Evaluation of Slope Stability in an Urban Area as a Basis for Territorial Planning: A Case Study

Featured Application: The application of a methodology for evaluating susceptibility to landslides and landslides, based on the experience of several researchers, has allowed the generation of a susceptibility map. This susceptibility map presents a correlation with the geophysical data, drilling and pits, which validates the application of the methodology carried out. Also, it has provided a basis for security measures and territorial planning. Abstract: Slope stability is determined by pre-conditioning and triggering factors. The evaluation of the stability by scientiﬁc criteria provides crucial input into land-use planning and development. This work aimed to evaluate the slope stability of “Las Cabras” hill (Duran, Ecuador) through geological and geotechnical analysis and a susceptibility assessment that allowed the deﬁnition of areas potentially susceptible to landslide and detachment for land planning recommendations. The methodology included (i) analysis of background information about the study area; (ii) ﬁeldwork, sampling and laboratory tests; (iii) assessment of susceptibility to landslides and detachment through a theoretical– practical evaluation (using suggestions by various authors); (iv) a safety factor assessment employing the simpliﬁed Bishop method; and (v) analysis of the relationship between susceptibility and stability. Sixteen geomechanical stations were evaluated. Of these, seven stations are characterised as category III (medium susceptibility), six stations as category IV (high susceptibility) and three stations as category V (very high susceptibility). According to the susceptibility zoning map, 58.09% of the total area (36.36 Ha) is in the high to very high susceptibility category. The stability analysis based on 16 critical proﬁles shows that three of these proﬁles have safety factor values of less than one (0.86, 0.82 and 0.76, respectively), and two proﬁles have values close to one (1.02 and 1.00). The northern area is conditioned mainly by a vertical slope with an outcrop of fractured and weathered sandstones, thereby favouring rockfall. The landslide vulnerability in the case of the southern zone is principally conditioned by the fact that the slope and dip are parallel. The described characterisation and susceptibility analysis provide a basis for security measures and territorial planning.


Introduction
A landslide is the movement of a mass of rock, debris or soil down a slope under the influence of gravity [1,2]. They are considered serious natural geological hazards in many areas of the world [3][4][5]. Specifically, landslides are the second most notable geological disasters identified by the United Nations Development Program [6]. In general, landslides are controlled by several pre-conditioning factors (e.g., morphology, lithology, structural environment, vegetation and land use) and are induced by different triggers (e.g., heavy rain, earthquakes, volcanic eruptions and marine storms) [7][8][9][10][11]. Furthermore, as development expands into unstable hillside areas under the pressures of increasing population and urbanisation, human activity, such as deforestation or excavation of slopes for roads and construction sites, becomes an essential trigger of landslides [12][13][14][15].
As the most common natural geological hazard in mountainous areas, landslides often cause significant economic loss and human casualties [16,17]. For this reason, the evaluation of these phenomena has been a primary scientific duty in order to establish the zoning of the analysed territory and to identify the main objects exposed to risk [18,19]. These undertakings have received increasing amounts of attention from the scientific community in recent decades [20][21][22]. Various qualitative, quantitative and empirical approaches have been proposed in the scientific literature to assess hazards and risks arising from landslides, rockfalls or slope instability [23][24][25][26][27][28].
One of the most often used methods is susceptibility mapping. The first landslide maps were prepared in the 1970s [29,30], and their elaboration still implies a certain degree of interpretation and correlations with various factors (e.g., topography, geomorphology, geotechnical properties and vegetation) [31][32][33]. Susceptibility maps provide valuable information for disaster mitigation work and land planning strategies [34]. This approach yields a more precise sustainability assessment that includes identifying high-vulnerability areas, risk analysis, security arrangements and stabilisation [35,36].
The area studied in this contribution is "Las Cabras" hill, Duran, in Southwest Ecuador. The hill has a small population with the necessary infrastructure (e.g., houses, roads and electrical network). Nowadays, there are 1540 houses, of which 88.38% (1361 houses) are inhabited. The access roads are in poor condition, making it difficult for waste collection vehicles and water tank trucks to enter. In recent years, signs of instability (e.g., deterioration, fracturing and minor wall instabilities) have been reported on the slopes of "Las Cabras" [37,38]. This potential instability of the terrain could cause severe mass movements over time, affecting the population.
Our work aimed to evaluate the slope stability of "Las Cabras" hill (Duran, Ecuador) through geological and geotechnical analysis and a susceptibility assessment that allowed the definition of areas potentially susceptible to landslide and detachment for land planning recommendations. A zoning map with the most unstable areas is intended to provide a basis for present and future territory planning.

Setting of "Las Cabras" Hill, Duran
Duran canton consists of three parishes (Eloy Alfaro, El Recreo and Divino Niño) and has an area of 59 km 2 . This canton is part of the Province of Guayas in Ecuador. Duran is located 5 km from Guayaquil. Generally, it has a flat relief with a few isolated elevations, such as that of "Las Cabras" hill [39] (Figure 1a). "Las Cabras" hill has an area of 0.36 km 2 and an approximate height of 80 m. According to the Population and Housing Census in Ecuador (INEC, by its acronym in Spanish) [40], 11,868 people live in the area.
The climate is tropical (sub-humid), with temperatures ranging between 20 and 27 • C. According to the annual averages between 1985 and 2013 [41], precipitation can be between 800 and 1000 mm/year. The climate is tropical (sub-humid), with temperatures ranging between 20 and 27 °C. According to the annual averages between 1985 and 2013 [41], precipitation can be between 800 and 1000 mm/year.
Water supply is delivered three days a week through pipes. The area has a local sewer system, but there is no description of its condition. The houses have septic tanks and latrines, and there is no official control over their operation. Rainwater circulates downstream through channels deepened parallel to the stairs on the slopes of the hill. Urban planning (construction of houses or essential services) is seriously lacking in the area.
The geology of the study area ( Figure 1b) is represented mainly by the Cayo Formation (Upper Cretaceous), which consists of breccias, microbreccias, sandstones, shales even clays and argillites [42]. In general, these lithologies alternate and form centimetre to meter-thick bodies. The geological structure dips southwards with a slope of 15 to 25 degrees [43]. Significant colluvial soil development has taken place on top of the base rock, and some soils show signs of the current movement and instability (rotational rupture). A small part (in the southern part of the study area; see Figure 1b) corresponds to the Guayaquil Formation. Guayaquil Fm. consists of silicified shales and flint nodules that alternate with brown tubaceous siltstones [44] and sandstones with calcareous cement that belong to the Cayo Fm. [45].

Methodology
The methodology followed in this study ( Figure 2) consisted of three phases: (i) analysis of background information about the study area; (ii) field survey and geological-geomechanical studies; and (iii) landslide susceptibility zoning. Water supply is delivered three days a week through pipes. The area has a local sewer system, but there is no description of its condition. The houses have septic tanks and latrines, and there is no official control over their operation. Rainwater circulates downstream through channels deepened parallel to the stairs on the slopes of the hill. Urban planning (construction of houses or essential services) is seriously lacking in the area.
The geology of the study area ( Figure 1b) is represented mainly by the Cayo Formation (Upper Cretaceous), which consists of breccias, microbreccias, sandstones, shales even clays and argillites [42]. In general, these lithologies alternate and form centimetre to meter-thick bodies. The geological structure dips southwards with a slope of 15 to 25 degrees [43]. Significant colluvial soil development has taken place on top of the base rock, and some soils show signs of the current movement and instability (rotational rupture). A small part (in the southern part of the study area; see Figure 1b) corresponds to the Guayaquil Formation. Guayaquil Fm. consists of silicified shales and flint nodules that alternate with brown tubaceous siltstones [44] and sandstones with calcareous cement that belong to the Cayo Fm. [45].

Methodology
The methodology followed in this study ( Figure 2) consisted of three phases: (i) analysis of background information about the study area; (ii) field survey and geologicalgeomechanical studies; and (iii) landslide susceptibility zoning.

First Phase
The first step was to compile relevant literature (articles, reports and press releases) related to the study area. Data provided by the project "Studies and proposals for the stabilisation of "Las Cabras" hill" were also collected [33]. This phase focused on developing the detailed topography of "Las Cabras" hill using GPS Satellite GNSS S82T equipment, and a SOKKIA SET 630 total station. An inventory was completed of the constructions in the area (buildings, roads, water supply and septic tanks) to consider whether these could influence the terrain's instabilities. Finally, the technical and social context was analysed to consider the causes and impacts of the problem (instabilities).
Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 23 Figure 2. The proposed methodology for this study.

First Phase
The first step was to compile relevant literature (articles, reports and press releases) related to the study area. Data provided by the project "Studies and proposals for the stabilisation of "Las Cabras" hill" were also collected [33]. This phase focused on developing the detailed topography of "Las Cabras" hill using GPS Satellite GNSS S82T equipment, and a SOKKIA SET 630 total station. An inventory was completed of the constructions in the area (buildings, roads, water supply and septic tanks) to consider whether these could influence the terrain's instabilities. Finally, the technical and social context was analysed to consider the causes and impacts of the problem (instabilities).

Geological Characterisation
The fieldwork at "Las Cabras" hill focused on studying the terrain's morphology and obtaining measurements of heading and dip of strata and other geological structures. Three geological sections were made in the study area, and a representative stratigraphic column.
Based on the identification of areas of geological interest, 18 vertical electrical soundings (VESs) were planned using the Schlumberger method [46,47]. The VESs carried out allowed estimating the thickness of the layers at depth. To verify the information obtained from the VESs, strategic points were established for drilling and pits.
Three rotational type boreholes were drilled with sample recovery. Six VESs close to the perforations were chosen to correlate them. Additionally, eight pits were made (excavations of at least one meter deep) to analyse in detail the materials (rocks and soils) present (see Figure 3). These actions allowed an adequate geological interpretation of the study area through the correlations of geoelectric profiles, the geological field survey and the geological drilling record.

Geological Characterisation
The fieldwork at "Las Cabras" hill focused on studying the terrain's morphology and obtaining measurements of heading and dip of strata and other geological structures. Three geological sections were made in the study area, and a representative stratigraphic column.
Based on the identification of areas of geological interest, 18 vertical electrical soundings (VESs) were planned using the Schlumberger method [46,47]. The VESs carried out allowed estimating the thickness of the layers at depth. To verify the information obtained from the VESs, strategic points were established for drilling and pits.
Three rotational type boreholes were drilled with sample recovery. Six VESs close to the perforations were chosen to correlate them. Additionally, eight pits were made (excavations of at least one meter deep) to analyse in detail the materials (rocks and soils) present (see Figure 3). These actions allowed an adequate geological interpretation of the study area through the correlations of geoelectric profiles, the geological field survey and the geological drilling record.

General Geomechanical Study
The geomechanical study addressed the characterization of 16 stations (ordered set of geomechanical observations) in the study area ( Figure 3). The number of stations and location depend on the favourable terrain conditions (existing outcrops and accessibility) and representativeness concerning their susceptibility to landslide or detachment (explained in Section 3.3). Eight geomechanical rock stations and eight soil stations were focused on in the study (Figure 4a,b). The number of stations studied was limited due to budget constraints, access permits and terrain. Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 23

General Geomechanical Study
The geomechanical study addressed the characterization of 16 stations (ordered set of geomechanical observations) in the study area ( Figure 3). The number of stations and location depend on the favourable terrain conditions (existing outcrops and accessibility) and representativeness concerning their susceptibility to landslide or detachment (explained in Section 3.3). Eight geomechanical rock stations and eight soil stations were focused on in the study (Figure 4a,b). The number of stations studied was limited due to budget constraints, access permits and terrain.  Eight rock samples were taken using the combo and chisel or easily removable blocks. Laboratory tests were carried out on each of the collected samples to evaluate their physical-mechanical properties. Simple linear compression, resistance, density, cohesion and angle of internal friction were tested on the samples. All tests were carried out in a specialised rock mechanics laboratory. Eight rock samples were taken using the combo and chisel or easily removable blocks. Laboratory tests were carried out on each of the collected samples to evaluate their physicalmechanical properties. Simple linear compression, resistance, density, cohesion and angle of internal friction were tested on the samples. All tests were carried out in a specialised rock mechanics laboratory.
It is important to note that the linear compressive strength was determined by the "standard" method using cores with a height/diameter ratio of 1.5. The "Brazilian test" method was used for tensile strength. According to [48,49], the Brazilian test is a simple indirect test method to obtain the tensile strength of brittle material such as concrete, rock or rock-like material, in which a thin circular disk is diametrically compressed to failure. Cohesion and internal friction angle values were established using Mohr-Coulomb stress circles [50,51] from compressive and tensile strength values. To verify the results, we used former data obtained in the same area for other projects. These physical-mechanical properties were used for the determination of the stability conditions [21] 3.

Specific Geomechanical Study
For this experimentation phase, the geomechanical rock stations, previously defined, were used for a specific analysis. A set of geomechanical parameters was used to characterise rock quality, such as the rock quality designation (RQD), the geological strength index (GSI), the degree of weathering and the lithostructural groups. These geomechanical parameters made it possible to characterise the conditions of landslide and detachment susceptibility that currently exist in "Las Cabras" hill.

Rock Quality Designation (RQD)
According to Deere [52], the RQD is a modified core recovery percentage, in which all pieces of sound core over 100 mm (4 in.) long are summed and divided by the length of the core run. The RQD index is an index of rock quality "in that problematic rock that is highly weathered, soft, fractures, sheared and jointed is counted against the rock mass" [52] (Table 1). For field observations, we applied the methodology given by Palmstrom [51] (Equation (1) and Table 2), where the RQD parameter is estimated using a correlation with the volumetric joint count (Jv).
where Si is the average spacing for the joint sets Table 2. Correlation between RQD and volumetric discontinuity according to Palmstrom [53].

Weathering Degree, Geological Resistance Index (GSI) and Lithostructural Group
The study of the weathering degree, which implies reduced resistance, altered physical state and variation of the tension state on the slope or slopes, is necessary for the engineering-geological evaluation [54][55][56]. The weathering degree was determined by visual observation and by the method proposed by Suárez [57], based on the International Society of Rock Mechanics (ISRM) reference value [53,58] (Table 3). Table 3. Weathering degree based on ISMR [53].

Term
Description Weathering Degree

Fresh
No visible sign of rock material weathering. Perhaps slight discolouration on major discontinuity surface. I

Slightly weathered
Discolouration indicates weathering of rock material and discontinuity surfaces. All rock material may be discoloured by weathering.

Moderately weathered
Less than half of the rock material is decomposed or disintegrated into the soil. Fresh or discoloured rock is present either as a continuous framework or as a core stone.

Highly weathered
More than half of the rock material is decomposed or disintegrated into the soil. Fresh or discoloured rock is present either as a continuous framework or as a core stone.

IV
Completely weathered All rock material is decomposed or disintegrated into the soil. The original mass structure is still largely intact. V

Residual soil
All rock material is converted to the soil. The mass structure and the material fabric is destroyed. There is a significant volume change, but the soil has not been significantly transported.
VI GSI is a system for characterizing the geomechanical properties of the rock mass through easy identification by visual evaluation of geological properties in the field [57]. The rock mass was characterised with the GSI using the table given by Hoek [59,60]. Finally, the different materials were classified according to the lithostructural groups established by Nicholson and Hencher [61]: strong massive rock (I), strong discontinuous rock (II), composite rock (III), tectonically weakened rock (IV), weak granular rock (V), karst rock (VI), anisotropic rock and ground-like rock (VII).

Detachment and Landslide Susceptibility
The theoretical-practical evaluation procedure used in this phase was based on the criteria of several authors, such as Ambalagán [62], Suárez [57], González, [63], Nicholson [61] and Blanco [33]. The geomechanical characteristics were defined (Tables 4 and 5) by assigning values attained following expert criteria. The selected parameters were lithology, geological structure, morphometry, discontinuity, water presence, vegetable cover, seismic activity and weathering rank, which were evaluated from 0 to 4 (Supplementary Table S1) based on the conditions observed in the field (see definitions of the parameters in Supplementary Table S2).  Table 5. Main parameters and weights assigned to soil in the susceptibility coefficient estimation, based on [21,57,61,63,64].

Parameter Weight
Soil This method allows the susceptibility levels to detachment and landslide (Tables 6 and 7) to be defined. The susceptibility level is a qualitative value (I to V), and it is related to the susceptibility coefficient (SC) estimated [21,64]. Thus, the grade and susceptibility levels for rocks are: I for SC ≤ 5.0; II for 5.0 < SC ≤ 10.0; III for 10.0 < SC ≤ 15.0; IV for 15.0 < SC ≤ 20.0; and V for SC > 20.0. The grade or susceptibility levels for soils are: I for SC ≤ 5.0; II for 5.0 < SC ≤ 8.0; III for 8.0 < SC ≤ 12.0; IV for 12.0 < SC ≤ 16.0; and V for SC > 16.0. As a result, the susceptibility map of instability (detachment and landslide) was prepared from the geomechanical evaluations obtained using the geostatistical kriging tool that allows the values to be interpolated.

Safety Factor Assessment
Slope stability was assessed by the 2-dimensional stability program (SLIDE), which is based on the limit equilibrium calculation method [65]. Profiles were drawn according to the geomechanical stations and detailed topography. These were analysed in a static analysis (considering only the geotechnical characteristics of the terrain and the topography) and a pseudostatic analysis (considering the seismic activity of the study area).
We used seismic activity values established by the Ecuadorian Construction Standard (NEC-15, by its acronym in Spanish) [66]. According to [61], the study area is considered a zone of high seismic intensity, with a peak ground acceleration (seismic acceleration) of 0.40 g. However, in the pseudostatic analyses, 60% of the acceleration (i.e., 0.24 g) must be considered. Simplified Bishop methods were used to calculate the safety factor (SF). The simplified Bishop method uses the method of slices to discretise the soil mass and Appl. Sci. 2021, 11, 5013 9 of 22 determine the SF. This method satisfies the vertical force equilibrium for each slice and overall moment equilibrium about the centre of the circular trial surface. Since horizontal forces are not considered at each slice, the simplified Bishop method also assumes zero interslice shear forces [67,68]. The parameters input in the software SLIDE: the number of slices was 25 with a maximum number of iterations of 50. In this analysis, water table values were not considered.
Based on the susceptibility levels to detachment and landslide and the safety factor for rocks and soils, the obtained results were evaluated to establish the viability of the applied methodology.

General Characterisation
Fieldwork on "Las Cabras" hill revealed inefficient development of essential services such as sanitary sewers, storm sewers, access roads and informal human settlements on surface watercourses. These could be considered instability triggers in the study area. The sanitary sewer system that fails to cover the needs of the entire population is of particular importance. Sixty percent of the houses were found to have septic tanks, and the other 40% had latrines. However, there is evidence of discharge escape towards the surface and infiltration into the ground in both cases.
The topographic work yielded a detailed, up-to-date topography of "Las Cabras" hill with UTM coordinates, which allowed us to draw the topographic plan of the study area ( Figure 5b). The topographic work yielded a detailed, up-to-date topography of "Las Cabras" hill with UTM coordinates, which allowed us to draw the topographic plan of the study area (Figure 5b).

Stratigraphy
The field data obtained show a sedimentary succession that consists of metric brecciated and microbrechified bodies formed by angular ridges of sizes ranging from centimetric to decimetric. It has a shale composition. Glauconite is present, as are mafic volcanic fragments and medium and fine-grained sandstones. The shale interval is characterized by the presence of intercalations of those medium and fine-grained sandstones. We also saw in the shales parallel laminations and normal and inverse gradations. Subsequently, and with net contact, a succession of centimetre and decimetric layers of medium-grained sandstones, siliceous chert shales and fissile shales are present. This layer is overlaid by a set of unconsolidated colluvium/breccia with intercalations of medium-grained sandstones and shales. Colluvial material appears on the top of the sedimentary succession ( Figure 6).  The field data obtained show a sedimentary succession that consists of metric brecciated and microbrechified bodies formed by angular ridges of sizes ranging from centimetric to decimetric. It has a shale composition. Glauconite is present, as are mafic volcanic fragments and medium and fine-grained sandstones. The shale interval is characterized by the presence of intercalations of those medium and fine-grained sandstones. We also saw in the shales parallel laminations and normal and inverse gradations. Subsequently, and with net contact, a succession of centimetre and decimetric layers of medium-grained sandstones, siliceous chert shales and fissile shales are present. This layer is overlaid by a set of unconsolidated colluvium/breccia with intercalations of medium-grained sandstones and shales. Colluvial material appears on the top of the sedimentary succession ( Figure 6).

VESs
Eighteen VESs have been interpreted with IPI2win software (versión 3.0.1), and resistivity curves were adjusted to represent the strata with an error of less than 6%. Table  8 shows the lithologies based on the resistivities determined in the VESs. The interpretations of the 18 VESs can be seen in Supplementary Table S4. Table 8. Geoelectrical interpretation of the lithology based on VESs. In the geological studies carried out in the pits, stratigraphic columns were constructed (an example is seen in Table 9).

VESs
Eighteen VESs have been interpreted with IPI2win software (versión 3.0.1), and resistivity curves were adjusted to represent the strata with an error of less than 6%. Table 8 shows the lithologies based on the resistivities determined in the VESs. The interpretations of the 18 VESs can be seen in Supplementary Table S4. In the geological studies carried out in the pits, stratigraphic columns were constructed (an example is seen in Table 9).
In Table 9 (representative of the eight pits), we can see that the column reaches a depth of 2.50 m. In general, the levels determined in this section correspond to (from base to top): sandstones and microbreccias with glauconite, medium-grained sandstones, medium-grained sandstones and dark shales and soil. Information from all pits is presented in Supplementary Table S5. Table 9. Stratigraphic column of pit P08.  In Table 9 (representative of the eight pits), we can see that the column reaches a depth of 2.50 m. In general, the levels determined in this section correspond to (from base to top): sandstones and microbreccias with glauconite, medium-grained sandstones, medium-grained sandstones and dark shales and soil. Information from all pits is presented in Supplementary Table S5. Table 10 shows the lithologies based on the samples recovered from the boreholes and the depths reached in each one. The materials obtained from the drilling are mainly sandstones and shales from the Cayo Formation. A more detailed description of each drilling can be seen in Supplementary Table S6.

VESs and Drilling Correlation
Three correlations have been made between VESs and perforations using kriging interpolation. One of the correlations between drilling D1 and the VESs close to it (VES3 and VES4) is presented in Figure 7. Two other correlations have been made with drilling D2 with VES1 and VES8 and drilling D3 with VES10 and VES11. These can be seen in Supplementary Figures S4 and S5. Table 10 shows the lithologies based on the samples recovered from the boreholes and the depths reached in each one. The materials obtained from the drilling are mainly sandstones and shales from the Cayo Formation. A more detailed description of each drilling can be seen in Supplementary Table S6.

VESs and Drilling Correlation
Three correlations have been made between VESs and perforations using kriging interpolation. One of the correlations between drilling D1 and the VESs close to it (VES3 and VES4) is presented in Figure 7. Two other correlations have been made with drilling D2 with VES1 and VES8 and drilling D3 with VES10 and VES11. These can be seen in Supplementary Figures S4 and S5.

Description of Materials and Physical-Mechanical Properties
The materials defined from pits and outcrop characterisation are generally sandstones and shales from the Cayo Fm. The data in the columns can be extrapolated to the geomechanical stations. Of the 16 geomechanical stations defined, results of the physicalmechanical characterization of rocks were obtained.

Description of Materials and Physical-Mechanical Properties.
The materials defined from pits and outcrop characterisation are generally sandstones and shales from the Cayo Fm. The data in the columns can be extrapolated to the geomechanical stations. Of the 16 geomechanical stations defined, results of the physicalmechanical characterization of rocks were obtained. Table 11 demonstrates that sandstone is the most common rock type, according to the characteristic of the Cayo Fm. The rock samples were of fair quality (270-685 Kg/cm 2 ). The results of the geomechanical characterisation of rock stations are shown in Table  10. The study in the geomechanical soil stations revealed a moderate to residual weathered soil degree. Additionally, these stations are characterized by no water to minimal surface water, and from little vegetation to being largely covered by adequate vegetation. In the case of the characterised soils, it was found that the predominant soil is represented by clayey colluvial soil, with weathered sandstone blocks of medium grain (Table 12).

Geomechanical Characteristics of Soil and Rock Stations
The results of the geomechanical characterisation of rock stations are shown in Table 10. The study in the geomechanical soil stations revealed a moderate to residual weathered soil degree. Additionally, these stations are characterized by no water to minimal surface water, and from little vegetation to being largely covered by adequate vegetation. In the case of the characterised soils, it was found that the predominant soil is represented by clayey colluvial soil, with weathered sandstone blocks of medium grain (Table 12).  The results of the geomechanical characterisation of rock stations are shown in Table 13. The weathering degree values are in categories II and III, and the GSI values are between 70 and 60. The RQD values are between 55 and 70%. Lithostructural group values are in categories II and III.

Detachment and Landslide Susceptibility
According to the proposed susceptibility classifications, Tables 14 and 15 present the  overall mass movement susceptibility assessment (Tables 6 and 7). Figure 8 shows the zoning of the study area.

Detachment and Landslide Susceptibility
According to the proposed susceptibility classifications, Tables 14 and 15 present the overall mass movement susceptibility assessment (Tables 6 and 7). Figure 8 shows the zoning of the study area.   In soils (Table 12), four stations (GS05-GS07, GS09) were found to belong to landslide susceptibility category III, one (GS03) to category IV and three (GS01, GS11 and GS12) to category V. In rocks (Table 13), three stations (GS04, GS08, GS10) fall into category III, and five stations (GS02, GS13-GS16) into category IV. Regarding the areas in Figure 7, 22.80% of the total area has low susceptibility, 19.11% medium susceptibility, 48.54% high susceptibility and 9.55% very high susceptibility.

Stability Assessment
The 16 critical profiles can be seen in Figure 9. category V. In rocks (Table 13), three stations (GS04, GS08, GS10) fall into category III, and five stations (GS02, GS13-GS16) into category IV. Regarding the areas in Figure 7, 22.80% of the total area has low susceptibility, 19.11% medium susceptibility, 48.54% high susceptibility and 9.55% very high susceptibility.

Stability Assessment
The 16 critical profiles can be seen in Figure 9.   Figure 9) in the SLIDE program in static and pseudostatic conditions. For the stability assessment, the geotechnical parameters of Table 16 were used, which were obtained from the results of Table 11. In some profiles, colluvium's presence was taken into account-its values of density, cohesion and friction angle present in a previous work were used [33].
(a)   Figure 9) in the SLIDE program in static and pseudostatic conditions. For the stability assessment, the geotechnical parameters of Table 16 were used, which were obtained from the results of Table 11. In some profiles, colluvium's presence was taken into account-its values of density, cohesion and friction angle present in a previous work were used [33]. Table 17 shows the comparison of the susceptibility analysis and the safety factors (SF) of the stability analysis in the SLIDE program (obtained from critical profiles in Figure 5). The safety factor was found to be between 0.76 and 2.64. Regarding susceptibility levels, the profiles CP10-CP16 range from high to very high susceptibility, while the profiles CP01-CP09 range from low to high.

Stability Assessment
The 16 critical profiles can be seen in Figure 9.   Figure 9) in the SLIDE program in static and pseudostatic conditions. For the stability assessment, the geotechnical parameters of Table 16 were used, which were obtained from the results of Table 11. In some profiles, colluvium's presence was taken into account-its values of density, cohesion and friction angle present in a previous work were used [33].
(a)   Table 17 shows the comparison of the susceptibility analysis and the safety factors (SF) of the stability analysis in the SLIDE program (obtained from critical profiles in Figure  5). The safety factor was found to be between 0.76 and 2.64. Regarding susceptibility levels, the profiles CP10-CP16 range from high to very high susceptibility, while the profiles CP01-CP09 range from low to high.

Analysis of Results and Discussion
The sedimentary succession of "Las Cabras" hill (Duran) is characterised by the alternation of breccias, strongly fractured and weathered fine-grained silty sandstones, shales and argillites. The rock mass has a high weathering degree and both mechanical and chemical deterioration, which favours the formation of clay and residual soils. The instability observed on different slopes of "Las Cabras" hill is due to-among other causesthe dipping of the layers in the direction of the slope. This means that they are susceptible to different types of mass movement, as indicated by several authors [3,4].
Susceptibility was assessed ( Figure 7) through a combination of methods, in which several geomechanical parameters were considered [21,57,61,63,64]. This allowed a specific categorisation (low to very high) of the landslide and rockfall conditions in the study area. The stability analysis demonstrated that different zones present an unstable equilibrium. Three of the studied profiles obtained values lower than one (profiles CP10, CP12 and CP13), and two profiles obtained values close to one (profiles CP15 and CP16). According to the NEC-15 [66], Melentijevic [69] and Morante et al. [21], safety factors (SF) less than one represent instability.
The zones classified with high and very high susceptibility ( Figure 8) correspond to layers of shales and fractured sandstones. These zones were validated with the low resistivities (between 0 and 45 Ωm) in the first 15 m of the VESs carried out (Supplementary Table S4). Additionally, from the results of Table 15 (comparing SC and SF pseudostatic ), it can be observed that they agree with one another well. For example, the CP10 profile has a very high SC that agrees with the obtained SF of 0.76. Additionally, the CP04 profile has a high SC and agrees with the SF obtained of 1.25 (in Supplementary Figure S6 can be seen the zoning map using the safety factor). Thanks to these comparisons, the reliability of the applied expert methodology can be established. This methodology considers the experience of the researchers and the geological-geotechnical characteristics, the relief and the environmental characteristics of the study site. This method has been used to assess landslide and detachment susceptibility in areas of sub-vertical and vertical slopes, and urban areas or sites of direct or indirect impact on populations or heritage sites. [21,54,56,64].
The obtained results provide a basis for stabilisation measures, solutions and territorial planning that guarantee safety. As a guideline of territorial planning, if it is not possible to ensure security in a given area, it must be reorganised. Figure 11 indicates the areas that must be given top priority. Two zones were identified where human settlements are present and the landslide susceptibility is very high: area 1 (2.93 Ha) and area 2 (0.54 Ha). The stabilising measures should be selected so that they also generate added value. These works must start on the crown of the slope and proceed downwards. We do not propose to vacate houses, except those few that are particularly unsafe and are removed from the rest of the buildings. Solutions are proposed for both the southern and northern sides of "Las Cabras" hill. For the northern side, injected anchors (using gunite and Ø25 mm rods) and mechanical drains are proposed to reduce the water table in that area, and The stabilising measures should be selected so that they also generate added value. These works must start on the crown of the slope and proceed downwards. We do not propose to vacate houses, except those few that are particularly unsafe and are removed from the rest of the buildings. Solutions are proposed for both the southern and northern sides of "Las Cabras" hill. For the northern side, injected anchors (using gunite and Ø25 mm rods) and mechanical drains are proposed to reduce the water table in that area, and to be applied in combination with stairway-like surface channels to evacuate rainwater ( Figure 12). The stabilising measures should be selected so that they also generate added value. These works must start on the crown of the slope and proceed downwards. We do not propose to vacate houses, except those few that are particularly unsafe and are removed from the rest of the buildings. Solutions are proposed for both the southern and northern sides of "Las Cabras" hill. For the northern side, injected anchors (using gunite and Ø25 mm rods) and mechanical drains are proposed to reduce the water table in that area, and to be applied in combination with stairway-like surface channels to evacuate rainwater ( Figure 12).  Although a detailed study of triggers is beyond the scope of this study, a general approach is presented. Three possible factors were identified. Seismic activity is significant in the study area (up to 0.24 g [66]) and is considered a possible triggering factor. Furthermore, two anthropic factors, namely, the presence of houses in inappropriate places (without territorial planning) and a sewer system in poor condition, must also be considered as potential triggers. These anthropic factors were not considered in the current stability analysis.
In summary, the problematic situation in Las Cabras hill is mainly due to the instability characteristics of the rock mass (deterioration, fracturing and in some cases the dipping of the layers) and the action of anthropic effects. This situation is conditioned by:

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Instability of some slopes, due to, among other causes, the dip of the layers in the direction of the slope, the degree of fracturing and deterioration of the topmost ground layers. This facilitates erosion due to water flow through fissures, with the consequent destabilisation of the ground and buildings.

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The terrain's morphology, inadequate location and technical deficiencies of some of the constructions. Additionally, most buildings were constructed without any design or planning, which resulted in a chaotic distribution. • Malfunction of the natural drainage system, which is clogged due to uncontrolled constructions. The lack of additional water drainage channels.

Conclusions
The diagnosis of the current situation regarding the stability of "Las Cabras" hill was established through a geological-geomechanical study that included evaluating stability and susceptibility to landslide and detachment and the calculation of safety factor. The obtained results revealed a slope, on the northwest side, of very high to high susceptibility to landslide and detachment (more than 60% of the total area) and unstable areas (with SF less than 1). On the other hand, the most critical areas were the profiles CP10 to CP16 because the dip of the layers has a general south-southwest trend, which coincides with the slope of the terrain. It is recommended in critical sectors to leave buffer strips at the foot of these sectors and implement monitoring and protection measures.
As that part of the hill consists of competent and moderately competent rocks, three factors were considered to affect the possibility of mass movement: (a) problems with drainage (both rainwater and sewage); (b) negative anthropic actions (e.g., housing in inappropriate places, with direct and indirect consequences); and (c) the existing fracturing and weathering degrees of the rock.
Possible solutions to compensate for instabilities are based on controlling erosive processes on the rock masses that could move or slide. Solutions are proposed for both the north and south sides, including the implementation of injected anchors, mechanical drains and stairway-type surface channels to evacuate rainwater. These measures necessarily depend on territorial regulation and require constant monitoring due to the presence of buildings in the affected areas.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available in article and supplementary material.