N115, Km 68+850 to Km 69+000, Near Bucelas – Slope Stabilization
Gonçalo David
Student, Instituto Superior Técnico, Av. Rovisco Pais, 1,1049-001 Lisbon, Portugal;
ABSTRACT:
This work studied the theme of Slope Stability, based on a certain set of written works on proper methods to apply in certain situations, such as in containment works. The study aimed, in particular, the implementation of a Slope stabilization work. Over six months, we had the opportunity to experience the course of a work from the beginning to the end, and all decisions and setbacks that have emerged derived from several factors, climatic and construction, among others. Over the course of the work, there has been two specific construction technologies in particular an implementation of a curtain of bored piles on the ground, supported by buttresses and other, related to the execution of micropiles, as passive reinforcement solution. The methodology used was through direct contact with the completion of the work By employing the computer software Plaxis, it was estimated deformation values and elaborated a comparative analysis. The results showed that accompanied intervention contributed to improving the stability on the slope, proven by comparative analysis of the displacements obtained in Plaxis, for interventions.
KEY-WORDS: Slope stability, bored piles wall, micro-piles, modeling.
1. INTRODUCTION
The population mobility nowadays has been of increasing importance to the governing organisms. The population growth along with the better life conditions, based on the each time more frequent everyday technological innovations made the families turn their concerns towards the comfort and well-being domains, which were unreachable before. Adding to that fact, the matter of an existing time pressure on today’s population, life style, leads to a big time spent on travels as well as a big percentage of the day life. This way, it’s important to maintain the roads pathways in best operation for its users. Thus, this study portrays the monitoring of the execution work for the stabilization of a road platform, in the proximity of Bucelas, between Km 68+850 and 69+000, through re-profiling of the slope, with the execution of a curtain of bored piles, complemented by reinforcements of the existing estabilization structures, using passive solutions, materialized by micro-piles. The same slope had already been target of interventions before, which did not fully solve the stability problem. A comparative analysis, based on estimated values through numeric modeling, was also made, in order to confirm the improvements imposed in the slope by the new solution, relative to the previous interventions.
2. CASE STUDY: N115, Km 68+850 TO Km 69+000, Near Bucelas – Slope Stability
2.1. GENERAL FRAMEWORK
The slope in study, is located near Bucelas, Loures, Portugal, on a national road, EN115, concerning the Km 68+850 to 69+000, as shown on . The job intends to re- profile the slope, in order to improve its overall stability.
EN115 N
ZONE IN STUDY
Figure 1 - Description of the zone being studied (GOOGLE MAPS)
The intervention on the slope consisted of a group of 3 proposed solutions, on which one of them was new construction and the other two, reinforcements of the existing structures. The new construction dictated the execution of a curtain of bored piles, with buttresses composed by HEB 160 profiles, both founded on the competent ground. Regarding the reinforcements, both using micro-piles, replacing the existing anchored structures, wherein the load verified a continued loss, as will be seen later on. On Figure 2 are represented, as plant, the localization of the solutions.
Estrada
Figure 2 - Description of the group of solutions to execute, as plant.
2.1. RESTRICTIONS 2.2.1 Geological and Geotechnical Nature
According to the geological investigation works, made over the various phases of the intervention on the slope, all of them were able to confirm that the existing ground in the zone affected by instability belonged to the Mesozoic Era, corresponding to the Late Jurassic, denominated by J4-5. The present ground was composed by marl and limestone, showed a stratification composed by the alternating of marl levels and marly clays, with limestone essentially marly. It was also observed that the solid limestone was fragmented by two families of fractures: one of which with sub-parallel direction to the stratification, but with opposed incline, and the other with a virtually perpendicular direction to the ones before. It was still possible to verify that the existence of the fractures allowed the percolation of water on them. The geotechnical characterization allowed to resume the following values which characterize the geotechnical zoning obtained, Table 1, and used to future analysis. The geotechnical zoning divided the soil in 4 distinct layers. The superficial layer constituted by landfills (ZG1), with approximately 1 meter thick, supported by a layer of clay (ZG2), of varying thickness, until around 7,5 meters deep. The solid competent and stable marly limestone (ZG4), was found under the layer of clay, its thickness being unknown. It was still possible to verify the insurgence of a layer composed by marl ZG3, of varying thickness between 0,5 to 1 meter, on the interface between the layer of clay and the solid marly limestone.
Table 1 - – Description of the parameters obtained in geotechnical zoning.
Aterros CLAY MARL MARLY LIMESTONE Constitutive Model Hardening Soil ZG1 ZG2 ZG3 ZG4 γ kN/m3 17 19 22 24
풓풆풇 2 푬ퟓퟎ kN/m 2000 12000 120000 8000000 c’ kN/m2 1 20 30 1500 φ’ ° 20 22 23 33
2.2.2 Engaging neighborhood
As mentioned earlier, the intervention of stabilization on the slope is bounded by EN115, as well as by existing structures that were executed on the previous projects along the sides and even a drainage network composed by drainage trenches. Therefore, the intervention to be executed on the slope accounted for all the existing structures, minimizing the impact on the structures and in the optimal functioning of the EN115, as observed on Figure 1.
2.2. BRIEF HISTORY
As mentioned before, the present case of study had already been target of two interventions, 1st and 2nd phases, which will be described then. At beginning of 2001, the slope in study registered significant movement, and even leading to the interruption of traffic on said road, as illustrated on Figure 3.
Figure 3 - Boundary of the zone where the incident occurred.
The main cause of the incident occurred was the water, which by infiltrating the terrain formed a flow and raised the interstitial pressures, lowering the effective tensions on the terrain, as well as the softening of the soils, leading to the diminishing of the resistance to the soil cut. On Figure 4, it is possible to observe the destabilizing action of the water on the slope.
Alrota Slope RIVER
Actual position of tree Original position of EN115 Original position of tree Actual position of EN115
Water Flow
Figure 4 - Triggering action of the water destabilization.
A set of solutions mainly of hydraulic character was executed, between 2001 and 2005, corresponding to the 1st and 2nd phases, constituted by anchored retaining structures, along the passage of the road platform, network drainage, constituted by trenches, of forwarding and capture of the water, and finally, a retaining structure formed by gabions, as seen on Figure 5.
Figure 5 - Executed solutions project of 1st and 2nd phases.
2.3 PROJECT 3RD PHASE
Taking into account the efficiency of the previous structures, corresponding to the 1st and 2nd phases considering the use of ground anchors and drainage trenches, since the main destabilizing factor was the water. Although, the present monitoring on the ground anchors, showed a continuous evolution on the loss of load, as Figure 6 proves, from the end of the intervention until November 2010.
LOAD (kN)
Figure 6 - Description of the evolution of load on the anchors.
This way, facing what was exposed before, the solution of intervention on the project of 3rd phase was restricted by not considering the ground anchors, since the solutions before did not show the ability to improve the system of slope stabilization.
Therefore, the solution of intervention proposed by the project of the 3rd phase, integrated the realization of a curtain of bored piles, of φ1000 diameter, with the centers spaced by 2 meters, with buttresses, both solutions founded on the competent ground ZG4. The curtain of bored piles was completed by the installation of gabion baskets on the space between the buttresses, as well as a drainage system, composed by vertical drains, and containment and water boilers. On the other hand, reinforcement solutions were still part of said intervention, using micro-piles founded on the competent ground (ZG4), of the anchored structures, which were still losing load. On Figure 7 are illustrated the solutions executed by the 3rd phase project.
Estrada
Figure 7 - Set of integrating solutions on 3rd phase.
2.5. MODELING
A numeric modeling analysis was also executed, with the intent of comparing the solutions imposed on the slope, by the projects of the 1st and 2nd phases and the 3rd phase, to confirm if there was any improvement imposed by the solution of the 3rd phase. The comparative analysis based on the displacement values of the road platform, estimated by the software Plaxis, in which were defined the geometry shown on Figure 8. The referred model is composed by the soil layers, parameterized according to the values on Table 1, and by the existing structures on the slope, executed over all the interventions of which it was target.
ZG1 ZG2
ZG4 ZG3
Figure 8 - Geometry of the model used on the numeric modeling.
Regarding the solution finalized by the intervention of 1st and 2nd phases, it was obtained a maximum displacement, on the road platform of around 3 cm. In this modeled solution, the effect of the ground anchors was not taked into account for since, as it was already mentioned, were losing load. This maximum displacement confirms that this stabilizing solution did not solve the ground stability problem of the slope. The deformed and the value of maximum displacement are shown on Figure 9.
Figure 9 - Obtained deformation at the end of the intervention of the 1st and 2nd phases. Scale amplified 100 times. Maximum displacement of 27,07E-3 m.
On the other hand, an analysis at the end of the intervention of the 3rd phase was also executed, so that the displacement values of the road platform could be compared. From this solution, the elements executed on the 3rd phase reinforcements of micro- piles in PA2 and the curtain of bored piles, with buttresses, lead to a very low value of maximum road platform displacement, as illustrated on Figure 10Erro! A origem da referência não foi encontrada..
Figure 10 - Obtained deformation at the end of the intervention of the 3rd phase. Scale amplified 100 times. Maximum displacement of 31.82E-6 m.
In order to obtain a better comparison of the values obtained, the Figure 11 represents the graphic that correspond to the evolution of the displacement values, for the 3 previous interventions.
Total Displacement 30.00
25.00
20.00
15.00 1st and 2nd phases 10.00 3rd phase Displacement [mm] 5.00
0.00 0 2 4 6 8 Time (step)
Figure 11 - Graphic relating the total displacements on the road platform. It is possible to observe on the previous figure, the solution imposed by the intervention of 1st and 2nd phases, presents an evolution of the displacements with tendency to increase, contrary to what happens with the solution of the 3rd phase, on which the displacements tend to stabilize. This way, it is possible to prove that the 3rd phase solution is effective on the improvement to the slope stabilization, mainly at the road platform level.
3. MAIN CONCLUSIONS
The numeric analysis proves that the solution executed at the 3rd phase imposed a significant improvement on the slope overall stability, proven by the significant difference between the displacements obtained by the modeling, to the interventions in study, and even adds the fact that the displacements on the road platform at the 3rd phase present tendency to stabilize in a very low value, contrary to what happens with the previous intervention (1st and 2nd phases).
Another important conclusion to be retrieved from this study is the fact that on the distinct interventions that the slope was target of, the main restriction that led to the intervention was different, being on 1st and 2nd phases the water action and on the 3rd phase, the continuous loss of load at the ground anchors.
It should also be pointed out, the fact that a good instrumentation and observation plan is an important element to manage the geotechnical risk, not only during the course of the job, but also extended to the post construction period, to be able to prevent new occurrences and take timely measures, minimizing the damage caused.
4. REFERENCES
David, G., EN115, Km 68+850 A Km 69+000, Proximidade De Bucelas – Reabilitação Do Sistema De Estabilização Da Encosta, Dissertaçao, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, 2014.