Narrow trenches are a common technique for the installation of utility pipelines of small diameter. The excavated soil is not always appropriate as landfill and, in those cases, an appropriate soil from somewhere else (ex. a borrow pit or another construction site) should be used instead (classical solution, CS). Another common solution is to use a controlled low-strength (cementitious) material (CLSM) as backfill instead of compacted soil. However, both solutions lead to increased raw material consumption, waste generation, need for transportation, and CO₂ emissions. In an attempt to address these issues, researchers developed an eco-trench (ECO) that reuses the excavated soil of narrow trenches to produce a controlled low-strength material to be used as landfill. Although technically viable, the sustainability of this solution versus the traditional solution has not been properly addressed. Hence, this paper aims to develop a method for the sustainability assessment of trenches. The Sustainability Index of Trenches (SIT), based on the MIVES decision-making method, enables the assessment and prioritisation of different types of trenches according to sustainability criteria. Criteria, indicators, weights and value functions were specifically defined based on seminars with experts in the field of utility services and construction. A case study was performed in which four types of trenches (CS, CS with recycling CS + R, CLSM and ECO) were assessed and prioritised according to SIT. ECO resulted in the most sustainable alternative with a SIT of 0.80 out of 1 followed by CS + R, CS and CLSM with SITs of 0.63, 0.40 and 0.38 respectively. The sensitivity analysis showed consistent results in different scenarios. These findings demonstrate the capability and reliability of SIT as a decision-making tool for the evaluation of the sustainability of different construction processes for trenches and the prioritisation of the most suitable solution for different situations.