Professor Anna Giacomini

Fragmentation of rocks upon impact during rockfall is a phenomenon that is poorly understood, scarcely researched and difficult to predict. However, to adequately predict the outcomes of rockfall events, it is essential to know whether a given block is likely to fragment given the impact conditions and what will be the outcome of the fragmentation process; that is, the number, size and trajectory of fragments. To date, there is no model or data that can be used to fully answer these questions. This paper presents the first theoretical model that can predict the fragmentation survival probability of brittle spherical blocks upon dynamic impact (i.e. drop tests) from the statistical distribution of material properties, determined from a range of standard quasi-static tests. Considering that survival probabilities tend to follow a Weibull distribution, the model focuses on predicting the two Weibull parameters, commonly known as the shape parameter (m) and the scale parameter (here, the critical kinetic energy). The model is based on theoretically-derived conversion factors used to turn the critical work required to fail disc samples in quasi-static indirect tension into the critical kinetic energy to cause failure of spheres at impact in drop tests. The mechanistic conversion factors specifically account for the shape and size of the specimens tested and the increase of strength under dynamic loading (strain rate effect). Three series of drop tests were conducted (on spheres of three different diameters) and complemented by extensive material characterisation testing in order to validate the novel predictive model. The variability of material properties was characterised, and it was found that the material strength found by the characterisation tests generally follows a Weibull form, but the survival probability distribution of the drop tests seems to be linear. The predicted conversion factors were first compared against their experimental counterparts before validating the prediction of survival probability of the spheres upon dynamic impact (in drop tests). It was found that it is possible to predict the survival probability of artificial rock of three different diameters (50 mm, 75 mm, 100 mm) and two different strengths upon impact solely from the statistical information coming from Brazilian tests and with an average relative error of less than 9%.

This paper presents a new methodology for estimating the expected energies and first impact distances at the base of a rock cliff, subject to the geometry and properties of the cliff and the representative block being known. The method is based on a sensitivity analysis, conducted by means of kinematic simulations and carried out for a large range of input parameters and their combinations, taking into account the uncertainty associated with their estimate. The proposed approach is validated by comparing predictions to experimental data and shows great potential for a quick qualitative hazard assessment.

In rockfall science, the bullet effect refers to the perforation of a rockfall mesh by a small block traveling at high speed. To date, there is still no comprehensive experimental data set investigating the underlying mechanisms of such effect. The bullet effect illustrates the fact that the capacity of a rockfall mesh depends on the size and speed of the impacting block. This paper presents the results of an experimental study on the effect of block size and mesh geometry (aperture and wire diameter) on the mesh performance. The results clearly show that the amount of energy required to perforate the mesh drops as the blocks get smaller. They also suggest that the mesh performance reaches a maximum and reduces to zero when the mesh cannot sustain the static load imposed by very large blocks. The outcome of the first series validates an analytical model for mesh perforation, making it the first simple model capturing the bullet effect. A second series of tests focused on the effect of mesh geometry and it was found that decreasing the mesh aperture by 19¿% improves the performance by 50¿% while only an extra 30¿% could be gained by increasing the wire diameter by 33¿%. The outcomes of the second series were used to discuss and redefine a dimensionless geometrical parameter G* and to validate a simple power type equation relating the mesh characteristics and the mesh performance.

From a consideration of the concepts of geological weathering and structure, it can be expected that rockfall hazards should be characteristically different in different geological environments. This paper tests this idea by looking at the geometric characteristics of rock fragments formed on natural slopes in four different geological environments in Eastern Australia, where rockfall phenomena are often characterised by rolling of pre-detached debris. By measuring the three principle dimensions and making a systematic assessment of the shape characteristics of samples of rock debris in significant geological environments, it is found that the distributions of size and shape for the surface debris are statistically different. From the results, it is shown that the size and shape of debris is directly controlled by the rock type, its weathering characteristics and the structure of the parent rock mass. The severity of rockfall hazards is shown to be relatively lower in areas of Tertiary basalt, as the size of rolling fragments is limited by closely spaced fracturing inherited from its formation and the tendency to deteriorate further as it weathers deeply and rapidly. It is also lower in areas of Palaeozoic volcanics, since these tend to produce relatively angular fragments with higher proportions of fragments that are inherently more resistant to rolling. By contrast, thickly bedded sandstones form larger blocks with a larger proportion of shapes that are more prone to rolling. The size distribution of fragments is shown to be well approximated by a log-normal statistical distribution, and using the data provided in this study, it is possible to generate the size and shape data needed to undertake a stochastic assessment of rockfall trajectories in different geological environments. © 2013.

This paper reports an analysis procedure for the evaluation of the features of the motion of blocks detaching from a steep rock wall and traveling down the slope below. Starting from the execution of real scale rock fall tests, carried out on two slopes having different morphology and lithology, the paper describes the methodology used for test interpretation and a procedure for the evaluation of the parameters best suited to the description of rock fall motion. The influence of the parameters assessed on the prediction of the rock fall trajectory was also investigated using two-dimensional and three-dimensional numerical models. These models were calibrated by means of a back analysis of the in situ tests, which also allowed the evaluation of the uncertainties involved in the parameters experimentally estimated.

Rockfall can cause loss of lives and significant damage to infrastructure and requires adequately designed protection structures to reduce the risk to an acceptable level. In Australia, rockfall draperies are more and more used as a passive protection structure although some research is still required to fully characterize the performance of draperies. Full-scale experimentation on rockfall drapery systems is very expensive and time consuming so that numerical methods are a useful and more economical alternative in order to better understand the behaviour of such systems. This paper presents a realistic 3D model of a chain-link drapery system developed with the commercially available finite element package of ABAQUS. The dynamic response of the system was simulated by incorporating the elastoplastic constitutive model in the explicit procedure of the solution. The developed model was calibrated by comparing the numerical results with the results of the laboratory-scale experiments. Preliminary results showed that the calibrated model was capable of predicting the system response with reasonable accuracy. In future, the model will be used in order to conduct parametric studies on different aspects of rockfall drapery design including slope material properties, number and spacing of anchors, impact energy, slope inclination, height of drapery system and block size.

Reliable prediction of shear strength for engineering scale in-situ discontinuities is still problematic. There is currently no consensus or a satisfactory method to estimate shear strength, account for surface variability and manage the effects of the recognised ¿scale effect¿ phenomenon. Shear behaviour of a discontinuity rock mass greatly depends upon the rock joint roughness, which is generally concealed within the rock mass. As such, the limited amount of accessible surface information complicates even further the exercise of shear strength prediction. Often, small size specimens (e.g. rock core) are recovered to conduct experimental tests and predictions can be made from analysing traces. In this paper, three different shear strength prediction approaches were followed and their relative performances were compared. The results reveal that the prediction of shear strength from recovered sub samples can be significantly variable. The application of Barton¿s empirical model to four selected traces, produced a large scattering of results, with the prediction highly dependent on the trace used. Conversely, the application of Casagrande and co-workers¿ stochastic approach on the same traces, produced the least scatter and provides statistical data to quantify variability and uncertainty.

This paper investigates the factors affecting the shear strength of granular materials by means of the tilt box test. The test involves the titling of a specifically designed split-box and measuring the angle at which the upper half of the box slides off. This angle can be interpreted as the peak friction angle of the sample for a low stress state. Experimental tests and numerical analyses using the Discrete Element Method (DEM) are carried out and compared. The study is carried out for two different materials, a rounded and an angular aggregate. It is shown that rolling resistance is needed to match the experimental results if only spheres are used but this is not the case for non-spherical particles. © 2015 Taylor & Francis Group, London.

This work consists of a preliminary numerical study on the impact behaviour of rock blocks on muckpiles using the Discrete Element Method (DEM). A contact law using a linear hysteresis model for normal contacts and a Mohr-Coulomb model for tangential contacts is used. In addition, two dissipative laws are introduced consisting of a moment transfer law and normal viscous damping law. One of the aims of this study is to investigate if the contact law enables a realistic modelling of the rebound characteristics of a block during impact on a muckpile. A parametric study is carried out in order to investigate the influence of the constitutive model parameters and the presence of clumps. Numerical predictions are compared to experimental results. It is shown that clumps in the muckpile are essential to reproduce realistic results. © 2015 Taylor & Francis Group, London.

Numerical simulation of collapse of tunnels in weak rocks is one of the challenging problems in rock mechanics. Collapse can impose technical and financial difficulties during the construction of underground excavations. This phenomenon, particularly in shallow excavations, may lead to propagation of a caved zone toward the surface and formation of chimney subsidence. Chimney subsidence has numerous catastrophic consequences such as the adverse environmental impacts and damage to surface structures in vicinity of excavations. Due to the importance of subsidence in civil engineering, the main goal of this study is to propose an appropriate method for preventing the formation of subsidence in shallow tunnels in weak rocks. The Dolaei road tunnel is investigated as a case study, where a large chimney subsidence has occurred. The geology of the area and the geostructural characteristics of the rock mass surrounding the tunnel are briefly reviewed in this paper. Two-dimensional as well as three-dimensional numerical simulations are conducted to study the tunnel stability. Moreover, the effects of staged excavation and pre-supporting method on controlling the ground movements are studied. Outcomes of this study indicate that employing forepoling and staged excavation can be a practical remedy to control the ground movements and prevent the formation of chimney subsidence.

This paper presents a discrete modelling approach which allows the simulation of rockfalls behind drapery systems. The falling rock is represented by a rigid assembly of spheres whereas the slope is represented by triangular elements. Energy dissipation during impact on the slope is considered via friction and viscous damping. The drapery is represented by a set of spherical particles which interact remotely. The numerical model is used to investigate the efficiency of a drapery system. Various simulations with two different block sizes are performed and the numerical predictions of simulations with drapery are compared to simulations without drapery in order to assess the performance of the protection system.

Ground subsidence due to mining is a very complicated geomechanics phenomenon, particularly in the longwall mining method. Due to the adverse consequences of subsidence on surface structures, facilities, and environment, subsidence prediction has attracted interest in geotechnical engineering over the past few decades. Previous research has indicated that the thickness and inclination of seam, depth of working panel, and ground characteristics are the main factors which should be considered in estimating the subsidence. Methods for predicting the subsidence are: empirical, analytical and/or numerical. The geotechnical characteristics of ground affect the caving, collapse, and bulking of goaf and the extent of the cavity toward the surface. The main aim of this paper is to investigate the mechanism of ground surface subsidence based on analytical and numerical simulations. Analytical solutions are based on the theory of elasticity. The Distinct Element Method, which can simulate jointed and stratified rock mass, is used for numerical modelling of subsidence due to mining. In this paper, numerical and analytical results are obtained for two different cases of isotropic and transverse isotropic (layered) strata. Outcomes of this study indicate that the weak planes and stratification not only play a significant role in geotechnical behavior of the rock mass, but they affect the mechanism of ground movement. Results also show that considering the real structure and properties of ground will lead to more accurate estimation of measured subsidence in the field.

The paper investigates the soil-structure interaction between a clay embankment of the new high speed railway network Milan-Naples (Italy) and its clay foundation reinforced with different length driven concrete piles and geotextile. The embankment has been placed next to the old railway line. A monitoring system has been installed in a section of the embankment from the beginning of the construction of the new line. Numerical simulations describing both the old and new embankments (for a given section) have been carried out, by using Abaqus computer code, in order to back analyse the monitoring data. The analysis has been carried out in effective stress conditions, referring to long term conditions, to analyse the engineering problem to reduce the final embankment settlement. The results of the numerical simulations put into evidence that the model can reproduce the experimental data regarding the settlements below the new embankment and the interaction between the new embankment supported by piles and the old railway line embankment. © 2007 Taylor & Francis Group.