Landslides are one of the natural hazards, which have significant negative effects on both humans and the environment. Thus, slope stability analyses and stabilization processes are necessary to obviate or mitigate landslides. In this study, the effect of groundwater level fluctuations and the construction of a building (i.e., a recently built church) on slope stability was investigated on the eastern slope of the Avas Hill, at Miskolc, in Northeast Hungary. Soil movements and groundwater levels were monitored and geological and slope stability models were constructed. Furthermore, the possibility of constructing a retaining system was evaluated to minimize the detrimental effects of both groundwater level fluctuations and the construction of the church. The findings showed that the fluctuation in groundwater levels had a destructive effect on slope stability due to pore-water pressure, which decreased the soil strength of the slope and slope stability. On the other hand, the church added an external load onto the underlying soil leading to an increase in slope instability. Hence, we suggested constructing retaining structures such as gravity retaining walls to increase the soil shear strength and enhance slope stability in the long term.
Natural hazards, such as landslides, volcanic eruptions, floods, and earthquakes, are natural phenomena that occur without any human interference. They have significant negative effects on both humans and the environment as well. Landslides, which are defined as mass rock, debris, and slope processes are the most common natural hazard in mountainous regions . In the last few years, many disasters resulting in thousands of deaths, injuries, and economic losses were caused by landslides . They occur when a part of the natural slope becomes unstable, and moves when it fails to carry its weight. Therefore, it is imperative to investigate slope stability for interpreting the landslide mechanism, and to avoid its occurrence. Many factors govern the stability of a slope. They can be categorized into two main groups: natural and artificial factors [3,4]. One of the natural triggering factors is water, which is represented by variations in groundwater levels , snowmelt , or heavy rainfall . The groundwater level plays a significant role in the evaluation and interpretation of landslide stability. Numerous landslide studies have revealed that water affects slope stability [8,9,10,11]: increasing groundwater levels and consequently increasing pore water pressure, groundwater exfiltration, uplifting due to hydraulic pressure, and the effect of water on landslide plasticity are ways in which water affects slope stability. The artificial factor is extensively represented by anthropogenic interactions, which are related to increased population, plantation, deforestation, quarrying, mining, and construction activities as a result of growing urbanization [12,13]. Then, the removal of deep-rooted vegetation binding colluviums to the bedrock followed by construction processes on the slopes are the principal manmade factors threatening landslide stability. Root fibers increase the shear strength of the soil reducing slope deformation [14,15]. Additionally, these roots also reduce rainwater infiltration and delay the rising of the groundwater table .
Hence, many soil improvement and stabilization processes were suggested to avert or mitigate this hazardous phenomenon by increasing the strength of the soil. These stabilizing techniques can be mechanical [19,20], chemical [21,22,23,24], or biological [17,25,26,27]. Additionally, structural techniques such as retaining systems [28,29], stabilizing piles , and drainage tunnels  are used to mitigate soil instability.
Retaining systems are one of the common structural techniques to enhance soil stability [32,33]; they can hold back and strengthen the soil that has a problem in consolidation. These systems consist of a retaining wall constructed to retain the vertical or slightly vertical banks of earth materials or any other material. There are many types of retaining walls such as gravity walls, cantilever walls, fort-counter walls, anchors walls, and piled walls.
Hungary is a country that faces the threat of landslides . The Avas Hill in the Northeast of Hungary is an area where plenty of landslides occur. Several landslides have occurred here earlier. Soil movements continue now too in this area , particularly, in the Northeast of the Bükk Mountains, at Miskolc, which also is the case study area of this project (Figure 1). In the last decades, as a result of landslides, soil movements were detected in Avas Hill with less than 2.5 mm/year displacement negatively affecting the upper part of the soil [36,37]. Although this displacement rate may not be enough to constitute a danger, any higher levels of surface movement could trigger landslides and endanger surrounding buildings. Recently, the landslides in Avas Hill have caused large fractures and joints in some buildings. They have also tilted trees and fences. However, the effects of groundwater, construction, and retaining systems on slope stability in this area have not been discussed earlier in any study. Hence, this study aims to comprehensively investigate the effects of groundwater fluctuations (natural factor) and constructing processes (artificial factor) on the slope stability of Avas Hill, at Miskolc, in the Northeast of Hungary. Moreover, we seek to reduce the soil movement effect and thereby increase slope stability by constructing gravity-retaining walls. For this purpose, the following approaches were taken: (1) monitoring the levels of groundwater in wells and the rate of soil displacement; (2) evaluation of slope stability on a finite slope using GEO5 software; and (3) evaluation of groundwater fluctuations, constructions, and the effect of retaining systems on slope stability using GEO5 software.
1.1 Location and climate
The study area is located between latitudes 48° 05′ 03″ N and 48° 5', 15″ N and longitudes 20° 46′ 31″ E and 20° 47′ 05″ E (Figure 1). It is part of the Avas Hill, at Miskolc city, in Northeast Hungary, which was studied geologically, hydrogeologically, and geotechnically earlier . It lies 124 m above sea level. The climate of Miskolc is classified as temperate and warm. The rate of rainfall in Miskolc is remarkable, as precipitation has occurred during the driest month as well. The average annual precipitation is about 551 mm/year. The maximum amount of rainfall occurs in July with an average of 83 mm; otherwise, the minimum amount of rainfall occurs in January with an average of 26 mm. Figure 1 shows eight observation wells (51, 52i, 53i, 55, 64i, 65, 66i, 68), which were used to measure the changes in soil displacements and groundwater fluctuation. Wells of 51, 52i, 53i, and 55 belong to the fifth line, while 64i, 65, 66i, and 68 wells are related to the sixth line.
1.2 Geological setting
Generally, Avas Hill’s basement consists of Mesozoic rocks (Middle–Upper Triassic Bükkfennsík limestone), which are known here only from boreholes with a depth of about −100 to −200 m (Figure 2). This limestone is subjected to strong faulting and folding tectonics, which appears most prominently in the central part of the Bükk Mountains [38,39].
The Triassic limestone is covered with angular unconformity by Karpatian (upper part of the Lower Miocene) brackish and shallow marine deposits after a long weathering and erosion period. These deposits are clayey, silty, and sandy beds intercalated by thin layers of rhyolitic tuff, tuffite, and brown coal (Salgótarján Formation). The thickness of the Carpathian deposits ranges between 50 and 100 m. These deposits outcrop only on the western side of the Avas Hill. The Avas main mass, which overlain the Karpatian deposits, is built up of Middle Miocene (Badenian–Sarmatian) rhyolitic pyroclasts mixed with clastic sediments or pure volcanic material (Sajóvölgy Formation). This formation has a thickness between 55 and 100 m. The dip angle of layers is between 5 and 12 degrees toward S and SE. Sajóvölgy Formation’s lower part consists of rhyolitic tuffs, while the upper part consists of highly variable beds of mostly andesitic tuff. The depositional and structural character of the andesitic sequence indicates that it was formed mainly in the continental (terrestrial, lacustrine, and fluvial) environment based on the presence of flora (Acer trilobatum [Brongt.], Carpinus grandis [Ung.], Phragmites oeningensis [Brongt.], and Salix varians [Göpp.]) in the andesitic tuffs .
The Sajóvölgy Formation is overlaid by Pannonian deposits (Edelény Formation), which were only found in artificial exposures and drill holes. The thickness of the Pannonian deposits ranges between 2 and 30 m. The paleoenvironment of the Pannonian sequence was lacustrine, and fluvial, as indicated by the clayey–silty lithofacies and cross-bedded lenses in the sand, respectively . Quaternary rocks cover the Pannonian sequence with 1–25 m in thickness. They are composed of aeolian and fluvial deposits, which originated from the weathering process of the andesitic tuffs (the upper part of Sajóvölgy Formation). Moreover, clastic sediments such as gravel and silty sand beds can also be found. These clastic sediments mainly originated from the metamorphic rocks, with a lesser amount of fragments from the limestones or the local volcanics of Bükk Mountains.
2.1 Monitoring of soil movement and groundwater fluctuation
The field investigations were carried out in Avas Hill, at Miskolc city, to obtain the data required to analyze slope stability. The slope monitoring system installed on Avas Hill consists of eight small, shallow diameter observation wells to measure the changes in soil displacements (52i, 53i, 64i, and 66i) and groundwater fluctuation (51, 52i, 53i, 55, 64i, 65, 66i, 68) using inclinometer and groundwater devices, respectively (Figure 3). The groundwater level measurements were carried out weekly correlating with rainfall, and inclination measurements per month. Rainfall is one of the most common factors controlling these groundwater variations. Therefore, it was investigated for the short (per hour) and long period (per month) . Based on these data and other previous data [36,37,40,44,45] that identified the lithology and general mechanical soil properties of the observation wells in the study area, we can analyze the slope stability under these conditions with other simulated parameters, for instance, the changing of the groundwater levels, construction effect (church), and soil properties.
2.2 Geological modeling
Based on the core drilling results, the geological model was constructed with two cross-sections of the fifth and sixth lines of observation wells. Golden Software Surfer 2020 was applied to illustrate the engineering geological model of the research area. Before using the program, the Global Positioning System (GPS) coordinates of the studied wells were converted from the World Geodetic System (WGS 84) to Unified National Projection (EOV), based on the Baltic Sea as a standard sea level for Hungary. The input data included the elevations of the ground surface, Quaternary clay, and Pannonian clay based on data obtained from the observation wells. Furthermore, the groundwater level was measured and linked with the previous elevation data (Table 1).
|Well no.||GPS coordinates||Groundwater level (m) (Oct., 2020)|
|WGS 84||EOV||Elevation (m)|
|Latitude||Longitude||(Y, m)||(X, m)||Ground-surface||Quaternary clay||Pannonian clay|
|51||48° 05′ 06.65″ N||20° 46′ 44.11″ E||778998.32||306054.61||172.822||167.489||163.389||168.392|
|52i||48° 05′ 05.86″ N||20° 46′ 48.18″ E||779082.82||306032.01||167.64||166.640||158.14||166.050|
|53i||48° 05′ 05.53″ N||20° 46′ 51.53″ E||779152.25||306023.2||160.200||159.200||150.100||155.900|
|55||48° 05′ 06.07″ N||20° 46′ 55.74″ E||779239.54||306042.16||150.876||149.276||140.176||145.656|
|64i||48° 05′ 10.72″ N||20° 46′ 34.82″ E||778803.28||306176.31||188.976||187.476||184.176||182.626|
|65||48° 05′ 10.68″ N||20° 46′ 43.18″ E||778975.91||306178.13||174.995||173.395||170.855||171.525|
|66i||48° 05′ 10.79″ N||20° 46′ 48.58″ E||779088.15||306184.84||165.202||162.602||155.702||161.942|
|68||48° 05′ 10.00″ N||20° 46′ 55.70″ E||779235.58||306163.64||150.876||149.276||140.176||144.996|
2.3 Evaluation of slope stability on a finite slope
The factor of safety (FS) is generally an essential parameter to evaluate slope stability. Table 2 shows the ranges of custom minimum total safety factors (F) as reported by Terzaghi and Peck  and recommended in the Canadian Foundation Engineering Manual (1992) . The upper values of these total safety factors relate to normal loads and service conditions, while the lower values can be applied for maximum loads and worst geologic conditions.
|Failure type||Category||Safety factor range|
|Earth retaining structures, excavations||1.5–2|
|Exit gradient, piping||2–3|
Principally, the FS of the slope is the ratio between the resisting force and the driving force, and can be obtained from the following equation (1):
where c, σ, and ϕ denote cohesion, normal stress at rupture surface, and angle of internal friction, respectively.
Table 3 shows the limited equilibrium methods of slope stability utilized in the GEO5 program, which was applied to calculate the values of FS based on different analytical methods such as Fellenius/Petterson , Bishop , Spencer , Janbu , and Moregenstern-price . These methods were classified based on the shape of the slip surface, which is known as a failure surface or surface of sliding where the soils or rocks slip. The slip surface has four types: circular, non-circular, transitional, and compound [54,55]. FS calculations were conducted based on the Bishop Optimization method under the following conditions: before and after the construction of the church and retaining system, as well as groundwater dropping.
|Method type||Assumption||Slip surface||References|
|Fellenius/Petterson||Acting forces on the sides of slices are neglected||Circular|||
|Bishop||Acting forces on the sides of slices are horizontal without acting shear force between slices|||
|Spencer||The inter-slice forces are parallel with the same inclination. Normal forces are active on the base of the slice||Non‐circular|||
|Janbu||The inter-slice normal force location is defined by the thrust line|||
|Morgenstern-price||The inter-slice shear and inter-slice normal forces are related to each other. Normal forces act on the base of the slice|||
This program is a finite element program that was designed to find solutions to geotechnical problems. The main advantages of the GEO5 program are analysis of the slope situation, finding the problems, accuracy improving, and saving time. Besides, it can be used after a long time of work break to modify the structure design. GEO5 program has used standard information and knowledge in many countries based on common and well-known theories. The program has complaints with Eurocode 7 (EN 1997-1)  and LRFD (AASHTO standards) .
Generally, the Bishop method is the most widely used technique nowadays . It is used to identify the highest optimum critical slip surface of the slope. When it is used in the GEO5 program, accurate and satisfactory results can be obtained. Here, the settings of the program were adjusted based on the cross-sections of the fifth and sixth line observation wells. These cross-sections were obtained from the Golden Software Surfer 2020. Also, the standard of FS was set at about 1.5 in this study.
2.3.1 Evaluation of groundwater fluctuation and construction influences
The simulated parameters of layers (Table 4) associated with those of the groundwater levels and dimensions of the church (Tables 5 and 6) were used as input data in the GEO5 program to calculate the FS before and after the groundwater level fluctuations and construction effects of the church on the finite slopes, respectively. These parameters were obtained based on the standard values of soil types involved in the GEO5 program. These parameters, such as unit weight (γ), internal friction angle (ϕ ef), soil cohesion (c ef), and saturated unit weight (γ sat), are listed in Table 2. The church built on the eastern slope of the Avas Hill between the lines of the fifth and sixth observation wells affected slope stability (Figure 1). The groundwater levels and construction effect data were obtained based on the cross-sections of the fifth and sixth line observation wells. These cross-sections were obtained from the Surfer program.
|Parameters||Landfill||Quaternary clay||Pannonian clay|
|ϕ ef (°)||36.50||15.00||24.50|
|c ef (kPa)||0.00||5.00||14.00|
|γ sat (kN/m3)||20.00||20.50||18.50|
|Well no.||Line No.|
|(X*, m)||(Z**, m)||(X*, m)||(Z**, m)|
*Represents the horizontal coordinate (distance) of a finite slope in the GEO5 program.
**Represents the vertical coordinate (elevation) of a finite slope in the GEO5 program.
|Length (m)||Width (m)||Height (m)||Magnitude (kN/m2)||Slope (°)|
2.3.2 Influence of the retaining system
Retaining systems include many types of retaining structures such as gravity walls, cantilever retaining walls, fort-counter retaining walls, anchors retaining walls, or piled retaining walls [58,59,60]. In this study, the gravity retaining wall was constructed to increase the shear strength of soil in the fifth and sixth lines of observation wells. This type is the most commonly applied due to its economical features, so it was selectively nominated. Concrete and gravel are the two components of this gravity retaining wall. Concrete is one of the most common and effective materials used to construct a shield against artificial or natural hazards [61,62,63,64,65].
The simulated parameters of the gravity retaining system input in the GEO5 program are related to the fifth and sixth line observation wells. The used parameters are listed in Tables 7 and 8. These parameters include foundation soil, frontfill, and backfill, which are composed of silty gravel (Table 7). Table 8 shows the groundwater level and parameters of the material used in the wall structure such as compressive strength (f ck ) and tensile strength (f ctm ) of concrete, yield strength (f yk ) of the reinforcing steel bars, and unit weight of the wall (γ). Figure 4 illustrates the simulated dimensions of the gravity retaining system based on the fifth and sixth lines.
|γ (kN/m3)||ϕ ef (°)||c ef (kPa)||γ sat (kN/m3)|
|Parameters||Line fifth||Line sixth|
|Material of wall structure||Concrete||f ck (MPa)||80||80|
|f ctm (MPa)||4.8||4.8|
|Reinforcing steel bars||f yk (MPa)||500||500|
|Foundation soil||Geometry||Thickness (m)||0.7||1|
|Offset left (m)||0.5||0.7|
|Offset right (m)||0.5||0.7|
|Soil type||Silty gravel||Silty gravel|
|Soil type||Silty gravel||Silty gravel|
|Soil type||Silty gravel||Silty gravel|
|Groundwater level||In front of construction (m)||4.5||4.5|
|Behind construction (m)||1.5||4.5|
3 Results and discussion
3.1.1 Soil movement
As shown in Figure 5, the results of the inclinometer measurements show that there is a type of soil movement with a displacement of nearly 2 mm/year in an oblique direction toward the NW–NE from April 2018 to November 2020. Compared to the measurements conducted in April 2018, the recent measurements (Figure 5) show that the soil movement was more effective on the uppermost first 5 m of the soil. In the first 3 m of the uppermost soil, the displacement rate is about 2 mm/year toward the NW direction of the two first meters and NE direction to the rest. After that, this rate decreases to zero value at about 5 m. After 5 m, the displacement becomes not active.
3.1.2 Groundwater fluctuation
Groundwater fluctuation is one of the main reasons for slope instability [66,67]. The hydrogeological map of the study area illustrates that groundwater is accumulated in the east–southeast of Avas based on data collected from the observation wells (Figures 6 and 7). The pore pressure of water reduces the soil effective stresses and consequently the soil shear strength . Subsequently, a severe hazard leads to the collapse of the ground surface and the construction.
Compared to the rainfall level, the monitoring data of underground water level does not indicate a connected aquifer. This is illustrated in Figure 7, where the levels of rainfall and underground water are not consonant. This can be attributed to both natural and artificial factors. The natural one is related to the lithological composition of the underlying impermeable soil, which is composed of clay and sandy clay (Figure 8), while the artificial ones (Figure 8) include leakage from the network pipeline, construction pits, or construction of the deep foundation . Additionally, the impermeable coverage of the asphaltic layer capping the road of the area plays an influential role in the changes in underground water levels. This can be assigned to its predominance of joints and tensional cracks (Figure 9), which contribute to the infiltration of surface water (i.e., rainfall, runoff, the water of freeze-thaw, etc.) into the ground . These cracks can be attributed to the weak strength of the upper landfill layer .
Although the infiltration is responsible for slope failure [16,69], this is not achieved in this study. This can be attributed to the artificial drainage systems (e.g., drainage ditches), which were constructed to reduce the infiltration of slope water into the ground (Figure 10).
3.2 Engineering geological modeling
Figure 11 illustrates the engineering geological modeling of the eastern side of Avas Hill, which was drawn with the knowledge of the two lines of observation wells (fifth and sixth) having 4.5–10 m depth. This modeling shows that the Avas Hill consists of two layers, Quaternary (mainly reddish-brown to brown clay) and Pannonia (dominantly brownish clay and sandy clay), according to GeoExpert Geotechnikai tervező és szakértő Kft, 2013. Also, it is deduced that the slope dips to the east, and the two geological profiles of the two lines of observation wells (fifth and sixth).
The modeling shows (Figure 11a) that the church is constructed in an erosive valley where erosion and weathering of soil take place. Subsequently, it is a disadvantage of engineering geology when the constructed buildings were established on the valley floor instead of the flat area. The thickness of the Quaternary clay layer varies in the two geological cross-sections ranging from 16.67–18.33 to 9.21–23.68 m toward the eastern direction of the fifth and sixth slopes, respectively (Figure 11b and c). The difference in the thickening of this layer between the two slopes (fifth and sixth) could reflect the variation in the topographic surface. The accumulated soil deposits on the downslope occurring as a result of soil movements can lead to the thickening of the Quaternary clay layer. In contrast, the erosion and weathering processes can lead to the thinning of this layer.
3.3 Evaluation of slope stability on a finite slope
3.3.1 Slope stability modeling
Based on the simulated results observed in Figure 12, the FS of the two slopes ranges between 1.29 and 1.47, and this range is less than 1.5 (the critical value of stable slope). So, these values indicate that the current states of the two slopes are unstable . Moreover, the critical slip surface is observed within a shallow depth below the Quaternary clay deposits in the fifth and sixth lines of observation wells based on the Bishop Optimization method using the GEO5 program (Figure 13). It indicates that the layers of Quaternary clay and landfill and their overlaying building (church) are threatened by the sliding on the Pannonian surface.
3.4 Influence of groundwater fluctuation on slope stability
Based on the simulated results obtained from the GEO5 program (Figure 14), the groundwater level falls 40 cm in depth as shown in Figure 15. After that, the ranges of FS at the two lines of fifth and sixth observation wells increase from 1.29–1.47 to 1.33–1.51 (Figure 14). Consequently, the slope stability increases due to the reduction in the pore-water pressure and the rise in soil friction. Additionally, the uplift pressure induced by the confined water decreases under the sliding surface. Moreover, the lithology of the soil (Quaternary–Pannonian sequence) is composed of highly plastic clay minerals (i.e., montmorillonite). As commonly known, montmorillonite has a destructive drawback on slope stability due to its low shearing strength and high swelling ability [71,72]. Furthermore, groundwater fluctuation has a significant effect on the radius of the critical slip surfaces . This can be observed from the radius critical slip surface at the fifth line observation wells when it decreases and moves from being tangential to the Quaternary–Pannonian interface to be within the Quaternary clay layer (Figure 15). This occurs when the groundwater level drops. Therefore, the volume of soil movements decreases on the slope (Figure 15).
3.4.1 Influence of construction on slope
After applying different analytical methods (Figure 16), the results obtained from the fifth and sixth lines show that the slope became unstable after constructing the church, and that the the FS of the slope is less than 1.5. However, before constructing the church, the FS was higher than the value of the critical standard. Therefore, the church decreased the shear strength of the soil resulting in a negative impact on slope stability. Figure 17 illustrates that the critical slip surface was located near the ground surface at the landfill layer before constructing the church. However, it shifted deeper into the underground reaching the Quaternary–Pannonian boundary of the sixth line observation wells after construction of the church. As a result, the volume of soil movements increased after construction of the church compared to what it was before.
3.4.2 Influence of different retaining structures
The simulated results (Figure 18) show that the construction of a gravity retaining wall system has enhanced slope stability. This can be observed by the increasing FS at the fifth and sixth lines of observation wells from 1.29–1.47 to 1.56–1.84 after the construction of the retaining systems (Figure 18). Figure 19 illustrates that the analysis models of the fifth and sixth lines of observation wells have the same FS value, which is equal to 1.83 based on the Bishop Optimization method . Moreover, the position of the critical slip surface has shifted from the lower to the upper part of the Quaternary clay layer reflecting the reduction in the volume of soil movements.
Based on the experimental and simulated results, the following deductions can be drawn:
The soil movement is more influential on the first 5 m of the soil: the displacement rate is equal to 2 mm/year toward the NW–NE direction. After 5 m, the soil movements are inactive.
There is no relationship between rainfall and underground water level fluctuation due to the following reasons:
Natural aspects related to the underlying soil composed of impermeable clay and sandy clay.
Artificial aspects included leakage from the network pipeline, drainage network, construction pits, or construction of the deep foundation. Otherwise, the cracked asphaltic layer covering the roads contributes to surface water percolation (i.e., runoff, rainfall, the water of freeze-thaw, etc.) into the ground.
The safety factor of the study area is less than 1.5 indicating that the slope is rather unstable. Besides, the critical slip surfaces are optimized below the Quaternary clay layer. This will create a hazard for the church built above these deposits.
The fluctuations in groundwater levels have both deleterious and beneficial impacts on slope stability in the study area. The deleterious one is due to groundwater levels rising owing to the pore-water pressure released in the soil and the swelling property of the montmorillonite composing the soil. The beneficial one is the decrease in soil movements due to falling groundwater levels.
The recently built structures have a pernicious effect on slope stability when it is constructed on the middle slope. These structures are susceptible to soil movements at any time. So, it is not recommended to build any structures on the slope except at the summit of the slope (i.e., the plateau area).
Based on the simulated results of slope models, the recently built structures can be supported against the soil movements on the slope by constructing retaining wall systems such as gravity retaining walls.
The authors are very grateful to Prof. György Less, Institute of Mineralogy and Geology, University of Miskolc, Hungary, for help in the field as well as Miss Udomp P. Udomp, Mr Arbi Ben Aoun and Mr Alaa E. Abbadi, Institute of Environmental Management, University of Miskolc, Hungary, for help in the software analysis. The authors are also grateful to the Embassy of the Arab Republic of Egypt and Egypt’s Office for Cultural and Educational Relations, Vienna.
Author contributions: E.H. and T.K. designed and led the study. E.H. and M.A.M. wrote the manuscript of this work. T.K., E.H., A.T., and M.A.M. analyzed the data of the article. A.T. helped and contributed to the fieldwork. All authors contributed to the writing and gave final approval of this paper
Conflict of interest: The author has not declared any conflicts of interest.
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