Hernán Patiño

Tailings are unconventional geomaterials that require dynamic characterisation due to seismic hazards at several storage facilities. Due to the anthropic origin of these materials, their dynamic properties differ from those reported for natural soils. In particular, the damping ratio is a relevant parameter that controls the dynamic response of tailings storage facilities. It can be estimated using different experimental methods. The objective of this research is to disclose the results obtained through laboratory tests in which the damping ratio was evaluated independently by Half-Power Bandwidth or the free-vibration decay methods. A comprehensive testing plan comprising resonant column tests and free-vibration decay tests was carried out on three types of tailings from the Riotinto mines (Huelva, Spain): Cerro Salomón Sand (CSS), High-Density Sludge (HDS), and Copper Lamas (CL). These tests were carried out under different effective consolidation pressures and torsional excitations. The results allowed the establishment of a series of relationships between the testing conditions and the identification of differences between the methods for tailings.

The characterisation of shear modulus degradation is essential for understanding the dynamic response of geomaterials. This article presents a modified hyperbolic model that evaluates the shear modulus for various angular strains and effective confining stresses. The model has been calibrated and validated using data from 108 resonant-column tests conducted on three different types of tailings from the Riotinto mines in Huelva, Spain. These tests were conducted on saturated samples that were consolidated at effective stresses of 50, 100, 150, 200, 250, and 300 kPa, accompanied by various combinations of torsional excitations to induce distinct angular strains. The results show that the hyperbolic model effectively predicts the shear modulus degradation in unconventional geomaterials, characterising the shear modulus under the testing conditions for the three types of Riotinto tailings. Additionally, the model can identify and confirm both the initial (or maximum) shear modulus and the reference angular strain as functions of the effective confining stress. The findings and model presented in this article contribute to enhancing the stability and resilience of geotechnical structures, including tailings storage facilities, that are subjected to dynamic loading, leading to safer designs and improved infrastructure performance.

The accumulation of tailings from ore extraction processes presents significant challenges in geotechnical engineering and environmental management. Tailings possess complex mechanical behaviour and unique characteristics, making them particularly sensitive to seismic activity. This study investigates the stiffness degradation of two non-plastic copper ore tailings collected from a seismically active region in Spain. An extensive experimental programme comprising 72 resonant-column tests was conducted to examine the dynamic behaviour of these materials under various isotropic stress conditions and shear strain levels. The findings were compared with those of other soils, including two well-characterised sands, providing a comprehensive analysis of normalised shear modulus and highlighting the distinct dynamic behaviour of the tailings. The experimental results offer a detailed analysis of the stiffness degradation, emphasising their unique dynamic behaviour and proposing a unified model to reliably describe the shear modulus degradation of these materials. The study underscores the importance of characterising tailings due to their inherent variability, enhancing the safety of tailings storage facilities in seismic regions.

In relation to the dynamic behavior of soils, the damping (D) and the dynamic shear modulus (G) has always acquired special relevance, since its evaluation has allowed the application of simplified models of dynamic soil behavior. They depend on many factors, being necessary to include the combination of monotonic (τ_o) and cyclical shear stress (τ_c) that are inevitably present under any structure. As part of an extensive testing programme, the objective of the present paper, besides the identification and classification tests, was to perform 139 simple cyclic shear tests with different levels of static stresses (τ_o) before the cyclic phase. This paper refers to the influence that the combination of monotonic (τ_o) and cyclic shear (τ_c) stresses has on the damping of a soft cohesive soil susceptible to sudden failure. Laboratory tests were carried out on undisturbed samples from the Port of Barcelona, in Spain. The samples were obtained at depths between 29 m and 52 m, that correspond to an effective vertical stress of (σ′_ov) between 277 kPa and 413 kPa, respectively. In general, the results indicate that: a) the combination of τ_o and τ_c controls the damping ratio (D), b) in the combined monotonic and cyclic case, it is possible to establish empirical functions of the type D = D(N, γ_c), D = D(N, G/σ′_ov) and D = D(N, u/σ′_ov), for different (τ_o/σ′_ov) ratios and c) it is possible propose a chart to evaluate the damping ratio as a function of N, G/σ′_ov and u/σ′_ov, for different (τ_o/σ′_ov) ratios.

The failure criterion of low-density volcanic rocks differs radically from that of conventional rocks by manifesting collapse under isotropic stress. In this way, the shapes of the failure model do not reveal a continuously increasing growth of deviating stress with the isotropic stress, but they reach a maximum value, after which they decrease until they vanish under the isotropic collapse pressure. As a consequence, engineering applications require the implementation of numerical codes and the resolution of associated numerical difficulties. This article presents the problem of the bearing capacity of a foundation on a low-density volcanic rock using the DLO (discontinuity layout optimization) numerical method. The analysis of results shows the ability of the DLO method to solve the numerical difficulties associated with the complex failure criteria, so that the convergence and stability of the solution can be achieved without generating high computational costs. Additionally, a discussion of the DLO results is also presented, demonstrating forms of failure on the ground following the real collapses in these volcanic materials. In addition, numerical validation was performed with the finite difference method, using FLAC, and with an analytical method using simplified configurations, obtaining good contrast results, with the DLO method performing better. In this way, an adequate and reliable resolution technique is provided to face the problem of bearing capacity in low-density volcanic rocks, overcoming limitations referred to in the technical literature regarding the difficulty of treating highly non-linear and non-monotonic numerical criteria, which allows the introduction of isotropic collapse failure.

This paper refers to cyclic shear strains (γ_c) and permanent shear strains (γ_p) of a soft cohesive soil, when both monotonic shear stresses (τ_o) and cyclic shear stresses (τ_c) are applied. The research is backed by an extensive experimental program with 139 cyclic simple shear tests that included identification and classification tests. These cyclic simple shear tests were conducted under different levels of stresses, τ_o, before the cyclic phase. Laboratory tests were carried out on undisturbed samples from the Port of Barcelona, located in Spain on the Mediterranean coast, and characterized by a monotonic strength (τ_max) approximately equal to 30% of the initial effective vertical stress (σ′_ov). The samples were taken at depths between 29 and 52 m and correspond to an initial effective vertical stress between 277 and 413 kPa, respectively. In general, the results indicate that: (a) the combination of τ_o and τ_c controls the generation of γ_c and γ_p, (b) it is not always true that when τ_o/σ′_ov + τ_c/σ′_ov ≈ τ_max/σ′_ov, the soil reaches failure cyclically, and (c) empirical relations useful for design can be established between γ_c, γ_p, and the number of cycles (N), for different relationships varying (τ_o/σ′_ov) between 0% and 25%.

The application of large-amplitude cyclic shear loads on soft soils results in large strains after a certain number of cycles. These strains are highly dependent on static shear load and cause stress–strain relationships with various characteristics that are difficult to model. Moreover, the characterization of suitable cyclic behavior in soft soils should include the effects of soil degradation, load history, and excess pore pressure. This article presents a model that is based on the mathematical theory of hysteresis. This model considers the response of soft cohesive soils under combinations of static and cyclic shear loads with large strains. In addition, the resulting model reproduces the shapes and values of the stress–strain laws that were obtained from simple shear tests on cohesive soil samples.