# Assessing the Sustainability of Alternative Shaft Construction Methods
## 5 Discussion Regarding Sustainability  
Until recently, the optimal design of a structure was defined as that which met the safety requirements at minimum cost. Due to the climate emergency, sustainability considerations have become increasingly important. Indeed, nowadays, it is indispensable to consider this perspective in the design stage, with the optimal solution being a compromise between cost and sustainability while meeting all safety requirements. However, this evaluation is difficult to perform since the quantification of costs, and particularly of CO\({}_{2}\) emissions, at the design stage requires knowledge of several factors. These include those whose associated prices/emissions are very volatile, such as fuel and human labor, which can only be adequately defined closer to the start of construction [30]. Moreover, there is still scarce and scattered information regarding the evaluation of CO\({}_{2}\) emissions related to specific activities, rendering their quantification even more difficult. Nevertheless, with the implementation, standardization [54; 55; 56], and generalization of LCA in different accessible platforms [19; 20; 31], this task, which is key when evaluating the sustainability of different construction processes, is becoming easier to perform. Using as a reference those platforms and previous studies [28; 30], it was possible to define a reasonable range of unit CO\({}_{2}\) emissions (i.e., in ton per m\({}^{3}\)) associated with the construction of diaphragm walls, application of shotcrete, and installation of PCL. Those values are presented in Table 3 and were used to calculate the CO\({}_{2}\) emissions associated with each of the solutions studied in the previous  
Figure 8: Influence of the EBS shaft excavation method on the settlements. Figure 7: Influence of the EBS shaft excavation method: (**a**) horizontal displacements; (**b**) hoop force; (**c**) bending moments. section. As expected, the unit values regarding the diaphragm walls are substantially higher, being mostly affected by the steel reinforcement required. The difference between shotcrete and PCL is mostly related to the concrete composition and with the rebound of material associated with the shotcrete application, which produces higher emissions when compared to the PCL. The CO\({}_{2}\) results presented in Table 3 reflect those differences, with the SBE analyses presenting much higher CO\({}_{2}\) emissions, with a maximum of 16,835 t being estimated for the worst-case scenario, which corresponds to the reference analysis (SBE-Ref). In contrast, the PCL solution (EBS-Seg) is responsible for the minimum CO\({}_{2}\) emissions (maximum of 1267 t), as can be more easily seen in Figure 9.  
Also, in Table 3, the engineering requirements related to the displacements (vertical and horizontal) and the lining forces (hoop force and bending moments) are presented. According to a review performed by [PERSON] (2018), all analyses met the displacement thresholds for both vertical and horizontal movements, with the highest values predicted being smaller than 0.25% and 0.20% of the shaft height (H) for vertical and horizontal displacements, respectively. However, analyses where an excavation step higher than 1 m (EBS-0.35-2 m and EBS-0.35-3 m) was applied exceeded the recommendations of [PERSON] and [PERSON] (2018), which defined a threshold of 0.15% for horizontal movements. In terms of lining forces, it is also clear that the analyses with a shotcrete thickness of 0.75 (EBS-0.75) and 1.5 m (EBS-1.50) are not acceptable since the magnitude of the predicted bending moments far exceeds the capacity of the shotcrete, which is usually just reinforced with fibers. The forces predicted for all other cases are within the capacity of the support solutions considered in this study.