Stefano Utili

Modeling the evolution of natural cliffs subject to weathering: 2. Discrete element approach

The evolution of slopes subjected to weathering has been modeled by assuming Mohr-Coulomb behavior and by using a numerical approach based on the discrete element method (DEM). According to this method, soil and/or rock are represented by an assembly of bonded particles. Particle bonds are subject to progressive weakening, and so the material weathering and removal processes are modeled. Slope instability and material movement follow the decrease of material strength in space and time with the only assumption concerning the weathering distribution within the slope. First, the case of cliffs subject to strong erosion (weathering-limited conditions) and uniform weathering was studied to compare the results of the DEM approach with the limit analysis approach. Second, transport-limited slopes subject to nonuniform slope weathering were studied. Results have been compared with experimental data and other geomorphologic models from the literature (Fisher-Lehmann and Bakker–Le Heux). The flux of material from the slope is modeled assuming degradation both in space and time.

A combined DEM-FEM numerical method for Shot Peening parameter optimisation

A novel combined DEM-FEM method for Shot Peening optimisation is presented.The model provides a rapid method to investigate the effect of the peening parameters.A new algorithm was implemented accounting for impact dependent coefficients of restitution.The minimum area required in the analyses to obtain true residual stresses was determined. A numerical modelling approach capable of simulating Shot Peening (SP) processes of industrial interest was developed by combining the Discrete Element Method (DEM) with the Finite Element Method (FEM).In this approach, shot-shot and shot-target interactions as well as the overall shot flow were simulated efficiently using rigid body dynamics. A new algorithm to dynamically adapt the coefficient of restitution (CoR) for repeated impacts of shots on the same spot was implemented in the DEM code to take into account the effect of material hardening. Then, a parametric study was conducted using the Finite Element Method (FEM) to investigate the influence of the SP parameters on the development of residual stresses.Finally, a two-step coupling method is presented to combine the output of DEM simulation with FEM analyses to retrieve the Compressive Residual Stresses (CRS) after multiple impacts with the aim to evaluate the minimum area required to be modelled to realistically capture the field of residual stresses. A series of such coupled analyses were performed to determine the effect of peening angle and the combination of initial velocity and mass flow rate on CRS.

Designing Tunnel Support in Jointed Rock Masses Via the DEM

A systematic approach of using the distinct element method (DEM) to provide useful insights for tunnel support in moderately jointed rock masses is illustrated. This is preceded by a systematic study of common failure patterns for unsupported openings in a rock mass intersected by three independent sets of joints. The results of our simulations show that a qualitative description of the failure patterns using specific descriptors is unattainable. Then, it is shown that DEM analyses can be employed in the preliminary design phase of tunnel supports to determine the main parameters of a support consisting of rock bolts or one lining or a combination of both. A comprehensive parametric analysis investigating the effect of bolt bonded length, bolt spacing, bolt length, bolt pretension, bolt stiffness and lining thickness on the tunnel convergence is illustrated. The highlight of the proposed approach of preliminary support design is the use of a rock bolt and lining interaction diagram to evaluate the relative effectiveness of rock bolts and lining thickness in the design of the tunnel support. The concept of interaction diagram can be used to assist the engineer in making preliminary design decisions given a target maximum allowable convergence. In addition, DEM simulations were validated against available elastic solutions. To the authors’ knowledge, this is the first verification of DEM calculations for supported openings against elastic solutions. The methodologies presented in this article are illustrated through 2-D plane strain analyses for the preliminary design stage. More rigorous analyses incorporating 3-D effects have not been attempted in this article because the longitudinal displacement profile is highly sensitive to the joint orientations with respect to the tunnel axis, and cannot be established accurately in 2-D. The methodologies and concepts discussed in this article, however, have the potential to be extended to 3-D analyses.

Analytical Solutions for the Construction of Deeply Buried Circular Tunnels with Two Liners in Rheological Rock

The construction of underground tunnels is a time-dependent process. The states of stress and strain in the ground vary with time due to the construction process. Stress and strain variations are heavily dependent on the rheological behavior of the hosting rock mass. In this paper, analytical closed-form solutions are developed for the excavation of a circular tunnel supported by the construction of two elastic liners in a viscoelastic surrounding rock under a hydrostatic stress field. In the solutions, the stiffness and installation times of the liners are accounted for. To simulate realistically the process of tunnel excavation, a time-dependent excavation process is considered in the development of the solutions, assuming that the radius of the tunnel grows from zero until its final value according to a time-dependent function to be specified by the designers. The integral equations for the supporting pressures between rock and first liner are derived according to the boundary conditions for linear viscoelastic rocks (unified model). Then, explicit analytical expressions are obtained by considering either the Maxwell or the Boltzmann viscoelastic model for the rheology of the rock mass. Applications of the obtained solutions are illustrated using two examples, where the response in terms of displacements and stresses caused by various combinations of excavation rate, first and second liner installation times, and the rheological properties of the rock is illustrated.

Investigation of rock fragmentation during rockfalls and rock avalanches via 3-D discrete element analyses

This paper investigates the characteristics of dynamic rock fragmentation and its influence on the postfailure fragment trajectory. A series of numerical simulations by discrete element method (DEM) were performed for a simple rock block and slope geometry, where a particle agglomerate of prismatic shape is released along a sliding plane and subsequently collides onto a flat horizontal plane at a sharp kink point. The rock block is modeled as an assembly of bonded spherical particles with fragmentation arising from bond breakages. Bond strength and stiffness were calibrated against available experimental data. We analyzed how dynamic fragmentation occurs at impact, together with the generated fragment size distributions and consequently their runout for different slope topographies. It emerges that after impact, the vertical momentum of the granular system decreases sharply to nil, while the horizontal momentum increases suddenly and then decreases. The sudden boost of horizontal momentum can effectively facilitate the transport of fragments along the bottom floor. The rock fragmentation intensity is associated with the input energy and increases quickly with the slope angle. Gentle slopes normally lead to long spreading distance and large fragments, while steep slopes lead to high momentum boosts and impact forces, with efficient rock fragmentation and fine deposits. The fragment size decreases, while the fracture stress and fragment number both increase with the impact loading strain rate, supporting the experimental observations. The fragment size distributions can be well fitted by the Weibulls distribution function.

Analytical Solutions for Tunnels of Elliptical Cross-Section in Rheological Rock Accounting for Sequential Excavation

Time dependency in tunnel excavation is mainly due to the rheological properties of rock and sequential excavation. In this paper, analytical solutions for deeply buried tunnels with elliptical cross-section excavated in linear viscoelastic media are derived accounting for the process of sequential excavation. For this purpose, an extension of the principle of correspondence to solid media with time varying boundaries is formulated for the first time. An initial anisotropic stress field is assumed. To simulate realistically the process of tunnel excavation, solutions are developed for a time-dependent excavation process with the major and minor axes of the elliptical tunnel changing from zero until a final value according to time-dependent functions specified by the designers. In the paper, analytical expressions in integral form are obtained assuming the incompressible generalized Kelvin viscoelastic model for the rheology of the rock mass, with Maxwell and Kelvin models solved as particular cases. An extensive parametric analysis is then performed to investigate the effects of various excavation methods and excavation rates. Also the distribution of displacements and stresses in space at different times is illustrated. Several dimensionless charts for ease of use of practitioners are provided.

Novel Approach for Health Monitoring of Earthen Embankments

AbstractThis paper introduces a novel modular approach for the monitoring of desiccation-induced deterioration in earthen embankments (levees), which are typically used as flood-defense structures. The approach is based on the use of a combination of geotechnical and noninvasive geophysical probes for the continuous monitoring of the water content in the ground. The level of accuracy of the monitoring is adaptable to the available financial resources. The proposed methodology was used and validated on a recently built, 2-km-long river embankment in Galston (Scotland, United Kingdom). A suite of geotechnical probes was installed to monitor the seasonal variation of water content over a 2-year period. Most devices were calibrated in situ. A novel procedure to extrapolate the value of water content from the geotechnical and geophysical probes at any point of the embankment is shown. Desiccation fissuring degrades the resistance of embankments against several failure mechanisms. An index of susceptibility is ...

Numerical Simulation of the Collapse of Granular Columns Using DEM

In this paper, the collapse of plain strain dry granular columns was investigated by means of 3D DEM (Discrete Element Method) simulations. In the literature, 2D DEM analyses have been previously performed and showed to be unable to replicate the experimentally observed mass flow and final run-out distances. This is mainly due to the inability of 2D simulations to replicate the three dimensional motion of real particles. Spherical particles and a simple contact model based on linear springs, dashpots and frictional sliders were employed in the presented simulations. A rolling resistance model governed by two micromechanical parameters was added in order to indirectly account for the effect of particle non-sphericity on the angular moment equilibrium of the granular assembly. Calibration of the rolling resistance model leads to predictions of run-out distances in quantitative agreement with the available experimental data.

DEM modeling of cantilever retaining excavations: Implications for lunar constructions

Purpose – Retained excavation is important for future lunar exploratory missions and potential human colonization that requires the construction of permanent outposts. Knowledge in excavation obtained on the earth is not directly applicable to lunar excavation because of the low lunar gravity and the non-negligible adhesive van der Waals interactions between lunar regolith grains. The purpose of this paper is to reveal how the gravity level and lunar environment conditions should be considered to extend the knowledge in earth excavation response to lunar excavation. Design/methodology/approach – Two-dimensional discrete element method simulations were carried out to investigate the respective effect of gravity level and lunar environment conditions (high-vacuum and extreme temperature) on retained excavation response. A novel contact model was employed with a moment – relative rotation law to account for the angularity of lunar soil particles, and a normal attractive force to account for the van der Waals...

A Numerical Investigation of Quasi-static Conditions for Granular Media

A numerical campaign by means of the Discrete Element Method (DEM) was carried out in order to investigate the accuracy of the assumption of quasi-static (QS) conditions in DEM experiments. The dimensionless inertial number, , was considered in order to assess the inertia of the system at both the peak and critical states of the material. DEM triaxial tests were run in a 3D periodic cell at increasing inertial number (e.g. loading rate). The classical Hert-Mindlin no-slip solution was employed at the contact level and grains were assigned with realistic properties of silica sands. Furthermore, a realistic wide particles size distribution was adopted.Macroscopic observations show that the QS limit is not approached even for very small inertial numbers, i.e. I≈10−5, in contrast with the threshold identified by Roux and Combe (2010), i.e. I≤10−3. Nevertheless, the rate dependency of the mechanical response should be interpreted as a result of local dynamic effects (micro-inertial effects) rather than an indication of a time-dependent behaviour. Interestingly, such behaviour was only visible at very small strains, i.e. less than 3%, suggesting that different mechanisms may dominate at the particulate level at different stages of shearing.The combined influence of inertia and confining stress was then studied in detail at the critical state. Results show that the influence of the applied pressure is non-negligible on the critical void ratio. Nevertheless, the opposite and counterintuitive behaviour is exhibited (i.e. higher void ratios for higher pressures) when a constant inertial number I≤10−3 is adopted. A modified definition of limiting inertial number, proportional to the speed of sound, is proposed which appears to produce results in agreement with the principles of critical state theory.