Dr. Villalobos-Jara, Felipe

The technique of soil nailing has been increasingly used in stabilization works of slopes and excavations. With this, the use of numerical modelling tools in soil nailing projects is becoming increasingly present in Geotechnical companies. This paper includes a case study of a soil nailing wall instrumented in Concepción city, which consists of an excavation of 15 m height in a residual soil of completely decomposed granitic rock. The numerical model was calibrated, comparing the results of the field instrumentation with the numerical estimates provided by the FEM-RS2 software, based on the two-dimensional finite element method and considering an elastic perfectly plastic model. In this way, the strength reduction factor of the geotechnical structure was obtained, which was compared with the overall factor of safety obtained by limit equilibrium analysis. In addition, through the numerical simulation, it was possible to realize an analysis of the loads on the nails, total displacements of the vertical wall, and compare them with the numerical results. The analysis of the results made it possible to confirm the capacity and usefulness of the FEM-RS2 software in the development and elaboration of soil nailing projects.

The study of materials with large particle size has been a great challenge in geotechnical engineering. Despite the current work around the world using coarse-grained materials CGM in rockfill dams and mining waste rock dumps, for instance the geotechnical characterization of these materials is still an important issue in geotechnical engineering practice which deserve more research. There are standards covering CGM in a few particular applications and scaling methods have been proposed to deal with large particle sizes. However, scaling methods are appropriate only under certain conditions. The scalping techniques consist in a simple approach for the geotechnical characterization of CGM. In this article, the scalping techniques analysed are divided in: the scalping method, the matrix method and the scalping/replacement, which are studied in detail in terms of its effectiveness, focusing on the geomechanical characterization of CGM. As a main conclusion, these three techniques are limited in its use under small scalping ratios (3 < r < 8) which is the ratio of maximum particle size of both original and scalped gradation. Finally, recommendations for the use of percentages and ratio of scalping are provided.

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.

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.

The development of a monitored district heating piping system has allowed the study of axial displacement variations in a buried pipeline. This piping system includes four instrumented sections of piping within an in use district heating network. There are also different conditions under testing such as thickness of expansion cushions, temperature ranges and bedding soil types. The pipe axial displacements were on-line monitored by means of extensometers in six positions along each of the four sections of the pipeline. Measured maximum pipe axial displacements were 24 and 25 mm in the corners of the 41 m long monitored pipelines, while estimated values were 23 mm using current recommendation procedures and 27 mm using calibrated commercial computer programs. One temperature unloading-reloading caused displacements to not return to the same values as before, but around 3 mm smaller. Therefore, several unloading-reloading temperature cycles may affect the pipe deformation behaviour in the short and long term. © 2021 Published with permission by the ICE under the CC-BY 4.0 license

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%.

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.

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).