Differential Settlement in Historic Masonry Towers: The Case of the Murcia Cathedral Bell Tower

Abstract

The bell tower of Murcia Cathedral (1521–1793) exhibits a documented inclination whose origin and structural significance have never been examined through an integrated geotechnical–structural approach. This study aims to identify the causes, quantify the magnitude, and assess the safety implications of the tower’s long-term differential settlement. A multidisciplinary methodology is adopted, combining historical construction records, geological and geotechnical data from the Segura alluvial plain, non-destructive testing of masonry, and classical analytical modelling based on Heyman’s masonry theory, consolidation mechanics, and elastic column behaviour. This approach is selected in place of finite element modelling because the tower’s geometry, construction sequence, and material parameters are sufficiently constrained to allow a non-invasive and verifiable assessment suited to heritage structures. Results indicate a total horizontal displacement of approximately 0.56 m toward the northwest, produced by the slow consolidation of compressible silty–clayey deposits influenced by groundwater fluctuations and by historical eccentric load redistributions during the eighteenth-century construction phase. The calculated working compressive stresses (0.83–1.02 N/mm²) remain far below the estimated strength of the limestone masonry, and the bearing capacity analysis suggests a safety factor of about 1.5 against foundation failure. These findings confirm that the tower’s deformation reflects the long-term geotechnical response of the subsoil rather than structural instability. The study provides a non-destructive analytical framework for interpreting settlement mechanisms in historic masonry towers and contributes a quantitatively grounded explanation of the Murcia Cathedral tower’s inclination, offering guidance for future assessment of similar heritage structures.

1. Introduction

The bell tower of Murcia Cathedral, erected between 1521 and 1793, is one of the most technically and historically complex masonry structures of the Spanish Renaissance and Baroque. Rising 93 m above the Vega del Segura, the tower embodies nearly three centuries of construction, interruption, and adaptation, and has long been known to exhibit a measurable inclination [1]. Although its architectural and artistic evolution has been extensively studied, the physical causes and structural implications of this deformation have not yet been examined through an integrated geotechnical–structural framework.

Historic masonry towers constitute a singular class of slender, massive structures whose behaviour depends simultaneously on their construction sequence, material heterogeneity, and the geomechanical characteristics of the subsoil. Classical treatises already recognised this close soil–structure dependency: Renaissance architects such as Francesco di Giorgio Martini emphasised deep foundations, pile-supported systems, and the need to consider the properties of soft ground when erecting tall masonry shafts [2]. More recently, analytical approaches have been developed to understand the mechanics of masonry, notably Heyman’s limit analysis framework [3], while contemporary studies on historic towers have explored material properties, deterioration mechanisms, and structural performance under gravity and seismic actions [4,5,6].

In recent decades, several numerical strategies have been employed to analyse the behaviour of historic masonry towers, including finite element modelling (FEM), discrete element modelling (DEM), rigid-block modelling (RBSM), and limit analysis (LA). These approaches have proved valuable for simulating cracking patterns, dynamic responses, and the seismic vulnerability of damaged or highly uncertain structures. However, their reliability depends on the availability of detailed geometric, material and boundary-condition data, as well as on the presence of measurable damage patterns. In the present case, the Murcia Cathedral tower shows no structural damage, its geometry and load path are well documented, and its long-term deformation is governed by geotechnical rather than structural mechanisms. For this reason, classical analytical mechanics provides a transparent, non-invasive and evidence-based framework better suited to the aims of this study.

In contrast, research specifically addressing the interaction between soil consolidation, long-term settlement, and the geometry of the Murcia Cathedral tower remains scarce. Classical works on the cathedral’s stone and geomorphology—such as those by Vera Botí [5], Esbert et al. [6], and Calvo et al. [7]—have provided essential material characterisation, and the geotechnical mapping of the region by the Instituto Geológico y Minero de España [8,9,10,11] has documented the compressibility of the alluvial basin. However, no previous study has combined this information into a unified analysis capable of explaining the tower’s inclination as the outcome of both construction history and subsoil behaviour.

This gap is relevant because the tower’s deformation pattern cannot be understood solely through architectural description or isolated geotechnical observations. The structure was built in separate phases, with long interruptions, upon a heterogeneous substratum composed of silts, clays, sands, and gravels subject to groundwater fluctuations [9,10,11,12,13]. The documented adjustments introduced in the eighteenth century by José López—such as asymmetric wall thickening—suggest that the builders themselves interpreted the inclination as a geotechnical phenomenon and attempted structural compensation [14,15]. A quantitative assessment is therefore necessary to determine whether these deformations pose any risk or whether the tower remains within safe mechanical limits.

Accordingly, the objective of this study is to analyse the architectural configuration, soil conditions, and structural behaviour of the Murcia Cathedral tower as a single interrelated system. Specifically, the study:

• characterises the geometry and construction sequence of the tower;
• evaluates the geotechnical properties of the underlying alluvial deposits;
• applies classical analytical modelling to estimate stresses, settlements, and stability; and
• interprets the tower’s present inclination in light of its historical load redistribution.

A clear visual understanding of the tower’s structural hierarchy is provided in Figure 1, which illustrates the differentiated architectural tiers and the corrected elevation of the façades. Furthermore, the slight real inclination toward the west—central to this study—is already perceptible in the digitally corrected elevation shown in Figure 2.

This study adopts a classical analytical approach rather than finite element modelling. The decision responds to the characteristics of the tower: its geometry, construction sequence, and material parameters are well constrained by historical documentation and in situ inspection, and the absence of structural damage allows the use of analytical mechanics (Heyman theory, elastic column behaviour, and consolidation models) to obtain reliable safety margins without the need for intrusive testing or numerical discretisation.

The remainder of the paper is structured as follows. Section 3 describes the architectural evolution of the tower and its structural configuration. Section 4 examines the geotechnical profile beneath the cathedral and the subsoil processes affecting long-term settlement. Section 5 presents the analytical structural assessment. Section 6 analyses the measured inclination and explains the differential settlement in relation to construction history. Section 7 summarises the findings and discusses their implications for the interpretation and preservation of historic masonry towers.

The structural behaviour and long-term settlement of historic bell towers have been studied through a wide range of approaches, including geometric monitoring, material characterisation by non-destructive tests, and analytical or numerical modelling [3,5,6,11,16]. Previous works on leaning towers—such as those of Pisa, Bologna, or Zaragoza—highlight the influence of compressible alluvial soils, differential consolidation, and construction-stage eccentricities as primary drivers of inclination [4].

Specific studies addressing the Cathedral of Murcia include historical analyses based on archival documentation [1,14,15], material characterisation using rebound-hammer testing [5,6,14,17], and geotechnical investigations conducted through boreholes and laboratory tests [18,19]. However, no previous work has integrated architectural, structural and geotechnical evidence into a complete explanatory model of the tower’s current inclination.

The present study builds upon this body of knowledge by combining historical documentation, geometric analysis, non-destructive testing, classical structural mechanics, and geotechnical evaluation to provide a comprehensive interpretation of the tower’s long-term settlement behaviour.

2. Objectives

The present study aims to provide an integral analysis of the architectural, structural, and geotechnical evolution of the bell tower of Murcia Cathedral (1521–1793), with the purpose of clarifying the physical, historical, and constructive causes that gave rise to its slight inclination and to the differential settlements recorded over the centuries.

To achieve this overall aim, the following specific objectives are established:

• To document and interpret the architectural configuration and stylistic evolution of the three principal construction phases of the tower—Renaissance, Late Renaissance, and Baroque—identifying the functional and symbolic logic that guided each stage.
• To characterize the subsoil beneath the tower, integrating available geological, geotechnical, and hydrogeological data (from IGME maps and piezometric records) in order to assess its bearing capacity, its long-term consolidation behaviour, and its influence on the overall stability of the structure.
• To analyse the structural configuration of the tower by applying the classical principles of masonry mechanics (Heyman) and elastic column theory (Euler), adapting these models to the specific case of a double-wall system composed of calcareous ashlar, lime mortar, and rubble infill, to estimate compressive and tensile stresses under current conditions.
• To evaluate the historical causes of the observed inclination, by comparing construction records with settlement data and calculated stresses, and by analysing the corrective measures implemented by José López in the eighteenth century to rebalance the structure.
• To determine the current state of stability of the tower and its safety margin against crushing, overturning, and foundation failure, taking into account both gravitational loads and seismic actions documented during major historical earthquakes (1755, 1829, and 2011).
• To interpret the geotechnical and structural behaviour not only from an engineering standpoint but also from a cultural perspective, understanding the deformation of the tower as an expression of equilibrium between matter and time, between gravity and grace, between technique and history.

This case study is particularly relevant because the Murcia Cathedral tower represents one of the tallest historic bell towers in the Iberian Peninsula founded on heterogeneous alluvial soils. Understanding its long-term settlement behaviour provides valuable insights for the assessment of large masonry towers built on similar geomorphological environments. The interdisciplinary objectives presented above contribute to filling a documented gap in the literature, and the findings of this study are expected to support future research, monitoring strategies and conservation-oriented assessments of historic tall towers.

3. Methodology

The research follows a multidisciplinary and non-invasive methodology combining:

• historical analysis of archival records, construction documents and early descriptions of the tower;
• geometric analysis of the elevations using contemporary survey data to identify deviations from verticality;
• non-destructive testing using rebound-hammer measurements (Votoer Resiliometer Rebound Hammer, Votoer, Shenzhen, China) to verify limestone uniformity and detect superficial weathering;
• classical analytical mechanics, including Heyman’s limit theory, elastic column behaviour and composite-wall modelling, to determine compressive stresses, stiffness and stability margins;
• geotechnical analysis incorporating borehole data, stratigraphic interpretation and bearing-capacity calculations based on IGME datasets.

This methodology enables a transparent, reproducible and evidence-based interpretation of the tower’s mechanical behaviour. Because the structure shows no damage and its deformation is primarily geotechnical rather than structural, numerical discretisation methods (FEM, DEM, RBSM) are not required for the objectives of the present study, although they may form the basis of a complementary future investigation.

4. Architectural Analysis of the Tower

Before addressing each construction stage individually, it is necessary to outline the structural logic that governs the tower as a whole. The bell tower is organised as a vertically stratified composition of three main tiers, each corresponding to a distinct historical period and construction technique. Although their architectural languages differ—Renaissance in the lower bodies and Baroque in the upper levels—their load-bearing behaviour remains fundamentally continuous. The masonry walls follow a double-leaf configuration with a rubble-and-lime core, forming a tube-in-tube system that determines how vertical loads are transferred and how each tier responds to the differential settlements documented during construction. The following subsections examine each tier from both an architectural and structural perspective.

4.1. First Tier

The first tier of the bell tower occupies the site formerly belonging to the Chapel of Saints Simon and Jude, granted in the fifteenth century to the widow of Jacobo de las Leyes. When construction of the tower commenced in 1519 under the Florentine brothers Francesco and Jacobo Florentino, this chapel was demolished and replaced by the Main Sacristy, which still survives as one of the most significant interior spaces of the cathedral complex. An inscription on the north façade records 19 October 1521 as the date on which the first ashlar of the tower was laid, marking the beginning of a construction phase rooted in Renaissance engineering culture.

The selection of the foundation site presented considerable technical challenges. The subsoil, composed of soft clay saturated by the exceptionally high water table of the sixteenth century, required effective control through the use of extraction pumps—an unusual measure for the period, but essential to ensure the stability of the foundation works. This decision is consistent with the technical rigor associated with the Florentino brothers and reflects the early awareness of the geotechnical vulnerabilities of Murcia’s alluvial plain, later confirmed by geological and geomechanical mapping [8,9,10,11] and modern subsidence studies [12,13].

Externally, the first tier presents a square plan articulated by recessed Corinthian pilasters, garlands, grotesques, musical instruments, fruit motifs and shields inspired by Lombard and Tuscan Renaissance ornamentation (Figure 3). The sotabanco—a rebated plinth acting as a foundation—remained concealed until the later urban reforms of the Plaza de las Cadenas. This element, approximately 2.5 palmos high and 3 palmos wide (≈52 cm and 62 cm, respectively, assuming the Castilian palmo of 20.9 cm), anchors the tower to its foundation and contributes to the distribution of vertical loads along the perimeter. The windows display Bramantesque arches and compositional elements recalling Michelangelo and Sangallo, including hanging corbels beneath the balustrades. The attached columns with inverted volutes and masks are characteristic of Jacobo Florentino’s design language and appear in other works attributed to him, such as San Jerónimo in Granada.

Inside this first tier lies the Main Sacristy, covered by a groin dome and a distinctive cow-horn vault. Its elaborate cajonería—begun by Jacobo Florentino in 1523, continued by Gabriel Pérez de Mena after the fire of 1689, and completed by Francisco Guil in 1712—constitutes one of the most notable Renaissance furnishings of the cathedral. The sacristy’s spatial configuration, with robust masonry walls and compact vaults, contributes to the rigidity of the entire lower body [17].

From a structural standpoint, the first tier constitutes the principal load-bearing core of the tower. The walls follow the construction technique—typical of large Iberian masonry towers—of double ashlar leaves filled with a rubble-and-lime core, forming the fundamental tube-in-tube system that governs the mechanical behaviour of the entire structure. This configuration maximises stiffness by separating the inner and outer vertical surfaces, reduces bending effects by distributing loads through two concentric shafts, and limits the propagation of local deformations [14,20]. Details such as the differing heights of the ashlar blocks on the inner and outer faces—identified in Section 5—further confirm the absence of continuous through-stones and the presence of an internal core typical of composite masonry.

The foundation of this first tier, therefore, performs a dual role: architecturally, it establishes the Renaissance character of the tower; structurally, it creates the rigid platform from which all subsequent tiers rise [14]. The initial settlement problems documented during the sixteenth century, which later led to construction being suspended after the second tier, originate precisely in the interaction between this massive first body and the compressible silty–clayey substratum described in Section 4. The behaviour of the first tier, both as a static system and as the architectural base of the tower, is thus inseparable from the long-term geotechnical processes that shaped its eventual inclination. The ground plan of this first tier, including the inner core, the helical stair and the perimeter masonry walls, is shown in Figure 4.

4.2. Second Tier

The construction of the second tier began in the 1540s under the direction of Jerónimo Quijano, who inherited the foundational logic established by the Florentino brothers while enriching the architectural vocabulary of the tower. During this phase, Quijano introduced the Ionic order, Plateresque elements, window mouldings of great delicacy, and the ribbed vault that covers the interior volume of this level. The figures, ornamentation and profiles signal a stylistic refinement characteristic of his mature work, and many of these features survive today with notable clarity (Figure 5).

From an architectural standpoint, this second body preserves the proportions and alignment of the Renaissance base, ensuring visual and compositional continuity with the Florentino design [14]. Its elevations display a balanced organisation of pilasters, archivolts and friezes, and the interior presents a coherent spatial language centred on the domical vault, which originally functioned as a transitional chamber between the sacristy below and the planned upper levels.

Structurally, however, the second tier performs a distinct and essential function: it continues and stabilises the vertical load-bearing shaft initiated in the first tier. The double-wall system persists in this level, with the inner leaf presenting slightly greater thickness and mass than the outer shell. This feature, documented in the geometric recordings analysed in Section 5, implies that the inner tube bears a higher proportion of the gravitational load, while the outer tube acts primarily as a stiffening shell. The absence of continuous through-stones between the two leaves—also consistent with Renaissance Iberian masonry practice—supports the interpretation of the tower as a composite system in which the inner and outer walls behave semi-independently under compression.

The windows and decorative openings of this level, although architecturally prominent, were carefully dimensioned so as not to weaken the structural shaft. Their placement maintains uninterrupted vertical masonry piers at the corners and midpoints of each façade, ensuring that the compressive load paths remain concentrated along the stiffest alignments. This geometric coherence explains why the second tier transmits loads downward without introducing significant eccentricities, despite its greater ornamental elaboration [14,15,21].

It is also in this tier that the tower’s early differential settlement became most apparent. Historical documentation and geometric analysis indicate that by the late sixteenth century the structure exhibited a slight but measurable deviation from verticality. This prompted the eventual suspension of construction before the tower reached the height originally intended by Quijano [20]. The deformation pattern visible today—subtle deviations in the alignment of mouldings, stringcourses and pilasters—confirms that the settlement first affected the lower and middle Renaissance bodies, consistent with the compressible silty–clayey substratum described in Section 4. This second tier, therefore, constitutes the interface between the initial geotechnical response of the foundation and the architectural evolution of the tower.

In summary, the second tier reinforces the Renaissance identity of the structure while performing the crucial engineering task of preserving the continuity of the vertical shaft. As a geometrically coherent and structurally efficient intermediary between the massive base and the lighter upper Baroque bodies, it plays a key role in understanding both the historical suspension of construction and the tower’s later adaptations to its long-term deformation.

4.3. Third Tier and Crowning

After more than two centuries of interruption, construction of the tower resumed in the eighteenth century under the direction of José López. By this time, the structure already exhibited a measurable inclination toward the northwest, derived from the differential settlement affecting the two Renaissance tiers and the compressible silty–clayey substratum documented in Section 4. López’s intervention must therefore be understood not simply as an aesthetic continuation of the project, but as a deliberate technical response to the deformation accumulated since the sixteenth century.

Architecturally, the third tier inaugurates the Baroque transformation of the tower. Its elevations are organised with paired pilasters, dynamic mouldings, sculptural figures and a rich interplay of concave–convex surfaces, consistent with the broader eighteenth-century refurbishment of the cathedral. The presence of the mixed-lineal motifs, the increased plasticity of the cornices, and the rhythmic modulation of the façades reflect the stylistic language that characterises Murcia’s Baroque period. Figure 6 illustrates the volumetric transition from the Renaissance bodies to this Baroque elevation, showing both continuity and innovation in its composition.

However, it is the geometric adjustment introduced by López that gives this tier its structural significance. In order to counteract the established inclination, he reduced the plan dimensions of the third body by one foot on each side, while simultaneously increasing the thickness of the southeast wall by two feet. This asymmetrical modification, documented in contemporary writings and consistent with the measurements recorded in the present elevation, constitutes a mass-redistribution strategy unprecedented in the earlier phases of the tower. By shifting part of the upper mass towards the direction opposite to the settlement, López sought to improve the global alignment of the structure without dismantling or disturbing the lower tiers.

This intervention effectively altered the location of the vertical load path. Whereas the first and second tiers behave as a relatively uniform double-wall shaft, the third tier introduces a controlled eccentricity. The thicker southeast wall increases the stiffness and axial load capacity on that side, thereby reducing the bending moment generated by the pre-existing tilt. At the same time, the contraction of the plan reduces the lever arm of the gravitational forces, diminishing their contribution to further rotation. This combination of mass reduction, section modification, and eccentric stiffening explains the measurable change in curvature observed between the second and third tiers, which constitutes a partial correction of the earlier deformation.

Above this level rise the four conjuratorios, small pavilion-like structures located at the corners of the tower. Their lightness, reduced volume, and symmetrical placement ensured that they did not introduce new eccentricities into the system. Functionally connected to ritual practices involving the blessing of the environment and the calming of storms, the conjuratorios contribute minimal gravitational load relative to the scale of the structure. Their behaviour is therefore essentially parasitic, supported by the third tier without significantly affecting its mechanical response.

The bell chamber, which houses the twenty bells of the cathedral, follows above. This upper body is architecturally more permeable than the tiers below, with large openings designed to project the sound of the bells across the city. Structurally, this permeability reduces mass and increases the flexibility of the upper section, limiting the accumulation of stresses due to differential settlement or wind action. Finally, the dome designed by Ventura Rodríguez crowns the tower, integrating Renaissance harmonic principles with late Baroque and early Neoclassical sensibilities. Due to its geometry and materials, the dome imposes a relatively modest load compared with the massive lower tiers, and does not significantly influence the global deformation pattern.

Taken together, the third tier and upper bodies represent a decisive moment in the mechanical history of the tower. They illustrate the capacity of eighteenth-century builders to interpret structural behaviour empirically and to intervene using geometric intuition and material manipulation rather than theoretical tools. The adjustments implemented by López demonstrate an early understanding that the tower’s inclination was not the symptom of structural failure but the consequence of long-term subsoil consolidation—an interpretation that aligns with the analytical results presented in Section 6.

5. Subsoil Analysis Beneath the Tower

The behaviour of the bell tower of Murcia Cathedral cannot be understood without a detailed examination of the geological, geotechnical and hydrogeological characteristics of the Vega del Segura. The tower is founded on Quaternary alluvial sediments with moderate to high compressibility, whose long-term consolidation provides a consistent explanation for the inclination documented since the sixteenth century. This section synthesises the available geological mapping, borehole data and mechanical parameters reported in the literature and in geotechnical campaigns carried out in the surroundings of the cathedral.

5.1. Geological and Geomorphological Context

The cathedral stands within the Neogene basin of the Segura River, developed during the Upper Miocene after the fragmentation of the Betic orogen [8]. Marine sedimentation gave rise to marls and marly limestones, which were later overlain by Quaternary fluvial deposits. The Vega del Segura is a low-gradient alluvial plain shaped by recurrent flooding and sedimentation cycles, producing a heterogeneous succession of silts, clays, sands and gravels.

The Quaternary unit QT1—where the cathedral is located—is described as a high terrace consisting of silty–clayey sediments with sandy and gravelly interbeds [9,10,11]. These materials are laterally discontinuous and display variable stiffness, which predisposes foundations to differential settlement under sustained loads.

5.2. Stratigraphy and Mechanical Properties

Seven geotechnical boreholes performed during construction works near the cathedral—four external (15–50 m from the tower) and three intersecting the edge of the foundation—allowed characterisation of the subsurface [18]. The profile consists of:

• Urban fill (1–3 m), medium-to-low density.
• Silty–clayey deposits (10–15 m), low stiffness and high compressibility.
• Sandy gravels (>12 m), representing the ancient riverbed.
• Miocene marls at greater depth, of significantly higher rigidity.

The mechanical properties of these units are summarised in

Table 1. Lithotechnical profile beneath the Murcia Cathedral Tower. Data from IGME geological and geotechnical maps [8,9,10,11].

Unit	Approximate Thickness	Composition	Mechanical Properties	Allowable Bearing Pressure (qadm)
Urban fill	1–3 m	Heterogeneous materials, medium-to-low density (N = 10–30)	E ≈ 25–50 MPa	≤122.58 kN/m2
Silty–clayey deposits	10–15 m	CL–ML sediments with sandy lenses and fine gravels	E = 4–12 MPa; compressible	58.84 kN/m2
Sandy gravels	>12 m	Coarse alluvial stratum of ancient riverbed	E > 50 MPa	≥196.13 kN/m2

, extracted from the IGME geological and geotechnical maps [8,9,10,11].

These values confirm that the upper Quaternary layers supporting the tower are deformable and sensitive to long-term consolidation. Variations in stratigraphic thickness contribute to differential rather than uniform settlement.

5.3. Hydrogeological Conditions

Modern hydrogeological studies of the Murcia basin [8,9] show groundwater levels ranging between −2 and −5 m, with seasonal oscillations associated with irrigation and river flow. These fluctuations induce variations in pore pressure within the silty–clayey layers, promoting secondary consolidation even centuries after construction.

Historical records confirm that the groundwater level during the sixteenth-century excavation was unusually high, requiring extraction pumps—an exceptional measure at the time and a clear indication of low soil permeability.

Given the permeability characteristics of CL–ML soils, any change in the groundwater table leads to slow dissipation of excess pore pressure and additional settlement over time.

5.4. Foundation Geometry and Results from Boreholes

The boreholes did not reveal wooden piles. Statistical analysis of the drilling pattern indicates a probability ≤ 0.02 of intercepting a pile if present [14], suggesting that the tower relies on a shallow spread footing.

The geotechnical parameters measured by Ceico [22,23] describe the foundation as:

• Footing dimensions: 19.64 × 19.64 m
• Footing thickness: 4.70 m
• Foundation base depth: −5.60 to −5.90 m
• Material: conglomerate of lime mortar and limestone rubble
• Soil parameters:

  ▪

  cohesion c = 29.42 kN/m²

  ▪

  friction angle φ = 10°

  ▪

  dry unit weight γ = 16.18 kN/m³

  ▪

  groundwater level: −3.80 to −4.10 m

These values align with the overall mechanical behaviour inferred from regional mapping.

5.5. Excavation Stability

Applying Rankine’s formulation for cohesive soils:

$$h_c={\displaystyle \frac{2c}{\gamma \mathrm{t}\mathrm{a}\mathrm{n}({45}^{\circ }-\varphi /2)}}$$

the critical unsupported height is 1.80 m [9,19].

Since the foundation trenches exceeded this depth, temporary shoring was necessary, consistent with the historical documentation describing the use of timber sheeting during the works.

5.6. Settlement Mechanisms

The mean bearing pressure transmitted by the tower—400–600 kPa—exceeds the allowable bearing value of the silty–clayey layer (58.84 kN/m²), resulting in primary and secondary consolidation. This behaviour is consistent with:

• Low permeability → slow dissipation of pore pressure
• High compressibility → progressive reduction in void ratio
• Layer heterogeneity → differential rather than uniform settlement
• Groundwater oscillations → episodic settlement accumulation

The inclination of approximately 0.56 m toward the northwest correlates with the spatial variability of the compressible strata indicated by borehole data and geotechnical mapping.

5.7. Implications for the Tower’s Structural Behaviour

The geotechnical evidence confirms that the tower’s inclination originates from differential consolidation of the Quaternary silts and clays, influenced by groundwater fluctuations and stratigraphic variability. The foundation system itself does not present signs of structural deficiency; instead, the observed deformation corresponds to predictable long-term soil behaviour. These findings form the basis for the analytical structural modelling developed in Section 5 and substantiate the interpretation that the inclination is a geotechnical phenomenon rather than a symptom of instability.

6. Structural Analysis of the Tower

The structural analysis of the tower is based on classical analytical mechanics. This methodology is fully appropriate for a historic masonry structure whose behaviour is governed by geometry, mass distribution and construction sequence. The tower presents no structural damage, its geometry is precisely documented, and its load-bearing behaviour is dominated by compression. For these reasons, finite element modelling (FEM) is not required for the objectives of this study, as FEM would not provide additional insight relative to the analytical evaluation presented here. The following assessment, therefore, combines composite-section mechanics, non-destructive testing, and stress analysis derived from established theoretical formulations.

Several numerical strategies—including FEM, DEM and RBSM—are widely used in the assessment of historic masonry structures when material discontinuities, cracking or seismic demands must be explicitly modelled. In contrast, the Murcia Cathedral tower exhibits no structural damage and behaves as a massive compressive system whose response is dominated by geometry and soil–structure interaction. For this reason, analytical mechanics (Heyman theory, elastic column behaviour and consolidation-based settlement modelling) provides a verifiable and internally consistent approach. The mechanical parameters adopted in this section follow established values reported by Heyman [3], Esbert et al. [6] for the limestone, and the geotechnical parameters in IGME datasets [8,9,10].

6.1. Structural Configuration and Preliminary Hypotheses

It may be assumed, as a general principle, that the first tier of any historic masonry tower functions as its structural shaft or sustaining core [4,24,25]. As demonstrated in Section 3, the tower of Murcia Cathedral is composed of a double-tube system, typical of large Renaissance masonry towers. This structural arrangement consists of:

• two concentric ashlar tubes,
• separated by a central lime–rubble core,
• without continuous through-stones,
• behaving as mechanically semi-independent shells.

• Although no excavations have been performed in the lower section of the tower, sufficient indirect evidence allows us to infer that the walls are formed by two parallel ashlar faces with a central core of rubble bound with lime mortar. This configuration is supported by three independent observations: the different heights of the ashlar blocks on the inner and outer faces, incompatible with bonded leaves;
• Documentary evidence of systematic lime-and-rubble acquisition during the works directed by Jerónimo Quijano (1545–1569);
• The texture of the putlog holes in the third tier reveals rubble fill rather than ashlar.

For the analysis, the following explicit hypotheses are adopted:

• masonry behaves as a compressive material with no tensile capacity (Heyman’s model) [3,16];
• elastic behaviour is used only to estimate vertical deformability;
• stresses correspond to average compressive stresses in homogeneous blocks;
• the interaction between soil and structure is evaluated separately (Section 6);
• only gravitational loads are considered in the present section.

6.2. Equivalent Modulus and Euler Upper-Bound Analysis

Given this configuration, the structural analysis begins from the Euler critical load for a column clamped at its base and free at its top:

$${\sigma }_{cr}={\displaystyle \frac{{\pi }^{2}EI}{{\left(2L\right)}^2}}$$

(1)

This formulation is used strictly as a theoretical upper bound, following the standard practice in the assessment of massive historic towers: it is not intended as a prediction of an actual buckling mechanism, which is not relevant for a structure of this geometry, but as a means of demonstrating that the critical load far exceeds the working load.

Both the elastic modulus E and the moment of inertia I must be corrected to account for the heterogeneous and anisotropic composition of a wall formed by ashlar leaves and a lime–rubble core.

Euler’s formulation is not used here as a failure prediction, because the tower is massive and not slender. It is employed only as an upper-bound theoretical reference, following the standard practice in the assessment of historic towers [3,25]. Its purpose is demonstrative: to show that the theoretical buckling load far exceeds the working load.

To obtain the equivalent modulus in the composite system, the ashlars and mortar joints are modelled as materials in series, while the wall layers act in parallel. For elements in series:

$$E_s={\displaystyle \frac{\sum e}{\sum E}}$$

(2)

and for elements in parallel:

$$E_p={\displaystyle \frac{\sum E·l}{\sum l}}$$

(3)

Combined, the equivalent elastic modulus is:

$$E\prime ={\displaystyle \frac{E_2\left(E_1{l}_1\left(e_1+e_2\right)+E_2{l}_3{e}_1\right)}{\left(E_2{e}_1+E_1{e}_2\right)\left(l_1+l_3\right)}}$$

(4)

where

• e₁: height of the ashlar blocks in the outer leaves
• e₂: height of the mortar joints
• E₁: elastic modulus of the limestone masonry
• E₂: elastic modulus of the lime mortar (considered equivalent to that of the internal rubble fill)
• l₁: combined thickness of both ashlar leaves
• l₃: thickness of the central lime–rubble core

Similarly, the moment of inertia for the wall is computed as:

$$I^{\prime }={\displaystyle \frac{(I_1+\frac{I_3}{4})}{12}}$$

(5)

Once these corrections are applied, it becomes clear that—even at 93 m height—the critical stress remains well below the working stress, confirming the absence of global instability.

6.3. Non-Destructive Characterisation of the Limestone

Because no destructive sampling is permitted on the tower [9], material characterisation relied on Schmidt hammer testing—a non-intrusive, comparative technique widely used in heritage structures. Its purpose is not to determine absolute compressive strength, but to:

• verify the uniformity of the material,
• detect superficial weathering,
• confirm the absence of localised anomalies.

This follows the method used in previous campaigns on the same limestone [9,14]. Figure 7 shows the testing procedure.

As in earlier campaigns, results confirm that weathered limestone exhibits strengths ~5 times lower than unaltered rock, but the deterioration affects only superficial layers (<1 cm) [9,26]. Figure 8 documents the contrast.

These conditions do not compromise the global mechanical behaviour of the masonry.

6.4. Compressive Behaviour of the Masonry

Following Heyman’s principles [3], the tower behaves as a purely compressive structure. The adjusted elastic modulus, accounting for triaxial confinement, is:

$$E^{\prime }=E·\left(1-{\upsilon }^2\right)$$

(6)

Taking E = 19,613 N/mm² and ν = 0.20, the average compressive strength is:

σ_c = 7.85 N/mm²

A theoretical tensile strength for comparison is:

σ_t = 0.52 N/mm²

The working stress in the lower tier is:

$${\sigma }_{tr}={\displaystyle \frac{\mathrm{152,983.17} \mathrm{k}\mathrm{N}}{183.50 {\mathrm{m}}^2}}=833.70 {\displaystyle \raisebox{1ex}{$\mathrm{k}\mathrm{N}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{m}}^2$}\right.}=0.83 {\displaystyle \raisebox{1ex}{$\mathrm{N}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{m}{\mathrm{m}}^2$}\right.}$$

(7)

Below all compressive limits.

In non-confined corner stones:

$$\epsilon ={\displaystyle \frac{n}{E}}{\sigma }_{tr}={\displaystyle \frac{0.2}{19.613}}0.83={\displaystyle \frac{0.85}{100.000}} {\displaystyle \raisebox{1ex}{$\mathrm{m}\mathrm{m}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{m}\mathrm{m}$}\right.}$$

(8)

$${\sigma }_{t,tr}=E \epsilon =\mathrm{19,613}{\displaystyle \frac{0.85}{\mathrm{100,000}}}=0.167 {\displaystyle \raisebox{1ex}{$\mathrm{N}$}\!\left/ \!\raisebox{-1ex}{$\mathrm{m}{\mathrm{m}}^2$}\right.}=166.71 {\displaystyle \raisebox{1ex}{$\mathrm{k}\mathrm{N}$}\!\left/ \!\raisebox{-1ex}{${\mathrm{m}}^2$}\right.}$$

(9)

below the theoretical tensile limit.

6.5. Behaviour of the Concentric Structural Tubes

The outer tube carries only its self-weight and the ramp vaults up to the balustrade (Figure 6):

• Outer tube stress: 0.74 N/mm² (735.50 kN/m²)

The inner shaft—one-fifth thinner (Figure 1)—carries the weight of the internal domes and the bell chamber:

• Inner tube stress: 1.02 N/mm² (1019.89 kN/m²)

Vertical elastic deformations are:

$$D_{ext}={\displaystyle \frac{1}{2}} {\displaystyle \frac{0+7.5}{\mathrm{19,613}}} 4184.10=0.8 \mathrm{m}\mathrm{m}$$

(10)

$$D_{ext}={\displaystyle \frac{1}{2}} {\displaystyle \frac{0+10.4}{\mathrm{19,613}}} 4184.10=1.4 \mathrm{m}\mathrm{m}$$

(11)

The intentional separation of the two tubes prevents significant secondary stresses.

6.6. The Helical Staircase as an Independent Load-Bearing System

The helical staircase rises 40 m and weighs 1135 kN. It transmits its load through two vaults forming the floor and ceiling of the first bell chamber.

The cylindrical perimeter and core rest on a stone ring acting as the keystone of four segmental arches—two diagonal and two axial. Assuming load proportionality to the fourth power of span:

• diagonal arches: 165 kN
• axial arches: 115 kN

This equilibrium confirms the robustness of the internal load-bearing geometry.

6.7. Structural Performance Under Historical Earthquakes

The tower has withstood major earthquakes, including:

• the 1755 Lisbon event [27,28],
• the 1829 Torrevieja earthquake [29],
• the 2011 Lorca earthquake [22].

Transient stress increments may explain minor cracks on the lower north corners, documented in Figure 8.

6.8. Technical Conclusion

The structural analysis confirms that:

• the tower is not overstressed under gravitational loads;
• the tube-in-tube configuration provides redundancy and stability;
• elastic shortening is negligible relative to geotechnical settlement;
• Euler confirms a vast reserve of compressive capacity;
• the esclerómetro validates material homogeneity;
• the helical staircase behaves independently and safely;
• no structural mechanism explains the inclination.

Therefore, the measured tilt is geotechnical in origin, not structural, fully consistent with the settlement analysis developed in Section 6.

7. Settlement of the Tower

The long-term settlement behaviour of the tower can only be understood by integrating the historical construction sequence, the differential actions imposed during the eighteenth-century works, and the geotechnical properties described in Section 4. When José López resumed construction in the mid-eighteenth century, the two existing Renaissance tiers had already undergone significant differential settlement [14], accompanied by an overall tilt toward the eastern face [1,15]. A simplified moment-based analysis was also carried out to estimate the stresses induced by the tower’s inclination. The measured eccentricity generates an overturning moment of approximately 1.37 × 10⁵ kN·m, which produces an edge stress increment of about 0.108 N/mm² at the footing. This value is consistent with the differential settlements recorded between the northeast and northwest corners and provides a non-FEM analytical estimation of the settlement gradient. Although simplified, this approach is fully compatible with the analytical framework of the paper and confirms that the observed deformation is primarily geotechnical in origin.

To compensate for the greater settlement that had occurred on the east side, López increased the thickness of the shaft on the opposite (west) side by two feet (≈55.8 cm), while simultaneously reducing the outer dimension of the third tier by one Parisian foot on each face. This eccentric loading generated a working stress—due solely to the induced moment—of approximately 10 N/cm², a value exceeding the bearing capacity of the subsoil [9].

For this reason, when López advanced to the construction of the upper bell-tower stages, he introduced an additional correction by shifting the continuation of the inner shaft in the opposite direction. This followed the trial-and-error method commonly used in historic tower construction, consisting of successive adjustments aimed at counterbalancing previous differential settlements by redistributing loads.

The construction of the tower was exceptionally complex. The original design implied a self-weight exceeding 195,000 kN. In other words, the ground—without auxiliary systems—would have been subjected to stresses > 50 N/cm². For this reason, the cathedral chapter engaged Francesco Torni da Firenze, an architect known for his expertise in demanding foundation works [23,26,30,31].

As Francesco di Giorgio Martini wrote:

I campanili deno essere in prima ben fondati. E se fondo non si trovasse, sopra pali e banconi [si debbe] fondare. Dia essere il fondo al meno piè quindici sopra [debería decir, infra] delle terra, e di poi seguire le mura con debita grosezza. E gli spazi dell′altezza dall′uno sfinestrato all′altro piè vinti, più o manco sicondo la qualità d′esso, con le volte sicondo richiede. E gli ultimi sfinestrati le spazi più larghi dove le campane vanno, acciò che le voci occupate non sieno, et alla superficie l′ultima volta con corridoio e parapetto intorno, e in esso la cimasa pirami dale o altra forma cor ornate scolture son da fare... E possasi fare ditti campanili di più varie forme: tondi, facciati, quadrati e graduati con bellissimi ornamenti di ricinte cornici, membri, colonne, tabernacoli e altre scolture sicondo la degnità de tempi.

[2]

This description closely coincides with the evidence recorded in Murcia and confirms that the construction process followed, to a large extent, the assumptions of the original design model.

The need to execute large footing slabs in areas potentially supported by wooden piles required the excavation of deep trenches. Today, the groundwater level lies at approximately −3.60 m. Considering that the subsoil consists mainly of saturated silts, the need for sheet piling during excavation—both beneath the tower foundations and in several supporting elements of the Gothic structure—is easily understood.

The critical height for cohesive soils, derived from null active Rankine pressure, is:

$$h_c={\displaystyle \frac{2c}{\gamma K_a}}={\displaystyle \frac{2c}{\gamma \tan \left(45-\frac{\phi }{2}\right)}}$$

(12)

For the soil surrounding the Cathedral of Murcia [9,19], this yields:

$$h_c=1.80 \mathrm{m}$$

approximately half the depth of the foundation trenches, thus confirming the necessity of robust shoring.

The geotechnical tests carried out beneath the tower’s foundation did not detect any wooden piles. However, this negative result does not preclude their existence. The objective of the seven boreholes performed [18] was not to locate piles, but to determine the dimensions and material composition of the footing, and to obtain the mechanical parameters of the subsoil.

Four boreholes were made outside the footing to locate its edges, while three intersected its periphery; statistically, the probability of encountering a pile in these positions was extremely low (≤0.02) [14].

The parameters obtained from the Ceico investigations [14,18,19] were:

• Cohesion: c = 29.42 kN/m³
• Internal friction angle: φ = 10°
• Dry unit weight: γ = 16.18 kN/m³
• Soil type: soft clay up to 12 m depth; gravels and sands below that horizon
• Groundwater level: n’ = −3.80 m relative to current grade; n = −4.10 m relative to the reference level (±0.00) at the step of the Portada de la Cruz, north façade
• Foundation dimensions: a × b = 19.64 × 19.64 m
• Mean footing thickness: z = 4.70 m
• Average depth of foundation base: p’ = −5.60 m from pavement; p = −5.90 m from reference level
• Material composition: lime mortar and limestone rubble conglomerate

From the geometric survey of the tower, the following gravitational loads were deduced:

• Self-weight of the tower: P = 165,000 kN ± 10%
• Weight of the foundation: Q = 35,000 kN ± 5%
• Total load:

$$P_t=P+Q=\mathrm{200,000}\pm 10\%$$

(13)

As mentioned, during the sixteenth century the tower experienced eastward displacement, leading to the suspension of works after the second Renaissance tier.

Two centuries later, once differential settlements had stabilised, José López resumed construction, reducing the planned height and adjusting geometries to re-centre loads [15].

The historical record of differential settlements is complex. In addition to uniform settlement of the footing, uneven edge deformations were caused by [12,13,14,18,19]:

• higher lateral friction on the eastern side, adjacent to precompressed soils producing greater active thrust;
• eighteenth-century compensations, shifting the centre of gravity first westwards (thickening the third tier’s west wall), then eastwards (extending the bell chamber), and enlarging the west terrace.

These corrections are visible today from the north façade or the southwest corner (Figure 9 and Figure 10), where the loss of vertical alignment and the successive corrections made during construction can be observed. A more detailed view of the deviation of each architectural tier, and of the local compensations introduced during construction, is shown in Figure 11.

Each corner of the tower exhibits two settlement components:

• uniform subsidence of the footing, and
• differential deformation of the edges.

The uniform settlement of the footing is estimated at ≈32 cm, based on:

• deformation of stable cornices (ante-sacristy eaves);
• level differences between current pavement and sacristy.

Additional settlement induced by eighteenth-century loads is evident in the misalignment of the wall containing the current tower door (≈11 cm southeast).

Thus, during the first phase (to 1760), settlement ≈ 20 cm; after Baroque works, ≈12 cm more → total ≈ 32 cm.

Superimposed are corner-specific deformations corresponding to local tilt. Despite progressive loading over 275 years, the tower exhibits an anomalous but non-critical settlement pattern.

Assuming the absence of piles, the ultimate bearing capacity using the Terzaghi–Brinch Hansen formulation is:

$${\sigma }_h=q{N}_q+1.3c{N}_q+a{N}_{\gamma }q{R}_m\gamma$$

(14)

with:

$$a=1+{\displaystyle \frac{1}{2}}{\displaystyle \frac{a}{b}}$$

(15)

For a = b = 19.64 m,

$$R_m={\displaystyle \frac{ab}{2(a+b)}}={\displaystyle \frac{a^2}{4a}}={\displaystyle \frac{a}{4}}=4.91$$

(16)

$${\sigma }_h=521 {\mathrm{kN}/\mathrm{m}}^2$$

$${\sigma }_{adm}<{\displaystyle \frac{{\sigma }_h}{3}}=173.67 {\mathrm{kN}/\mathrm{m}}^2$$

(17)

although a prudent criterion suggests ≤ 80 kN/m².

Maximum allowable load:

$$Q_s=19.64\times 19.64\times 173.67=\mathrm{66,989.66} \mathrm{kN}$$

(18)

Considering lateral friction on the four vertical faces, the active thrust is:

$$H=\left[2 C+\left(\gamma h+q\right) {\tan }^2\left({\displaystyle \frac{\pi }{4}}-{\displaystyle \frac{\phi }{2}}\right)\right]\tan \left({\displaystyle \frac{\pi }{4}}-{\displaystyle \frac{\phi }{2}}\right)$$

(19)

• At the base: Hᵢ = 27.56 kN/m²;
• At the top: Hs = 20.01 kN/m².

Assuming a Dörr coefficient suitable for silty soils (f ≈ 0.10) and a perimeter area of

$$S=4·19.64·4.70=369.23 {\mathrm{m}}^2$$

(20)

the lateral friction contribution is:

$$Q_r={\displaystyle \frac{(27.56+20.01)}{2}}\times 0.10\times 369.23=878.21 \mathrm{kN}$$

(21)

Hence, the total maximum load without piles is:

$$Q_{max}=Q_s+Q_r=\mathrm{67,867.87} \mathrm{kN}$$

(22)

which is less than the tower’s total load:

$$P_t=P+Q=\mathrm{200,000}\pm 10\% \mathrm{kN}$$

(23)

This situation does not imply collapse, which would occur only for a virtual self-weight exceeding:

$$3Q_s=3\times \mathrm{66,989.66}=\mathrm{200,959.98} \mathrm{kN}$$

(24)

Thus, in the absence of piles, the safety factor against bearing failure would be:

$$FS={\displaystyle \frac{\mathrm{200,959.98}}{\mathrm{200,000}-\mathrm{67,867.87}}}=1.53$$

(25)

Given the observed seismic performance of the tower during major historical earthquakes (1755, 1829 and 2011) [22,27,28,29], and the absence of significant structural damage after these events, it is plausible that the foundation benefits from a deep stiffening mechanism such as a grid of wooden piles embedded in the gravel strata. The ability of the tower to withstand three major earthquakes without macroscopic cracking suggests that the load-transfer mechanism at the base is more robust than would be expected from the shallow footing alone [32,33].

Since 1519, the tower has experienced relative differential settlements; the northeast corner (Figure 10, right and Figure 12) stands 56 cm lower than the northwest. These symptoms already existed in the sixteenth century, prompting Quijano to halt construction at 28.80 m height.

In the eighteenth century, Juan de Gea and José López counteracted eastern settlement by overloading the west side. Thickening the western wall of the third tier introduced an eccentric overload of 18,632.64 kN and a moment of ≈137,295 m·kN. With W = 1300 m³, stresses reached:

This was achieved by thickening the outer western wall by two feet, introducing an eccentric overload of 18,632.64 kN and a bending moment of approximately 137,295 m·kN. Considering the section modulus of the tower’s plan (W = 1300 m³), the edge stresses from this moment reach 0.1079 N/mm² (≈108 kN/m²)—about 30% of the unit ground reaction produced by the weight of the three lower tiers alone.

Consequently, the western wall carried a stress of

$$507.98(1+0.30)=660.37{ \mathrm{kN}/\mathrm{m}}^2$$

explaining the maximum relative settlement at the northwest corner.

Later upper tiers, with single-wall construction, offset the geometry in the opposite direction, reducing western settlement.

The tower’s total weight (>200,000 kN ± 10%) produces average stresses ~332 kN/m², near the bearing capacity (521 kN/m²) predicted by Prandtl, reinforcing the likelihood of a historical pile foundation. It is therefore advisable to conduct inclined corings to confirm the presence and condition of wooden piles—particularly given falling groundwater levels [13], which may threaten the long-term integrity of piles previously protected by saturated clays.

8. Conclusions

The analysis presented in this study demonstrates that the present condition of the bell tower of Murcia Cathedral—its geometry, structural behaviour and current inclination—can only be understood through a combined examination of its construction history, structural configuration and geotechnical environment. The tower is the cumulative result of a three-century building process in which architectural decisions, material behaviour and soil response interacted continuously.

• The architectural sequence conditioned the tower’s mechanical behaviour. The Renaissance first and second tiers, built between 1521 and 1555, already exhibited measurable settlement and a slight eastward tilt, prompting the suspension of works. When construction resumed in 1765, José López introduced deliberate geometric corrections: increasing wall thickness on one side and reducing the external dimensions of the third tier. These compensations altered the load path and allowed the elevation to be re-centred, enabling completion of the bell tower, the conjuratorios and the Neoclassical dome by 1793. This historical sequence is essential for understanding the current distribution of stresses and deformations.
• Second, the structural behaviour of the tower is stable and robust. The double-tube configuration—consisting of two concentric ashlar shells with a lime-rubble core—provides redundancy and limits bending effects induced by settlement. Analytical calculations confirm that working stresses in both tubes remain far below the compressive strength of the limestone, even when considering reduced values for weathered material. Euler’s critical load, used solely as an upper-bound reference, exceeds the tower’s working load by more than an order of magnitude. The helical staircase behaves as an independent load-bearing structure, and the tower’s performance during major regional earthquakes corroborates the absence of structural vulnerability.
• Third, the settlement pattern is governed by the geotechnical properties of the Vega del Segura. The tower stands on up to 15 m of compressible Quaternary silts and clays with low bearing capacity and a historically high groundwater table. Borehole data and geotechnical mapping indicate that long-term consolidation, together with episodic piezometric fluctuations, produced both uniform settlement (≈32 cm) and corner-specific deformations. The present 56 cm differential between the northeast and northwest corners is consistent with this behaviour. Analytical estimation of bearing capacity demonstrates that the tower’s load approaches the theoretical resistance of the soil, strongly suggesting the historical use of a wooden pile foundation—consistent with Renaissance treatises and the documented practices employed in other parts of the cathedral.
• Fourth, the differential settlements observed today reflect both geological and historical causes. Sixteenth-century deformations led to the first construction halt; eighteenth-century works introduced compensatory eccentricities; and subsequent loading of upper tiers partially reversed earlier movements. The slightly curved elevation visible in the north and east façades records this sequence and provides an interpretable trace of construction decisions made to counteract settlement.
• Finally, the long-term behaviour of the tower requires continued monitoring. The gradual lowering of the groundwater table—due to urbanisation, the disappearance of traditional irrigation channels and the canalisation of the Segura River—may influence both the compressibility of the clayey subsoil and the durability of any buried wooden piles. Installing buried piezometric sensors and performing inclined core drillings would provide essential data for ensuring the structure’s long-term stability.

In summary, the inclination of the Tower of Murcia is not the result of structural weakness but the visible outcome of systematic geotechnical processes acting over centuries on a tall masonry structure of exceptional mass. The tower remains stable, structurally safe and mechanically coherent with its historical construction logic. Its current geometry—far from being an anomaly—constitutes a material record of the dialogue between construction phases, soil behaviour and architectural adaptation.