Abbas Taheri

Pre-Peak and Post-Peak Rock Strain Characteristics During Uniaxial Compression by 3D Digital Image Correlation

A non-contact optical method for strain measurement applying three-dimensional digital image correlation (3D DIC) in uniaxial compression is presented. A series of monotonic uniaxial compression tests under quasi-static loading conditions on Hawkesbury sandstone specimens were conducted. A prescribed constant lateral-strain rate to control the applied axial load in a closed-loop system allowed capturing the complete stress–strain behaviour of the rock, i.e. the pre-peak and post-peak stress–strain regimes. 3D DIC uses two digital cameras to acquire images of the undeformed and deformed shape of an object to perform image analysis and provides deformation and motion measurements. Observations showed that 3D DIC provides strains free from bedding error in contrast to strains from LVDT. Erroneous measurements due to the compliance of the compressive machine are also eliminated. Furthermore, by 3D DIC technique relatively large strains developed in the post-peak regime, in particular within localised zones, difficult to capture by bonded strain gauges, can be measured in a straight forward manner. Field of strains and eventual strain localisation in the rock surface were analysed by 3D DIC method, coupled with the respective stress levels in the rock. Field strain development in the rock samples, both in axial and shear strain domains suggested that strain localisation takes place progressively and develops at a lower rate in pre-peak regime. It is accelerated, otherwise, in post-peak regime associated with the increasing rate of strength degradation. The results show that a major failure plane, due to strain localisation, becomes noticeable only long after the peak stress took place. In addition, post-peak stress–strain behaviour was observed to be either in a form of localised strain in a shearing zone or inelastic unloading outside of the shearing zone.

Experimental study on degradation of mechanical properties of sandstone under different cyclic loadings

An experimental investigation was carried out on the Hawkesbury sandstone to study changes in mechanical rock behavior (i.e., strength and deformability) during various cyclic loading conditions under uniaxial and triaxial testing situation. Axial load, confining pressures and axial and lateral deformations were measured continuously from start of loading until the post-peak state. Cyclic tests were carried out at different confining pressures, stress levels and unloading amplitudes. In cyclic loading tests, a relatively uniform accumulation in axial, lateral and volumetric strain is observed followed by a rapid strain increase as it heads toward failure. This rapid accumulation in strain occurred on average at approximately 65 % of the cumulative axial strain. During systematic cyclic loading a continuous degradation in the tangent Youngs modulus, Etan, until beginning of large plastic deformation was observed. The rate of stiffness decrease increased rapidly during few cycles before the failure. Poissons ratio showed a similar trend of behavior and increased continuously with cyclic loading. Moreover, Etan values measured during interval cyclic loadings were affected by two factors: (1) stiffening by cyclic loading because of elasto-viscoplastic properties; and (2) damage effects that have taken place during cyclic loading, in particular, if cyclic loading begins close to the monotonic peak strength. Depending to the stress level at initiation of cyclic loading with respect to the monotonic peak strength, either of these two factors could become significant and affect stiffness of rock during or after a cyclic loading. Finally, it was found that damage increases with an increase in unloading stress level and unloading amplitude, and fatigue life of a confined specimen is longer than a sample without confinement.

Rock Drilling Performance Evaluation by an Energy Dissipation Based Rock Brittleness Index

To reliably estimate drilling performance both tool–rock interaction laws along with a proper rock brittleness index are required to be implemented. In this study, the performance of a single polycrystalline diamond compact (PDC) cutter cutting and different drilling methods including PDC rotary drilling, roller-cone rotary drilling and percussive drilling were investigated. To investigate drilling performance by rock strength properties, laboratory PDC cutting tests were performed on different rocks to obtain cutting parameters. In addition, results of laboratory and field drilling on different rocks found elsewhere in literature were used. Laboratory and field cutting and drilling test results were coupled with values of a new rock brittleness index proposed herein and developed based on energy dissipation withdrawn from the complete stress–strain curve in uniaxial compression. To quantify cutting and drilling performance, the intrinsic specific energy in rotary-cutting action, i.e. the energy consumed in pure cutting action, and drilling penetration rate values in percussive action were used. The results show that the new energy-based brittleness index successfully describes the performance of different cutting and drilling methods and therefore is relevant to assess drilling performance for engineering applications.

Local Damage and Progressive Localisation in Porous Sandstone During Cyclic Loading

Stress-induced deformation problems are often encountered in civiland mining-related rock engineering projects. In a typical situation, rock disturbance under external loading by mechanical excavation or blasting methods induces the rock to undergo a complex history of the response states of load and deformation. For instance, a number of loading, unloading and then reloading sequences can be anticipated at a certain location ahead a tunnel face. As the tunnel advances forwards, excavation promotes disturbed zones ranging from failed, damaged, disturbed and undisturbed zones (Martin 1997; Martin and Read 1996). Damage effects insights (Xu et al. 2004) and the mechanical properties (Eberhardt et al. 1999; Taheri et al. 2016a, b) of quasi-brittle rocks under uniaxial compression within the pre-peak stress–strain regime are commonly found in the literature. However, very limited studies have been undertaken on the stress–strain behaviour of rocks during post-peak cyclic loading condition. This is mainly due to the major difficulties with measuring properly postpeak stress–strain quantities associated mainly with a suitable testing method and rock brittleness (Munoz et al. 2016a, b) contrary to conventional testing methods (Wawersik and Brace 1971; Wawersik and Fairhurst 1970). In addition, post-peak cyclic loading damage evolution, i.e. the characteristics of non-localised uniformly distributed damage and localised damage, associated with localisation of strains and stiffness degradation have not been addressed yet. In this regard, very few investigations into the local deformational characteristics of rocks under uniaxial cyclic loading limited to both pre-peak regime (Dautriat et al. 2011; Hao et al. 2010; Zhang et al. 2013) and 2D DIC (Song et al. 2013, 2016; Xu et al. 2004) have been conducted. Therefore, the present study focuses primarily on studying strain localisation evolution and damage evolution characteristics of quasi-brittle porous sandstone subjected to compressive cyclic loading in post-peak regime. To do so, firstly, an innovative laboratory testing method was designed to investigate progressive damage in postpeak cyclic loading. Secondly, 3D DIC was implemented to study the insights of strain localisation evolution and damage evolution throughout the cyclic tests.

Fracture Evolution Around a Cavity in Brittle Rock Under Uniaxial Compression and Coupled Static–Dynamic Loads

To experimentally investigate the stability of underground excavations under high in situ stress conditions, several rock samples with a mini-tunnel were prepared and subjected to monotonic axial and coupled static–dynamic loading until failure. Mini-tunnels were generated by drilling circular or cubic cavities in the centre of granite rock blocks. Strain gauges were used to monitor the deformation of the mini-tunnels at different locations, and a high-speed camera system was used to capture the cracking and failure process. We found that the dynamic crack initiation stress, failure mode and dynamic crack velocity of the specimen all depend on the pre-stress level when the sample is under otherwise similar dynamic disturbance conditions. The crack initiation stress threshold first increased slightly and then decreased dramatically with the increase in the pre-stress value. The specimens were mainly fractured by tensile cracks parallel to the compression line under lower pre-stress, while they were severely damaged with additional shear cracks under higher pre-stress. Furthermore, the propagation velocity of the primary crack was significantly larger than that of the subsequent cracks. The effect of applying different amounts of static pre-stresses on the velocity of the primary tensile crack was similar to that observed for the crack initiation stress threshold; however, it did not affect the velocity of the secondary and subsequent tensile cracks.

A sulphonated oil for stabilisation of expansive soils

ABSTRACT The efficiency of a commercially manufactured sulphonated oil (SO) agent in treating a highly expansive soil was investigated through an extensive experimental program. A total of six SO to water mass concentrations, i.e. 0.25, 0.5, 0.75, 1, 1.25 and 2.5%, were examined. The test program included swell–load oedometer, unconfined compressive strength and cyclic wetting and drying tests. SO-stabilisation amended the soil’s mechanical behaviour through improvements achieved in swelling and strength characteristics. The reduction in swelling potential and swelling pressure was dependent on SO concentration, while the effect of curing time was found to be insignificant. A similar dependency was concluded for the unconfined compressive strength and stiffness of the stabilised soil. Both dependencies suggested an SO concentration of 1.25% capable of yielding an optimal stabilisation scheme. Results of the cyclic wetting and drying tests indicated that the beneficiary effects of SO-stabilisation at optimum concentration, particularly in ameliorating the adverse effects of swell–shrink-related volume changes and to some extent increasing the strength, are strongly preserved under the influence of alternate wetting and drying.

Rock cutting performance assessment using strain energy characteristics of rocks

Abstract The intrinsic specific energy, that is the minimum energy consumed by pure cutting action to cut unit volume of rock, along with the rock strain energy, that is the absorbed energy by unit volume of rock in uniaxial compression, were used together to assess rock cutting performance. A series of rock cutting tests using a single polycrystalline diamond compact (PDC) cutter under steady conditions, and uniaxial compressive tests with a single-cyclic loading, otherwise monotonic loading were carried out on rock samples. Different rock types having unconfined compressive strength values of 9–249 MPa were carefully tested to determine the intrinsic specific energy and strain energy values. The results suggest that stress and strain characteristics of the rock obtained from uniaxial compressive tests, in terms of absorbed energy, can be related to cutting performance. Therefore, it was concluded that the strain energy can be used to assess the intrinsic specific energy to evaluate drilling performance in PDC cutting.

A New Method to Simulate Stress–Strain Relations from Multiple-Step Loading Triaxial Compression Test Results

Multiple-step loading (ML) on a single specimen can be used to determine the peak compressive strengths at different confining pressures (σ′hS) of cement-mixed gravel (CMG) very similar to those obtained by single-step loading (SL) drained triaxial compression (TC) tests. However, only the unload/reload stress–strain relations at different σ′hS (except for the primary loading one at the first step) can be obtained from a ML test and the reloading relations become softer with an increase in the negative irreversible axial strain increment that has taken place during respective immediately preceding unloading regimes. This effect gradually decreased during reloading while it totally disappeared once large-scale yielding started. An empirical equation was developed to derive undamaged reloading stress–strain curves (URCs) by removing the damage effects from ML TC test measured reloading curves (MRCs). A new simple method was developed in the framework of proportional rule to simulate primary loading curves (PLCs) at different σ′hS from the MRCs from a given ML TC test using a correlation factor. A practical procedure for simulation of PLCs in a ML test were established and applied to generate PLCs in a ML test increasing σ′h and a ML test decreasing σ′h. The method was validated by comparing the PLCs simulated from the results of a pair of ML tests increasing and decreasing σ′h with those measured in a set of SL TC tests at different σ′hS.

Long-term prediction of rockburst hazard in deep underground openings using three robust data mining techniques

Rockburst phenomenon is the extreme release of strain energy stored in surrounding rock mass which could lead to casualties, damage to underground structures and equipment and finally endanger the economic viability of the project. Considering the complex mechanism of rockburst and a large number of factors affecting it, the conventional criteria cannot be used generally and with high reliability. Hence, there is a need to develop new models with high accuracy and ease to use in practice. This study focuses on the applicability of three novel data mining techniques including emotional neural network (ENN), gene expression programming (GEP), and decision tree-based C4.5 algorithm along with five conventional criteria to predict the occurrence of rockburst in a binary condition. To do so, a total of 134 rockburst events were compiled from various case studies and the models were established based on training datasets and input parameters of maximum tangential stress, uniaxial tensile strength, uniaxial compressive strength, and elastic energy index. The prediction strength of the constructed models was evaluated by feeding the testing datasets to the models and measuring the indices of root mean squared error (RMSE) and percentage of the successful prediction (PSP). The results showed the high accuracy and applicability of all three new models; however, the GA-ENN and the GEP methods outperformed the C4.5 method. Besides, it was found that the criterion of elastic energy index (EEI) is more accurate among other conventional criteria and with the results similar to the C4.5 model, can be used easily in practical applications. Finally, a sensitivity analysis was carried out and the maximum tangential stress was identified as the most influential parameter, which could be a guide for rockburst prediction.

Swell–Shrink–Consolidation Behavior of Rubber–Reinforced Expansive Soils

This study examines the effects of two types of recycled tire rubber of fine and coarse categories on the swell-shrink-consolidation behavior of a highly expansive soil mixture. Each of the two rubber choices were incorporated into the soil at four different content levels (i.e., rubber to dry soil mass ratio) of 5, 10, 20, and 30 %. The experimental program consisted of consistency limits, compaction, swell consolidation, swell-shrink, and unconfined compression tests. Improvement in the swell-shrink-consolidation capacity was in favor of higher rubber contents; however, when excessively included, it raised strength concerns. The swell-shrink-consolidation properties were also rubber size-dependent, meaning that the rubber of coarser sizes often outperformed finer rubber. In terms of strength, however, the two rubber types promoted similar results with marginal differences. The results of the unconfined compression tests were cross checked with the swell-shrink-consolidation properties to arrive at the optimum stabilization scenarios. A maximum rubber inclusion of 10 %, preferably the rubber of coarser category, proved to satisfy the stabilization objectives (i.e., decrease in the swell-shrink-consolidation capacity as well as maintain or improve the strength) and thus was deemed as the optimum choice. Where context changes and the strength and stiffness are not a primary concern, higher rubber inclusions of up to 20 % may also be considered acceptable.