Numerical modelling of ground-tunnel support interaction using bedded-beam-spring model with fuzzy parameters

The study of the ground-tunnel interaction by introducing a predetermined degree of variation (fuzziness) in some parameters of the chosen model is presented and discussed. This research comes from the consideration that tunnel model parameters and geometry are usually affected by a degree of uncertainty, mainly due to construction imprecision and the great variability of rock mass properties. The research has been developed by using the fuzzy set theory assuming that three model parameters are affected by a certain amount of uncertainty (defined by the so-called membership functions). The response of the numerical model is calculated by solving the fuzzy equations for different shapes of the membership functions. In order to investigate the effects of some model parameters, and to provide a simple procedure and tool for the designers, a study on the effect of tunnel boundary conditions, based on a fuzzy model, has been carried out using a simple but well known and widely used design method such as the bedded-beam-spring mode

Tunneling in heterogeneous ground: an update of the PBE code accounting for the uncertainty in estimates of block quantities from site investigations

Hard rock blocks embedded in a soft matrix can be found in many geological units. When tunneling in these heterogeneous formations, many technical problems can be caused by the presence of cobbles and boulders. These risks depend on the excavation method selected, and typically increase as the block content, dimension and frequency increase. Therefore, successful tunneling projects require cobble-boulder properties to be accurately predicted, in order to select the best tunneling method and design the most appropriate cutterhead. A recent and extensive statistically-based methodology was developed to estimate the probability of encountering rock blocks located totally or partially within the tunnel excavation area, resulting in a free executable code, named PBE. A significant limitation of this code is that it requires, among other input parameters, the block content expected within the tunnel zone. This parameter can be estimated from 1D (i.e. borings/scanlines) or 2D (i.e. geological maps, outcrops, photographs) measurements. However, the block quantity inferred from these measurements is generally affected by a high magnitude of error. In this paper, an extension of the PBE is presented, that accounts for the uncertainty in estimates of block quantities from site investigations. The updated version of the executable code is also provided

Sviluppo di un software per l'interpretazione dei dati ottenuti con cella CSIRO

This article deals will the development of a software that allows the operator to evaluate the complete state of stress in the rock mass in the neighborhood of the point where the cell CSIRO overcoring is carried out. For this purpose the theory presented in the '60s by Y. Hiramatsu and Y. Oka (Hiramatsu Y. and Oka Y., 1967) has been considered

Geomechanical characterization of an Italian complex formation with a block-in-matrix fabric

Melanges are chaotic and heterogeneous geological mixtures of strong rock blocks embedded in a weaker finer-grained matrix. These complex formations, often referred to as “bimrocks” (block-in-matrix rocks) and “bimsoils” (block-in-matrix soils), are characterized by a high spatial, dimensional and lithological variability. Such a variability, together with the presence of rock inclusions of different lithologies and dimensions, makes the collection of representative high-quality specimens for laboratory tests very challenging. As a consequence, the determination of the geomechanical properties of such geomaterials is extraordinarily problematic. In this paper a preliminary characterization of an Italian sedimentary chaotic melange is performed by means of several laboratory tests. In dry conditions it looks like a bimrock but it is very sensitive to water and will transform into a bimsoil if it comes into contact with water. Since conventional drilling methods could not be employed to collect the pecimens, a manual coring had to be used. The irregular lumps obtained were then cut with a specially made cutting machine. Point load tests and uniaxial compression tests were performed on the melange specimens. Atterberg limits, grading curves and mineralogical analyses were further carried out on the matrix material, in order to characterize it. The paper comments on both the procedure of these tests and the results obtained

Slope stability in heterogeneous rock masses with a block-in-matrix fabric

Heterogeneous rock masses composed of strong rock blocks embedded in a weaker matrix belong to complex formations and are often referred as BIMRocks (Block-In-Matrix-Rocks). The high variability of such rock bodies (spatial, dimensional and lithological) and the extraordinary problematic determination of their geomechanical properties make both the planning and the modelling of engineering works in and on these geomaterials ex-tremely challenging. The aim of this study is to investigate the stability of theoretical slopes in bimrocks. In or-der to determine how rock inclusions may influence the overall behaviour of these rock units, different block orientations and several Volumetric Block Proportions (VBP) are considered. To take the inherent spatial and dimensional variability of the blocks into ac-count, many heterogeneous slope models are generated for each block orientation and VBP examined. In order to do this, a stochastic approach is used, which provides for an acceptable statistical validity of the results

Effects of block shape and inclination on the stability of melange bimrocks

A wide range of heterogeneous geological units composed of strong rock blocks enclosed in a bonded matrix of fine texture exists worldwide. Such geomaterials belong to geotechnically complex formations and are often referred to as bimrocks (block-in-matrix rocks) or bimsoils (block-in-matrix soils), as a function of their matrix characteristics and the interface strength between the matrix and blocks. Stability problems occurring in such complex geomaterials have been analysed almost exclusively by means of deterministic approaches and with the aim of investigating the effects of variable block contents on their mechanical behaviour. However, bimrocks and bimsoils can present very different internal block-in-matrix arrangements and properties according to their forming process and, consequently, significantly dissimilar mechanical behaviours. Therefore, the aim of this paper was to statistically investigate and compare the stability of theoretical slopes in the most widespread bimrock formations, i.e. sedimentary and tectonic melanges. These formations are characterised by substantial differences in their rock inclusion geometry. To this aim, a great number of 2D slope models were generated to enclose blocks with variable shapes, dimensions, arrangements, inclinations and contents. To obtain statistically based results, fifteen configurations were analysed for each block content and geometrical configuration considered. The results obtained indicate that block shapes and orientations significantly affect the stability of slopes in bimrocks only when the block contents are greater than 40%. Moreover, it is demonstrated that blocks inclined 0° to the horizontal provide the most tortuous and irregular failure surfaces and, consequently, the highest safety factors

A Statistically Based Methodology to Estimate the Probability of Encountering Rock Blocks When Tunneling in Heterogeneous Ground

Strong rock blocks embedded in a weaker soil matrix are found in many geological units. When tunneling in ground containing cobbles and boulders, extremely challenging conditions can be encountered. Such inconveniences may be avoided by means of appropriate tunneling methods and cutterhead designs, which require the content, frequency, and size of rock blocks to be predicted as accurately as possible. Several approaches have been developed to estimate the block fraction of heterogeneous geomaterials for excavation. However, the estimation of cobble–boulder quantities both all along the tunnel and only partially embedded within the tunnel face remains a critical issue. This study develops a methodology for the estimation of the probability of encountering blocks partially or totally contained within the tunnel excavation area, wherein the area of intersection with the tunnel face is greater than the given critical values. For this purpose, a statistical approach has been implemented in a Matlab routine. The potential of this code is that it provides extremely useful and statistically based information that can be used for making a more rational choice regarding tunneling technique and in terms of designing a suitable cutterhead in order to avoid technical problems during tunnel excavations in heterogeneous ground. The executable code is provided

A simple procedure to calibrate soil parameters for slope stability modelling: the Langhe (1994) case study

Shallow landslides triggered by rainfall represent common geotechnical hazards in Italy. In this context, the hundreds of landslides which occurred in the Piedmont Langhe area in November 1994, still identified among the most extensive areal event in the last 30 years in Italy, are investigated. Exploiting data from the surveys of Campus et al. (1998), here we calibrate simple soil water and mechanical properties (i.e., the saturated permeability and cohesion values) to overcome the large uncertainty affecting the determination of parameters to be used as inputs for physically based soil slip models. This work aims to contribute to the development of reliable soil data inventory that may be of direct interest in slope stability modelling, following Vannocci et al. (2020). The analysis was conducted on a small number of cases selected from a sample of 238 observed landslides, to which geometries and geotechnical features were attributed from a regional database. The calibration was performed using a simple hydrological model, i.e. that of Rosso et al. (2006), since it allows a reasonable check on the sensitivity of soil parameter values to the instability conditions. For saturated conductivity, a variation range was obtained, whose upper limit referred to the so-called bucket model. Assuming that part of the rainfall contributes to surface runoff, a lower permeability value was derived using the proportional flow method ψ, consistently with the real dynamics of the processes. Soil cohesion was calibrated by mechanical analysis based on the infinite slope theory, by targeting the Safety Factor SF to assume the value 0.99. When comparing locally calibrated parameters and the reference ones found in the database some differences arise; in particular, in several cases, based on calibrated values, SFs quite lower than 1 were derived. It must be pointed out that the calibration procedure allowed us to characterize shallow soils, made up of remolded and often vegetated soil, while the regional dataset provide information on undisturbed soil samples, typically collected at depths greater than those of interest. The possibility of getting reliable soil parameters to be used in physically based modelling of shallow landslides is a complex task. Here we use a calibration method to obtain meaningful saturated hydraulic conductivity and cohesion values, compatible with the observed instabilities. The implications of the differences found between the calibrated parameters and those published in the regional dataset will lay the foundations for subsequent investigations, as this analysis will be part of the research framework of the RETURN Extended Partnership Project

A reliability-based method for taking into account snowfall return period in the design of buildings in avalanche-prone areas

For many natural hazards, design codes include simple approaches, based on semiprobabilistic methods, which allow the engineer to check the structural safety of the construction. This framework implements the socio-economic optimization of the resources to be used to build construction works. At present no simple approaches have been formulated for snow avalanche hazard. Recent research focuses on fully probabilistic assessment of structural safety, aimed at the estimation of the failure probability of the element at risk. This strategy requires a large amount of data and complex simulation techniques. In order to propose the implementation of snow avalanche hazard into the framework of modern design codes, we suggest an approach based on different avalanche scenarios with different snowfall return periods depending on the consequences of failure of the designed construction

The failure of the Stava Valley tailings dams (Northern Italy): numerical analysis of the flow dynamics and rheological properties

Tailings dams are made up of mining residue deposits, and they represent a high risk, in terms of mechanical instability. In the event of collapse, the tailings in such dams may be released and flow over long distances, with the potential risk of extensive damage to property and life. The traditional geotechnical assessment of tailings facilities has mainly concentrated on the stability of tailings dams, while relatively few studies have investigated the flow of tailings released after dam failure. In this context, it is possible to state that, if the complex rheological behaviour of the tailings material is captured correctly during the flow, numerical modelling can be used to contribute to a better comprehension of the flow characteristics and for the assessment of the possible extension of the impact area. Considering the wide range of possible rheological behaviour that tailings flows can assume (from laminar to turbulent), this paper presents the new version of a computer model, which was designed to simulate the motion of rapid flow movements across 3D terrain. This new version integrates the existing rheological kernel (Frictional, Voellmy) with two new rheological laws (Bingham and Turbulent), and adds the possibility of changing the rheological properties of the flowing mass during the propagation process. The code has been applied to the disastrous flow that was caused by the failure of a pair of tailings impoundments in the Stava Creek Valley (Italy) in 1985. Since different interpretations on this flow behaviour already exist in literature, and since a large number of changes in the rheological values along the run-out path have been proposed to recreate its dynamics, new simulations, carried out with different rheological combinations, are presented and discussed here in order to obtain a better understanding of the flow dynamics and to identify the rheology that reproduces the phenomenon that occurred with the fewest possible changes in the rheological values along the runout path. The latter aspect is particularly important when numerical analyses are used for prediction purposes. The great rheological flexibility of the new code has allowed the Voellmy rheology and a combination of its parameters to be identified as the most suitable to describe the Stava flow, even where the run-out path presented critical characteristics