Journal of Environmental Management 341 (2023) 118020 Contents lists available at ScienceDirect Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman Research article Environmental and economic impacts of combining backfill materials for novel circular narrow trenches I. Josa a, A. Petit-Boix b, M.M. Casanovas-Rubio c, P. Pujadas d, e, A. de la Fuente f, * a Department of Civil, Environmental & Geomatic Engineering, University College London, WC1E 6BT, London, United Kingdom b Sostenipra, Institute of Environmental Science and Technology (ICTA), Unidad de Excelencia “María de Maeztu” (CEX 2019-0940-M), Universitat Autònoma de Barcelona (UAB), 08193, Bellaterra, Barcelona, Spain c Department of Management, Universitat Politècnica de Catalunya (BarcelonaTECH), 08028, Barcelona, Spain d Department of Project and Construction Engineering, Universitat Politècnica de Catalunya (BarcelonaTECH), 08028, Barcelona, Spain e Group of Construction Research and Innovation (GRIC), Universitat Politècnica de Catalunya (BarcelonaTECH), 08222, Terrassa, Barcelona, Spain f C3S, Department of Civil and Environmental Engineering, Universitat Politècnica de Catalunya (BarcelonaTECH), 08034, Barcelona, Spain A R T I C L E I N F O A B S T R A C T Keywords: Over the last few years, several policies and new technological solutions have targeted the construction sector Trench with the aim of reducing the sector’s impacts on the environment. Among the different technological advances Pipe proposed, the reuse of materials in construction has been reported as a promising solution for an increase in LCA sustainability and circularity. In particular, a type of cities’ undergrounds assets for which materials’ reuse is LCC Circular economy being explored are trenches for protecting services (i.e., water and gas transport pipelines, and optic fibre and other telecommunications services). Nonetheless, the economic and environmental benefits and impact of this type of system is still insufficiently quantified. In this research study, the economic and environmental impacts of four scenarios of trenches were assessed by using Life Cycle Costing (LCC) and Life Cycle Assessment (LCA). The four alternatives analysed consisted of: (1) the classical solution; (2) the classical solution with the reuse of soil; (3) the control low-strength material, and (4) the eco-trench. The results allowed concluding that in the eco- trench system, for which all material is reused, the environmental and economic impacts could be reduced by more than 80% and 50%, respectively. A parametric study for which the dimensions of the trenches were varied, permitted to reinforce these results and to quantify the impact’s change along with the width and depth of the trench. Overall, this study provides a comprehensive view of the high-impact potential of reusing material for the construction of trenches in cities. The outcomes allow also remarking that the eco-trench system could be an attractive and advantageous solution for urban infrastructure stakeholders, both from an economic and envi­ ronmental perspective. 1. Introduction housing, and transportation. Therefore, sustainable construction solu­ tions need to ensure that demands are met and, at the same time, that The construction industry is one of the most polluting sectors in the their environmental impacts are reduced (The World Bank, 2020). world, generating 38% of energy-related greenhouse gas emissions Among all the necessary infrastructure in cities, current centralized gas, (Giunta, 2020; Nagireddi et al., 2022; United Nations Environment water and wastewater services translate into thousands of kilometres of Programme, 2021). While decreasing the construction demand would underground pipelines in need of maintenance and upgrades due to directly entail a decrease in such environmental impacts, the trends in aging infrastructure and connections with new residential areas (World urbanisation growth indicate that more infrastructure will be needed in Economic Forum, 2014). the coming years. At present, 55% of the population lives in urban areas In order to limit interference with other services or traffic during (The World Bank, 2020), and by 2045 it is predicted that this figure will construction, some authors have proposed trenchless pipeline systems increase by 1.5 times. This implies decision-makers need to ensure that (Jung and Sinha, 2007; Kaushal et al., 2020). In spite of their social all citizens have access to basic services, such as water and sanitation, benefits, these systems can be costly, which is why narrow trenches are a * Corresponding author. E-mail address: albert.de.la.fuente@upc.edu (A. de la Fuente). https://doi.org/10.1016/j.jenvman.2023.118020 Received 19 December 2022; Received in revised form 30 March 2023; Accepted 24 April 2023 Available online 15 May 2023 0301-4797/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by- nc-nd/4.0/). I. Josa et al. Journal of Environmental Management 341 (2023) 118020 more common technique to install small-diameter flexible pipelines in and, consequently, the reuse of the soil being possible without cities. Similar to trenchless systems, this technique causes limited compromising the construction process efficiency and productivity or interference during construction. However, they generate large amounts the performance of the pavement. Moreover, this solution allows of trench arisings. Once the trench is excavated, backfill material is used avoiding the final layer of asphalt, which entails an additional advan­ to fill up the trench and to support the pipe and surface elements. tage in terms of execution (no overlaps are required), logistics, and Nevertheless, the extracted material does not necessarily meet the schedule. quality and mechanical requirements to be used as pipeline support or To confirm whether these designs result in reduced environmental embedding material. For this reason, there exist several trench designs impacts and economic costs, previous research has applied a life cycle that ensure the functionality and protection of the pipeline under perspective to the system to shed light on hotspots from raw material varying conditions, which comes at different environmental and eco­ extraction to the end of life. These studies have especially focused on nomic costs due to the nature of the selected materials (e.g., Petit-Boix assessing pipe materials using life cycle assessment (LCA) (Petit-Boix et al., 2016a, 2018a). How to select suitable trench designs and backfill et al., 2016b), but no research has been yet done to assess the backfill materials is still a critical question that needs to be further aligned with material. To assess the degree of sustainability of existing trench de­ current environmental policies and concepts. For instance, integrating a signs, Casanovas-Rubio et al. (2019) developed the sustainability index circular economy perspective into urban planning can help take for trenches (SIT) based on the multi-criteria method MIVES (Pujadas advantage of current urban trends in circular construction and imple­ et al., 2017, 2019; Roigé et al., 2020) and applied it to a case study. Their ment overarching principles such as reuse, lifetime extension and analysis highlighted that there exist differences between the types of eco-design (Ghisellini et al., 2016; Kirchherr et al., 2017; Petit-Boix and trenches regarding the economic and environmental aspects. However, Leipold, 2018). To what extent novel designs with circular backfill the social impacts were very similar for all the alternatives. Thus, the materials reduce the environmental and economic impacts of cities re­ study presented herein focuses on the economic and environmental di­ mains to be seen, which demands a first step toward exploring common mensions. Considering the sheer volume of material and, thus, the designs and their alternatives. impact of the backfilled material in narrow trenches, scarce references The classical solution (CS) for a trench uses common soils and can be found in the technical literature regarding the environmental manufactured unbound granular materials as backfill and two top impact associated with resource consumption of these backfill materials layers, one of concrete and one of bituminous asphalt to restore the alternatives. Therefore, there is a clear need for determining the tech­ pavement surface (Blanco et al., 2014). However, several common de­ nically feasible configurations that result in an optimization of resources ficiencies may arise with this method (Casanovas-Rubio et al., 2019; and, consequently, in minimizing the life cycle environmental impacts Petit-Boix et al., 2016b). A poor selection of soils or inadequate and economic costs of the system. compaction may lead to settlements and other problems during the In light of the above, the main goal of this research is to quantify, service life of the conventional trench, such as critical load concentra­ analyse, and compare the environmental impacts and the economic tions in the pipe and pavement subsidence (Blanco et al., 2014). In this costs of equivalent backfill material alternatives that integrate circu­ context, the reuse of extracted soil is limited due to the difficulties larity principles by using LCA and life cycle costing (LCC); also, the study related to proper selection and separation and the risk of future aims at analysing the eco-efficiency of the alternatives by comparing the problems. environmental burdens and economic cost of backfills. By doing so, Alternatively, instead of compacted soil, the backfill used in narrow specific design conclusions and recommendations are defined, these trenches can be a cementitious material (i.e., made of a binder, aggre­ being meant to be useful for urban planners and urban assets managers. gates, water, and admixtures) known as controlled low-strength mate­ rial (CLSM) (Alizadeh, 2018; Blanco et al., 2014; Etxeberria et al., 2013; 2. Materials and methods Pujadas et al., 2015; Wu et al., 2016). This material is highly fluid to allow the filling of tight and restricted areas in which placing and This study used LCA and LCC for four types of trenches to identify compaction would otherwise be difficult; however, it is not excessively their environmental impacts and their representative costs during their fluid so that the material may remain in place in trenches with a slope. lifetime. LCA is a methodology to assess the environmental impacts of However, the above-mentioned solutions, the CS and the CLSM each process involved in the life cycle of a product or service from a trenches lead to significant raw material consumption and CO2 emis­ systems perspective as defined in the ISO 14040 and 14044 standards sions associated with material transportation among other environ­ (International Organization for Standardization [ISO], 2006a, 2006b). mental, economic, and social impacts. To overcome these drawbacks Similarly, LCC is a method for economic analysis that allows and promote the reuse of extracted soil, it may be necessary to change assessing the total costs of a system over its whole life span or over the the consolidated design and construction philosophy behind the narrow time during which the service is provided. Thus, an LCC involves an trench system. In this sense, Blanco et al. (2017) presented a more evaluation of costs such as purchase price and associated costs (e.g., sustainable solution, called eco-trench (ECO), which is based on the transport, installation), operating costs (e.g., energy, water use, main­ re-use of soil. tenance), and end-of-life costs (e.g., disposal). The framework for LCC The idea of reusing trench arisings has already been considered in the can be found in ISO 15686–5 (International Organization for Stan­ UK, where 100 million tonnes of this material are generated per year dardization [ISO], 2017), which includes the main principles, processes, (Maqbool and Amaechi, 2022) and several pilot experiences have been calculations, and definitions. carried out (Edwards et al., 2007). The so-called ECO trench consists in The following sections describe the material and methods utilised in re-using the trench arising as the main backfill and finishing the trench the LCA and LCC. The sections present the standard phases of an LCA, with a top layer of expansive concrete. This concrete contains a calcium including the definition of goal and scope, the life cycle inventory, the oxide admixture that generates a volumetric expansion during early ages life cycle impact assessment, and the interpretation. that reduces the likelihood of cracking due to shrinkage. Given its properties, this admixture has been commonly used to compensate 2.1. Goal and scope definition shrinkage. Nevertheless, in the case of trenches, not only is the volu­ metric expansion interesting but also the level of internal stresses The goal of this study was to assess and compare the environmental generated when the concrete is confined. This may favour the trans­ impacts of four types of trenches used to install flexible pipelines of small mission of stresses from traffic loads to the surrounding soil, thus, this diameter for the construction of utility services networks. In LCA relieving the stresses on the backfill material and the pipe. Conse­ studies, it is necessary to define a functional unit (FU), which is the quently, the restrictions related to the backfill selection are eliminated, reference unit that is used to quantify the performance of the product 2 I. Josa et al. Journal of Environmental Management 341 (2023) 118020 system. It is common practice to define the FU of systems involving pipes is particularly relevant in this study, as its impact has been seen to be in unit pipeline length (see, for instance, Du et al., 2013; Vahidi et al., relatively important in other studies (Hajibabaei et al., 2020). The 2016). Thus, the FU for this study is 1 linear metre of trench to install a maintenance of the trenches was considered to be negligible. The pipe with a diameter of 110 mm. Narrow trenches for the installation of nominal diameter of the pipe was considered to be 110 mm and remains flexible pipelines of small diameter are commonly used for the con­ the same for all the sizes of the trenches. However, the price and envi­ struction of utility services networks to generate minimum interferences ronmental impact of the pipe were not included in the analysis. Each with other urban services. In particular, this technique is frequently used design has its own particularities when managing backfill materials with natural gas pipelines in cities or small villages. (Casanovas-Rubio et al., 2019). The equivalent designs include (1) a classical solution (CS), (2) a classical solution with reuse (CS + R), (3) a controlled low-strength • Classical solution (CS). The excavated material is carried to a material (CLSM), and (4) an eco-trench (ECO) (Fig. 1). These solutions landfill. Afterwards, the trench is filled with a new filling material. were selected based on their relevance and designed to align with pre­ On top of the new material, a concrete layer is placed first. Then, vious studies to allow for comparability. In particular, the configuration 0.05 m of concrete is removed using a milling cutter. An extra width of the four types of trenches matches that of the trenches analysed in of 0.20 m at each side of the trench is also removed to allow for Casanovas-Rubio et al. (2019). An average lifespan of 50 years was overlapping with the rest of the pavement. The final step in the assumed in all designs, which were conceived in the state of develop­ production involves placing an asphalt layer of 0.05 m thick as ment of Mediterranean cities, specifically in Catalonia (Spain). wearing course. The LCA and LCC consist of equivalent system boundaries (Fig. 2) to • Classical solution with reuse (CS þ R). This solution is similar to enable a systemic integration of the environmental and economic im­ CS. The only difference is that, instead of using new filling material, pacts. The processes included are: (1) the trench excavation with a the soil that is extracted is then reused on-site. trencher considering five different types of ground, which applies to all • Controlled low-strength material (CLSM). The filling consists of a solutions; (2) filling of the trench with new or reused material; (3) load fluid mortar of low compressive strength for the whole height of the and transport of the waste from excavation to an authorised landfill; (4) trench (Blanco et al., 2014; Pujadas et al., 2015). After placing this compaction of the soil, and (5) surface finishing, which is different for material, 0.05 m of the top are removed using a milling cutter. An each alternative. All the trenches were considered to be located in an extra width of 0.20 m at each side of the trench is also removed to asphalt carriageway, which is a common situation in the urban areas allow for overlapping with the rest of the pavement. Finally, an where narrow trenches are usually located. The consideration of asphalt asphalt layer of 0.05 m thick as wearing course is placed. Fig. 1. The four types of trenches considered in the analysis. 3 I. Josa et al. Journal of Environmental Management 341 (2023) 118020 Fig. 2. System boundaries of the scenarios considered. • Eco-trench (ECO). The soil is excavated, and the material extracted and pigments used. Further information on the ECO can be found in is reused up until 0.15 m before the top. The top is then filled with a (Blanco et al., 2017). slightly expansive concrete. This concrete has a black pigment that allows obtaining the same colour as the rest of the asphalt pavement. In the study by Casanovas-Rubio et al. (2019), a single dimension In this solution, it is not necessary (neither for aesthetic or technical was considered for the trenches (i.e., 0.15 m width and 0.75 m depth). reasons) to add an extra width of 0.20 m at each side for an asphalt Nonetheless, in this study, different widths and depths were considered layer or overlapping due to the expansive properties of the concrete in order to analyse how results change from a parametric perspective. In 4 I. Josa et al. Journal of Environmental Management 341 (2023) 118020 particular, the studied dimensions are four widths (0.15, 0.20, 0.30, and including the bulking. For the ECO and CS + R, the volume that is 0.40 m), and nine depths (0.75, 0.80, 0.90, 1.00, 1.10, 1.20, 1.30, 1.40, transported to landfill is lower, given that part of it is reused in the and 1.50 m). This results in a total of 36 different trench dimensions, trench. The specific proportion that is taken to landfill for different di­ where the smallest one measures 0.15 m × 0.75 m, and the biggest one mensions of the trench can be found in the Supplementary material. measures 0.40 m × 1.50 m. In the ECO trench, the lime used was Link EVR, white F2000 and F2000-Ad from Cales de Pachs S.A. The used pigment was a micronized 2.2. Life cycle inventory (LCI) black iron oxide produced by a special process that provides a higher dispersability and brightness (NB-5970 model from Nubiola). The The LCI process involves developing an inventory containing the pigment is dusted on the surface with an amount of 50 g of pigment/m2 data corresponding to the input and output flows for the product system. of surface. In this study, data for the inventory was collected from local construc­ tion companies, construction databases and the life cycle database ecoinvent 3.3 (Ecoinvent, 2017) for background data. The public data­ 2.3. Impact and cost assessment base BEDEC, developed by the Catalan Institute of Construction Tech­ nology (ITeC), was used to obtain detailed information on the processes The life cycle impact assessment stage is the step where the impacts and materials for each of the four trenches (ITeC, 2019). This database are evaluated based on the LCI data. This impact assessment was carried provides technical and monetary information for a high number of el­ out to calculate the environmental impacts of each design. Such ements used in the construction sector. The reference year for the LCI assessment was performed using the ReCiPe (H) method (Huijbregts was 2019. et al., 2016) and the OpenLCA 1.9.0 software. Based on the dimensions modelled in each trench design, we created The LCA includes the mandatory classification and characterisation the LCI using the unitary processes included in the BEDEC database, steps. The following impact categories were selected: Global warming, which include material amounts and composition, energy consumption Stratospheric ozone depletion, Terrestrial acidification, Marine eutro­ of construction equipment and unitary prices for labour and phication, Freshwater eutrophication, Photochemical ozone formation, infrastructure. Mineral resource scarcity, Fossil resource scarcity, and Water use. These As for the transportation distances, we assumed 30 km for the impact categories represent the core environmental impact indicators of transport of backfill materials to the construction site and 10 km for the the standard EN 15804 + A2 (European Standards [EN], 2019), and they transport of waste soil to the landfill (Petit-Boix et al., 2014, 2016a). were selected based on their relevance to the cases studied and to allow The economic costs included were the direct costs (e.g., labour, for comparability with other similar studies. None of them was dis­ materials, plant, and auxiliary costs), whereas the indirect costs, over­ carded to avoid burden shifting. heads and industrial profit were excluded. In particular, the costs The cost analysis focuses on the total cost of the infrastructure. As the included were the ones corresponding to trench excavation; loading, operation and maintenance are excluded from the analysis, the costs are transportation, and controlled disposal of the waste to landfill; necessary reported for the entire lifespan of the system (i.e., 50 years). Given this backfill material; compacting; and surface finishing. The necessary la­ exclusive focus on direct costs of construction, the authors chose not to bour and energy were included in the prices used. A breakdown of these estimate the time-bound present value and annual equivalent costs. costs can be found in the Supplementary material. Excavation and backfill processes demand additional specifications. 2.4. Interpretation The price used for the excavation with the trencher was provided by Calaf Trenching S.L. for a trench of 0.35 m × 1.10 m. The excavation In the last step of the LCA, the information from the results of the LCI costs for other dimensions were calculated proportionally according to and the life cycle impact assessment are evaluated, and conclusions are the trench dimensions. Additionally, these costs were calculated extracted. In this study, the interpretation of the results was performed considering five different types of soils that are described in the Spanish primarily using the results for the two extreme dimensions. Namely, the standard (Ministerio de fomento, 2019), namely soft soil, compact soil, smallest (0.15 m × 0.75 m) and the largest (0.4 m × 1.5 m) sections were soft rock, medium rock, and hard rock. The considered bulking factors used to obtain an initial understanding of the results. for the excavation were defined as follows: 1.20 for soft soil, 1.30 for Some authors have highlighted that analyses in the context of un­ compact soil and soft rock, and 1.40 for medium and hard rock. derground infrastructure are prone to be affected by high uncertainties. Regarding the compacted soils, three different types of soil had to be Thus, in this study the data collection focused on several different di­ considered. The Spanish Government defines the following three types mensions and soil types instead of a single one, which was useful to of soil: selected, suitable, and tolerable (Gobierno de Espana, 2017). understand how results change with different assumptions (Hajibabaei Note that this definition and the specifications of each group may vary in et al., 2020). In particular, sensitivity analyses were performed on the other countries. This definition is relevant to this study because the cost alternative scenarios by changing the dimensions as described above. corresponding to compacting changes depending on the type of soil. Results from the sensitivity analyses were used to understand to what Given the five and three classifications for excavation and compac­ extent the main system parameters and assumptions may influence the tion respectively, the cost per linear metre of each type of trench was outcomes from the LCA and the conclusions drawn. calculated for each one of the 15 combinations of excavated ground and backfill material. In particular, the costs for each dimension were calculated, and then an (unweighted) arithmetic mean of the cost of the 2.5. Eco-efficiency assessment 15 combinations of excavated ground and backfill material was calcu­ lated for each type of trench. Using an unweighted mean carries the The eco-efficiency of the alternatives was assessed by combining the consequence that the cost utilised for the study does not consider the evaluation of the environmental and economic impacts. The assessment frequency of appearance of each combination of excavated ground and was performed following ISO 14045 (International Organization for backfill material. In the end, the process above yielded a total of 45 Standardization [ISO], 2012), which describes the principles and re­ tables, one for each size of the trench. Note that the CLSM trench has quirements for this type of analysis. mortar as filling and, thus, the classification of backfill material did not In this study, the same approach as in (Petit-Boix et al., 2018b) was apply. taken. Namely, the results of LCC were used to assess the monetary value Regarding the prices related to the landfill, it was considered that, for of the alternatives, and they were compared to the environmental the CLSM and CS, the waste corresponds to the whole volume excavated dimension by using a graph. 5 I. Josa et al. Journal of Environmental Management 341 (2023) 118020 3. Results and discussion The results in this work are in accordance with previous research on environmental impacts of pipe and road infrastructure. Moretti et al. In this section, the environmental and economic performance of the (2018) found that using low-impact procedures and using secondary raw trenches is presented in Sections 3.1 and 3.2, respectively. In each of the materials provide better overall environmental results in all impact sections, a sensitivity analysis to assess the difference in results when categories considered. Fathollahi and Coupe (2021) also found that varying the dimensions of the trenches is presented. An analysis of the lower necessary material requirements for the construction resulted in eco-efficiency is provided in Section 3.3. lower impacts in several stages of the life cycle. Petit-Boix et al. (2015) evaluated the environmental impacts of a sewer construction and found 3.1. Environmental life cycle assessment that using concrete as bedding material can have a significant envi­ ronmental impact during construction. Therefore, the selection of ma­ 3.1.1. Results of the LCA terials has an important effect on the final environmental consequences. The potential environmental impacts associated with the classical Several studies have discussed in the past the proportional impact solution (CS), classical solution with reuse (CS + R), controlled low- that the transportation phase has on the final environmental impacts. In strength material (CLSM), and the eco-trench (ECO) are shown in Fig. 3. this study, transportation had a low contribution to overall environ­ Comparing the alternative trenches, the results indicate that the eco- mental impacts (accounting for less than 4% in all impact categories, trench (ECO) has much lower environmental impacts (between 80% and except for fossil resource scarcity, where the impacts were around 7% of 97% less) than the other three solutions in all impact categories. This is the total impact). This aligns well with the results of Morera et al. directly related to two aspects. First, fewer new materials are needed for (2016), which showed that the impact from transport accounted only for this alternative, given that almost all the filling is reused except for the less than 4% of the total impacts; and of those in Hajibabaei et al. (2018), expansive concrete. This alternative avoids using asphalt, which con­ who found that the transportation stage represents, in average, less than tributes to the highest impacts in almost all impact categories and all 20% of the impacts. alternatives (Giunta et al., 2020; Praticò et al., 2020). Second, a lower use of new materials is linked to less energy and equipment needed for 3.1.2. Sensitivity analysis the stages of filling, compacting, and finishing. The results shown in the previous section corresponded to the two For the ECO trench, the backfill material is the main contributor to extreme cases of trench sections analysed. Fig. 4 shows the potential the total impact for impact categories Global warming, Marine eutro­ environmental impacts for the impact category Global warming. Note phication, Freshwater eutrophication, Mineral resource scarcity, and that similar graphs for the remaining impact categories can be found in Water consumption. In these impact categories, the process involving the Supplementary material. the backfill material represents between 54 and 90% of the total impact As the results of the smallest and largest trenches showed, the ECO (with a mean of 79% and 73%, and a standard deviation of 8% and 10% still shows the lowest environmental impacts for all the dimensions for the trenches with dimensions 0.15 m × 0.75 m and 0.4 m × 1.5 m, analysed, even for the largest sections. Regarding the highest environ­ respectively). mental impacts, these are achieved by the larger sections of the CLSM. Energy is the main contributor to the overall impacts for impact Below sections of 0.3 m2, some of the CS and CS + R alternatives have categories Stratospheric ozone depletion, Acidification, Ozone forma­ higher impacts than the CLSM. tion, and Fossil resource scarcity. Nonetheless, in this case, the contri­ In addition to the above, it can be observed that results for ECO are bution of each of the stages is not as extremely distributed as in the case less spread than those of CLSM (σECO = 6.02, σCLSM = 36.41). described above. In these impact categories, the diesel makes up be­ Besides, the results of CS and CS + R show similar spreadness (σCS = tween 36.7 and 71.3% of the total impact (with a mean of 46% and 57%, 23.81, σCS+R = 21.70). This indicates that the impact of reusing part of and a standard deviation of 9.2% and 8.9% for the trenches with di­ the excavated material does not bring significant benefits in Global mensions 0.15 m × 0.75 m and 0.4 m × 1.5 m, respectively). warming terms. System CLSM has the highest impacts in 7 out of 9 impact categories (i.e., Global warming, Stratospheric ozone depletion, Terrestrial acidi­ 3.2. Life cycle costs fication, Freshwater eutrophication, Photochemical ozone formation, Fossil resource scarcity, and Water use) in the case of the small trench, 3.2.1. Results of the LCC whereas the impacts are the highest in 8 out of 9 impact categories for The LCC results for the smallest and largest trench alternatives (0.15 the bigger trench (i.e., for Marine eutrophication as well). m × 0.75 m and 0.4 m × 0.15 m, respectively) are presented in Fig. 5. In this trench system, the finishing process (which involves the use of They have been broken up into representative life cycle stages, including asphalt) is responsible for the highest proportion of the impacts. More excavation, filling, compacting, finishing and end of life. specifically, for the case of the 0.15 m × 0.75 m trench, between 78.47% In the case of the smaller trenches (0.15 m × 0.75 m), the process of and 95.89% (average of 88.03% and standard deviation of 5.28%) of the finishing represents the largest cost in the CS, CS + R and CLSM solu­ total impact is given by this process, whereas for the 0.4 m × 1.5 m, it is tions (corresponding to a 52%, 63%, and 37% of the total cost respec­ between 48.85% and 86.71% (average of 69.71% and standard devia­ tively). This is not the case of the ECO, where the costs of the different tion of 10.89%) of the total impact. This is supported by the findings in stages are evenly distributed, from 19.14% to 32.54% of the total cost. Giunta et al. (2020) and Praticò et al. (2020). This is mostly because the finishing of the former three solutions re­ The trench system CS has the highest impacts in two impact cate­ quires extending a 20 cm layer to ensure adherence to the pavement. gories, namely Mineral resource scarcity and Marine eutrophication. In the case of CS, CS + R, and ECO, proportionally, the lowest costs Regarding Mineral resource scarcity, while the amount of asphalt (and correspond to the filling (less than 2.2%). Particularly, for CS + R and therefore its corresponding environmental impact) for CS, CS + R and ECO, this is because the necessary filling material is reused and thus no CLSM is similar, the impact of the backfill material for CS is higher. This costs are attributed to the filling stages. is because CS uses concrete, which is linked to high impacts in terms of Differently from the above three solutions, the filling for CLSM cor­ minerals depletion. responds to 25% of the total costs, given that the material used to fill the Regarding Marine eutrophication, the impacts are highest for CS trench has certain strength requirements that increase its corresponding when considering dimensions 0.15 m × 0.75 m, and for CLSM when price. The lowest cost in the case of CLSM corresponds to the compacting considering dimensions 0.4 m × 1.5 m. Nonetheless, the difference be­ process due to the fact that no compacting process is necessary. tween CS, CS + R and CLSM in this impact category are very low (less Looking at the overall costs, the alternative with the highest costs is than 5% and 10% for the smaller and bigger sections, respectively). the CLSM. Nonetheless, the second most expensive alternative is CS, 6 I. Josa et al. Journal of Environmental Management 341 (2023) 118020 Fig. 3. Potential environmental impacts for the four alternatives (CS: classical solution; CS + R: classical solution with reuse; CLSM: controlled low-strength material; ECO: eco-trench). 7 I. Josa et al. Journal of Environmental Management 341 (2023) 118020 Fig. 4. Potential environmental impacts (for impact category Global warming) of different areas of the trench for each of the 4 alternatives (CS: classical solution; CS + R: classical solution with reuse; CLSM: controlled low-strength material; ECO: eco-trench). Fig. 5. Costs per linear metre of a 0.15 × 0.75 m and a 0.4 × 1.5 m trench for each of the 4 alternatives (CS: classical solution; CS + R: classical solution with reuse; CLSM: controlled low-strength material; ECO: eco-trench). with a difference of less than 2% with respect to CLSM. 3.2.2. Sensitivity analysis In the case of the larger trenches (0.15 m × 0.75 m), the costs As it was done in the LCA, a discussion is included here on how the represent, on average, an increase of 40% with respect to the smaller costs change together with changes in the areas of the trenches. Fig. 6 trenches (0.4 m × 1.5 m). For these dimensions, the process of finishing shows the costs per linear metre of different areas of the trench for each represents the highest proportion of the cost only for the CS + R of the four alternatives. As can be seen, there exists a linear relationship (contributing by 40.07% of the total cost). The most expensive process is between the area of the section of the trench and the life cycle cost. the end of life for CS (38.61% of the total cost) due to the higher volume As the results for the smallest and biggest sections showed, the of material that is loaded in trucks and transported to landfill, the filling alternative with the lowest costs is always ECO if the section remains the for the CLSM (35.34% of the total cost), and the excavation for the ECO same, followed by CS + R. Again, this is because of the great number of (41.86% of the total cost). materials that are reused. The area remaining the same, ECO has lower Concerning the overall costs, the most expensive alternative is still costs than CS + R because it does not require the milling and asphalt CLSM. However, in this case, the difference with respect to the next most layer operations. expensive alternative, CS, is higher (12%). The highest costs correspond to the CLSM trench, followed by the CS solution. These two alternatives do not reuse soil on site. Therefore, the costs increase owing to the borrowing and transportation of materials, 8 I. Josa et al. Journal of Environmental Management 341 (2023) 118020 also the least efficient alternatives. Nonetheless, it needs to be observed that only the 0.40 m × 1.40 m and 0.40 m × 1.50 m are the least efficient ones. For smaller dimensions, while the CLSM still has higher environ­ mental impacts, the range of the cost is the same as the CS. There is a range of sections where the CS, CS + R and CLSM could yield similar environmental impacts and costs if the section is not the same. It corresponds to the range between 15 and 45 €/m. Having said this, it needs to be mentioned that the results shown are for the same assumptions regarding transportation distances. Trans­ portation distances not only could influence the total environmental impacts, but also the costs. Additionally, the relationship between changes in environmental impacts and costs in comparison to trans­ portation distances is not necessarily linear. Thus, this is acknowledged as a limitation of this study. While this study focused on economic and environmental impacts, an important aspect that might be hindering the deployment of more environmentally and economically friendly alternatives are social is­ Fig. 6. Costs per linear metre of different areas of the trench for each of the 4 sues. Some examples of such issues could be culture and tradition- alternatives (CS: classical solution; CS + R: classical solution with reuse; CLSM: related aspects, the training of workers, or regulations that do not controlled low-strength material; ECO: eco-trench). allow for more innovative solutions. While previous studies have analysed some social impacts arising transportation of waste, and landfill costs. from these elements, the indicators used were limited. For instance, Casanovas-Rubio et al. (2019) analysed the social impact of the same typologies of trenches studied here using two indicators: inconveniences 3.3. Eco-efficiency to the surroundings and occupational risks. They concluded that these social indicators did not have a significant influence on the overall Fig. 7 shows the results corresponding to the impact category of sustainability of the trenches. Nonetheless, a more in-depth social Global warming and the costs per linear meter of the different trenches impact assessment could provide more information. Therefore, it is analysed. The graph can be interpreted as the eco-efficiency for the suggested that further studies focus on carrying out thorough social different alternatives, where the bottom left part of the graph represents analyses using, for instance, Social LCA (S-LCA). the most efficient solutions (low environmental impact and low costs), and the top right part of the graph depicts the least efficient solutions 4. Conclusions and recommendations (higher environmental impact and costs). The impact category of Global warming has been chosen here given its particular relevance. Eco- The study presented in this paper consists in a comprehensive life efficiency graphs corresponding to the remaining impact categories cycle assessment and life cycle costing of the most representative and can be found in the Supplementary material. relevant current trench systems. The aim of the paper was to compare As observed, the most efficient solutions correspond to the ECO the environmental impacts of four systems of trenches for pipe instal­ trenches. For the smaller sections, the results show that they have the lation (i.e., the classical solution, the classical solution with part of the lowest costs and environmental impacts. However, for bigger di­ materials reused, a solution with controlled low-strength material, and mensions, their costs are comparable to those of the smallest dimensions the eco-trench, in which most of the material is reused). of CS + R, CS, and CLSM. In economic terms, results indicated that the eco-trench (ECO) yields The least efficient trenches correspond to the CLSM trenches. The between 30 and 63% lower costs with respect to the other three alter­ previous sections showed that they have the higher impacts both in the natives for all the dimensions. In general, the finishing process leads to LCC and the LCA of this solution and, therefore, it is evident that they are the highest costs. Regarding environmental impacts, results indicated Fig. 7. Global warming and costs per linear metre of different areas of the trench for each of the 4 alternatives (CS: classical solution; CS + R: classical solution with reuse; CLSM: controlled low-strength material; ECO: eco-trench). 9 I. Josa et al. Journal of Environmental Management 341 (2023) 118020 that the eco-trench (ECO) had lower impacts (from 80,1% to 97,4% less) 00734), and the Spanish Ministry of Economy, Industry and Competi­ in all the impact categories considered. This was mainly due to (i) tiveness for funding the “María de Maeztu” program for Units of reusing materials and not using asphalt; (ii) few processes involved. The Excellence in R&D (CEX 2019-0940-M). M.M. Casanovas-Rubio ac­ highest impacts were attained by the solution with controlled low- knowledges the Serra Húnter programme. P. Pujadas acknowledges the strength material (CLSM), which was found to have the worst impacts support of Catalan agency AGAUR through its research group support in 7 out of 9 impact categories. These higher impacts were strongly program (2021 SGR 00341) and the Serra Hunter program. related to high quantities of asphalt necessary. To address the limitations of this article, future studies could focus on Appendix A. Supplementary data improving the methodology used by modelling the uncertainties of some stochastic variables (assumed as deterministic for this research), and Supplementary data to this article can be found online at https://doi. conducting a consequential LCA instead of an attributional one. org/10.1016/j.jenvman.2023.118020. Furthermore, in this study, the average of the different types of exca­ vated and compacted materials was used in the LCI. Instead, future References studies could assess the environmental and economic dimensions of each type of material. Alizadeh, V., 2018. Influence of cementing paste volume on properties of controlled low strength materials. J. Mater. Civ. Eng. 30 (3) https://doi.org/10.1061/(asce) Additionally, there are several research avenues that could be mt.1943-5533.0002200. addressed in the future that were beyond the scope of this study. First, it Blanco, A., Pujadas, P., Cavalaro, S.H.P., Aguado, A., 2014. 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