# Practical Considerations in the Design of Passive Free Piles in Sliding Soil
## 7 Conclusions  
This study has aimed to advance some practical considerations in the design of single free-head piles for the stabilisation of shallow translational landslides, such as in the case of quiescent landslides in stiff formations that may undergo reactivation. On account of this, only the failure modes involving the formation of plastic hinges in the pile have been considered.  
The model pile, 1200 mm in diameter, is embedded in landslide with a thickness of D and an inclination of \(\beta\) = 10\({}^{\circ}\). It has a total length of D + L, in order to study the collapse modes B1, BY and B2 as defined by some previous studies on flexible pile [3; 11]. For this purpose, some 3D FEM numerical models have been built, and simplified constitutive models have been implemented for both the soil (Mohr-Coulomb) and the pile (elastic-perfectly plastic), in order to apply the practical design approaches provided by Eurocode [23] for reinforced concrete.  
The main results obtained are the following:  
* All the numerical models show maximum values of both shear stress, V\({}_{\text{d}}\), and normal stress, N\({}_{\text{d}}\), at the slip surface, whereas their bending moment is here low or negligible;
* Unless particular structural solutions are used, the classic arrangement of the reinforcements in a large diameter pile implies that the design shear strength of the pile, V\({}_{\text{ult,d}}\), is reached sooner than the simple-bending strength, M\({}_{\text{ult,d}}\), even for piles with a diameter greater than that considered in this study and with a comparable percentage of reinforcement. This means that the mechanisms BY, B1 and B2, associated with the formation of plastic hinges, could be preceded by a shear failure mechanism. Therefore, if particular solutions aimed to increase the shear strength of a bored pile are not envisaged, it might be more appropriate to limit the pile-soil interaction until the yield moment, M\({}_{\text{y,d}}\), is reached, so that the shear force mobilised in the pile does not exceed the ultimate design shear resistance, V\({}_{\text{ult,d}}\);  
Figure 14: Model 1 of Figure 7a that reaches the design yield moment M\({}_{\text{y,d}}\): variation in the strength ratio \(\mu\) due to small variations in the slope inclination with respect to the reference value of \(\beta\) = 10\({}^{\circ}\).  
* For the piles under study, the head displacements necessary to reach the design yielding moment, \(\text{M}_{\text{y,d}}\), in the most stressed cross-section, are as low as 30 cm, whereas those necessary to reach the design collapse moment, \(\text{M}_{\text{ult,d}}\), do not exceed 70 cm. These displacements may vary in the function of both D/L and \(\text{E}_{\text{sup}}/\text{E}_{\text{inf}}\) ratios;
* The strength offered by a pile is provided not only by the design shear strength, but also by the design normal load due to the lateral friction developing along the pile length D when in contact with the moving ground. This axial load could be mobilised even for small slope inclinations (\(\beta=10^{\circ}\) in this case) and for horizontal displacements of the pile head compatible with those required for reaching the design yielding moment, \(\text{M}_{\text{y,d}}\), in the most stressed cross-section;
* Finally, in a free-head flexible pile, the net soil pressure reaches the limiting passive pressure only in limited sections of the pile.  
Conceptualisation, P.C.; writing P.C.; software, C.P.; validation C.P. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors are very grateful to and thank the company Sinergo Spa, via Ca Bembo, 152, 30030 Martellago, Venice, Italy, for allowing the use of the MIDAS FEA NX 2021 v1.1 code.  
The authors declare no conflicts of interest.