Manuela Cecconi

The hermitage of Cerbaiolo (Tuscany, Italy): stability conditions and geomorphological characterization

The Italian cultural heritage is one of the most noticeable in the world; an important part of it is subjected to natural hazard and risk conditions. Religious buildings, due to their location on impervious sites are particularly exposed to landslides. In this paper we study the hermitage of Cerbaiolo (Tuscany, central Italy). The hermitage, founded in the 8th century as a Benedictine monastery is now a Franciscan Women Institute seat. The buildings are on the edges of a limestone plate overlapped to a clayey formation. Falls, toppling and slides are present along the limits of the plate due to the geomechanics properties of the rock and to the contrast between the two lithological complexes. In order to identify the main joint sets affecting the rock mass a structural-geological survey was carried out and four sets are identified. The rock plate was classified according to Bieniawski, Barton and Geological Strength Index approaches. The outcomes confirm the predisposing factors to mass movements, corresponding to the intensity and characteristics of fracturing of the rock mass. With regard to the slope stability analysis, a failure mechanism involving sliding along a single plane (plane failure) was assumed on the first approximation. The analyses take into account the presence of tension cracks as an indicator factor for the instability phenomenon. The results of stability analyses, performed in static and seismic conditions, indicate a widespread instability condition. The dip slope direction, the properties and type of discontinuities and the local variation of composition influence the hazard assessment.

Seismic Displacement Based Design of Structures: Relevance of Soil Structure Interaction

In this paper a brief summary of the seismic design method known as “direct displacement – based design” (DDBD) is presented, with some discussion of the appropriate seismic design input to be used, on the applicability to retaining structures and on the relevance of including nonlinear soil-structure interaction in the DDBD and the tools to account for it, with reference to shallow foundations.

Hydraulic Heave Failure in EC7: Suggestions for Verification

Hydraulic failure by heave is one of the most dangerous limit states in geotechnical engineering practice. Hence, the margins of safety associated to this failure mechanism have traditionally been quite high. Nowadays, with the advent of the Eurocodes, the approaches to verify heave failure have been substantially modified to take into account “partial” factors, as opposed to the traditional “global” safety factor. In the paper it is demonstrated that the two approaches given in the present version of Eurocode 7 do not provide a consistent evaluation of safety against failure by heave and may lead to both over-conservative and un-conservative design. An alternative approach is given to eliminate such inconsistencies, and to provide more reasonable levels of safety. Finally, two simple examples are presented to compare the different approaches and to highlight possible limitations in their use for design.

Seismic displacement-based design of embedded retaining structures

The paper presents an application of the seismic design method named “Direct Displacement Based Design” (DDBD), first introduced in 1990s in the field of earthquake structural engineering, and gaining due attention in the recent years. The method can be applied to a wide range of structural types, including geotechnical structures. The fundamental philosophy behind DDBD is that structures should be designed to achieve a specified performance level under a certain seismic intensity. In the paper, DDBD is described with considerable discussion on its applicability to flexible earth retaining structures in coarse grained-soils. Particular attention is given to the evaluation of the equivalent damping ratio of the wall/soil system, since it sensibly affects the results of the procedure. A simplification of the design process is proposed in order to provide a seismic demand curve, in terms of active/passive thrust, which is dependent on the system ductility. A numerical example of application of the method is provided in the paper.

Hydraulic failure of diaphragm walls: a possible methodology for safety improvement

The paper presents a methodology aimed at reducing—or even avoiding—the risk of heave and uplift failures of reinforced concrete diaphragm walls. The method is based on the simple concept of increasing the drainage capacity of the embedded portion of the retaining walls. The behaviour of a strutted excavation in a cohesionless soil below groundwater is examined by means of two distinct series of numerical analyses, respectively focused on the hydraulic and mechanical behaviour of the retaining system. It is shown that, for some frequent cases, such methodology is capable to improve the safety of the retaining system with respect to both hydraulic and geotechnical limit states.

Non-dimensional analysis for rock slope plane failure in seismic (pseudostatic) conditions

This study examines plane failure analysis of rock slopes and discusses its effect on seismic actions in a non-dimensional form, incorporating external loads transferred by anchors and stability indicators such as the depth of tension cracks and the failure plane dip. The results of limit equilibrium analyses, in terms of attained level of safety, are summarized in stability charts which can be used for design and assessment. Following guidelines established by Eurocode EC7, the stability is assessed through the R/E ratio, i.e. the ratio of the resisting shear forces (R) acting on the sliding surface and those promoting sliding (E). The proposed stability charts can be applied to various common practical situations, in order to provide a preliminary - but quantitative - assessment of the rock slope stability. As an example of an application, the paper focuses on the stability of a rock mass located at the bottom of the southern slope of the “Sasso di Pale” mountain (Central Italy), at the outskirts of the village of Pale. As many other historical towns in Central Italy, due to their location close to rock masses, this site is particularly exposed to instability phenomena such as plane failure and rock wedge failure. The structural-geological survey allowed us to identify the main joint sets affecting the behavior of the rock mass and, accordingly, we examined the plane failure mechanism involving sliding along a single plane. Stability verifications in pseudostatic conditions have been performed by adopting the proposed charts, consistent with Eurocodes prescriptions.

Collapse upon Wetting of Lime Stabilised Pyroclastic Soils

Compacted soils are commonly used in the construction of geotechnical works such as road embankments and earth dams. Several investigators have highlighted the wetting-induced behaviour of compacted soils resulting in collapse of the soil skeleton with increase of volume strains. The addition of lime, with the subsequent physic-chemical reactions, generally reduces the collapse potential of the stabilised soil if compared to the not treated one. In the paper, soil collapse has been investigated by means of soaking tests under constant stress on reconstituted not treated and lime stabilised pyroclastic soils. The soil collapse has been analysed for stress levels corresponding to pre-yield, yield and post-yield condition. The experimental results show the relevant dependency of the collapsible behaviour on the initial state of the compacted samples (void ratio, water content), the treatment parameters (curing time) and the reference stress level.