Dr. Villalobos-Jara, Felipe

The geotechnical characterization of coarse granular materials such as very coarse-grained soils, rockfills, mining waste rocks and related materials is one of the key themes in geotechnical engineering but least studied and developed. Although there are some geotechnical standards and accepted geotechnical practice, there is not a standard for size-scaling, which is a critical step in advanced stage engineering (i.e. detailed design) on large structures involving this kind of materials. Several size-scaling techniques are available for use, with advantages and disadvantages. Among these, the parallel gradation method PGM (also known as homothetic grain size distribution), is one of the current practices and used for more than 50 years, but surprisingly just a few studies have corroborated its capability, and under specific material types. This work assesses a detailed database covering the development of this method from its first uses up to now. The application of this method is analysed based mainly on the material maximum internal friction angle and the Marsal’s particle breakage index (Bg).

The effect of rock weathering on geotechnical parameters can become substantial in geotechnical design. The weathering degree (WD) has usually been divided into five levels from fresh rock to completely weathered rock. In this study, the above five WDs are adopted to analyse the variation of several regularly used geotechnical properties. To that aim, a series of laboratory tests on a Chilean granitic rock in different stages of weathering was carried out. The results of the tests have been analysed by normalizing each geotechnical parameter with respect to the value obtained for slightly weathered samples instead of for fresh rock samples as is usually adopted. In this way, it was found that the unconfined compressive strength reduces steadily and considerably with each WD, as has been previously reported. Conversely, the modulus of deformation and P-wave velocity tend to stabilize the rate of reduction for highly and completely weathered rock samples, which does not agree with the trend and higher rate reductions found by other researchers. This stabilization reduction rate with WD was also found for compaction density, friction angle and Poisson's ratio.

Rockburst events have been a serious problem for many years in many mines worldwide, and in particular at El Teniente mine in Chile. El Teniente is the largest copper mine in the world, located in the Andes Cordillera where high stress levels are present due to intensing mining activity in addition to complex geology. Consequently, the study and management of the rockburst threat are necessary. In this work, the case of the Diablo Regimiento (DR) mine within El Teniente is studied. The energy capacity of dynamic support systems is determined for different tunnel geometries based on two kinetic methodologies, using data from DR. Initially, rockburst potential is determined by means of a stress analysis around different tunnel geometries through the boundary elements method. In the first methodology a yielding zone (YZ) is estimated for each excavation geometry using the finite element method FEM. The second methodology involves the definition and determination of a critical strain energy (SE) around each excavation geometry using a FEM numerical analysis. In both cases, peak particle velocity PPV is estimated by a scaling law, which is subsequently adjusted due to tunnel amplification effects. According to the results, and knowing the working energy capacity applied in DR mine, it was found that the values of energy capacity for the rock dynamic supports were better estimated by the YZ-PPV approach than by the SE approach.

The limit equilibrium method LEM is widely used for static and pseudo-static soil nailing designs. Soil nailed wall stability is usually evaluated based on a global factor of safety FSG under a predefined failure mechanism. When appropriate failure surfaces are adopted, FSG should reduce with the soil nailed wall inclination β for different nail geometries (length L, inclination α and diameter D), soil–nail strength rs, soil cohesion c’ and angle of friction ϕ’. However, nail spacing S can change this trend since FSG increases with β under certain combinations of β and S. In this study the nail spacing effect has been evaluated using LEM assuming a bilinear failure surface with two rigid blocks and the Morgenstern-Price method where the failure surface is neither linear nor circular. However, it was found that FSG increases with β for S < 2.00 m, which can lead to potentially unsafe designs. Alternatively, the finite element method FEM was chosen including the strength reduction factor SRF methodology which is equivalent to FSG under failure conditions. It was found that results from FEM represent a significant improvement respect to those from LEM because curves in a FSG-β-S plot follow a logical trend as with the other parameters (L, α, D, rs, c’ and ϕ’). Finally, it is recommended to choose FEM instead of LEM in soil nailing designs. In case of using LEM, results should be carefully assessed, in particular for steep walls and close nail spacing.

Anisotropy is an important characteristic of rocks, especially distinguishable in metamorphic rocks. Transverse isotropy is a particular case of anisotropy where foliation planes are distributed in the rock mass. Anisotropy can also originate from mineral foliation where minerals are oriented in a preferential direction. This inherent anisotropy can affect the rock strength significantly. Here the effect of foliation on the anisotropic strength of a phyllite is experimentally investigated. Phyllite specimens with defined foliation orientations are prepared and a series of laboratory tests carried out. Results for tensile strength and unconfined compressive strength are analysed. Maximum values of tensile strength and unconfined compressive strength are found for β = 0 and 90° and a significant reduction of strength is found for β = 45°. Compressive triaxial tests are conducted under confining pressures up to 20 MPa. The Hoek–Brown failure criterion is found to capture the experimental results in a better form than the Mohr–Coulomb criterion. Anisotropy indexes are adopted to evaluate the anisotropy effect on strength. Confinement is found to reduce the effect of anisotropy on phyllite strength. Cohesion and angle of shearing resistance were also found to be affected by the stress level, and further influenced by β.

Renewable energy has become a relevant alternative to solve energy and environmental problems worldwide. Characterization of geothermal resources is fundamental for an efficient and sustainable extraction of heat. In this study, thermal conductivity, 𝜆, volumetric heat capacity, C, and thermal diffusivity, 𝛼, of two soils from Chile, namely Bío Bío sand and Maicillo residual soil, were analysed. Bío Bío sand is a uniform and clean sand that mainly consists of basaltic particles, whereas Maicillo soil is a silty and clayey sand with a high quartz content. Samples were tested with a thermal needle probe at varying water contents and densities. Measurements to obtain 𝜆, C and 𝛼 were undertaken. These results allowed the patterns and relationships between geotechnical and heat transfer parameters to be analysed. It was found that dry soil conditions led to the lowest values of 𝜆, while saturated soil conditions led to the highest values. Moreover, Maicillo soil has higher capabilities to transfer heat than Bío Bío sand and can reach up to 50% greater 𝜆 for dense soil and saturated conditions. This is due to its quarzitic and clayey mineralogy and non-uniform grain-size distribution. The results from this research represent an important contribution to industrial applications.

Anisotropy is a crucial characteristic of metamorphic rocks whereby minerals oriented in a preferential direction can originate mineral foliation. Inherent anisotropy can affect the rock behaviour significantly. Transverse isotropy is a particular case of anisotropy where foliation planes are distributed in the rock mass. Therefore, in this study the anisotropy effect on the elastic properties of a foliated phyllite is considered. To this end, a series of laboratory tests was programmed. Triaxial tests were carried out under confining pressures up to 20 MPa. Elastic parameters such as elasticity modulus, E, and Poisson's ratio, ν, are assessed from triaxial test results as well as from ultrasonic tests, where compression and shear wave velocities are determined. Empirical relationships for elastic parameters are suggested as a function of foliation angle, β, and confinement. In addition, anisotropy indexes are adopted to evaluate results, comparing when possible with those of previous works. It was found that E increases with confinement; however, the effect of anisotropy on E reduces with confinement. It was also found that ν is affected by β but not by confinement.

The dynamic properties of coarse granular soils have been much less studied than in sands. From a database of 14 gravel samples subjected to cyclic triaxial tests, available relationships are studied and new proposed to estimate the normalised shear modulus G/Gmax and the damping ratio D as a function of shear strain γ. The effect of confining stress, fines content, uniformity coefficient and loading frequency on the variation of G/ Gmax and D versus γ is analysed. It is obtained that G/Gmax is dependent on confinement, but not on loading frequency. 85.6% of the data converge in an error band of less than 25% for the proposed formulation. The damping D does depend on fines content as well as confinement and loading frequency. The proposed formulation for D has a 56% probability of having errors less than 25%.

In this work, a coupled macroelement model for shallow foundations under cyclic loading has been developed. Coupling a hypoplastic constitutive model with an uplift plasticity model and a Mohr-Coulomb interface model can substantially improve the prediction of the foundation response. The yield surface adopted in this work is an ellipsoid and the flow rule can be chosen associated or non-associated. The couplings are achieved by introducing new variables that control the dominance of one model on the total response. Initial validation is carried out against numerical FEM simulations for shallow foundations in loose and dense sand subjected to monotonic and symmetric cyclic loading. Further validation against experimental data is performed by comparing the results obtained from caisson foundations in loose and dense sand subjected to non-symmetric and symmetric cyclic loading. The proposed macroelement model shows satisfactory results because it captures not only the limit loads, loading and unloading stiffness of load-displacement curves, but also complex cyclic settlement variations, in particular foundation uplift.