Geotechnical Engineering Online Library

Pillar stability is one of important aspects for underground mines. Generally, the stability of the pillars is evaluated empirically based on case studies and site-specific rock mass conditions in mines. Nevertheless the empirical approach applicability can sometimes be constrained. The numerical-based approaches are potentially more useful as parametric studies can be undertaken and, if calibrated, can be more representative. Both empirical and numerical approaches are dependent on the strength evaluation of the pillars while the strain developing in the pillars is seldom taken into consideration. In this paper, gypsum and sandstone samples were tested in laboratory with different width-to-height ratios (W/H) to adapt the strain evaluation method to the laboratory-based pillars. A correlation was then developed between the strain and the width-to-height ratio for pillar monitoring purposes. Based on the results, a flowchart was created to conduct back analysis for the existing pillars to evaluate their stability and design new pillars, considering the strain analysis of the existing pillars with the W/H ratios modelled.

This paper presents the mechanical behaviors of the lacustrine deposit, a representative soil in Bogotá, Colombia. Initially, the physical characterization of the deposit is performed via laboratory tests (grain size distribution, scanning electron microscopy, Atterberg limits and water content). This characterization intends to explain the special characteristics of the mechanical behaviors of this soil. Then, various triaxial tests are carried out with controlled loading path, strain rate change, relaxation, extensile stress, and cyclic loading. The test results reveal the shape of the yield curve for Bogotá soil (in a natural state), and also show that an increasing effect of the strain rate depends on the liquid limit. This effect is also preserved with extensile stresses (which are poorly studied in soil mechanics). Finally, other effects, such as the loss of structure in the reconstituted samples and the effect of shear modulus at low strains, are studied for Bogotá soil.

Weakly consolidated reservoirs are prone to sand production problem, which can lead to equipment damages and environmental issues. The conditions for sand production depend on stresses and properties of rock and fluid. Accurate sand volume estimation is, however, still a challenging issue, especially for reservoirs in weak formations. The weak reservoirs containing viscous or heavy oil are mainly discovered in shallow depths in Kazakhstan, with moderate temperature and pressure. Many prediction models developed for open-hole completions where the reservoir materials usually possess certain strength are not applicable for the local reservoirs where the materials are significantly weaker even if casing is used to support the wellbore with oil produced through the perforation tunnels. In this context, a prediction model was proposed where the volume of the produced sand was estimated as the volume of the plastic zone of the failed materials surrounding the perforation tunnels. The model assumes an evolving truncated conical shape for the damage zone and takes into account stress distributions and shear failure in this zone. Then, the proposed model was used to estimate sand volumes in 20 wells during oil production with sequential increase of flow rates. The predictions match well with the measured sand volumes in a local oil field. Finally, a sensitivity analysis was conducted on the model performance. It shows that the permeability of the plastic zone was the most significant controlling factor in the prediction results.

In this context, a new boundary element algorithm based on the time-convoluted traction kernels is employed to evaluate the spatially varying earthquake ground motions of the Pacoima dam in the USA subjected to SH, SV and P incident waves. An accurate three-dimensional (3D) model of the dam canyon is implemented into the computer code BEMSA to investigate the seismic response of the dam. The analyses are performed in time domain with a linearly elastic constitutive model for the medium. This modeling procedure has been validated by the results reported in the existing literature. According to the results of this study, the response of the dam to earthquake waves is generally influenced by predominant frequency of the incident motion, surface topography, relative distance of observation points, and type of the incident seismic wave. For the cases considered, the incident SV wave has led to the maximum amplification of incident motions, especially at the left side of the dam. The results indicate that the proposed procedure can be employed for accurate prediction of a dam response during an earthquake.

Carbon capture, utilization and storage (CCUS) is considered as a very important technology for mitigating global climate change. Carbon dioxide (CO2) injected into an underground reservoir will induce changes in its physical properties and the migration of CO2 will be affected by many factors. Accurately understanding these changes and migration characteristics of CO2 is crucial for selecting a CCUS project site, estimating storage capacity and ensuring storage security. In this paper, the basic principles of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) technologies are briefly introduced in the context of laboratory experiments related to CCUS. The types of NMR apparatus, experimental samples and testing approaches applied worldwide are discussed and analyzed. Then two typical NMR core analysis systems used in CCUS field and a self-developed high-pressure, low-field NMR rock core flooding experimental system are compared. Finally, a summary of the current deficiencies related to NMR applied to CCUS field is given and future research plans are proposed.

Rolling dynamic compaction (RDC), which employs non-circular module towed behind a tractor, is an innovative soil compaction method that has proven to be successful in many ground improvement applications. RDC involves repeatedly delivering high-energy impact blows onto the ground surface, which improves soil density and thus soil strength and stiffness. However, there exists a lack of methods to predict the effectiveness of RDC in different ground conditions, which has become a major obstacle to its adoption. For this, in this context, a prediction model is developed based on linear genetic programming (LGP), which is one of the common approaches in application of artificial intelligence for nonlinear forecasting. The model is based on in situ density-related data in terms of dynamic cone penetrometer (DCP) results obtained from several projects that have employed the 4-sided, 8-t impact roller (BH-1300). It is shown that the model is accurate and reliable over a range of soil types. Furthermore, a series of parametric studies confirms its robustness in generalizing data. In addition, the results of the comparative study indicate that the optimal LGP model has a better predictive performance than the existing artificial neural network (ANN) model developed earlier by the authors.

This study aims to develop several optimization techniques for predicting advance rate of tunnel boring machine (TBM) in different weathered zones of granite. For this purpose, extensive field and laboratory studies have been conducted along the 12,649 m of the Pahang – Selangor raw water transfer tunnel in Malaysia. Rock properties consisting of uniaxial compressive strength (UCS), Brazilian tensile strength (BTS), rock mass rating (RMR), rock quality designation (RQD), quartz content (q) and weathered zone as well as machine specifications including thrust force and revolution per minute (RPM) were measured to establish comprehensive datasets for optimization. Accordingly, to estimate the advance rate of TBM, two new hybrid optimization techniques, i.e. an artificial neural network (ANN) combined with both imperialist competitive algorithm (ICA) and particle swarm optimization (PSO), were developed for mechanical tunneling in granitic rocks. Further, the new hybrid optimization techniques were compared and the best one was chosen among them to be used for practice. To evaluate the accuracy of the proposed models for both testing and training datasets, various statistical indices including coefficient of determination (R2), root mean square error (RMSE) and variance account for (VAF) were utilized herein. The values of R2, RMSE, and VAF ranged in 0.939–0.961, 0.022–0.036, and 93.899–96.145, respectively, with the PSO-ANN hybrid technique demonstrating the best performance. It is concluded that both the optimization techniques, i.e. PSO-ANN and ICA-ANN, could be utilized for predicting the advance rate of TBMs; however, the PSO-ANN technique is superior.

Pavements constructed over loosely compacted subgrades may not possess adequate California bearing ratio (CBR) to meet the requirements of pavement design codes, which may lead to a thicker pavement design for addressing the required strength. Geosynthetics have been proven to be effective for mitigating the adverse mechanical behaviors of weak soils as integrated constituents of base and sub-base layers in road construction. This study investigated the behaviors of unreinforced and reinforced sand with nonwoven geotextile using repeated CBR loading test (followed by unloading and reloading). The depth and number of geotextile reinforcement layers, as well as the compaction ratio of the soil above and below the reinforcement layer(s) and the compaction ratio of the sand bed, were set as variables in this context. Geotextile layers were placed at upper thickness ratios of 0.3, 0.6 and 0.9 and the lower thickness ratio of 0.3. The compaction ratios of the upper layer and the sand bed varied between 85% and 97% to simulate a dense layer on a medium dense sand bed for all unreinforced and reinforced testing scenarios. Repeated CBR loading tests were conducted to the target loads of 100 kgf, 150 kgf, 200 kgf and 400 kgf, respectively (1 kgf = 9.8 N). The results indicated that placing one layer of reinforcement with an upper thickness ratio of 0.3 and compacting the soil above the reinforcement to compaction ratio of 97% significantly reduced the penetration of the CBR piston for all target repeated load levels. However, using two layers of reinforcement sandwiched between two dense soil layers with a compaction ratio of 97% with upper and lower thickness ratios of 0.3 resulted in the lowest penetration.

In many engineering applications such as mining, geotechnical and petroleum industries, drilling operation is widely used. The drilling operation produces sound by-product, which could be helpful for preliminary estimation of the rock properties. Nevertheless, determination of rock properties is very difficult by the conventional methods in terms of high accuracy, and thus it is expensive and time-consuming. In this context, a new technique was developed based on the estimation of rock properties using dominant frequencies from sound pressure level generated during diamond core drilling operations. First, sound pressure level was recorded and sound signals of these sound frequencies were analyzed using fast Fourier transform (FFT). Rock drilling experiments were performed on five different types of rock samples using computer numerical control (CNC) drilling machine BMV 45 T20. Using simple linear regression analysis, mathematical equations were developed for various rock properties, i.e. uniaxial compressive strength, Brazilian tensile strength, density, and dominant frequencies of sound pressure level. The developed models can be utilized at early stage of design to predict rock properties.

Recently, stress-based dilatancy criteria have become essential tools to design underground facilities in salt formations such as gas storage caverns. However, these criteria can depend critically on the volumetric strain measurements used to deduce the dilatancy onset. Results from conventional triaxial compression tests can show different volumetric behavior depending on the loading conditions, as well as on the measurement techniques. In order to obtain a quantitative understanding of this problem, an experimental program was carried out and the testing procedure was investigated numerically under homogeneous and heterogeneous stress states. The experimental results showed that the deviatoric stress corresponding to the dilatancy onset was significantly dependent on the measurement techniques. With a heterogeneous stress state, the simulation results revealed that the strain measurements at different scales (referred to as local, hybrid or global) can provide different volumetric results with moderate to significant deviations from the idealized behavior, and hence different onsets of dilatancy. They also proved that, under low confinement, tensile stresses can take place within the compressed specimen, leading to a great deviation of the dilatancy onset from the idealized behavior. From both experimental and numerical investigations, the difference in sensitivity to the measurement techniques between the deviatoric and the volumetric behaviors is explained by the relatively small values of the volumetric strain. The non-ideal laboratory conditions have more impact on this strain than on the deviatoric one. These findings can have implications for the interpretation of the dilatancy behavior of rock salt, and hence on the geomechanical design aspects in salt formations.

Helical anchors are commonly used in Brazil for guyed transmission towers subjected to static and cyclic wind loads. In most cases, these anchors are installed in tropical residual soil, a micro-structured material in which the shear strength is provided by soil bonding. During installation of a helical anchor, as the helical plate moves downward into the ground, the soil penetrated is sheared and displaced. Consequently, in this type of soil, anchor installation affects the soil shear strength significantly associated with a bonded structure. However, the cyclic responses of helical anchors in this type of structured soils are rarely reported. To address this problem, tests were conducted in a Brazilian residual soil to investigate the monotonic, cyclic and post-cyclic performances of single-helix anchors. Field tests used two instrumented single-helix anchors installed in this typical residual soil of sandstone, which is frequently observed in large areas in the southern Brazil. The testing results indicate that the disturbance caused by the anchor installation affected the monotonic uplift performance markedly. The results of cyclic loading tests also show no significant degradation of helix bearing resistance and reduced displacement accumulation with increasing load cycles. This is perhaps due to the soil improvement caused by previous loading, which then increases the stiffness response of the anchor.

Deep underground excavations within hard rocks can result in damage to the surrounding rock mass mostly due to redistribution of stresses. Especially within rock masses with non-persistent joints, the role of the pre-existing joints in the damage evolution around the underground opening is of critical importance as they govern the fracturing mechanisms and influence the brittle responses of these hard rock masses under highly anisotropic in situ stresses. In this study, the main focus is the impact of joint network geometry, joint strength and applied field stresses on the rock mass behaviours and the evolution of excavation induced damage due to the loss of confinement as a tunnel face advances. Analysis of such a phenomenon was conducted using the finite-discrete element method (FDEM). The numerical model is initially calibrated in order to match the behaviour of the fracture-free, massive Lac du Bonnet granite during the excavation of the Underground Research Laboratory (URL) Test Tunnel, Canada. The influence of the pre-existing joints on the rock mass response during excavation is investigated by integrating discrete fracture networks (DFNs) of various characteristics into the numerical models under varying in situ stresses. The numerical results obtained highlight the significance of the pre-existing joints on the reduction of in situ rock mass strength and its capacity for extension with both factors controlling the brittle response of the material. Furthermore, the impact of spatial distribution of natural joints on the stability of an underground excavation is discussed, as well as the potentially minor influence of joint strength on the stress induced damage within joint systems of a non-persistent nature under specific conditions. Additionally, the in situ stress-joint network interaction is examined, revealing the complex fracturing mechanisms that may lead to uncontrolled fracture propagation that compromises the overall stability of an underground excavation.

One of the promising methods for rock cutting technology is the use of high-speed water jets. In order to improve the cutting capacity of water jets without increasing the hydraulic power of equipment, pulsed water jets are basically used to increase the rock cutting efficiency. However, there are no mature recommendations for selection of rational parameters, and the relationship between indicators of rock cutting efficiency and parameters of pulsed water jet is still not established. In this context, we aimed at developing a generalized equation for calculating rock cutting efficiency, in which all the major parameters in consideration of rock cutting process are included. Then, a calibration of the rational parameters of rock cutting by pulsed water jets was conducted. The results are likely helpful for increasing productivity and reducing energy consumption.

The stability analysis of an abandoned underground gypsum mine requires the determination of the mine pillar's strength. This is especially important for flooded abandoned mines where the gypsum pillars become saturated and are subjected to dissolution after flooding. Further, mine pillars are subjected to blast vibrations that generate some level of macro- and micro-fracturing. Testing samples of gypsum must, therefore, simulate these conditions as close as possible. In this research, the strength of gypsum is investigated in an as-received saturated condition using uniaxial compressive strength (UCS), Brazilian tensile strength (BTS) and point load index (PLI) tests. The scale effect was investigated and new correlations were derived to describe the effect of sample size on both UCS and BTS under dry and saturated conditions. Effects of blasting on these parameters were observed and the importance of choosing the proper samples was discussed. Finally, correlations were derived for both compressive and tensile strengths under dry and saturated conditions from the PLI test results, which are commonly used as a simple substitute for the indirect determination of UCS and BTS.

In the present study, unconfined compressive strength (qu) values of two lime-treated soils (soil 1 and 2) with curing times of 28 d, 90 d and 360 d were optimized. The influence of void/lime ratio was represented by the porosity/volumetric lime content ratio (η/Liv) as the main parameter. η/Liv represents the volume of void influenced by compaction effort and lime volume. The evolution of qu was analyzed for each soil using the coefficient of determination as the optimization parameter. Aiming at providing adjustments to the mechanical resistance values, the η/Liv parameter was modified to η/LivC using the adjustment exponent C (to make qu-η/Liv variation rates compatible). The results show that with the decrease of η/LivC, qu increases potentially and the optimized values of C were 0.14–0.18. The mechanical resistance data show similar trends between qu and η/LivC for the studied silty soil-ground lime mixtures, which were cured at ambient temperature (23 ± 2) °C with different curing times of 28–360 d. Finally, optimized equations were presented using the normalized strengths and the proposed optimization model, which show 6% error and 95% acceptability on average.