# Practical Considerations in the Design of Passive Free Piles in Sliding Soil
## 1 Introduction  
A landslide stabilisation can be carried out with various solutions which aim to increase the strength against sliding, or to reduce the destabilising actions on soil mass or with both approaches. The use of single piles of a large diameter stressed transversely to their axis is one of the possible ways to increase the mobilised strength to sliding in a translational type of landslide of moderate thickness. Referring to the calculation methods of a single pile embedded in sliding ground, the rigid-plastic approach was adopted by various authors [1; 2; 3; 4]. Such studies may consider both drained and undrained soil strength, as well as the type of constraint at the pile head (free, fixed) and the finite or unlimited strength of the pile. The interaction with piles arranged at a fixed spacing in sliding ground has also been considered [5; 6; 7; 8; 9; 10]. The use of numerical models [11; 12; 13; 14; 15] has allowed the study of a non-linear interaction process in relation to the variability of the geotechnical parameters. Although numerical three-dimensional analyses are in principle the most rigorous approach, decoupled methods have been widely used in numerical modelling, so that both the slope and the piles are analysed separately. For this purpose, the design approach is simplified in three main steps:  
* Computing the additional force needed to reach a given level of stability;
* Computing the shear strength that each pile may supply;
* Selecting the number of piles able to provide the required level of stability. This procedure refers to the ultimate state only and gives no indication on pile response prior to the ultimate state, nor on displacements required to achieve the ultimate state. To overcome this limitation, a model of rigid passive piles was proposed, embedded in a landslide with both subgrade reaction modulus and strength linearly increasing with depth [16; 17]. In addition, a model of flexible piles embedded in cohesive layered soils was proposed, where both the subgrade reaction modulus and the strength are constant with depth [18]. The purpose of these studies is to analyse the pile response with regard to the soil movement, so that the mobilised pile strength can be evaluated not only at the ultimate but also at the intermediate states. Finally, the displacements approach and the hybrid methods [19; 20; 21; 22] aimed to predict the degree of safety of the slope in relation to the design displacement. The rigid-plastic analysis carried out by [3] examined the case of a free-head single pile embedded in a horizontal translational landslide in which the soil is characterised by undrained strength. The author identified six possible failure mechanisms of the pile-soil system, three of which were related to the case of a pile having infinite stiffness and strength, for which failure is localised in the soil (type A, B, and C), while, in the cases of the other three, the pile collapses through the formation of one or two plastic hinges (type B1, BY, B2). Another author [11] made considerations similar to those of [3] and defined the following failure mechanisms:  
1. Flow mode, when the landslide is shallow and the unstable soil becomes plastic and flows around the pile;
2. Short-pile mode, when the slide is relatively deep and the length of pile in the stable soil is relatively shallow; the sliding soil carries the pile through the stable soil layer and full mobilisation of soil strength in the stable layer occurs;
3. Intermediate mode, when soil strength in both the unstable and stable soil is fully mobilised along the pile length;
4. Long pile failure, which occurs when the pile itself yields because the maximum bending moment reaches the yield moment of the pile cross-section; this mode can be associated with any of the three modes of soil failure listed above, although experience suggests that it is most likely to occur with the intermediate mode.