Macroelement modeling of SSI effects on offshore wind turbines subject to large number of loading cycles

In this paper, the hypoplastic macroelement formulation proposed by [1] has been modified in order to extend its range of applicability to offshore structures subject to cyclic loads with very high number of cycles, with particular attention to fatigue phenomena and cyclic displacement accumulation. A series of FE analysis has been performed to model the soil–foundation interaction processes of a prototype of offshore wind turbine, for which the geometrical characteristic of the superstructure and foundation, the soil conditions and the predicted environmental (wave and wind) loads were known. The study, carried out in parametric form, has allowed to better understand the role played by the modified cyclic part of the macroelement model in reproducing the shake–down effects as observed in small–scale model tests.Peer ReviewedPostprint (published version

Characterisation of ground thermal and thermo-mechanical behaviour for shallow geothermal energy applications

Increasing use of the ground as a thermal reservoir is expected in the near future. Shallow geothermal energy (SGE) systems have proved to be sustainable alternative solutions for buildings and infrastructure conditioning in many areas across the globe in the past decades. Recently novel solutions, including energy geostructures, where SGE systems are coupled with foundation heat exchangers, have also been developed. The performance of these systems is dependent on a series of factors, among which the thermal properties of the soil play one of major roles. The purpose of this paper is to present, in an integrated manner, the main methods and procedures to assess ground thermal properties for SGE systems and to carry out a critical review of the methods. In particular, laboratory testing through either steady-state or transient methods are discussed and a new synthesis comparing results for different techniques is presented. In-situ testing including all variations of the thermal response test is presented in detail, including a first comparison between new and traditional approaches. The issue of different scales between laboratory and in-situ measurements is then analysed in detail. Finally, thermo-hydro-mechanical behaviour of soil is introduced and discussed. These coupled processes are important for confirming the structural integrity of energy geostructures, but routine methods for parameter determination are still lacking

Three-Dimensional Modeling of Soil-Structure Interaction for a Bridge Founded on Caissons under Seismic Conditions

In recent years, the urgent need to increase the safety standards of viaducts and bridges—under static and dynamic loading conditions—has required the development of advanced modeling approaches able to accurately predict the expected behavior of such infrastructures in a reliable manner. This paper presents a comparison between the adoption of a simplified modeling approach, widely used in the current practice, where the response of the structural system neglects the effects of the soil-structure interaction (SSI) phenomenon (considering the base of the structure fixed at the ground surface) and a rigorous modeling approach that considers the full 3D problem with all the components of the system (superstructure, foundation, and soil), through a finite element model. The pier of a real-world viaduct in central Italy was considered, with the aim of starting from a specific case study with foundation characteristics that are frequently found in viaducts in Italy, to obtain results that can be generalized to a wide range of similar types. Its behavior was evaluated both in the dynamic range of small oscillations and in the field of the seismic response to low and strong motion events. The results show that, in terms of seismic demand, the fixed-based model appears more conservative, but it significantly underestimates both elastic and residual displacements and rotation

A Generalized Newmark Method for the assessment of permanent displacements of flexible retaining structures under seismic loading conditions

In recent years, much attention has been paid to performance-based design of flexible retaining structures, focusing on the evaluation of the permanent deformations of the soil-structure system caused by given seismic loads, rather than on the assessment of conventional safety factors determined by comparing seismic actions and system resistance (typically based on limit equilibrium methods). While only a few examples of fully coupled, dynamic numerical simulations of flexible retaining structures adopting advanced cyclic/dynamic models for soils can be found in literature, a number of recent works have proposed simple modications of the classical Newmark method to assess the permanent displacements of the structure at the end of the seismic excitation. Most of the aforementioned works refer to cantilevered diaphragm walls, for which the failure mechanisms at limit equilibrium are relatively simple to describe. However, this is not the case for anchored or propped flexible structures, where the velocity eld at failure under a pseudo{static seismic load is quite complex and can be affected by the plastic yielding of the wall upon bending. In this work, upper- and lower-bound limit analysis FE solutions are used as a basis for the development of a Generalized Newmark Method, based on the accurate evaluation of the critical accelerations for the retaining structure and the corresponding failure mechanisms. It can be shown that, under two reasonable simplifying assumptions, a Newmark-like scalar dynamic equation of motion can be derived which, upon double integration in time, provides the magnitude of the permanent displacements associated to each failure mechanism, as provided by limit analysis. This procedure allows the reconstruction of the full permanent displacement eld around the excavation, not just the evaluation of horizontal soil movements at selected points. The application of the method to a number of selected prototype excavations demonstrates the potentiality of the proposed approach, which can be extended easily to other complex geotechnical structures

Physical and numerical modelling of the response of slopes under different rainfalls, inclinations and vegetation conditions

We present the results of an experimental and numerical study focused at quantifying the effect of different crucial factors in slope evolution, such as: rainfalls, inclinations and vegetation conditions. To this aim a small-scale slope was constructed in the laboratory and an experimental program was design to understand the slope response under the following conditions: a) different inclinations, obtained through a system of hydraulic pistons; b) different rainfall conditions (in intensity and duration), obtained through an artificial rain simulator; c) presence / absence of a vegetated cover. The first set of tests allowed defining a trend for the percentage of rainfall infiltration within the slope as a function of the steepness. The second set of tests allowed observing the bare slope behaviour at failure; while the third set demonstrated the beneficial effect of a grass carpet. All the laboratory experiments were integrated by numerical simulations, using the SEEP/w FE code for transient infiltration analyses and the SLOPE/w code for stability analyses. The back-analyses, based on the classical Equilibrium Limit Methods, allowed estimating the enhanched shear strength provided by the presence of the roots, relevant in the practice for remedial works based on Soil Bio-Engineerin

Numerical Model of Energy Foundation Behavior: The Prototype of a Geothermal Micro-pile☆

Abstract This paper presents the results of a numerical study performed in the design stage of an innovative geothermal technology – the geothermal micro-piles – recently developed and currently under testing at the University of Perugia for the exploitation of low enthalpy geothermal energy in existing buildings. In this investigation, micro-piles are equipped with a primary circuit of a traditional GSHP system, where the circulation of a heat carrier fluid (i.e. glycolic water) permits a thermal interaction with the surrounding soil. The numerical study has been performed to simulate the thermal behavior of such prototype and to obtain useful information concerning its functioning under real operating conditions. The first results of the study showed that this new technology can provide a thermal flux comparable with the one provided by traditional geothermal piles

A Case-Study of Sustainable Countermeasures against Shallow Landslides in Central Italy

Traditional technical solutions for slope stabilization are generally costly and very impacting on the natural environment and landscape. A possible alternative for improving slope stability is based on the use of naturalistic engineering techniques, characterized by a low impact on the natural environment and being able to preserve the landscape identity and peculiarities. In this work, we present an application of such techniques for slope stabilization along a greenway located in central Italy, characterized by an extraordinary natural environment. First, 22 potentially unstable slopes have been identified and examined; then, among these, two standard type slopes have been selected. For both of them, an appropriate naturalistic engineering work has been proposed and stability analyses have been carried out. These have been performed by considering different piezometric conditions and using two different approaches: (a) a classical deterministic approach, which adopts deterministic values for the mechanical properties of the soils neglecting any uncertainty, and (b) a probabilistic approach that takes into account a statistical variability of the soil property values by means of their probability density functions (PDFs). The geometry of each slope derives from a digital model of the soil with 1 meter resolution, obtained through Light Detection and Ranging (LiDAR) survey provided by the Italian Ministry of the Environment. The soil mechanical characteristics and their PDFs are derived from the geotechnical soil property database of the Perugia Province. Results show an increase in slope stability produced by the adopted countermeasures measured in terms of Factor of Safety (Fs), Probability of Failure (PoF) and efficiency

A Heuristic Method to Evaluate the Effect of Soil Tillage on Slope Stability: A Pilot Case in Central Italy

Among the various predisposing factors of rainfall-induced shallow landslides, land use is constantly evolving, being linked to human activities. Between different land uses, improper agricultural practices can have a negative impact on slope stability. Indeed, unsustainable soil tillage can modify the mechanical properties of the soils, leading to a possible increase of the instability phenomena. However, the effects of soil tillage on slope stability are poorly investigated. To address this topic, the PG_TRIGRS model (a probabilistic, geostatistic-based extension of TRIGRS) was applied to a cultivated, landslide-prone area in central Italy, thoroughly studied and periodically monitored through systematic image analysis and field surveys. A heuristic approach was adopted to quantitatively evaluate the effect of soil tillage on the mechanical properties of the soil: after a first run of the model with unbiased parameters, the slope stability analysis was carried out assuming several percentages of reduction of the effective soil cohesion to mimic an increasing impact of soil tillage on the strength conditions. Then, a comparison between observed landslides and the spatial distribution of the probability of failure derived from the application of PG_TRIGRS was carried out. A back analysis with contingency matrix and skill scores was adopted to search for the best compromise between correct and incorrect model outcomes. The results show that soil tillage caused a 20 to 30% reduction in soil cohesion in the analyzed area