Case study on the Restoration of Minceta Fortress

Introduction
City walls of Dubrovnik form an irregular quadrilateral ending at each corner with strong forts.
Tower Minčeta is the strong fort located in the North wall ending, facing toward the land. The
name derives from the name of the Menčetić family, who owned the ground the tower was
built upon. Minčeta, the highest point in the Dubrovnik defense system, is a large round fort
with a massive base in the form of a huge covered outer wall. The fort is topped with a great
Gothic crown that spreads over the side of the fort instilling the sense of power. The gothic
crown is of negligible strategic importance and is more decorative in nature.

A view of the Minceta fortress

History
The tower originally constructed as a four-sided fort in 1319 and takes its name from that of
the landowners of the time: the Menčetić family. Immediately after the fall of Constantinople
to the Turkish Ottoman Empire in 1453, the tower was added to by Italian sculptor and architect
Michelozzo di Bartolomeo Michelozzi who built a round tower adapted for warfare with 6-
meter-thick walls and protected gun ports. A disagreement ensued between Michelozzi and
local government officials which prompted his return to Italy but the tower was completed by
Giorgio da Sebenico, otherwise known as Juraj Dalmatinac who added stability to the bottom
of the tower as well as its iconic Gothic crown.
 a b

(a) Dubrovnik, with the location of the Minceta fortress marked. (b) A view of the Minceta fortress from the
north-west.

Basic data of the fortress

Dubrovnik is one of the UNESCO World Heritage Sites in Croatia. Minceta fortress is a unique
symbol of Dubrovnik being located in the north-western corner of famous city walls. Top of
the fortress is the highest and the most prominent part of the city walls. With its position and
size, it dominates the north-western part of the old city having been highly important point of
its defense in history. Dubrovnik is located in a seismic zone with possible catastrophic
earthquakes, some of which have already occurred in its history. Minceta fortress has been
damaged and rehabilitated several times in its centuries of history. At the end of the eighteenth
century, the deep cut for the city road was excavated, that significantly destabilized the fortress.
Fortress has some safety and stability issues in the present state, although at first sight, it is in
a relatively good state because it has been partially renovated throughout the whole period.

View from the South of the city walls and the city. The present shape of the walls was defined in the 14th
century.
Minceta Fortress Today
Completed in 1464 and even though the 750 steeps, winding steps to the top are a challenge
for even the fittest cultural tourist, when we get to the top of Minceta Tower and see the
sensational views of the old town of Dubrovnik to one side and out into the Adriatic Sea to
the other, we will understand that the climb was worth it. We can also visit the museum in the
excavated basement. During the summer another flag is set next to it, so-called Libertas flag
annotating the slogan and symbol of the old Dubrovnik Republic, with ‘libertas’ being the
Latin word for freedom. After two years of construction works worth 3 million kunas, the
public gained access to the restored interior of the Minčeta fortress after 550 years, together
with a part of the western wall and Ironworks Museum. The central idea was to restore the
Minčeta in a way so visitors would not notice anything was done.

Night view of the Minceta fortress

Structure
Main parts of the fortress are the old tower, the pre walls and the new tower. Only the interior
of the old medieval tower, with an inside height of 12 m, and the south facade are still
preserved. Pre walls were built in a circular form with an outside diameter of 35 m and a height
of 16.3 m. Pre walls have three levels. Zero level has central and peripheral corridors and five
radially placed casemates. First level has a peripheral corridor and nine casemates. Second
level is infilled with earth and stone. Above the second level, there is the terrace surrounded by
the pre walls, at a height of 3.4 m, and a thickness of 2.8 m, with nine openings for guns and
the walkway.
 a b

Vertical cross section (a) and floor plan at the zero level (b) of the Minceta fortress.

New tower rises from the middle of the pre walls, with a diameter of approximately 18 m and
a height of 7.3 m. Bottom part of the tower is inclined, the central part is upright and the upper
part protrudes outward as a cantilever. Domed atrium of the tower has a diameter of 8 m and a
height of 7 m. Terrace at the top of the new tower is surrounded by a defensive wall with four
holes for cannons and two guard houses.

Views of the interior of the fortress

Study of the Fortress Damage and Safety

Minceta fortress was built over a long period of time. Design of the first medieval tower was
made in 1319. Tower was completed in 1343. Largest part of today’s fortress was built in the
fifteenth century. First, the old square tower was strengthened. Zero level of the tower was
infilled with stones and a lime mortar in 1453, while the upper part of the tower was raised to
3 m. Tower was rounded during 1455 by the pre walls on the three levels surrounding the
existing tower. Height of the old tower was probably reduced during the building of the new
tower. Second level of the pre walls was infilled in 1646 due to the strengthening of the fortress
defence. A large portion of Dubrovnik was destroyed in 1667 during a strong earthquake of
intensity X. Massive city walls and the fortifications did not collapse in the earthquake for the
most part, but suffered extensive damages. Exact damages of the Minceta fortress were not
specified. Obviously, the fortress damages were not extensive compared to the damages of
other buildings having been noted for repairs. Immediately after the earthquake, a fire followed
lasting for 20 days. In total, approximately 3000 people died, which was then half of the city’s
population.
Road around the city was breached at the end of the eighteenth century, which significantly
destabilised the fortress. Landslides soon followed along the slopes of the deep cut, which were
stabilised by the three stone struts across the cut and by the slope protection with the stone
cladding. Wider Dubrovnik region was badly damaged in the disastrous Montenegro
earthquake in 1979, of intensity IX-X. Earthquake’s power in Dubrovnik was of intensity VII.
Additionally, the Dubrovnik region was strongly damaged during the ston earthquake in 1996,
of intensity IX. It is not known whether the Minceta fortress suffered any damages from these
earthquakes. In the twentieth century, there were no significant restoration works on the
Minceta fortress. Joint repairs on the facade of the fortress were performed on several occasions
in the last 15 years. Larger and better-quality restoration works were carried out in 2002. High-
quality repairs to the stone cantilevers on the top of the new tower were completed in 2009.

Main Observed Defects and Damages of the Fortress

Rock layers below the fortress near the deep cut have an unfavourable slope inclination toward
the road threatening the stability of the fortress. Numerous cracks in the walls and the vaults of
the old tower were observed. South wall of the tower is very slender and its stability is
questionable. At the zero level of the fortress, numerous cracks in the vaults and the walls of
the patrol corridors and casemates were also observed. Wide cracks in the ceiling of the corridor
and the walls were probably caused by shifts in the underlying rock. Sliding of almost half of
the pre walls towards a deep cut was caused by this shift. Breaking apart of the pre walls and
their slipping towards the deep cut are associated by the direction and the width of the cracks.
a b

c d
 e

Some defects and damages of the fortress. (a) The deep cut on the northern side of the fortress. (b) Typical cracks
in the vaults of the patrol corridors and casemates at the zero level. (c) The broken piece of the wall corner in the
peripheral corridor at the zero level. (d) Typical cracks in the pre wall around the gun opening above the terrace.
(e) The marked cracks in the north-western facade of the fortress.

On the first level of the fortress, similar cracks to the zero level were observed. Wide cracks
and their directions in the northern and western parts of the pre walls show the sliding towards
the cut. Cracks were also observed in the pre walls around the gun openings. Cracks extend
along the entire height of the wall in several places. Consequently, the outer surfaces of the pre
walls are cracked in several locations from the bottom to the top being fractured into several
separate parts. New tower is visually well preserved in relation to the rest of the fortress because
it was recently repaired. Joint grouting between the stone blocks and the replacement of the
damaged stone cantilever on top of the tower have been involved in rehabilitation activities.
Cracks in the terrace on the top of the tower were also observed. Breaking apart of the tower
into separate parts has been possible.

Field and Laboratory Works

Extensive field and laboratory works were performed to determine the mechanical properties
of the fortress masonry and the top of the foundation rock at the zero level, as well as the
geotechnical and geophysical properties of the rock below the tower. More details can be found
in. Compressive strength of the top of the rocks below the tower are in the range of 15 to 60
MPa, and the compressive strength of the stone blocks of the fortress are 60–90 MPa. Infill
between the external stone blocks of lime and crushed stone has a compressive strength of 10–
15 MPa. Compressive strength of the mortar in the joint between the stone blocks is 12–17
MPa. Values were determined by the standard test procedures for determination of compressive
strength of masonry units and mortar HRN EN 772–1:2015 and HRN EN 1015–11: 2019,
respectively. Mechanical properties of the rock mass required for the static and dynamic
analysis of the fortress were determined by the geotechnical and geophysical investigations.
Rock layering toward the deep cut was observed, which is very unfavourable. Relaxation of
the rock mass after the excavation of the deep cut was noticed.

Stress-Strain and Safety Analysis of the Fortress

Stress-strain state and safety of the fortress before and after the road breaching on the north
and west sides of the fortress were analysed. Fortress and the underlying rock were included in
the geometry model. Stability of the underlying rock mass toward the deep cut was analysed.
Relevant load cases and actions were considered (dead load, temperature variations, modal
analysis, and static and dynamic earthquake analysis). Strongest earthquakes expected to occur
at the location of the fortress were counted for a return period of 475 years and the maximum
design spectral acceleration is 0.3 g according to EN 1998 (EC 8). In the dynamic analysis, the
fortress was analysed for the Stone earthquake from 1996. Different models used a different
approximation of the geometry (1D, 2D, 3D) and stiffness of the structure, as well as different
constitutive models of the masonry and the foundation rock (linear and nonlinear).
Used nonlinear computational model simulates the main nonlinear effects of the masonry and
rocks, such as the opening and closing of cracks in tension, the nonlinear behaviour in
compression, the tensile and shear stiffness of the cracked material, the nonlinearity on the
contact, and the anisotropy of the materials. Macro model for the masonry and the rock was
adopted. Cracks were modelled as smeared. adopted discrete elements between the fortress and
the rock can simulate the sinking, the uplift, and the sliding of the interface surface. effect of
the large displacements was also included. dynamic equation was solved by an implicit
Newmark algorithm. adopted numerical model has been verified using experimental and
numerical tests of other models and can be considered reliable.

a
 b

The graphical presentation of the adopted 2D nonlinear numerical model: (a) finite elements, (b) constitutive
model for basic materials, (c) constitutive model for contact elements, (d) possible crack patterns
Some results of finite element analyses. (a) The displacements for the nonuniform temperature distribution of
10°C at the south side and 0°C at the north side. (b) The vertical stresses for the dead load (MPa). (c) The vertical
stresses for the combination of the dead load and the earthquake forces for an acceleration of 0.3 g (MPa),
equivalent static analysis. (d) The first four mode shapes and eigenvalues of the numerical model.
 Table 1: Material parameters of rock and masonry used in the numerical analysis

Main Finding of the Study

Numerous cracks in the walls and vaults of the fortress were observed, particularly on the pre
walls. widest cracks are at the locations of the wall openings. old tower is heavily damaged,
and the stability of the south wall is especially threatened. In a structural sense, all of the
openings in the body of the fortress are the weak spots. corridors and casemates at the zero and
first level are almost overlapping, resulting in continuous weakening and vulnerability of the
fortress to vertical splitting in the direction of the weaknesses. Large horizontal cracks in the
ceiling of the peripheral corridors at the zero level and the vertical crack over the entire northern
facade likely indicate the breaking apart of the northern pre walls into the blocks. Fundamental
rock below the fortress near the deep cut is cracked and adversely layered. At that location, the
rock sliding surfaces are probably already formed. Existing drainage system at the top of the
fortress tower and at the terrace of the pre wall top has not been well constructed, which has
allowed water to leak into the fortress interior wider zones of the cracks in the masonry of the
fortress may occur only for the temperature actions. Fortress cannot withstand the maximum
design ground acceleration of 0.3 g and exceeds the stability design requirements. Deep cut
along the western and northern edges of the fortress significantly reduced its stability. In regard
with the Stone earthquake considered in this study, the fortress did not collapse, but it was
strongly damaged. Calculated maximum compressive stress in the masonry of the fortress for
Stone earthquake was approximately 8.0 MPa, and the maximum displacement of the fortress
top was approximately 77 mm.
Some parts of the fortress suffered severe structural damage under the adopted earthquakes
(lintels above the opening, some vaults, the southern wall of the old tower, the outdoor
cantilevered walls of the pre walls above the second level, etc.). Rock massif near the deep cut
is not safe against the sliding, even for only gravity loads. An imminent risk of the collapse of
the rock massif was prevented by the previous construction of three struts such as the stone
arches. next strong earthquake could be fatal for the stability of the slopes of the deep cut and
the fortress. An adequate increasing of the rock massif stability near the deep cut is urgently
needed. Restoration and strengthening of the fortress are urgently required. Water leaking into
the fortress on the top of the tower and on the terrace of the pre wall top must be prevented.
The stresses (MPa) in the assumed sliding surface of the rock for the dead load: (a) normal stresses, (b) shear
stresses
Some results for time history analysis under the Stone earthquake. (a) Some global displacements for the Stone
earthquake at 5.85 s and t 6.20 (s). (b) The principal tensile strains (cracking zones) at 6.2 s for the Stone
earthquake, view from the north. (c) Some characteristic displacements of the north-western facade of the
fortress over time for the Stone earthquake.

Restoration of the Fortress

Three alternative strengthening solutions for the foundation rock below the fortress near the
deep cut are proposed: (I) the tunnel, (ii) the additional struts, and (iii) the rock slope
stabilization by the ground anchors.
(I) Tunnel structure would support the opposite slopes of the deep cut and stabilize the cliff
below the fortress, which would provide a permanent and reliable solution. The original
appearance of the fortress and the city walls on that location before the road excavation
would be restored. A new city area of approximately 3000 m2 for parking and other
facilities would be created.
(II) In addition to the three existing stone struts, four new struts should be built. Solution
would be visually perceived as a tunnel. It would be more rational than the tunnel
construction but without the possibility of creating a valuable city area, and it would be
less favourable than the conventional tunnel.
(III) Strengthening of the cut slope by concrete beams and ground anchors would be
visually unacceptable and insufficiently reliable. In addition, the unfavourable seismic
effect of the deep cut on the fortress stability would not be reduced, and the durability
of such a solution would be limited due to the potential danger of the corrosion of the
ground anchors in a wet environment. Solution is considered as the least acceptable
among the proposed ones.
The assumed global cracking pattern, breaking apart, and sliding rock surface. (a) The overlapping of the
corridors and casemates at the zero and first levels. (b) The assumed global breaking apart of the fortress. (c)
The assumed sliding rock surface and the breaking apart of the pre walls.
The restoration of the fortress and its allowed limited strengthening included the following
main works:
(I) Grouting of the cracks in the walls and vaults that are wider than approximately 2 mm.
(II) Jointing between the stone blocks along the cracks and in the locations of the worn
joints using lime mortar.
(III) Stiffening of the south wall of the old tower by upgrading the southern boundary
city wall.
(IV) The Construction of soft diaphragms for stiffening the fortress at the level of
the open terraces on top of the pre walls and at the level of the terrace on the tower top,
with the improvements of the system for precipitation water drainage. All rehabilitation
solutions have been previously consulted by the city administration and conservators-
restorers.

The position of the glass fibre strands for stiffening the tops of the fortress: (a) the terrace on top of the pre walls,
(b) the terrace on top of the new tower.

Conclusions
Historic Minceta fortress in Dubrovnik has been severely damaged, with the risk of collapse
when it is subjected to even moderate earthquake action. The initial stability reduction of the
fortress was caused by the deep excavation along its north-western edge. Adverse layering of
the stone cliffs at that location and the gravity load of the fortress caused the shift of the fortress
toward the deep cut and the formation of the sliding surface of the rock toward the deep cut.
Some of the fortress damage is probably due to the effects of temperature and earlier
earthquakes. Urgent improvement of the stability of the foundation rocks below the fortress is
necessary. For this purpose, the construction of a common tunnel instead of the deep cut is
proposed as an optimal solution. Proposed solution would restore the terrain to the original
level creating valuable urban areas. Fortress should be repaired by an adequate grouting of the
masonry along the cracks, jointing between the stone blocks along the cracks and by stiffening
the weakest parts of the fortress structure. All the restoration works were consulted by the city
administration and conservators’ restorers.
The following characteristic stress-strain and safety states of the fortress in its history were
analysed by finite element method:
(I) The original state before excavation of the deep cut and without fortress damages (the
state A)
(II) The state after excavation of the deep cut and without fortress damages (the state B)
(III) The state after excavation of the deep cut and with the existing damages of the
fortress (the state C).
(IV) 'The state without the effect of deep cut (foundation rock was fixed), and the
restoration and limited strengthening of the fortress (the state D).

Submitted by Samira Ali

B.Sc in Interior Design
Semester 6