1 Monte Gordo’s slope, Vila Franca de Xira Analysis stabilizing solutions Rita Nunes Department of Civil Engineering, Instituto Superior Técnico, Universidade Técnica de Lisboa – Portugal Abstract: The growing occupation of urban space, as a result of population growth, both in urban centers and in remote areas, results in the search for new areas for buildings deployment, characterized by geological and geotechnical scenarios with limited potential to it. This fact induces changes on the ground, leading to formation of slopes whose stability must be confirmed by using calculation methods for his resistance investigation. Associated with this, the implementation of an assertive Land Management Plan for each region has an extreme importance for the correct occupation of urban space. This work stems from a case study concerning the instability phenomenon experienced by a slope situated on the Monte Gordo Hill, in Vila Franca de Xira, which began with the construction of a block of buildings at its base. Since that time the problems associated with its implementation led to the need to carry out successive interventions in the slope, having been executed to date two stability solutions, as well as the need to ensure close monitoring of their behaviour. Thus, it was proceeded the analysis of the slope instability phenomenon face to the various interventions made, in order to look for the interaction between them and the behaviour of both the slope and the buildings located at its base and verifying which were the effect of the last ones on slope instability mitigation through two commonly used in practice calculation methods, the finite element and the limit equilibrium method. Keywords: Slope stability, stabilizing solutions, limit equilibrium, finite element, instrumentation; modelling Introduction The increase in population density that exists in society, both in urban centres and in peripheral regions, mean that the problem of shortage of space occupied by cities have significant consequences, and lead to look for new areas for implementation of other buildings and services, in order to meet the populations needs. Logically, over time, the majority of urban areas occupied the zones that had better geological and geotechnical characteristics and, therefore, the unavailability of surface space induce the need to build in more adverse areas. Thus, the stability analysis of the slope formed by the change in ground areas has an extreme importance to prevent failure. As such, every region must have a specified urban planning in order to have the correct definition of the construction areas. Over time, it has been developed a range of studies that have the objective to development methods which allow the evaluation of slope resistance. Completed this analysis is necessary to apply a preventive measure, in order to suspend the possible slope instability and the recurrence of the phenomenon. This work arises from the occurrence of an experienced instability phenomenon in a slope located in the Monte Gordo Hill in Vila Franca de Xira, which began with the construction of a block of buildings at his base. Since then, the problems associated with buildings implementation headed to the need to carry out successive slope interventions, allied to the weak interested field features and man-made interference. Therefore, it was also necessary to ensure close monitoring of their behaviour. It were executed two slope stabilization solutions in total. The second one was developed since the previous one did not improve a positive action to prevent the instabilization process. This case was widely mediatic, given the presented buildings excessive deformations and their precarious security conditions, as a result of this phenomenon, affecting human lives and material goods. The main purpose of this work was to evaluate slopes instability phenomenon faced by the various interventions preconized in it, trying to find the cause effect relationships between them and the slope behaviour experienced over time. Once the events were enhanced by buildings implantation at the slopes base, it was also considered a detailed analysis of this area, in order to find what was his effect in the slope experienced growing deformations mitigation, as a result of all actions and the preconized stability solutions on it. Thus, in order to perform this analysis, it was took into account the results of instrumentation placed throughout the slope and buildings area, and to complete the slopes evaluation performance and the perception of instability phenomenon, it was proceeded the numerical modelling of the various actions that the slope was subject, by considering two methods commonly used in practice for the evaluation of slope stability, the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM). The first, by the finite element method through the Plaxis 2D program, which allows an evaluation of the stress and strain experienced by the ground, and the second by the program GeoStudio-Slope/W, which is based on the method of limit equilibrium with very assertive results. Considering the first mentioned, it was held the comparison of the slope movement results and the deformed configuration of the building throughout the various actions taken over time to that obtained and inferred upon the effective instrument registration and the observed on site. With a view to slope stability analysis for the various stages, the lower safety factor associated with the slip surface caused was gauged considering the two foregoing methodologies, and it was also looked for the ascertainment’s of the two used methods inherent dissimilarities, considering the obtained in the bibliography. In order to approach the slope behaviour and buildings deformed configuration with the really denoted, it was performed a back analysis for the optimization of future interventions if the instability phenomenon remained. Finally, it was realized a technical and economic analysis, with the proposed of an alternative solution, which would be performed in a first instance considering both of the preconized slope stability solutions, in order to assess the advantages and the importance of an assertive analysis of all the possible consequences in long term. Slope stability analysis A slope can have a naturally origin or be anthropogenically performed, lying in both cases in equilibrium in nature, with a certain degree of stability. According to (1), a stable slope exists if the capacity of the soil is higher than the required to equilibrium, and therefore, the major cause for the soil mass instability is not to comply this condition. This could be achieved in two main ways: by decaying soil resistant capacity, for example, the increment of pore pressure conditions, or by increasing shear strength required to balance caused by applying a loading or a ground movement in the slope, for instance. Thus, there is an extreme difficulty to isolate a single cause for instability, and an amount of possible classifications of the slope failures mechanism. In agreement to (2), "(...) in slope stability analysis all the failure mechanisms should be considered. The soil mass enclosed by the sliding surface should be treated as a rigid body or as several individual rigid bodies moving simultaneously (...) ". In line with (3), to evaluate slope stability, it is necessary to take into account the soil shear strength, the slope geometry, the installed pore water pressure, and the load conditions to which is subjected. Thus, for the correct slope dimension, it is defined a relation, the safety factor (FS), which evaluates whether the maximum soil shear strength (τ available ), along a slip surface given by least resistance between particles, are superior to the shear stresses mobilized for the required equilibrium (τ mobilized ), caused by loads, as the soil mass weight or a seismic action, which tend to lead to local slope rupture. Hence, the value for which the soil shear strength has to be reduced, so that it is in balance with the shear stress required for the equilibrium (FS = τ available τ mobilized ⁄ ). When the slope is on the verge of rupture, the FS obtained takes the value of unity. The slip surface correspondent to
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Monte Gordo’s slope, Vila Franca de Xira
Analysis stabilizing solutions
Rita Nunes
Department of Civil Engineering, Instituto Superior Técnico, Universidade Técnica de Lisboa – Portugal
Abstract: The growing occupation of urban space, as a result of population growth, both in urban centers and in remote areas, results in the search
for new areas for buildings deployment, characterized by geological and geotechnical scenarios with limited potential to it. This fact induces changes
on the ground, leading to formation of slopes whose stability must be confirmed by using calculation methods for his resistance investigation.
Associated with this, the implementation of an assertive Land Management Plan for each region has an extreme importance for the correct occupation
of urban space. This work stems from a case study concerning the instability phenomenon experienced by a slope situated on the Monte Gordo Hill,
in Vila Franca de Xira, which began with the construction of a block of buildings at its base. Since that time the problems associated with its
implementation led to the need to carry out successive interventions in the slope, having been executed to date two stability solutions, as well as the
need to ensure close monitoring of their behaviour. Thus, it was proceeded the analysis of the slope instability phenomenon face to the various
interventions made, in order to look for the interaction between them and the behaviour of both the slope and the buildings located at its base and
verifying which were the effect of the last ones on slope instability mitigation through two commonly used in practice calculation methods, the finite
Introduction The increase in population density that exists in society, both in urban centres and in peripheral regions, mean that the problem of shortage of space occupied by cities have significant consequences, and lead to look for new areas for implementation of other buildings and services, in order to meet the populations needs. Logically, over time, the majority of urban areas occupied the zones that had better geological and geotechnical characteristics and, therefore, the unavailability of surface space induce the need to build in more adverse areas. Thus, the stability analysis of the slope formed by the change in ground areas has an extreme importance to prevent failure. As such, every region must have a specified urban planning in order to have the correct definition of the construction areas. Over time, it has been developed a range of studies that have the objective to development methods which allow the evaluation of slope resistance. Completed this analysis is necessary to apply a preventive measure, in order to suspend the possible slope instability and the recurrence of the phenomenon. This work arises from the occurrence of an experienced instability phenomenon in a slope located in the Monte Gordo Hill in Vila Franca de Xira, which began with the construction of a block of buildings at his base. Since then, the problems associated with buildings implementation headed to the need to carry out successive slope interventions, allied to the weak interested field features and man-made interference. Therefore, it was also necessary to ensure close monitoring of their behaviour. It were executed two slope stabilization solutions in total. The second one was developed since the previous one did not improve a positive action to prevent the instabilization process. This case was widely mediatic, given the presented buildings excessive deformations and their precarious security conditions, as a result of this phenomenon, affecting human lives and material goods. The main purpose of this work was to evaluate slopes instability phenomenon faced by the various interventions preconized in it, trying to find the cause effect relationships between them and the slope behaviour experienced over time. Once the events were enhanced by buildings implantation at the slopes base, it was also considered a detailed analysis of this area, in order to find what was his effect in the slope experienced growing deformations mitigation, as a result of all actions and the preconized stability solutions on it. Thus, in order to perform this analysis, it was took into account the results of instrumentation placed throughout the slope and buildings area, and to complete the slopes evaluation performance and the perception of instability phenomenon, it was proceeded the numerical modelling of the various actions that the slope was subject, by considering two methods commonly used in practice for the evaluation of slope stability, the Limit Equilibrium Method (LEM) and the Finite Element Method (FEM). The first, by the finite element method through the Plaxis 2D program, which allows an evaluation of the stress and strain experienced by the ground, and the second by the program GeoStudio-Slope/W, which is based on the method of limit equilibrium
with very assertive results. Considering the first mentioned, it was held the comparison of the slope movement results and the deformed configuration of the building throughout the various actions taken over time to that obtained and inferred upon the effective instrument registration and the observed on site. With a view to slope stability analysis for the various stages, the lower safety factor associated with the slip surface caused was gauged considering the two foregoing methodologies, and it was also looked for the ascertainment’s of the two used methods inherent dissimilarities, considering the obtained in the bibliography. In order to approach the slope behaviour and buildings deformed configuration with the really denoted, it was performed a back analysis for the optimization of future interventions if the instability phenomenon remained. Finally, it was realized a technical and economic analysis, with the proposed of an alternative solution, which would be performed in a first instance considering both of the preconized slope stability solutions, in order to assess the advantages and the importance of an assertive analysis of all the possible consequences in long term.
Slope stability analysis A slope can have a naturally origin or be anthropogenically performed, lying in both cases in equilibrium in nature, with a certain degree of stability. According to (1), a stable slope exists if the capacity of the soil is higher than the required to equilibrium, and therefore, the major cause for the soil mass instability is not to comply this condition. This could be achieved in two main ways: by decaying soil resistant capacity, for example, the increment of pore pressure conditions, or by increasing shear strength required to balance caused by applying a loading or a ground movement in the slope, for instance. Thus, there is an extreme difficulty to isolate a single cause for instability, and an amount of possible classifications of the slope failures mechanism. In agreement to (2), "(...) in slope stability analysis all the failure mechanisms should be considered. The soil mass enclosed by the sliding surface should be treated as a rigid body or as several individual rigid bodies moving simultaneously (...) ". In line with (3), to evaluate slope stability, it is necessary to take into account the soil shear strength, the slope geometry, the installed pore water pressure, and the load conditions to which is subjected. Thus, for the correct slope dimension, it is defined a relation, the safety factor (FS), which evaluates whether the maximum soil shear strength (τavailable),
along a slip surface given by least resistance between particles, are superior to the shear stresses mobilized for the required equilibrium (τmobilized), caused by loads, as the soil mass weight or a seismic
action, which tend to lead to local slope rupture. Hence, the value for which the soil shear strength has to be reduced, so that it is in balance with the shear stress required for the equilibrium (FS =τavailable τmobilized⁄ ). When the slope is on the verge of rupture, the
FS obtained takes the value of unity. The slip surface correspondent to
2
the lowest FS is determined by a significant iterative process, where after a particular surface configuration is defined, the shear stresses necessary for the equilibrium are calculated. Although, it must be make an adequate analysis because the critical slip surface determined may not be the most adverse situation for the phenomenon. Notwithstanding, according to (4), it is possible to admit in a plane strain condition, the critical section of the slope that has the most damaging conditions. The limit equilibrium method (LEM), is the most widely method used in practice for the stability analysis because his results are from data observation and the interpretation of real ruptures. This method considers only static principles that satisfy the balance of forces and moments from a potentially unstable soil mass, for the determination of FS and to analyse the slope safety, and so it declines the slope displacements and strains compatibility. There are several MEL to the analysis, which the method of slices is the most commonly practiced, because it enables the division of the slip surface into slices, and therefore, allows the evaluation of significantly more complex geometry problems. The difference between the various formulations consists in the hypothesis that each one assumed for the equilibrium conditions of a slice, so that all the forces applied on her could be determined. The most prevalent limitation is the fact that LEM is purely based on static principles, and so, it does not consider a representative constitutive law of soil behaviour, so it decline the access on the compatibility of displacements and the generated soil deformations, consisting on the physical lack of the problem. Consequently, regarding (5), in one hand, associated with the fact of just being satisfied static balance, it isn’t possible to determine the stress distribution along the sliding surface, since it assumes that the ground failure follows the rigid-plastic Mohr -Coulomb criterion, and thus, it isn’t possible to determine the stress that represents the real field conditions. Despite this factors, the LEM has been widely used for slope stability analysis, as it allows to evaluate with a high degree of conservatism, the proximity of soil collapse, since their analysis has been considerably developed over time at their knowledge and calibration, and in his practical application. Currently, with the development of automatic calculation programs, it is possible to incorporate finite elements for reach the FS, and make a more refined stability analysis, which is reflected in better results. The FEM allows to overcome the inherent limitation in LEM formulation with respect to the influence of stress distribution in the slope, since it is possible to include a constitutive model for the nonlinear ground behaviour. With this, it is possible to include stress redistribution, compatibility of displacements and deformations, and analyse significantly more complex problems, such as heterogeneous materials, geometries, and the interaction between reinforced structures presented in the slope. Like LEM, the FS is the value that the shear resistance should be divided so that the soil mass reaches failure, and so the strength parameters are incrementally reduced till that happened. Yet, the complexity of computational problems increase in these methods, since the inherent nonlinear analysis iterations are themselves a function of the solution, so that a satisfactory FS is obtained with a relative simplicity to a particular case, which is not in discussed in LEM. o Limit equilibrium and finite element comparison
The inherent differences between the two methods lead to the fact that the FS is determined in a dissimilar way. Given the many studies conducted over time by several authors, it was accurate that, in general, the LEM increases the soil resistance, and so, the FS determined by the FEM is more conservative than the FEM, i.e., the FS obtained by the LEM is greater than FS by FEM. o Slope stabilization solutions
Once detected the potential slope instability situation, with the assessment of the failure mechanism causes and quantified the FS associated, it is necessary to develop a stabilization solution to avoid the sliding or to cut off the movement, increasing the security level. According to (6), the slope stabilization projects involve the following stages: diagnosis, treatment solution and monitoring of slopes behaviour. The slope stabilization techniques, attending to the FS definition, can be used to reduce the applied forces or to increase the resistant forces, whereas both aspects could be included on the same solution. Thus, five approaches to the slope stabilization treatment measures may be
considered. For the first approach, stabilization by changing slopes geometry is the most common, and for the second, the stabilization drainage, the inclusion of reinforcement, the execution of support structures and plant cover are frequently used. In this aspect, the performance of a back-analysis considering all cases involved is very important to prevent the possibility of future situations. Case study: Monte Gordo’s Hill slope, Vila Franca de Xira
o Cronological evolutions of the slope interventions
The case study is located in the Monte Gordo Hill, overlooking the town of Vila Franca de Xira, which is characterized by a limestone massive rock in its upper part and marl-limestone and stoneware complex characteristic from the Tagus zone. It has also a very steep topography and it is crossed by important tectonic accidents. Given this characteristics, it was installed a quarry to massive exploitation of the emerged rock massive. The slope was located approximately in the middle of the hill. Over time, it were being deposited limestone material from randomized blocks forming the rejected quarry, including the slope area. After the quarry inactivation, an illegal landfill material was created from other excavations, with very weak geotechnical characteristics, directly covering the massive rock formation, forming a layer of landfills and which coincides with the instable slope location. As it was expected, this action led to the origin of a sliding surface given the disparate mechanical characteristics and rigidity between the two materials involved. Indeed, the known instability problems began in the early nineties, with the construction of an urbanization at the slope base, composed by three blocks of buildings, which was responsible for his restarting movements in a first instance. Block B, containing three buildings, was the most affected by the phenomenon, mainly the middle one. At this time, it was also built a property, in the northeaster part of the slope, the Golden Stone Disco, whose access road was located parallel to that block of buildings, which was also affected. The topographic gap between them is 20 meters, which demonstrates the significant slope inclination. In Figure 1 it could be visualized an aerial photograph of the slope location. To implement the considered block of buildings, it was necessary to remove a large amount of ground, which was held without any design study or adequate containment structure. At the final of the construction, it was directly applied the local ground landfill at the buildings behind walls in a height of approximately 9 meters, which made a non-negligenciable impulse. Referring to the buildings stability project, it were carried out some gaps, which the most important for the study was the fact that the mentioned walls hadn’t been correctly designed. The building foundations consisted were directly assented on the weak materials.
Figure 1: Plan view of the location of the instable slope (7)
Completed the buildings execution, it were occurred several landslides in the slope, causing severe damage at the Disco, and respective access road, and also, in block B buildings. This was because of the existence of the referred slip surface, responsible for the instability phenomenon located overlying the buildings, in which they cause the increment of the movements in the contact between the two different substrates. Given the slope instability occurring, it was supplemented the instrumentation over the entire instable area, and so, to prevent the continuation of this phenomenon, it was materialized a stabilization solution comprising a geogrid reinforced embankment, along the entire length of the building block. This solution allowed, not only, the replacement of the unstable materials and the regulation of slopes, but also, to eliminate part of the ground that was directly supported by the
Golden Stone Disco
Disco acess road Instable slope
Vila Franca de Xira
Affected Buildings
3
buildings walls, about three meters of total height, which caused impulses to which it was not properly design. However, it was still remained an impulse of 6 meters height directly applied on the back wall. Similarly, it was carried out the slope drainage, because there were not preconized efficient elements over the entire length of the slope, so it was prevented water infiltration in buildings. Regarding the reinforced embankment constructive phasing, it was necessary to remove a significant and complex amount of ground from the slope for his materialization in about 20 m. This excavation was carried out without the use of any containment structures. It was also installed a peripheral drain at his base and at the contact ground foundation inside the embankment, constituted by crushed stone. Over time, the analysis of instrumentation devices placed allowed to verify a continuous increase of the remain of ground deformations, and the displacement problems maintained over years. Also buildings presented a very precarious situation due to their visual deformations, with a lack of structural and stability conditions. After the evaluation of the case, as reported by (8), it was concluded that the slope stabilization measures implemented previously did not have an efficient action, because they promoted only a superficial slope stabilization, rather than, its overall stabilization, because they did not have a significant effect in terms of the deeper slip surface, and so, the interaction between the slope movements and the buildings wasn’t dissociated. By the year of 2013, given the unaffordable situation of slope instability, affecting human lives and material goods, it was preceded a second stabilization solution that guarantees that all ground pressures acting on the buildings were eliminated through a controlled ground movement, until reach buildings foundation quota, approximately. This solution consists in the materialization of a bench at the referred depth, from which would be develop the excavation until face the Monte Gordo Limestone rock massive, which was covered by the landfill layer, and at the same time making controlled slopes. The massive would thus be exposed and suitably consolidated using covering solutions, with nailing and shotcrete. With this solution it was possible the independence among the problems that occurred in the buildings and the slope instability process. This stability solution was presently followed during the execution. Still, given the severe conditions, it was implemented a first phase of excavations, consisting in removing the superior three meters of the first solution, materializing a bench, and preconized a drainage system, so it could be reduce, albeit slightly, the acting impulses in the buildings, and thus favouring the slope stability. It was also possible with this action a better assessment about the properties of the materials concerned, principally the limestone massive rock. In this intermediate phase, according to (9), it was found that the peripheral drain located on the inside of the geogrids reinforced embankment was a collector element of water infiltration and percolation from limestone massive rock, which may have saturated the particular ground in the slip surface occurring at greater depths, since the materials were found wet and plastic. It was concluded that this final solution had a favourable effect in mitigating the phenomenon, as it was observed the decay of displacements in instrumentation, despite the slip surface continued to happen above buildings foundation ground. o Geologic and Geotechnical Scenario
Considering the local observations carried out by the designer, it was defined the geological plant of the instable slope area, as it could be seen in Figure 2. Also, in Figure 3 it could be observed the geological and geotechnical section regarded as the most critical along the entire length of the slope, represented in Figure 2, since it were observed the greatest displacement on soil and it included the building, which experienced the further damage and risky conditions face the instability phenomenon. The geomorphological conditions presented in slope area control the hydrogeological conditions. As it was explained later, considering the reports provided about the slope recognition process (10), it were identified two main geological and geotechnical units for the geotechnical zoning of the study area. They were constituted by the recent deposits and by the limestone rock massive, which is the upper Jurassic bedrock. The first one mentioned covers the second one, as already explained. The recent deposits, above the limestone rock massive, are composed by the construction waste landfill, essentially clayed soils, and the rejected limestone blocks from
the quarry (At1) and it was also considered, face its damage situation,
the geogrid reinforced embankment (At3), by the construction waste
landfill. The Jurassic bedrock consists of Monte Gordo limestones (CMG), and by
a marl-limestone (M) and a stoneware-marl (G) complexes, where the recent deposits settle. The characteristic NSPT value, defined in (10),
were 12 and 123 for the cohesive geological zones, At1 and M,
respectively, and 39 for G, which demonstrats the different mechanical
characteristics of the geological units refered.
Figure 2: Geological plant of the instable slope area (Adapted from (10))
By the fact that the Jurassic bedrock units were not directly observed, as they were covered by the recent deposits, the mechanical characteristics, principally of the limestone rock massive (CMG), like the cracking degree
and the geometry at greater depth, were unknown, and so, the intermediate intervention had a major importance to this knowledge. That way, it was defined a zone on slopes instable area by the designer that has more fragile characteristics and needed to be more accurately accompanied, which is represented by a yellow colour in Figure 2.
o Description of the accompanied slope final solution executed
and construction’s works evolution According to the established objective, which was to eliminate all the impulses of buildings walls, the final slope stabilization solution advocated a considerable earthmoving. The geometric definition of the excavations carried out had as baseline a bench materialized at the buildings behind walls, which corresponds to the location in depth of building foundations. So, involving all the slope instable area, above this bench, it was defined gentle slopes with predefined inclination and with vegetable and hydroseeding until found the face of the rock mass. Its surface would then be consolidated and properly coated with designed reinforced shotcrete, with metal fibres or electrowelded mesh, and nailing. Thus, up to that behind buildings bench, it were defined 4 intermediate benches, and respective slopes according to the above description, covering the whole instabilized slope area, having been set, yet, a bench next to the limestone massive rock undiscovered face, in order to make the link between several benches. It was also defined a surface drainage system, such as drains, both on the slopes and in rock mass. The excavations carried out assumed a special importance, as they will discover the limestone rock massive that has an inherent lack of knowledge, and so, the final solution has the particularly distinction of
𝐂𝐌𝐆
𝐆
𝐌
𝐀𝐭𝟏
𝐀𝐭𝟑
𝐌
𝐆
Figure 3: Geological and geotechnical site investigation of the critical
section on the slope (10)
𝐂𝐌𝐆
Block of buildings
𝐀𝐭𝟑 At1
𝐀𝐭𝟏/𝐌
/
𝐂𝐌𝐆 Cross failure 𝐀𝐭𝟐
Critical section
4
being closely linked to the real time geological and geotechnical conditions found at the site, as the work was being performed. The excavations carried out to find out the massive were executed in stages, with advances of about three meters in depth to be carried out safely. Taking into account the Figure 4, it was distinguished three zones to preconized a stabilization in the slope instable area.
In zone 1, the excavation was carried out with the materialization of the above described solution, and in an upper zone the slopes were defined with a variable declination in order to achieve better adaptation with the streets and ground optimization. In zone 2, which is the fragile area defined by the designer, because of the unknown geometric configuration and high complexity proximity of the limestone massive rock, the excavations were carefully executed. As such, at this stage of the work, the assertive and constantly monitoring of the designer took on an important key in adapting the solutions to be used on site to real geological and geotechnical conditions. Also, his presence was fundamentally to ensure the greater optimization of the treatment solutions and to guarantee that the excavations were carried out safely. In zone 3 were executed the slopes with intermediate benches explained previously, and these ones were stopped laterally with the in situ limestone massive rock that was being discovered in zone 2 In Figure 5 could be visualized photography’s taken during the construction evolution in the three zones considered.
In Figure 6 is displayed a global view of the final solution preconized in the slope.
o Monitoring plan and instrumentation evolution The high temporal horizon of slope instability phenomenon led to the implementation of numerous campaigns of prospection and monitoring, together with the interventions made, and so the evaluation of the monitoring results was performed with some difficulty. Since several
actions were taken in slope through the years, the analysis was carry out considering each slope preconized event, and the time break between them. So, with major conclusions, it was regarded two examination phases: a first, after stabilization solution using geogrids reinforced embankment and before the immediate intervention and the final solution, and after the last one. The detailed analysis of the numerous instrumentation results campaigns, placed on the slope over time, proved to be extremely important, since it enabled to complement the concluded previously in the chronological evolution of the slope instability. Moreover, it was possible to compare the established with numerical modelling performed afterward. Given the strong three-dimensional phenomenon component and his wide area on the slope, these monitoring plans relates to the perception of slip surfaces that could possibly overcome along the entire space, by analysing the ground deformations presented, in order to measure slope stability. In addition, the relative movements between the buildings were analysed, so that it could be perceive their behaviour face the actions taken over time in the slope, and therefore, the perception of their interaction with slope movements, associated with the mitigation of the instability phenomenon. The measuring devices with a higher relevance to the observation and perception of slope and building behaviour were inclinometers, arranged in the whole area, and topographic targets located, not only, in the slope, but especially, in buildings frontal and behind walls. To complement the monitoring analysis in place, it was also considered the visualization of the relative displacements between the buildings joints of block B, the most affected by the situation, to know the local relative building rotation and the displacements at the same direction wall and at its perpendicular direction. This measurements were carry out before the intermediate intervention, when the slopes deformations were very accented by the entities. Despite the fact that, in a first instance, it had been evaluated all the instrumentation installed in the significant study area, given its three-dimensional component, it was carried out a detailed analysis of the section considered the most critical (Figure 3). After stabilization solution using geogrids reinforced embankment and before the actual solution there was a significant succession of slope instrumentation campaigns and observations through the wide temporal horizon. For the topographic targets, since it was made countless campaigns with interruptions and new reference dates, it was considered all the observations made along slopes area to conclude about slope behaviour and buildings interaction. As regards to the inclinometers, the most important in the analisys, in the phase mentioned at the critical section, it was placed a device in the interior of the geogrid reinforced embankment, and particularly on buildings construction area, two inclinometers were located on opposite sides of the property. These last two allowed, not only, to make an assessment of possible slip surfaces that could form in a greater depth overlying their ground foundation, but also, to analyse the movements between slope and buildings interaction phenomena. After the final solution it were only just placed this inclinometric system at the slope base. However, the readings of the inclinometers placed at slopes base presented an insignificantly order of magnitude, given its local placement in a very advanced stage in the instability phenomenon, which was not compatible with the extremely deformations observed in the building. As such, it was considered to evaluate the tendency of readings taken during the various campaigns along the time. So, after an evaluation of these two devices observations it was deduced that the horizontal displacement data were composed by two separate phenomenon face the interaction of slopes movements and the buildings: one due to the relative rotation of the building and another given by differential settlement of the foundation ground. Regarding the two study phases mentioned and this compound behaviour inferred, it was verified that buildings experienced an increasing sliding in depth towards the higher slope declination and a differential settlement of the foundation ground bigger at building front rather than the behind. Concerning the buildings deformed configuration, it was obtained different rotation, as it could be seen in Figure 7 and Figure 8. By analysing the readings of the device placed in the geogrid reinforced embankment, it was observed the presence of a slip surface on the inside, thereof with a downward movement in the steepest slope direction.
Zone 1 Zone 2 Zone 3
Block of buildings
Access road
Cross failure
a)
b)
Figure 6: General view of the final solution
Figure 4: Extension area of the slope and the location of the different zones to the final solution’s definitions (Adapted from (9))
c) d)
Figure 5: Photography’s taken on slopes local of the different zones: (a)-Zone 1; b) Zone 3; c) and d) Zone 3
a) b)
5
With regard to the remaining active observations devices, the topographic
targets, after stabilization solution using geogrids reinforced embankment
and before the immediate intervention / final solution analysis phase, there
were a correlation between the results previously concluded in
inclinometers, both the topographic targets placed on the buildings
facades and spread over the entire area. Also, the observation of the
displacement of the joints between the lots of B block at buildings top
before the immediate action coincides with the previously building
clockwise rotation.
The explanation to all of these results will be taking at the comparison with
the defined numerical modelling.
Although it was not observed any more reading after the final solution
besides the inclinometric system displacements, it is considered that the
on-site observation of the coated building movement of extreme
importance, since it reflects the real deformed occurring therein.
Considering Figure 9 and remembering Figure 8, it was observed opposite
building configurations.
.
Numerical Modelling
o Analysis of the slope results during the intervention phases
Given the previously stated, it was evaluated the soil mechanical
behaviour against all intervention made over time in the slope by the EF
program (Plaxis 2D), in order to compare his experienced displacement,
in particular on the buildings construction area, to take into account the
perception of their interaction with the events and with the observed
instrumentation.
The critical section available (Figure 3) in modelling was the same as the
previously analysis. This program allows a fairly realistic approximation of
site conditions and its interaction with structures. To represent the history
of interventions in modelling, it was considered the following calculation
phases: initial ground before the buildings construction, excavation for his
implementation, buildings execution, excavation for the realization of the
first stabilization solution, preconisation of its, immediate intervention and
the final solution.
The behaviour of geotechnical layers considered were simulated by using
the Hardening Soil constitutive model for the soil layers, and for the
limestone rock massive the Linear Elastic constitutive model, since both
models are appropriate for simulating the materials response more
accurately. To have the better response and approximation between the
model and the previous concluded by analysing the compound
phenomena inclinometers deformation, the At1 landfill layer was divided
into two, At1 and At′1, with different thicknesses and different E.
In Table 1 and Table 2 the characteristic parameters for each layer and
model considered are presented. Table 1: Soil parameters (Hardening Soil Model)