# Assessing the Sustainability of Alternative Shaft Construction Methods
###### Abstract
[7], [PERSON] [8] and [PERSON] et al. [12]. These new technologies are mainly focused on replacing part of the cement in the production of concrete with low-carbon concrete materials (LCCMs) [12], such as ground granulated blast-furnace slag (GGBF) and fly ash (FA), among others [14; 15]. Other strategies consist of improving the energy efficiency of the production of concrete [7] and exploiting the potential of concrete recycling to increase the rate of CO\({}_{2}\) uptake [13]. Naturally, one of the best options for reducing concrete emissions is simply using concrete more rationally and efficiently in construction, which requires a better optimization of structures at the design stage and also an improvement in the construction methods employed [16; 17]. As pointed out by [PERSON] et al. [18] and [PERSON] et al. [19], this optimization should also include geotechnical works, where the comparison of different solutions through life cycle assessments (LCAs) would highlight the most climate-friendly design for that specific structure [19; 20]. Following that approach, a few studies have been published on the topic. For instance, [PERSON] et al. [21] analyzed the impact on carbon emissions of installing driven precast and bored piles; [PERSON] et al. [22] evaluated the emissions released by constructing two types of foundations of residential buildings, while [PERSON] [23] presented a study on the optimization of the concrete mixture in order to reduce the CO\({}_{2}\) emissions for large wind turbine foundations; [PERSON] et al. [24], [PERSON] et al. [25], and [PERSON] et al. [26] performed LCAs of different types of retaining walls, trying to establish the most eco-friendly solution for particular scenarios; [PERSON] et al. [27] calculated the emissions for two sections of the UK Channel Tunnel Rail Link; [PERSON] [28] and [PERSON] et al. [29] discussed, in detail, the emissions caused by different types of linings usually adopted in tunnels; and [PERSON] et al. [30] and [PERSON] et al. [31] evaluated the emissions for different design solutions of deep excavations. In all cases, it is evident that performing an LCA analysis can be very complex, particularly at the design stage, where there is no clear definition of all aspects involved, mainly those associated with construction works. The gap between design, construction methods and sustainability can be bridged by performing back-analysis studies, where the performance of the adopted design and of possible alternative solutions is evaluated to provide suggestions for better design in future projects. Naturally, this process is highly dependent on the quality of the back analysis and on the amount of data available to calibrate and validate the numerical model. Following this methodology, this study assesses the impact of adopting alternative shaft construction methods from both engineering and sustainability points of view. Using, as a reference, the case of the WA2 shaft, built as part of the Dublin Port Tunnel project (DPT) [32], a numerical model is developed incorporating all the geotechnical information available at the shaft site. After a rigorous calibration of the constitutive model adopted for the soil layers and its validation against instrumentation data, alternative sequences for shaft excavation are assessed. These include the simulation of the excavation employing the SBE (Support Before Excavation) technique--where the support, such as diaphragm walls, is installed prior to the excavation--and of the EBS method (Excavation Before Support)--where the support is only installed once excavation is completed, such as in the case of shotcrete lining or precast concrete segments (PCL) [33].