O. Buzzi

Evaluation of shear strength of rock joints subjected to cyclic loading

Variation of the shear strength of rock joints due to cyclic loadings is studied in the present paper. Identical joint surfaces were prepared using a developed moulding method with special mortar and shear tests were performed on these samples under both static and cyclic loading conditions. Different levels of shear displacement were applied on the samples to study joint behaviour before and during considerable relative shear displacement. It was found that the shear strength of joints is related to rate of displacement (shearing velocity), number of loading cycles and stress amplitude. Finally, based on the experimental results, mathematical models were developed for evaluation of shear strength in cyclic loading conditions.

Prediction of the bullet effect for rockfall barriers: a scaling approach

The so-called “bullet effect” refers to the perforation of a rockfall protection mesh by impact of a small block, which has a kinetic energy lower than the design value, where the design value is determined through tests with relatively large blocks. Despite playing a key role in the overall performance of a flexible rockfall barrier, this phenomenon is still poorly understood at present. An innovative approach for quantitatively characterizing this effect based on dimensional analysis is proposed in this paper. The analysis rests on a hypothesis that the relevant variables in the impact problem can be combined into three strongly correlated dimensionless parameters. The relationship between these dimensionless parameters (i.e., the scaling relationship) is subsequently investigated and validated by means of data generated with a finite element model. The validation process shows that the dimensionless parameters are apt and that the proposed scaling relationship characterizes the bullet effect with a reasonable level of accuracy. An example from the literature involving numerical simulation of a full rock barrier is considered, and satisfactory agreement between the calculated performance of the barrier and that predicted by the established scaling relationship is observed.

Laboratory Investigation on High Values of Restitution Coefficients

Restitution coefficients are used to quantify the energy dissipation upon impact when predicting rock fall events. These coefficients can be determined in situ or in the laboratory. In any case, the usual values for the normal restitution coefficient kn are below unity. Values greater than one are quite rare, seen as unusual and barely explained. Previous experimental research conducted in Australia has shown consistent and systematic values of the normal restitution coefficient greater than one. This was tentatively explained by a combination of parameters such as low impacting angle, rotational energy and block angularity. The study presented in this paper aims at (1) identifying the critical parameters conducting to high kn values and (2) at explaining the associated motion mechanisms. The objective was reached with values of kn up to almost 2. In addition, the study has confirmed the significance of low impacting angle, rotational energy and block shape in this context.

Use of expanding polyurethane resin to remediate expansive soil foundations

Injection of expansive polyurethane resin can be used to remediate differential settlement issues. The resin is injected incrementally under a structure to achieve a desired foundation level, forming a composite resin–clay material. This solution is not well documented in the literature and some questions arise on the long-term performance of this solution. As injection is usually carried out in a settled soil mass that is dry and dessicated, rehydration of the soil after injection may lead to swelling of the leveled foundation and overlifting of the structure. Experimental research undertaken to investigate this rehydration issue and determine if there is a risk of overlifting in the long term is presented here. In situ and laboratory testing was performed to investigate the most fundamental aspects of the problems. This included the in situ injection of resin, study of resin propagation in the soil mass, influence of resin on the hydraulic conductivity of the soil mass, and large-scale swelling tests. T...

Perforation of Flexible Rockfall Barriers by Normal Block Impact

Flexible rockfall barriers are a common form of protection against falling blocks of rock and rock fragments (rockfall). These barriers consist of a system of cables, posts, and a mesh, and their capacity is typically quantified in terms of the threshold of impact (kinetic) energy at which the barrier fails. This threshold, referred to here as the “critical energy,” is often regarded as a constant. However, several studies have pointed out that there is no single representative value of critical energy for a given barrier. Instead, the critical energy decreases as the block size decreases, a phenomenon referred to as the “bullet effect.” In this paper, we present a simple analytical model for determining the critical energy of a flexible barrier. The model considers a block that impacts normally and centrally on the wire mesh, and rather than incorporate the structural details of the cables and posts explicitly, the supporting elements are replaced by springs of representative stiffness. The analysis reveals the dependence of the critical energy on the block size, as well as other relevant variables, and it provides physical insight into the impact problem. For example, it is shown that bending of the wire mesh during impact reduces the axial force that can be sustained within the wires, thus reducing the energy that can be absorbed. The formulas derived in the paper are straightforward to use, and the analytical predictions compare favorably with data available in the literature.

Strength of an Australian Coal Under Low Confinement

Experimental testing of brittle rocks has shown that both brittle and ductile behaviours can be observed, depending on the level of confinement applied to the specimen. In particular, brittle rocks fail in a brittle mode as long as the confining stress falls below the Mogi line (Mogi 1966). Spalling of rocks is associated with brittle failure and is known to occur under low confinement, i.e. in the vicinity of excavation walls (Stacey 1981; Martin et al. 1999; Cai and Kaiser 2013). Indeed, at low confinement, large tension cracks may develop parallel to the excavation boundary when the stress exceeds the crack initiation threshold, which may lead to rapidly propagating instabilities and formation of thin slabs. Such slabs can represent a significant hazard to the workforce in confined mining excavations. Increasing the level of confinement modifies the nature and propagation mechanism of the cracks that develop upon loading: at high confinement, short shear cracks develop and ultimately join to form a macroscopic shear band. Martin et al. (1999) showed that a single set of Hoek–Brown parameters failed to capture the two mechanisms and they distinguished Hoek–Brown frictional (for high confinement) and brittle (for low confinement) sets of parameters. Their proposed brittle criterion falls below the frictional counterpart reflecting a reduction in strength. Recently, Kaiser and Kim (2008) and Amann et al. (2012) proposed a non-convex criterion to capture the strength under both low and high confining pressures. However, some of the data they used involved a large degree of scatter (in Kaiser and Kim 2008) or not many points were obtained in the low confining range (in Amann et al. 2012). Considering the recent findings by Kaiser et al. and the lack of data in the literature about the strength of coal under low confinement, it has been decided to conduct a series of triaxial tests in order to mitigate this gap. Gaining a better understanding of the behaviour of the coal under low confinement is highly relevant for the stability of coal mine excavations.

A New Stochastic Approach to Predict Peak and Residual Shear Strength of Natural Rock Discontinuities

Natural discontinuities are known to play a key role in the stability of rock masses. However, it is a non-trivial task to estimate the shear strength of large discontinuities. Because of the inherent complexity to access to the full surface of the large in situ discontinuities, researchers or engineers tend to work on small-scale specimens. As a consequence, the results are often plagued by the well-known scale effect. A new approach is here proposed to predict shear strength of discontinuities. This approach has the potential to avoid the scale effect. The rationale of the approach is as follows: a major parameter that governs the shear strength of a discontinuity within a rock mass is roughness, which can be accounted for by surveying the discontinuity surface. However, this is typically not possible for discontinuities contained within the rock mass where only traces are visible. For natural surfaces, it can be assumed that traces are, to some extent, representative of the surface. It is here proposed to use the available 2D information (from a visible trace, referred to as a seed trace) and a random field model to create a large number of synthetic surfaces (3D data sets). The shear strength of each synthetic surface can then be estimated using a semi-analytical model. By using a large number of synthetic surfaces and a Monte Carlo strategy, a meaningful shear strength distribution can be obtained. This paper presents the validation of the semi-analytical mechanistic model required to support the new approach for prediction of discontinuity shear strength. The model can predict both peak and residual shear strength. The second part of the paper lays the foundation of a random field model to support the creation of synthetic surfaces having statistical properties in line with those of the data of the seed trace. The paper concludes that it is possible to obtain a reasonable estimate of peak and residual shear strength of the discontinuities tested from the information from a single trace, without having access to the whole surface.

Use of Hand-Spray Plaster as a Coating for Soil Bulk Volume Measurement

This paper presents a new coating material [hand-spray plaster (HSP)] used to measure the bulk volume of soil specimens having irregular shapes. The new method has been validated against two benchmark methods, namely, the wax and plastic bag methods, by conducting swelling and shrinkage tests on intact Maryland clay. The results show that the new method yields similar values of volume but with much reduced data scattering. The HSP method is also far easier to use than the other two methods. Finally, the stiffness of the coating has been measured and its restraining effect has been found to be negligible. Some of the benefits of using the HSP method are: (1) limited fluid retention by the specimen post-immersion for volume measurement, (2) reduced water-intake rate with elimination of cracking upon swelling caused by high-suction gradients, (3) absence of the restraining effect on specimens upon swelling, and (4) accurate determination of the swelling and shrinkage curves with only one specimen per curve.

An Indirect Approach for Correlation of Permeability and Diffusion Coefficients

Diffusion tests in porous media are quite sensitive and long lasting procedures compared to permeability tests, which are usually more reliable and of shorter duration. Both diffusion and advection phenomena are dependent on the tortuosity of the material tested. A relevant question is to know whether it is possible to correlate permeability tortuosity p and diffusion tortuosity d. Several diffusion and permeability tests have been performed on non-uniform sand specimens having different grain size distribution. For each specimen, both the permeability and diffusion coefficients have been measured and two tortuosity factors (permeability and diffusion) have been back calculated. A theoretical model has been proposed to estimate d from p for a non-uniform granular material. The maximum particle diameter dmax is used to determine the maximum hydraulic diameter dh-max using the Hydraulic Radius Theory (HRT) for a 3D arrangement of spheres of same diameter dmax. Then, a filling factor  is applied to dh-max in order to capture the fact that smaller grains tend to fill the voids present in between the bigger particles. The filling factor is based on the coefficient of uniformity Cu. Relatively good results are obtained so that this model allows estimating the diffusion properties from a simple permeability test rapidly and at a fraction of the diffusion test cost.

Calibration of a Coupled Model to Predict the Magnitude of Suction Generated by Osmotic Technique With PES Membranes and Temperature Effect

Using the osmotic technique to control soil suction requires a reliable correlation between concentration of soluble polyethylene glycol (PEG) and suction generated under isothermal condition. Experimental data to establish such correlation can be found in the literature but they tend to be incomplete and mainly pertain to cellulose membranes. Recent research has shown that polyethersulfon membranes (PES) are a better alternative than cellulose membranes in terms of durability and reliability. However, no complete calibration dataset is available for applying the osmotic technique to account for use of PES membranes and effects of different PEG molecular weights, concentrations and temperature. This study intends to fill such a gap by providing reliable and comprehensive experimental data. The response of PES membranes was investigated over a range of temperature varying from 20°C to 40°C and the results were used to calibrate a coupled model that allows a better prediction of suction generated by the osmotic technique. The influence of PEG molecular weight has been evidenced and can be incorporated in the model.