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 (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

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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

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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)

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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)

Parameters Geotechnical Zones

𝐀𝐭𝟏 𝐀𝐭𝟏′ 𝐆 𝐌

𝛄𝐮𝐧𝐬𝐚𝐭 (𝐤𝐍 𝐦𝟑⁄ ) 18 18 21 21

𝛄𝐬𝐚𝐭 (𝐤𝐍 𝐦𝟑⁄ ) 20 20 23 23

Type of material Drained

𝐜′(𝐤𝐍 𝐦𝟐⁄ ) 10 10 0 200

𝚽′(°) 32 32 39 25

𝚿(°) 0 0 0 0

𝐄𝟓𝟎𝐫𝐞𝐟(𝐤𝐍 𝐦𝟐⁄ ) 5500 15000 130000 200000

𝐄𝐨𝐞𝐝𝐫𝐞𝐟 (𝐤𝐍 𝐦𝟐⁄ ) 5500 15000 130000 200000

𝐄𝐮𝐫𝐫𝐞𝐟(𝐤𝐍 𝐦𝟐⁄ ) 16500 45000 390000 600000

𝐦(-) 0,5 Table 2: Massif of rocky limestone parameters (Linear elastic model)

Parameters 𝛄𝐬𝐚𝐭

(𝐤𝐍 𝐦𝟑⁄ )

𝛄𝐮𝐧𝐬𝐚𝐭

(𝐤𝐍 𝐦𝟑⁄ )

Type of material

𝐄𝐫𝐞𝐟

(𝐤𝐍 𝐦𝟐⁄ ) 𝛎′

𝐂𝐌𝐆 25 26 Drained 250000 0,15 To represent the buildings influence on ground behaviour, it was decided

to consider a simplification of their structural elements, simulated the

building as a rigid body, with the stipulation that in case of soil failure, it

could not be caused by loss of rigidity and strength of their walls and

foundation. The principally structural elements were the back and frontal

wall, the foundation and two subterranean caves. Regarding the geogrid

reinforced embankment, it was stipulated a homogeneous equivalent soil,

considering the defined in the executed project and applying resistance

and stiffness criteria’s. It was set the Mohr Coulomb constitutive model to

define the equivalent soil layers behaviour. Given the complexity of the

geometry and slopes problem, it was not carry out the generation of the

initial tensions by K0 procedure, as this is commonly applied to model

surfaces with horizontal geometries, and so it was considered an

additional calculation phase called Gravity Loading. The construction

phasing was conducted in order to approximate the calculation steps with

the events actually experienced in the slope and in the buildings.

Considering Figure 10, it can be seen the accumulated total

displacements experienced by the slope along the various stages

considered, and the accumulated vertical displacement in point A, which

a positive sign corresponds to a swelling and a negative to a settlement.

Where there is more variation compared to the total displacement

different events over time is in At1 landfill layer, verifying the

occurrence of swellings, with higher expression in more superficial

areas of the slope. Then, with regard to the implementation of various

actions on the slope and considering point A, in the phases

calculations in which are carried out excavations at buildings behind

wall, for example at the final solution, the soil features a swelling given

that upon removal the loading of the soil weight overlying occurs a

volumetric deformation variation by air outlet, forcing the particles to

rearrange themselves face to decompression occurred. In the phase

calculation in which are placed materials behind the buildings, in

particular in the construction of reinforced soil embankment geogrid,

an overload occurs on the ground which results in occurrence of a

settlement that is distributed throughout the area. This results showed

the consistency of the model.

Figure 7: Illustration of the building’s deformational behaviour after the geogrid reinforced embankment and before the immediate intervention

Figure 8: Illustration of the building’s deformational behaviour after the final solution

Figure 9: Building’s photography showing his deformed configuration

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Given the significant changes in the ground at the slope toe, due to

the location of the buildings, it was proceeded a detailed analysis in

this area, to evaluate their behaviour face to various actions

implemented, in order to infer the cause-effect previously measured.

To perform this analysis it was verified the finite element deformed

mesh through all the phases in order to observe buildings movement.

Nevertheless, it was found that the Plaxis 2D computer program, as

also verified in other studies, increases swellings experienced by soil,

and so, intensify the stress relief, which constituted a limitation to the

problem in question, since this phenomenon was significantly

enhanced in calculation phases that contained excavations.

Due to this fact, it was decided to analyse the building deformed

configuration in two approaches: a first one, taking into account the

displacements accumulation along the stages, and a second,

considering the elimination of the displacements, induced by the

excavations carried out behind the buildings, excluding the influence

of soil swellings. With the first line, it was possible to compare the

movements of the ground and the buildings between the various

actions occurred. Through the second line, it was conceivable to

analyse the effect that each individual action applied induced in slope,

and what was its influence on building behaviour in terms of their

rotation and deformed configuration, and therefore, in their interaction

with the slope movements for his stability.

The deformed configuration of the building in every phase,

considering both of approaches, is presented in Figure 11.

Overall, it was noted that the buildings behaviour was quite

susceptible to any action taken on the slope. Considering the

accumulated displacements approach, it was inferred that whenever

the ground was eliminated from the slope, the buildings experienced

a clockwise rotation in the highest slope declination, in all the phases.

Regarding buildings preconisation phase, it was noted that they

experienced a clockwise rotation in the steepest slope direction, and

their movement had an increasing progress along the same

declination. This movement is contrary to the impulse caused by the

ground, applied anthropogenically in buildings walls, with a 9 m

height, so it follows that after the finalization of the buildings

construction, they contributed beneficially to slope stability.

o Comparison of numerical modelling results and the analysed

by the instrumentation during the intervention phases

Indeed, it was proceeded a comparison between the results obtained in

modelling and the already exposed on instrumentation readings and site

observations, which has a significant complexity. As such, in the

evaluation was emphasized the comparison mainly concerned to the

buildings deformed configuration and the tendency of the ground

experienced deformations. In order to assess on the viability of each

action implemented in the slope, this analysis was carried out by taking

into account the phases considered in the monitoring evaluation, and it

was regarded the accumulated displacements experienced by the slope

on modelling, i.e., it was considered the effect that the previous phase

induced on the carried out stage. Overall, it was verified that the

displacements obtained in the numerical modelling showed a higher

magnitude to those observed in the readings monitoring. After stabilization solution using geogrids reinforced embankment and

before the immediate intervention / final solution

Regarding to numerical modelling, it was found a presence of a slip

surface within the geogrid reinforced embankment, which is coincident

with the already inferred previously in the monitoring. As for buildings

rotation, it was visualised the clockwise buildings rotation toward the

higher slope declination, both in instrumentation and simulation. 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. According to these

results, as it could be seen in Figure 12, it was concluded that the

modelling results were very approximate to soil behaviour and with the

buildings deformed configuration in consequence.

Taking into account the considered geotechnical zones, this

compound displacement, given by the buildings differential settlement

experience and their associated clockwise rotation in the highest

slopes declination, was attributed to the weak geological and

geotechnical characteristics of building ground foundation, which

enhance the experienced differential displacements, and also, to the

direct consequence of the potential slip surface formed between the

contact layer At1 and the layers M e G, due to their dissimilar stiffness

and resistance, combined with the high inclination of the slope and the

significant sub-vertical arrangement of the marl stratum to deeper

levels, that enhancing the kinematic slip of the buildings along the

referred least resistance surface.

Furthermore, the fact that the buildings architecture detain a greater

weight on their front side in comparison with its back part, increasing the

Máx. Total Disp.: 38 mm

𝛅𝐕𝐀: 4,8 mm

Geogrid embankment excavation Geogrid embankment construction

Intermediate intervention Final solution

Figure 10: Accumulated displacements on the slope considering each intervention phase in Plaxis 2D

Initial ground Building construction

Máx. Total Disp.: 52 mm

∆𝛅𝐕𝐀: -2,8 mm

Máx. Total Disp.: 70 mm

∆𝛅𝐕𝐀: 14,8mm

Máx. Total Disp.: 98 mm

∆𝛅𝐕𝐀: -0,8 mm

Máx. Total Disp.: 95 mm

∆𝛅𝐕𝐀: -0,3 mm

Máx. Total Disp.: 82 mm

∆𝛅𝐕𝐀: 19,9 mm

With accumulated displacement Without accumulated displacement

Building’s construction

Geogrid fill’s construction

Final solution Figure 11: Deformed mesh in Plaxis 2D on each intervention phase (Amplified 100 times)

A A

A A

A A

7

application of a differential loading on the foundation ground. Likewise,

the direct application of a considerable impulse of 6 meters of At1 landfill

in their back wall, which was not removed at the time of the implemented

stabilization solution, had an increased effect on its rotation. Also with the

same result, although it wasn’t supposed to, the own weight of the geogrid

reinforced embankment acted as an overload which increases the

impulse in buildings wall, and also added a larger shear stress in materials

involved, mainly in the slip surface, promoting the materials plasticity in

the contiguity area.

However, taking into account the eliminated displacements approach in

the numerical modelling in the implementation of the first stabilization

solution, after its excavation, since it could be obtained the impact of the

first solution on slopes movements and buildings interaction, the achieved

counter clockwise rotation comparing with the buildings construction

phase shown that the geogrid reinforced embankment interrupted the

initially buildings movement. Still, his action had only a superficial effect

because, in the long term, as it was mentioned before, it would increase

the sliding between the layers, due to its weight, acting as an overload

behind the building.

The time deformed configuration actually observed in the building on site

coincides with the measured in modelling, which confirmed the results.

Thus, it was concluded that the geogrid reinforced embankment

construction didn’t act favourably to prevent the continuing of instability

phenomenon, increasing it further, although, in the first instance, it had an

improvement in the stability conditions at a superficial level. It could be

inferred that the buildings deformed configuration induces the slope

experienced instability mitigation process, since their movements are

precisely contrary to all actions installed, although they had been the main

cause of the instability phenomena. After the final solution:

As it was seen on the previous phase, 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. It was verified, as it could be observed in the

scheme of Figure 13, the clockwise rotation of the building considering

the numerical modelling the accumulation of displacements, i.e., taking

into account the influence of previous events, instead of anti-clockwise

rotation noted instrumentation.

In the last two actions applied to stop the slope instability, it was where

the influence of the increase of soil decompression was denoted in a

greater way by the Plaxis calculation program, since it was made almost

by excavations, and therefore, it was found once again the highest

sensitivity of the model due to the buildings rotation, which amplified the

clockwise rotation. The significant time horizon associated with buildings

deployment, induces a vertical soil confinement in their proximity, being

less susceptible to the decompression by the realized excavations, which

justifies the anti-clockwise rotation seen in the instrumentation readings.

Though, they are indicative of a decompression of the ground and

therefore it could be deduced that they reflect the decrease of buildings

stability action in mitigating the instability slope mechanism, which was

deduced from the previously analysed stage, since the rotation

experienced by the property is precisely opposite, and thus, indicates the

favourable action in slope. In this monitoring approach, at the most, the

property reached verticality, which could not happen due to foundation

differential settlements as a result of slip surface at greater depths.

However, the clockwise rotation obtained from the calculation program is

not incorrect as to cause a decompression in the buildings behind wall, by

removing the background, it was also expectable this type of rotation. It

should also be considered that the deformed building visualized by the

movement of the joints between buildings on-site indicates the

permanence of clockwise movement, and it was coincident with the one

obtained by the program, which enhances the susceptibility of the site in

relation to the excavation made before. This local rotation demonstrates

that the final solution was not sufficient to inverse buildings configuration,

because of the wide application of the adverse actions directly on

buildings behind wall through a high time horizon.

As such, it was demonstrated the favourable performance of the final

solution in favour of dismantling the interaction between the movement of

the slope and the building, fostering the restoration of its stability and

safety since all the materials at buildings behind. Despite this fact, its

kinematic motion is maintained, since the slip surface is still between the

layers with different mechanical characteristics overlying the buildings. o Slope Stability evaluation on each intervention phase

As part of the stability analysis, FS was obtained using the FEM

calculation procedure Phi/C Reduction, where the resistance parameters

Φ’ e c’ are successively reduced until soil rupture occurs. (11). In LEM, it

was considered the slice method of Morgenstern-Price, that satisfies both

force and moment equilibrium, with a half sine function, and the circular

shape of the slip surface by the Grid and Radius analysis type. To

represent the Mohr-Coulomb failure criteria of the geotechnical zones, it

was took into account the soil strength parameters presented in Table 1,

and for the limestone rock massive it was considered 25 kN m3⁄ for γ,

107 kN m2⁄ , and 46° for Φ′. Due to the geometry complexity of the

problem, in Slope/W program, associated with the MEL methodology, it

was necessary to represent the building as a soil mass with properties

such that there was not possible the existence of a slip surface inside of

Figure 12: Illustration of the building’s deformational behaviour

considering modelling and instrumentation approaches

Building’s rotation referring to modelling

Building’s rotation referring to instrumentation

Figure 13: Illustration of the building’s deformational behaviour considering modelling and instrumentation approaches

Building’s rotation referring to modelling

Building’s rotation referring to instrumentation

8

the property, as regarded in MEF. The geogrids reinforced embankment

was modelled as in MEF.

The slope security verification to global ultimate limit state, as a result of

actions undertaken, was carried out by taking into account the philosophy

present in EC7 (2), considering the AC1-Comb.2. In Table 3 it is

presented the safety factors of the various phases obtained by the MEL

and the MEF. However, it must be said that the instability phenomenon in

question has a strong three-dimensional component, so instability may

occur in a single mass of soil, which makes the 2D analysis with some

uncertainty.

Attending to the ground in its initial state, it was found that their stability

was quite poor. As regards to the buildings construction phase, it was

considered that the LEM results obtained were not consistent, since it was

considered a high simplified simulation on models property. Table 3: Modelling safety factors

FS Plaxis 2D Slope/W

Startup ground 1,1 1,2

Building’s construction 1,5 0,6

Geogrid fill excavation 1 1,1

Geogrid fill construction 1,2 1,4

Intermediate intervention 1,3 1,7

Final solution 2,1 2,4

Attending to FEM results, the critical slip surface shown in Figure 14 is

located behind the buildings, which demonstrates, once again, their

favourable action to instability phenomenon.

Although they had fostered slope instability, the FS is greater than unity

and also bigger considering the grounds initial phase, so they were stable

in the first instance.

Advocated the first stabilization solution, the FS suffered a slight increase

compared to the excavation performed, but with an unsteady safety, since

its magnitude was near the rupture, and the corresponding slip surface is

on his inside. However, given the homogenization carried out to simulate

the embankment in the calculation models, this surface may not be

representative of reality.

Still, the FS for this solution is less than the determined in the buildings

construction phase, which is consistent with the previously concluded,

since the action of its weight potentiate the sliding at the interface between

deeper layers. As it could be seen in Figure 15, the location of plastic

points are at the contact between these two materials.

As for the solutions provided later there was a significant increase in FS,

since it withdrew the ground to buildings foundation quota, and so, as it

could be observed by the reduction of the plastic points at the interface

between the deeper materials, it may be concluded the stopever of

instabilization phenomenon progression

With reference to the FS obtained by the FEM, it could be seen that the

value determined by the LEM is higher than the measured by FEM, as

inferred in the bibliography, with a dissimilarity between 1% to 6%, except

in buildings construction phase, as it had significantly simplifications, and

the final solution. Despite the fact that FEM allow for greater consistency

in the results for more complex geometry problems, it was found

significant convergence problems, and so, LEM gave acceptable results,

as it could be seen in Figure 16 and Figure 17, by because the critical slip

surfaces and the correspondent FS don’t have a great difference.

o Back-analysis

In spite of all the impediments found in the observations carried out by the

instrumentation, it was performed a back analysis of the solution in order

to approximate the results obtained in numerical modelling with the

actually observed in real.

The analysis relates to the calibration of the model in question in an

attempt to emulate and improve their behaviour, particularly in the

property deformed configuration, through its relative rotation, given the

movements experienced by the slope. It is intended to also assess the

influence that the geotechnical parameters of interested layers holds the

slope behaviour, and movement associated with the building.

Thus, to calculate the relative rotation or building slope (θbuilding) it was

took into account the methodology used in risk analysis conducted by

Boscardin and Walker (12), in order to estimate the buildings response

face slope movements experienced in the simulation performed for each

calculation phase that corresponds to rotation due to settlement of a line

between two reference points in the structure, which were considered at

the extreme points of buildings behind and frontal walls, minus the slope

due to rigid body movement.

Conservatively, it was considered to define the settlements, the relative

displacements in the plane of the walls between expansion joints of lots

of block B at buildings top, displayed on the site prior to the intermediate

intervention. It was defined a value for the local relative rotation

(θbuildinglocal observation). Since it was found that on-site the deformed

configuration experienced had not changed over time against all slope

interventions, this θbuildinglocal observation was considered a reference value.

In Plaxis 2D modelling, the rotation (θbuildingInitial modelling

) was obtained

through accumulated vertical displacements of the buildings foundation in

their exterior walls, between the back and frontal walls at the same points

considered before applying the same methodology exposed. It must be

said that the displacements are defined at different locations, which takes

a significant approximation to this backanalysis.

Figure 14: Critical slip surface from Plaxis 2D for building’s construction phase

Figure 15: Plastic points from Plaxis 2D for geogrid reinforced embankment construction phase

Figure 16: Critical slip surface from Plaxis 2D for initial ground phase

Figure 17: Critical slip surface from Slope/w for initial ground phase

9

It was inferred that the relative rotation given by numerical modelling had

a higher value than that obtained by observation on site, and the buildings

deformed configuration had a very pronounced movement, so it was

considered that the soil parameters would be conservative compared to

what they really were in place. Therefore, it was preconized a parametric

analysis with a view to optimizing the geotechnical parameters and

improve the behaviour of At1, since it had presented greater

deformations in numerical modelling and in monitoring, and also, the

instability effectively occurred in this layer. It was found that the E value

was the most influential parameter in the slope displacements and in the

buildings deformed configuration so, it was defined a new characterization

of the model in order to obtained a new buildings rotation

(θbuildingChanged modelling

) to subsequently compare with the relative rotation

reference.

In Table 4 is presented the Hardening Soil model geotechnical

parameters of At1 layer used in back-analysis. All the other parameters

remain the same as the initially considered. Table 4: Modelling optimization of the Hardning Soil properties for 𝑨𝒕𝟏

Initial modelling Changed modelling

𝐀𝐭𝟏 𝐀𝐭𝟏′ 𝐀𝐭𝟏 𝐀𝐭𝟏

′

𝐄𝟓𝟎𝐫𝐞𝐟(𝐤𝐍 𝐦𝟐⁄ ) 5500 15000 15000 15000

𝐄𝐨𝐞𝐝𝐫𝐞𝐟 (𝐤𝐍 𝐦𝟐⁄ ) 5500 15000 15000 15000

𝐄𝐮𝐫𝐫𝐞𝐟(𝐤𝐍 𝐦𝟐⁄ ) 16500 45000 45000 45000

In Table 5 is presented the relative rotations obtained by the displacement

between the expansion joints and displacements in numerical modelling,

with the initial geotechnical parameters and with the changed at every

phase considered previously. It could be observed that the building

relative rotation suffered a decay with back analysis within the limitations

found. Observing Figure 18, it may be seen the attenuation of the

buildings deformed configuration, principally in their relative rotation,

comparing with the initially considered, and therefore, the backanalysis is

validated.

Table 5: Buildings relative rotation in back-analysis

𝛉𝐛𝐮𝐢𝐥𝐝𝐢𝐧𝐠 (°) (*10-3)

After the geogrid’s fill and before

the immediate intervention

Between the immediate

intervention and the final

solution

After the final solution

Local Observation 1,25

Initial modelling 2,96 2,99 4,24

Changed modelling 1,17 1,20 1,78

Alternative Solution

It was presented an alternative solution to the first executed in slope,

comprising more robust and heavier containment structures that would

achieve the identified slip surface at a greater depth, and therefore, in

detriment of the two stabilization solutions performed. Also, this proposed

solution aimed to carry out a technical and economic analysis, in order to

verify if a more expensive and with a significant quality control solution,

would be advantageous face to all the interventions performed to date to

prevent slope instability

The stabilization solution consisted in the materialization of two support

structures, one located at the slope top, and the other on the base, along

buildings behind walls, so as to intercept all possible slip surfaces at a

greater depth, removing the existing material between them, and

suppressing the unfavourable impulse performance on buildings walls. In

Figure 19, it could be seen the alternative solution.

The geometry of the calculation model remained unchanged to the

corresponding buildings construction phase, as it was only defined and

dimensioned the new elements that composed both structures. In terms

of geological setting and property characterization, the geomechanical

parameters adopted for ground modelling were identical to those used in

the previous solutions analysis, characterized by the Hardening Soil

constitutive model, but considering the changes made during the back

analysis in At1 layer.

The phase calculations were adopted to the solutions preconized, and

after that, it was possible to observe the total displacements on ground,

presented in Figure 20.

By analysing the obtained results, it was verified the ultimate limit state

and the serviceability limit state of both structures, according to (2) and

(13). As regards to the proposed solution security stability verification, it

was considered the methodology discussed in EC7 (3), and only the FEM

analysis. It was obtained an FS with a value of 4, which shows the

suitability of the alternative solution related with instability mitigation. It

was also denoted that the buildings deformed configuration remained the

clockwise rotation in the highest slopes declination, face the considerable

excavations occurred. Technical and economic analysis results

The analysed solutions have a substantially different characteristics and

guiding principles, not only, in technical terms, but also, in their

implementation and constructive suitability. Still, after each individual

study, it could be inferred that all solutions guarantee slopes stability,

since it was ensured the security considering EC7 (2), although with some

reserves.

The first solution preconized promoted only the superficial slope

stabilization, and it was not efficient, since it didn’t act on the rupture line

at greater depths, and further enhancing the sliding of material over time

due to its weight. The proposed solution, with robust containment

structures, would lead to high maintenance and exploitation, and an

associated strict quality control require with his performing, because of

the poor buildings security conditions. Yet, this solution presents a greater

Figure 19: Alternative solution

In

itia

l mo

del

ling

C

han

ged

mo

del

ling

After the geogrid reinforced embankments fill and before the immediate intervention

Between the immediate intervention and the final

solution

After the final

solution

Figure 20: Total displacements for the alternative solution (máx 23 mm)

Bored piles curtain Φ600//1,2m; L=22m

Metallic curtain profiles HEB 180//2m in quinquoncio

R. C. slab

2 micropiles levels Φ127//3m; L=16m Sealing bulb L=5m

i=15º

Figure 18: Deformed mesh in initial and changed modelling (Amplified 100 times)

10

safety from the first one executed, demonstrating its effectiveness. The

final stabilization solution that was realized face the inefficiency of the first

implemented

The economic analysis of constructive solutions was performed in an

approximate way and with some simplification, given their disparity

between and the three-dimensional component. So, it was took into

account an influence area of slope instability, considering the longitudinal

development of buildings blocks, and it was taken as a reference the

critical section analysed previously. In Figure 21 it is presented a

comparative chart of the economic value between all the solutions.

Comparing with the geogrid reinforcement embankment, the final solution

had a highest total cost associated. That was considered probable

because, not only, the slope intervention area had a greater increase over

the first stabilizing solution made, and also, it depended on the actual site

conditions founded, which had a great uncertainty associated. The

alternative solution composed with more robust elements was the most

economically unfavourable, as it was constituted by structural elements

most onerous, which explains the much more significant total final cost.

It was noted that the total monetary value from the set of the two solutions

made is less expensive regarding to the alternative total solution. Given

this diminutive disparity, it could be said that the proposal solution, more

costly and with higher maintenance, but with a direct resolution of the

slopes instability phenomenon problems, would be justifiable relatively to

the material and social costs associated with building the pre-collapse,

particularly as regards to peoples resettlement and indemnification, as

also to the buildings rehabilitee or even its demolition. Conclusions

The instability phenomenon analysis carried out in this study has shown

and demonstrate the causes and the resulting effects experienced in

slope, and consequently in the buildings located at his base, with a

relative assertiveness.

It was carried out that the MEF allowed an assertive results for reaching

out slope stability for more complex geometries.

In a case of this nature, and the implications as regards to the risk of life

and sensitive structures, the implementation of a Plan of Instrumentation

and Monitoring, complete and accurate, and a Plan of Geological and

Geotechnical Prospection, with the detailed definition of the geotechnical

layers and laboratory testing areas for measurement of its parameters,

would be an advantage to prevent and anticipate the events that took

place in the slope, and interpret in real time the geotechnical behaviour of

the ground, by his readings, promoting the safety of structures located at

its base and avoiding extreme consequences, which can last for a

significate time. As such, the definition of these plans mentioned should

be seen as an investment, with proactive and advantageous results,

rather than an additional cost to lead. In this aspect, a correctly specified

urban planning for the various regions would also be essential to perform

a pro-active management of the construction areas and to prevent cases

as this study. It must also be said that the illegal material deposition on no

appropriated and register areas for that proposed should be controlled,

despite the difficulty of this action.

Although it is considered the most critical section in a 2D analysis, the

problems of slope stability analysis have a strongly three-dimensional

component, such as in the case study, which constitutes a limitation of

these evaluations, since the probability of occurrence of a single mass of

unstable soil is very high. It was took into consideration the fact that the

program increase significantly the stress relief of soil, principally face the

decompression by excavations, which was an inherent limitation to the

case study, because all the solutions considered had this type of phase

in their construction.

Furthermore, the critical slip surface associated with the lower FS

obtained may not correspond to the most unfavourable situation, as

observed in the present case, since it was obtained other rupture lines

instead of the located between the deeper layers with different properties,

which demonstrates the importance of conducting an assessment of the

problem taking into account all the possible situations.

Particularizing the case study, it was concluded that the buildings

improperly constructed on the slope basis, although in a first instance had

driven the slope instability phenomenon, subsequently, acted as a

stabilizing element of the slope, which should not be the major objective

of an edification.

In this context, the redundancy of performing a work by infeasibility of

other previously executed, demonstrates the importance of conducting a

reliable analysis of the situation and intervention sites, especially with

regard to stability ratings, in order to cover all possible consequences and

to promote a greater economic and material properties. Also the assertive

analysis and choice of multiple stability solutions an instability problem

must be made to ensure adequate control on the surrounding structures,

in safe and economic conditions.

The present course of the last stabilizing solution, had shown the inherent

the perception of the various conditions that may be encountered on-site,

and the importance of the monitoring of projects to achieve the

requirement and quality executions over the various construction

processes associated, in order to optimize the solutions applied.

References

1. Aryal, K. P. (2006). Slope stability evaluations by limit equilibrium and

finite element methods. Dissertação de Doutoramento. Norwegian

University of Science and Technology.

2. Normalização, C.E. (2010). Eurocódigo 7: Projeto geotécnico - Parte

1: Regras gerais.

3. Abramson, L. W., et al. Slope Stability and Stabilization Methods.

Second edition, publicado por John Wiley & Sons, Inc.

4. Maranha das Neves, E. (1990). Methods of soil slope stability.

Constrains of the limit equilibrium methods for natural slopes.

Apresentado em European School of Climatology and Natural Hazards

Course, pp. 83-99.

5. Krahn, J. (2003). The 2001 R.M. Hardy Lecture. The limits of limit

equilibrium analysis. Canadian Geotechnical Journal, Vol. 40, pp. 643-

660.

6. Ortigao, J. A. R. and (2004), Sayao A. S. F. J. Handbook of Slope

Stabilization. Publicado por Springer.

7. Google, (s.d.)(2013). Obtido em 15 de Outubro de 2013.

8. LNEC, Laboratório Nacional de Engenharia Civil (2011). Relatório

307/2011 - Análise do processo de instabilização da Encosta do Monte

Gordo (na zona da Quinta de Santo Amaro) e dos aspetos geotécnicos

no comportamento dos edifícios do bloco B.

9. CENOR, Consulting Engineers (2013). Projeto de execução -

Estabilização e Consolidação do Talude na zona da Quinta de Santo

Amaro, Encosta do Monte Gordo (Vila Franca de Xira).

10. CENOR, Consulting Engineers(2013). Relatório de caracterização

geológica e geotécnica do talude situado na Encosta do Monte Gordo,

Vila Franca de Xira.

11. Plaxis, Reference Manual, (2012). Plaxis Reference Manual v.8.

12. Boscardin, M. and Walker, M. (1998). Ground Movement, Building

Response and Protective Measures. American Society of Civil Engineers.

13. Normalização, C. E. (2010). Eurocódigo 2: Projeto de estruturas de

betão - Parte 1-1: Regras gerais e regras para edifícios.

Figure 21: Comparing alternatives

Geogrid

Reinforcement

Embankment

493 902 €

Final

Solution

557 049 €

Alternative

Solution

1 189 805 €

Both of the

realized solutions

47%

Alternative

Solution

53%