# Assessing the Sustainability of Alternative Shaft Construction Methods
## 4 Shaft Construction Methods
### EBS Shaft Excavation  
To further assess the influence of the shaft construction method, additional analyses following the principles of EBS, i.e., support only installed after excavation, were carried  
Figure 5: Influence of the SBE shaft excavation method: (**a**) horizontal displacements; (**b**) hoop force; (**c**) bending moments. Figure 6: Influence of the SBE shaft excavation method on the settlements. out. The support provided by the diaphragm wall was replaced by shotcrete or pre-cast segmental lining, which are, isolated or combined, the typical lining solutions installed in EBS methodology [35]. In the performed analyses, the thickness of the shotcrete was reduced from 1.50 m to 0.35 m, with the latter value being in agreement with the thicknesses usually employed in shafts excavated with a similar methodology and in comparable materials [35]. For the segmental lining, a standard thickness of 0.35 m was adopted [35]. For ease of comparison, the elastic properties of the shotcrete were considered equal to those of the diaphragm wall, while for the segmental lining, a Young's modulus of 37 GPa was employed, reflecting the characteristics of prefabricated concrete. In the EBS methodology, a key factor is the size of the excavation step, since lager values of this quantity will cause larger soil decompression, leading to increased movements and instabilities (local or even global), while small steps introduce significant constraints in the excavation process, requiring higher construction time and costs, while diminishing the overall advantages of using EBS principles. To evaluate the effects of the type of support and of the excavation step, six analyses were performed, as outlined in Table 2.  
To simulate the EBS analyses, the construction sequence adopted in the numerical model was modified. Hence, after the generation of the initial stress state, the first excavation step was performed. In the following stage, a new excavation step was performed while installing the lining supporting the previously excavated step. This sequence was carried out until the full excavation depth was reached. The duration of each stage was adapted so that the whole excavation process would take the same 160 days defined for the SBE analyses. The horizontal displacements and lining forces obtained for the last excavation stage are plotted in Figure 7. As expected, the reduction in thickness of the shotcrete from 1.50 m to 0.35 m increased the horizontal displacements by a factor of 1.7, reaching a maximum of 37.5 mm in EBS-0.35 in comparison with 21.9 mm in EBS-1.50. For the analysis with a 0.75 m thick shotcrete support, the maximum displacement was 27.8 mm. In contrast, a significant reduction in the bending moments was observed, with an absolute maximum of 1395 kNm/m in the thicker case, in comparison to just 190 kNm/m in EBS-0.35. A reduction but of a smaller magnitude was also observed in the hoop forces (7025 kN/m against 6450 kN/m). The use of the stiffer segmental lining (EBS-035-Seg) leads to smaller horizontal displacements (29.9 mm) but slightly higher lining forces (6670 kN/m and 220 kNm/m) in comparison with the shotcrete analysis with the same thickness (EBS-0.35). The results also confirm that the excavation step is an important aspect. When its value is increased from 1 m to 3 m, an increase in horizontal displacements of about 35% is observed. In contrast, a reduction of approximately 7% in the hoop force and about 4% in bending moments is predicted. A similar behavior can be observed in terms of settlements (Figure 8), as a reduction in stiffness or an increase in the excavation step will increase the magnitude of the deformations, which can reach a maximum of 47 mm in the analysis where the support is less stiff and the excavation step larger (EBS-0.35-3 m).