Embodied Carbon Analysis of Microtunneling Using Recent Case Histories

Abstract

With increasing demand for sustainable underground infrastructure and pressure to reduce embodied carbon (EC), microtunneling (MT) has become an increasingly popular trenchless method of installing buried utility tunnels. Life-cycle analyses have shown that trenchless methods cause lower emissions than traditional open-cut construction. However, existing literature specifically considering MT is limited and fails to consider the impact of the entire construction process. In this paper, an approach for calculating the EC of MT is presented. The proposed approach is applied to three recent case histories in the United Kingdom through the development of a bespoke MT EC database in collaboration with industry partners. Total emissions across all three projects (870 m of pipeline) total 1,005 tCO2e. Production of materials and components is shown to account for an average of 68.5% of EC across the three projects, with most of these emissions coming from the key structural materials, namely concrete and steel. Sensitivity analyses demonstrate that the source and production method of steel products have a significant impact on EC. Site activities also make a significant contribution, accounting for an average of 20.5% of total EC. Normalization of the results suggests that MT produces less EC than open-cut pipeline installation and highlighted how increasing drive lengths and reducing the number of shafts can significantly reduce EC. One of the case studies is then used as an example to quantify how the reduction of intermediate launch/reception shafts can reduce overall EC. In this paper, an approach for calculating the embodied carbon (EC) of microtunneling (MT) is presented, defining a scope that will enable fair comparison of future projects. The presented methodology also provides a useful reference for readers to find sources for EC factors. The proposed approach is applied to three recent case histories in the United Kingdom through the development of a bespoke MT EC database in collaboration with industry partners. The presented results provide insights into the EC of MT projects in five key areas: (1) the relative contribution of different materials and different construction phases to overall EC, (2) the sensitivity of EC to the method of steel production, (3) the influence of on-site emissions, (4) the comparison of MT to traditional open-cut pipeline construction, and (5) quantifying the environmental benefits of minimizing the number of shafts.