Conservation at work in Istanbul : land walls

Metin et Zeynep Ahunbay, Université Technique d’Istanbul, Faculté d’Architecture

History

The land walls of Constantinople, a strong fortification from the reign of Theodosius II (408-450 AD), marks the western border of the ancient city. The strategical life of the wall lasted about thousand years, during which period it suffered from serious earthquakes and had to be repaired several times. Today, even in their ruinous state, the wall sections and towers bear testimony to the grandeur of the fifth century military architecture. Conceived as the most developed defense system of the ancient worls, Theodosian walls consist of three members:

  1. the inner wall with rectangular or polygonal towers,
  2. the outer wall with rectangular or U shaped towers,
  3. the moat.

According to the Turkish practice of cultural property ownership, municipalities own the city walls and they are responsible for the care of the historic walls within their territories. By the appeal of the Ministry of Culture, the historic peninsula, the walled city of Istanbul was included on the World Heritage List in 1985. This was followed by an interest which developed into organized action for the restoration of endangered monuments in the city. In 1986 a project to restore the city walls and gates was launched by the Municipality of Istanbul after an international campaign initiated by UNESCO for the safeguarding of the city’s cultural heritage. During the years 1986-88 Belgrade, Silivri and Mevlevihane gates were restored along with the wall sections between them. However, this restoration was not sensitive to the archeological character of the wall. Archeologists and architects started a wide ranging debate about the restoration theory employed in this restoration. Municipality administration considered these cristicisms and initiated a new project with a shift of attitude. Thus it was possible for a team of architects and engineers from Istanbul Technical University to start to work on a 200 meters long portion of this structure. The site under consideration is at the southern end of the seven kilometers long Land Walls, starting with the firts tower (T1) and ending at a point at the north of the sixth tower (T6). Originally, T1 was connected to the sea walls by a curtain wall, the remains of which were removed during the construction of the motorway in late 1950’s. The intercity railway which was introduced in the 1870’s pierced through the section of the wall between Tower 6 and 7, causing a severe break in this section of the wall.

The main wall was relatively well preserved between T1 and T6, but the outer wall and the moat were only perceptible above ground, due to changes in the topography and the damages caused by the crouching of industry since the end of the last century. The occupation of the area by tanneries lasted until 1990, when the Municipality of Istanbul finally succeeded in clearing the site. Thus, as part of the conservation activity for the ancient city, protective measures for this section of the wall was initiated by the Municipality of Istanbul in 1991.

Excavation and documentation

Clearance in the project area brought to light the 200m long section of the wall which was completely curtained by tanneries. The first three towers of the outer wall had disappeared completely, leaving no vestige above ground level. The wall surfaces had been stained with dyes or chemicals used in leather industry; there were several cavities and flues dug into the wall mass. A thick layer of earth was deposited above the rampart walk and towers, providing firm ground to several fully grown trees. The vegetation and the earth deposit had to be removed with care. On the eastern side of the wall, access to the towers was hardly possible, due to accumulation of about three meters high debris, which changed the ground level along the walls.

Excavation was conducted until the original ground and floor levels, were reached. For complete documentation and understanding of the structure, excavation continued within the towers, revealing interesting details about history of the structures.
The second step in the work was preparing surveys which provided the basis for further investigation on the wall, such as the analysis of different phases of construction and structural problems. They also provided the principal documents on which the result of the inspections of the present fabric were recorded, leading to decisions for conservation measures. Photogrammetry was employed for elevations. Professor Cankurt Ömerci and associated professor Oguz Müftüoglu from the Photogrammetry Department of Istanbul Technical University were responsible for 1/100 and 1/50 scaled elevations. 1/50 and 1/20 scaled plans and cross sections were produced by a team of architects specialized in conservation. The ruinous state of the walls and towers presented a complexity which required a combination of geodosic and direct measuring systems.

Conservation methodology

From the start, we planned to limit interventions to a minimum. Our objective was to strengthen the existing fabric, so that it would suffer less from climatic factors and also resist moderate earthquakes in the future. Professors Müfit Yorulmaz and Feridun Cili from the Faculty of Architecture (ITÜ) were responsible for structural inspections and interventions.

We preferred to use materials which would match the originals. This required research of the mortars, brick sizes and types of stones used in the original as well as the historic repair phases. Due to earthquakes and ravages of time, the wall had to be repaired several times in the past, using different mortar mixes, brick sizes and stones. Conservation of the historic sequence of these repairs also posed problems which had be tackled with care.

Materials and construction techniques

The inner wall is ca. 4.80-5 meters thick and is fortified by towers placed approximately 50 meters apart. The curtain wall was preserved to a maximum height of 12 meters. It consisted of a thick rubble core, contained between two facings built out of squared blocks, 30-50 centimeters deep. Brick bands consisting of five courses were laid at intervals of 7-11 courses of dressed stone. These ran through the whole thickness of the wall, binding the structure firmly. The towers are attached to the inner wall with their eastern sides. Their walls are 180-200 cm thick and exhibit the same constructive detail as the main wall, consisting of a rubble core between two facings.

As experts in building materials, professors Nihat Taydemir and Erol Gürdal from the Faculty of Architecture (ITÜ) examined the historic building materials. Dr. Ahmet Güleç from Istanbul Central Conservation Laboratory, analyzed mortar samples from different levels of the walls and towers. The composition of mixes for mortars were developed by laboratory research. In the fifth century mortar, hydraulic property was provided by the brick dust. In order to shorten setting time and improve pozzolanic activity, a low percentage of white cement was added to the new mortar mix. For medieval repairs, lime mortar with coarse sand and crushed brick had been used. A new mix was prepared with matching colour and composition to be used for repointing these sections.

The fifth century structure was built with sandstone quarried locally from the area just outside this part of the city wall. These quarries are no longer in function; they have been exhausted by continuous use over the centuries. Samples of matching sandstones were provided from two different sources. According to the results of laboratory investigation, sandstone from Kandira was selected for replacements and renewals.

Bricks measuring 38 x 38 x 4-5 cm were used in the 5th century construction. For later repairs, smaller and thinner bricks were employed. When we started this project, it was not possible to find solid bricks in the turkish market to match the original ones from the fifth century. We tried to persuade the brick industry in Turkey to produce solid bricks for our work. After spending a year for this cause, Kılıçoglu firm from Eskişehir could be persuaded to cooperate and produce solid bricks for the conservation of the Theodosian walls.

Structural observations and interventions

In this part of the walls, no significant damage due to soil conditions was detected. The wall and the towers were constructed together with the same stone courses and brick bands up to the level of the rampart and breakstwork. Above this level, the eastern walls of the towers sit on the main wall. This detail is visible in towers which still have preserved sections above the rampart level. According to our present knowledge, this kind of jointing is not appropriate, especially when the structure is founded upon a non-uniform soil and the differences in vibration periods as well as of torsional deformations generated by earthquakes, are expected. However, correction of this detail is out of question from the point of restoration principles. Apart from earthquakes, the structural problems related to the restoration of the walls and towers can be summarized as:

  • minor cracks on the walls and domes,
  • major cracks or discontinuities in the towers,
  • bulging,
  • cantilevered wall sections,
  • the loss of the eastern archways.

During the strengthening operation, it was aimed to restrict insertion of new structural members to the minimum so that the behaviour of the original construction would not be altered. Stitching the cracks by insertion of new bricks or stone blocks and using stainless steel clamps was also employed when the cracks ran through the wall thickness. Large cavities adjoining the cracks were filled with rubble and grout. Reinforced concrete beams were inserted at the top of the walls, to achieve bonding over major vertical cracks on the tower walls.

In some parts of the inner wall and towers, wall sections had bulged out due to overloading caused by quick repairs, or action of trees; their roots splitting massive sections of the wall. In such cases, numbering the revetment blocks and removing them until the core would be reinstated with new material was one solution.

Restoration work

The first tower had been restored in 1988, along with the western elevation of the curtain wall connecting it to the second tower, as part of the larger restoration project covering the three city gates (Belgrade, Silivri and Mevlevihane gates).

Our practical work on the wall started at the beginning of february 1992 by consolidation of the section between T1 and T2. On the western elevation of the wall, there were a number of lacunae, formed by action of the tanneries and a long period of neglect. The voids were filled with rubble and mortar and the facing was renewed with the original construction technique. The lower sections of the wall had its 5th century fabric still intact, but the upper sections had been subjected to several repairs during the Middle Ages. We tried to identify the different phases of construction, in an effort to put our small patches into the right context.

The eastern façade had been restored in 1988. Due to the presence of tanneries on the site, the western side was only partially treated. The battlements had been reconstructed but the top of the wall was in a very poor condition. It was necessary to repair the rampart floor to stop water penetration from the top. In the original design, the floor of rampart wall was inclined towards the east, so that it could be drained easily. The same detail was repeated in our project, with the insertion of a water-tight layer under the brick tiles.

Tower 2

The second tower (T2) presented several problems for its preservation. The octagonal prismatic form had been seriously damaged due to earthquakes and mutilation by the tanneries. The tower stood as an imposing figure, a remarkable ruin which should be preserved as it was. Yet its structural instability forced us to take measures against further losses. The gap in the first dome had to be filled. The structural considerations also made it necessary to raise the walls up to the base of the dome, at least. With this in mind, we worked on a 1/100 scale model to figure out the final form of the ruin after treatment. The cracks in the upper dome also presented problems; the wedge shaped portion at the north was stabilized by stitching, grouting and the insertion of a buttress next to the wall. A similar detail was deviced at the southeast corner, to back up the overhanging sections of the tower. During the strengthening operation, it was aimed to restrict insertion of new structural members so that the behaviour of the original construction would not be altered. Stitching the cracks by insertion of new bricks or stone blocks ans stainless steel clamps was adopted for minor cracks. Grouting was also employed when the crack ran through the wall thickness.

The vestibules of the towers were preceeded by brick arches, most of which have been lost in the course of time. This has resulted in the formation of cantilevered wall sections over the missing arches. In order to limit interventions to a minimum, we first considered the possibility of doing without rebuilding the arches. But during the first winter season we spent at the site, it was observed that, after heavy rains, the binding mortar weakened and stone blocks fell down, presenting life risk for the people around. Considering the difficulty of holding these sections of the facing in their cantilevered state by merely strengthening their binding mortars, we decided to reconstruct the supporting arches. After acquiring solid bricks in 1993, the vestibule arches were built.

The wall between Tower 2 and Tower 3

The wall between T2 and T3 had been seriously damaged on its eastern side. Only the middle section of the wall had its original facing preserved partially, while the rest suffered from stone removal and vegetation attck. There were about 1.5 m deep cavities on the wall surface. Structurally, it could be left as it was, but the risk of further disturbance by weather conditions and trees was challenging. Refacing the wall with new stone, as hab been done in previous restorations would create a striking contrast, due to the ratio of the newly built surface. Finally, a solution which will protect the wall from action of man and nature was adopted. In the lower section of the wall, ashlar facing was raised to approximately two meters from the ground, while in the region out of reach, a recessed, haphazard core surface filled in the voids. The execution of the core surface demanded special technique which we tried to accomplish with several masons on the work site. By placing the headers deep into the wall, it was aimed to establish a larger margin of safety in connecting the rubble core to the facing.

Tower 3

T3, which is a rectangular tower was in a very poor state of preservation. It has lost the vestibule arch, its southern wall, half of its west wall, part of the east wall and the barrel vault over the ground floor. The lower sections were from the fifth centuery, but it was evident that the tower had suffered from earthquakes, had been repaired and partially reconstructed. The north wall had a dangerous void, with immanent risk of collapse . The same was true for the eastern wall; only after putting up props, it was possible to start to work on it. Excavation revealed part of the round arch over the gateway into the ground floor of the tower. The vestibule arch had to be rebuilt in order to uphold the upper sections of the wall. The void in the northern wall was filled with fifth century constructive details, according to the evidence exhibited by the surrounding wall surface. The north and west walls were preserved in their existing height and a continuous reinforced concrete beam was placed at the top to achieve continuity and bonding between the two walls.

Tower 4

This tower is octagonal in plan and being almost totally reconstructed after an earthquake at the beginning of the 11th century, it is one of the well preserved towers in the land walls. The upper chamber and platform levels are intact. The most serious damage to the structure is the diagonal crack which starts above the window level on the W façade and runs down about 6 meters, making an angle of 45 degrees with the horizontal.

Restoration work in this section of the wall stopped at the end of 1993 after working on the wall section between Tower 4 and Tower 5. The conservation project for Tower 4 was completed but we did not have the chance to apply it. In our work, we tried to be true to the historic evidence exhibited by the fabric. Careful documentation was followed by protective measures. Restricting the additions to a minimum was our guideline in treating the invaluable entrusted to us. We hope that before long the conditions will improve and the work will commence again.



Citer ce billet
oui (1994, 3 décembre). Conservation at work in Istanbul : land walls. Observatoire Urbain d'Istanbul. Consulté le 25 juin 2024, à l’adresse https://doi.org/10.58079/slqj

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