Continuous repairing of the walls has contributed to their preservation to date. Yet structural damage, such as cracking, gaps and extreme deformations are evident in many parts of the ancient construction, just as in the more recently repaired sections. During the past 25 years, the condition of the walls has been systematically monitored and the problems noted have frequently been reported. During this time a strategy and programme of an interdisciplinary approach has been formed in order to deal with the problems. To begin with, primary emphasis was placed on documentation of the form and state of preservation of the walls, in order to build the necessary database for evaluating and estimating any future intervention.
The following projects were initially implemented for the protection of the wall:
Weeding the area from indigenous vegetation and cleaning accretions from the drainage system of the South Wall
The aim of these projects was to protect the Circuit Walls and the underlying limestone slope against the destructive mechanical action of the root system of indigenous vegetation, to remove unsafe rock pieces and reveal the current state of the monument. This project was also a prerequisite for the geometric documentation of the Circuit Walls and the Rock.
Geometric documentation of the walls
In 2009 orthophotographs were taken throughout the length of the walls. Moreover, a three-dimensional scanning of the Walls was carried out in order to fully and precisely conduct a geometric documentation of the existing state of the walls.
In 2006 geophysical investigations (electrical tomography, radar) on the walls were conducted by the laboratory of Applied Geophysics of the Aristotle University of Thessaloniki in collaboration with I.M.S. FORTH, in order to look into invisible surfaces. Horizontal and vertical tomographies were taken on specific positions of the south wall. After comparing the measurements and calibrating the materials it was possible to discern the limestone, the backfilling, the rock and a damp area. Geophysical monitoring, in order to detect the depth of backfilling, was carried out in 2014. An important undertaking of the same period, which enriched our knowledge related to the stratification of geomaterials on the Acropolis, was the geophysical investigation using the method of electromagnetic prospecting V.L.F. that observed the total length of 2300m during the period of 2014-2015.
This information is important, on the one hand, because it has revealed the location of areas on the wall where dampness was evident in the backfilling and on the other, to find the width of the wall.
In addition, a geotechnical investigation was carried out in 2015 in order to investigate the geotechnical characteristics of the buttressed artificial backfilling of the Acropolis rock and the fillings of the physical and mechanical properties of the geological limestone substratum. The Service dug two shafts, each 4m deep; the first one in the box grid excavation south of the Parthenon, adjoining the wall, the other one in the north wall near the Belvedere tower. However the nature of the artificial backfilling in the Acropolis terrace, a result of the extensive excavations conducted at the end of the 19th century did not allow the taking of unperturbed samples. Moreover, sensors taking measurements of geotechnical parameters were placed in the north shaft, near the Belvedere tower, in order to gather information on backfilling materials.
Instrumental observation of the wall and the rock
The intervention programme was concerned with the monitoring of structural damage in the walls. Following research on various methods of instrumental monitoring of the micro-movements, it was decided to use a combination of mechanical methods and other recording systems of advanced technology. For this purpose mechanical crack meters were installed on cracks in the walls. The information they provide is local but very useful, since they gather reliable information as to whether and to what extent the cracks are "active". Measurements were repeated in 2016 and 2017 without observing any serious deviations.
Non expanding INVAR wire
At the same time (2005) an underground non-expanding INVAR wire has been installed between the stable foundation of the Parthenon and the south wall. A network of observation points has also been established on the south and east walls for monitoring the micro-movements.
Topographic monitoring with the use of special topographic prisms
In the period 2006-2008 a high precision topographic monitoring was conducted in the north and east wall, aiming at reporting minor shifts of the walls in areas of imminent danger. The study was outsourced after a public tender and supervised by YSMA’s technical office. Measurements were obtained monthly from 52 special, permanently placed topographical prisms on the wall. Two steady pedestals were placed on the south slope of the Acropolis in order to successfully repeat the best possible relocations of the instrument. It should be pointed out that a topographical method can produce a complete picture of a large part of the wall, something very important for understanding its pathology and monitor the development of its existing damages. A long term topographical monitoring, conducted regularly during the years, can reveal more accurately the present evidence of minor shifts, especially when this information is compared with evidence produced by the optical fibres placed on the SE corner of the wall. A reconfigured topographic monitoring continues following the specifications imposed by its inclusion in the GIS data of the wall. In 2016 and 2017 the measurements were repeated but produced no significant deviations.
Optical fiber sensors
Additionally, in terms of monitoring the circuit wall, a pilot program including installation of optical fiber sensors has been realized. These sensors aim at the measurement of strains induced from mechanical or thermical actions in characteristic areas of the wall. Optical fiber sensors are the state of the art in strain monitoring and provide extremely high accuracy. They have been developed mainly around the SE corner of the wall as well as in a certain area of the North Wall in the proximity of Arryphorion filling, where the circuit wall appears intensely deformed.
Recordings until 2009 concerned temperature changes. In 2015 four fiber optics were placed, two inside and two outside, on the foundation of the west wall at the Pinakotheke of the Propylaia.
A Network of Accelerometers
The network of ten accelerometers, installed by the National Observatory of Athens, was updated and extended. Recording seismic events is constant, whereas a data base was created to assist the correlation of the parameters and the drawing of filtered information that describe the singularities of seismic waves and the way the rock and the monuments respond to them.
Apart from advanced technological methods, optical inspection remains a safe method of evaluating even the most problematic areas. Access to these areas is achieved with the use of a weightlifting platform for two people. By using this, surveying of the cross-section and the walls distortions is done with the practice of traditional methods. These works are conducted together with weeding, and cleaning of accretions in the drainage system that in some cases is also enhanced.
Intervention in the south wall – The area between the 6th and 7th buttress
An initial small-scale intervention was conducted in 2009 in order to restore a structural problem due to a deficiency in block support on the crown of the Wall in the area of Athena Nike. The works included taking safety measures for binding the blocks, dismantling dangerous blocks, cleaning the block joints, resetting and structurally restoring blocks by cementing and anchoring.
However, at the beginning of 2014, an instability problem appeared on the south wall of the Acropolis after a heavy downpour in February 2012, thus it was clear that an extensive intervention was imperative to begin. While planning the work of consolidating the rock masses and the walls of the Acropolis it was revealed that 12 areas of the walls demanded treatment to a greater or lesser extent.
The works started in the area between the 6th and 7th buttress of the south wall – a programme considered to be a “pilot project” for the rest of the interventions in the wall. The project, completed in July 2015, included works of archaeological cleaning, revealing the conservation state of the upper part of the wall in its full width of 6.85m, of laying asphaltic material and creating a drainage system directing rainwater runoff towards the nearby drain, of replacing damaged construction material, of filling gaps with the use of stone from Pitsa Korinthias and finally of consolidating construction mortars. During this project an endoscope, a thermal camera and other instruments, were used to investigate the structure of the wall. About 20 spolia of ancient scattered material were identified, of which the most important was part of a Roman inscription that was removed from the wall.
The beginning of the works on the walls and the rock widens the scope of activities of ESMA and YSMA. Despite the fact that the classical monuments, the walls and the rock of the Acropolis constitute a single monumental complex, fundamental differences in their structure impose a different methodological approach and consolidation. The work of restoring the wall, large parts of which incorporate later constructions due to successive repairs, demand investigation and use of other materials except marble, compatible with the construction mortars.