Intervention Works Conducted to Ensure the Stability of a Slope: A Sustainability Study
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1. Introduction
In the specialized literature specific to the field of civil engineering, there are numerous standards dealing with structural safety, as well as various studies and models for assessing their sustainability, most of which are specific to new constructions and are developed during the design phase. Achieving the desired level of ensuring a sustainable environment on a global scale requires taking the best decisions to protect the environment through the rational and productive use of economic resources, all while meeting society’s current needs without affecting future generations who will benefit from them directly. In this sense, the researchers’ attention must be directed to the old existing structures, which may or may not present some structural, aesthetic, or energetic vulnerabilities due to the age of the materials and equipment used, in order to meet the needs of the present in terms of their safety and exploitation. However, special attention needs to be given to the land under the structure in question, both in the case of new or old structures, known in geotechnical engineering as the foundation soil. This natural resource is indispensable for both structures, being the most ancient building material, to which the choice of its resistance and stability characteristics is not an option, only their improvement through various mechanical or chemical technological processes but which inevitably result in higher costs. In the last decade, with the understanding worldwide that it is vital in all fields of activity to find and apply effective solutions to reduce emissions, the concept has been extended to the total elimination of embodied energy consumption and greenhouse gas emissions into the atmosphere resulting from the consumption of building materials across all industries, starting with tracking the manufacturing process, transportation, and the equipment used for putting them into operation, and real interest has started to appear for the sustainability study of the soil layers in the construction field for railways, tunnels, dams, roads, and highways. Nevertheless, there is still a big gap in the specialized literature that directly targets the sustainability of soil foundations and the possible intervention works that must be conducted on them.
This paper carries out a sustainability investigation focused on four intervention works to ensure the stability of a slope. The analysis was carried out applying the Bob–Dencsak specific model, which presents a series of advantages such as the method’s focus on all three factors associated with sustainability, having a wide range of applications and consisting only of quantifiable parameters. The main purpose is to compare different solutions, in order to determine which is most efficient from a sustainable perspective. Thus, two intervention works have been explored which involved soil reinforcement with a geogrid in the configuration of the slope 2:3 and 1:1, where the sustainability index was obtained as SI1 = 0.920, for the first mentioned case, and SI3 = 0.951 for the second case, and another two intervention works of reinforced concrete: a retaining wall with a height of 2.50 m situated at the base of the slope, for which a sustainability index SI2 = 0.779 was obtained; and the second retaining wall of 6.40 m in height, which shows the lowest value of the sustainability index at SI4 = 0.573. Following the final values in the slope sustainability analysis, we can assert that the reinforced soil retaining walls obtained the highest sustainability scores, being much more sustainable than the ones using reinforced concrete. This can be highlighted by comparing the reinforced soil configuration with a slope of 2:3 and that of the 2.5 m reinforced concrete retaining wall, where approximately the same amount of filling material was used. As a consequence, it turned out that the reinforced concrete’s embodied energy is only 2.69 times, while the GHG gas emissions are 7.40 times higher than those generated by geogrids, resulting in an 18% more sustainable solution than the version with a reinforced concrete retaining wall.
3. Case Study
3.1. A Brief Description of the Geographic Location
The geotechnical investigation revealed that the foundation soil is composed of a package of cohesive materials like clay—sandy loam, brown-yellow color, in a state of consistency from plastic to hard, located beneath a layer of vegetable soil varying in thickness from 10 to 30 cm.
Slope stability analysis was performed with the Geostru Slope application, with multiple tactics concerning the shape of failure surfaces, using the circular surface (the simplest shape). To avoid the situation of ultimate equilibrium, an acceptable safety level of 1.50 was proposed. The step search was set to 30, with a number of 30 strips in order to have a reasonable time period for the stability analysis. The partial coefficients for soil geotechnical parameters were considered at 1.25 for the angle tangent of internal friction and for the effective cohesion, respectively, 1.40 for the undrained cohesion.
3.2. The Intervention Works Analysed on the Slope
The stability factor’s analysis was conducted using the computational application with imposed surfaces in Geostru Slope, which is based on the Finite Element Method (FEM).
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Reinforced soil, inclination of slope 2:3;
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Retaining wall H = 2.50 m;
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Reinforced soil, inclination of slope 1:1;
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Retaining wall H = 6.40 m.
The quantities of materials required for performing the intervention works were estimated for a section of 5 m in length, following the profile with the highest level difference, measuring nearly eight meters.
Thus, the analysis started with the retaining wall made of reinforced soil with geogrilles in the geometric configuration with the smoothest slope of 2:3, where it is necessary to bring and compact 424 m3 of local soil. Biaxial geogrids with a specific weight of 0.284 kg/m2 were used in both reinforced soil configurations, being arranged in layers along the entire height of the retaining wall, at distances of 50 cm between them, consolidating each layer on a 50 m2 surface. The required quantity was established taking into account the design length of 10 m, which was supplemented with the overlapped length required to secure the geogrid to the layer above over a distance of one meter.
The reinforced concrete retaining walls were designed with the C20/25 concrete class and an S500 reinforcement mark. The retaining wall with a height of H = 2.50 m, located at the base of the slope, is characterized by an elevation width be = 60 cm, foundation height hf = 70 cm, and foundation width B = 1.80 m. The retaining wall’s elevation and foundation are reinforced with ∅12/10 cm bars on both directions, forming closed edges both in the longitudinal and cross sections, obtaining 745.75 kg of iron for the analyzed intervention work.
In the last intervention work, we have a retaining wall with a height of H = 6.40 m located very close to the ecological landfill’s access road, which has the following geometric dimensions: elevation width be = 80 cm, foundation height hf = 1.00 m, and foundation width B = 3.00 m. And in this case, closed edges are formed on both sections from bars ∅14/10, obtaining a total steel amount of 2512 kg.
With the exception of the filler material that was brought to the site from a distance of maximum 10 km away, all materials were delivered to the site from the nearest local construction supplies warehouse, positioned 30 km away. All materials were delivered in trucks that could transport between 3.5 and 20 tons, which have the following coefficients: embodied energy EE = 4.60 MJ/tkm and GHG gas emissions EC-CO2e = 0.28 kgCO2/tkm.
4. Results and Discussions
The results of the sustainability study were obtained using the Bob–Dencsak specific model, thus calculating all the parameters in question. The ecological dimension is represented by the consumption of embodied energies (En) and the total amount GHG gas emissions (G) in the process of manufacturing and transporting the materials used in the intervention works. In the results of the calculation, these factors are given equal weight, accounting together for 40% of the sustainability indices’ value. The economic dimension of sustainability is expressed through the labor (W) and material costs (C) required to complete these interventions works, which also represent 40% of the final result, divided equally among the parameters within the dimension. The safety factor (SF) expresses the social dimension of sustainability, assigning 20% of the final value of each intervention work within the sustainability study.
where the reference values are: EnR = 126170.49 MJ, GR = 7090.68 kgCO2, CR = EUR 7879, WR = 213 man × h, and .
Thus, after performing the calculations, the following sustainability index values were obtained: SI1 = 0.920 for the reinforced soil, with a slope inclination of 2:3, SI2 = 0.779 for the retaining wall with the height of 2.50 m, SI3 = 0.951 for the reinforced soil, with a slope inclination of 1:1, and SI4 = 0.573 for the retaining wall with the height of 6.40 m.
The initial objective of this project was to ensure the resistance and stability of a slope that serves as an access road to an ecological landfill, where the intervention works involve the use of local ground that is used as a filling material to support the road. Thus, the first intervention analyzed was a retaining wall with unreinforced soil in the most stable geometric configuration, with a slope of 2:3, where the safety factor was found to be less than the minimum acceptable by the current regulations. In order to obtain an acceptable safety factor, the base of the slope was increased by adding a berm of 2 m wide at the midpoint of the retaining wall’s height, but also with an unfavorable result in terms of the resistance and stability of the slope, obtaining a value that is less than the 1.50 minimum acceptable value, which is a mandatory requirement. Each of the analyzed variants fulfill the condition of resistance and stability, trying to achieve a high value for the safety factor, a fact that plays a major role in raising the social dimension.
In the version with the retaining wall reinforced with biaxial geogrids, with a slope of 2:3, a safety factor value of FS = 2.18 was obtained, which was used as the social criterion’s reference value when analyzing the sustainability of the intervention works. The 2.50 m-high reinforced concrete retaining wall placed at the base of the slope reduces its base by approximately 2 m, which leads to a greater storage capacity, but it strengthens the slope by increasing the base’s stiffness, obtaining a satisfactory safety factor FS = 2.05, which is comparable to the reinforced soil with a slope of 2:3. Due to the retaining wall’s small dimensions, as well as the difference in filling material required to perform the intervention work, the total cost of the materials is lower than in the case of reinforced soil with a slope of 2:3, but taking into account all the parameters it offers a 18% lower sustainability index.
Investigating an additional decrease in the slope’s base, which directly implies an overall reduction in the slope’s stiffness due to its lack of massiveness, the geometric configuration of the reinforced soil retaining wall with a slope of 1:1 was analyzed, where a lower safety factor value FS = 1.65 resulted. Although it has the lowest stability factor of all the configurations that were examined, this intervention work provides the reference values for the economic and ecological dimensions because it requires 58 m3 less filling material than the reinforced soil retaining wall with a 2:3 slope configuration.
The geometric configuration of the slope with the smallest base, which provides an adequate total stiffness due to the large amount of concrete and steel bars used, is the intervention work of the reinforced concrete retaining wall with a height of 6.40 m, which offers a more than acceptable stability factor of FS = 2.01. Due to the large volume of reinforced concrete, it recorded the highest values for the embodied energy En = 258373.21 MJ and GHG gas emissions G = 24338.62 kgCO2, as well as the cost of materials and labor.
5. Conclusions
After carrying out the sustainability study of the four intervention works, it has been clearly observed that the best value SI3 = 0.951 was obtained for the reinforced soil with a biaxial geogrid, with a slope of 1:1. Based on the parameters’ values that were determined for this intervention work, it is important to draw attention to the optimal balance between the materials’ energy consumption and gas emissions as well as their costs, including labor, as these represent the study’s reference values. All of this supports the result, including the fact that the slope’s geometrical configuration is stable, even if it recorded the lowest value of the safety factor, which is 10% higher than the minimum accepted value.
The second option is represented by the reinforced soil retaining wall with a slope of 2:3, with a sustainability index value of SI2 = 0.920. Compared to the version with a slope of 1:1, there were increases in energy consumption of 13.5% and in the amount of gas released into the atmosphere of about 14% due to the filling material that must be brought additionally in order to achieve the slope geometrical configuration.
The third intervention work option from the perspective of sustainability, with an index value of SI3 = 0.779, is represented by the reinforced concrete retaining wall with a height of 2.50 m placed at the slope base. Following the ecological dimension, if we make a comparison with the version of reinforced soil with a slope of 2:3, there are significant increases of 23% in energy consumption, respectively 66% for GHG gas emissions released into the atmosphere. This is clearly underlined by the material values in terms of embodied energy and GHG gas emissions, where for the geogrid the following values were obtained En = 21230.97 MJ and G = 859.88 kgCO2 and respectively for reinforced concrete En = 57064.24 MJ and G = 6362.05 kgCO2.
The lowest value of the sustainability index SI4 = 0.573 was obtained by the reinforced concrete retaining wall with a height of 6.40 m. This result is highlighted by the values obtained by all of the studied parameters, specific ecological and economic dimensions, due to the large volume of reinforced concrete required, which unavoidably raises the total cost of the intervention work. Even though the storage capacity of the ecological landfill was not a criterion, for further research it should be noted that this intervention work provides the smallest base of the slope which offers the biggest storage capacity. This aspect can also be taken into account in the case of intervention works to stabilize slopes that have a limited base for various reasons, such as the presence of railroads nearby, or any type of structure, or even just the simple existence of flowing water.
As a final conclusion, we can state that compared to the retaining walls made of reinforced concrete, it can be clearly seen that the reinforced soil intervention works obtained the highest scores from the sustainability point of view. This points out the fact that using geogrids for soil reinforcement is a much more efficient solution from an ecological perspective, following energy consumption and gas emissions, but also from an economic aspect, analyzing both the cost of the materials used and the labor, compared to the case of reinforced concrete as a construction material. Ensuring environmental sustainability is an admirable activity that civil engineers should definitely perform, not only in the preservation or renovation of existing structures but also in the design of future infrastructure. What engineers design and build today will have a long-term impact on the environment and society.
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