthex or portico and the spiral stairway have survived.

The central part of the main church is cross shaped in plan, and over its square core rises the hemispherical dome (Figs 1 and 2), which stands on an almost cylindrical drum with 16 piers and 16 vaulted windows. The dome and its drum are curried by eight pendentives and eight arches (four semicircular and four squinches in the corners), forming an octagon and achieving in this way the transition from circle to square. Thus twelve piers, laying out in a square plan, provide support to the dome together with the groin vaulted arms of the cross, situated at a higher level. The interior of the church is decorated with mural mosaics. In Figure 1 a general view of the monument is presented.

Figure 1. General view of the monument from N-E, metallic buttresses are installed as urgent measures.

As one can clearly distinguish in Figures l and 2, the exterior face of the vertical perimeter walls are built according to the cloisonné masonry system, following two different types of masonry. In fact, there is a lower zone, built with large dimension stones (often reused old material), their length placed horizontally or vertically, in order to form crosses, and an upper zone constructed with smaller stones (Delinikolas et al. 2003). Using radar techniques and endoscopy

Figure 2. Past interventions to the monument: North façade. The different types of construction, mainly of the upper and lower part of the monument, are shown.

(Vintzileou et al. 2004), it was shown that in both cases a three-leaf masonry is used. As expected the width of each of the three leaves at the lower and the upper part is not the same due to the bigger stones used in the external leaf of the lower part. The existence of three-leaf masonry for the construction of the internal vertical walls was also ascertained (Côte et al 2008), while plain brick masonry was used in the construction of all the monolobe, bilobe and trilobe windows, and all the vaulted roof.

The various materials and construction techniques, the foundation and the soil condition, as well as the past modifications and local reconstructions taken place to remediate damages provoked due to strong earthquakes, affect significantly the mechanical and dynamic characteristics of the structure. The variations of temperature, settlement, creep and ambient vibrations have also a great influence on the seismic response of the monument. As a result, cracks occur and divide the whole structure in interacting parts with nonlinear behaviour.

Due to these aforementioned uncertainties in addition to the variability of the seismic excitation, even if detailed accurate finite element models and sophisticated analysis is developed, the estimation of the seismic safety of the structure is not enhanced.

In order to increase our understanding of the seismic structural response and to decrease the uncertainty of the seismic action, the use of an earthquake monitoring system is necessary. Moreover, the implementation of the recorded and analyzed data will be the basis for the further development and calibration of detailed numerical models, which will be used for the parametric study of the seismic behaviour before and after interventions.

Such monitoring systems have already installed in Hagia Sophia and Suleimaniye in Constantinople (Durukal, E. et al. 2003) and recorded the response during strong earthquakes, as part of the efforts for the determination of their earthquake worthiness.

The Katholikon of Dafni Monastery suffered severe damage during the strong Athens' earthquake of 1999

and supporting urgent measures were subsequently undertaken (Miltiadou-Fezans et al. 2003a), together with all the necessary investigations and studies, including also a first series of finite element analyses for the numerical verification of the existing damage of the structure (Miltiadou-Fezans et al. 2003b & 2004). This analysis helped for a better understanding of the structural behaviour of the monument and guided the design of the monitoring system. The monitoring system was installed in 2003 after the implementation of the aforementioned emergency measures and before the application to the masonry structure of any repair intervention. Thus, when the hydraulic lime grouting application started (Miltiadou et al. 2008), in June 2006, the system was functioning and had already recorded some weak earthquakes.

This paper presents the monitoring system that was installed to record the seismic response of the Katholikon of Daphni Monastery and the analysed data recorder during four earthquakes. Two of them took place before the grouting injections and the rest some months after the first phase of grouting. During this first phase, grouting was implemented to almost the half of the height of the perimetric walls. The digitized data during weak earthquakes which occurred after the application of the injections confirm the efficiency of grouting interventions and their effect toe the overall behaviour of the structure.

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