International Journal of Geomechanics

In this study, a constitutive model and numerical analysis were conducted on various types of rocks under triaxial compression to determine their softening response and volumetric strain. Experimental data sets corresponding to different types of rocks were used to determine the elastoplastic material parameters and disturbance parameters of the proposed model. The disturbed state concept was utilized based on the response of the material in two states. Relatively intact (RI) and fully adjusted (FA) responses were interpolated using a state function called a disturbance. The RI and FA parts were modeled based on nonlinear finite element analysis (NFEA) and the residual strength of the corresponding material, respectively. For the RI state, the elastoplastic behavior was simulated using a hierarchical single surface (HISS) plasticity model to account for factors such as continuous yielding, volumetric change and the stress path dependence of rock. The failure surfaces used for different types of rocks were extracted from a computer program and represented in this investigation. Furthermore, the contours of disturbance (D) were depicted and the failure pattern of corresponding rock was estimated based on the distribution of disturbance values. Verification of the numerical results with experimental data sets showed a good accuracy for various types of rocks under different confining pressures.

Reinforcement solutions can ensure that low geosynthetic-reinforced (LGR) embankments progressively subside instead of suddenly failing due to void collapse, which provides an early warning to allow surface refilling to be carried out. This study investigates the characteristics of the bearing capacity and failure modes of the LGR embankments overlying voids using large-deformation finite-element analysis and normalization method. The system as a whole has six distinct failure modes: the trapdoor, sidewall, combination of roof and sidewall, rotational roof, combination of roof and single sidewall, and slope failure modes. Five types of slip lines exist for the LGR embankment: the vertical, trapezoid, double-cambered, partial-vaulted, and vaulted slip lines. This study also presents the effects of influencing parameters (e.g., rupture strength of the reinforcement, location and size of the void, and height and shear strength parameters of embankment fills) on the ultimate bearing capacity of the system. Design charts are obtained to determine the required soil properties and evaluate the characteristics of the bearing capacity of the system.

Cost effectiveness is a major factor in most vacuum preloading projects, considering the significantly reduced time allotted to achieve a relatively high degree of consolidation. However, the shallow base and deep layer of newly dredged fills are always treated separately because the traditional vacuum preloading method can barely make sufficient improvement in the one-time treatment in these weak and complex grounds. Therefore, an improved vacuum preloading method was developed to consolidate newly dredged fills in the Lin Gang Industrial Zone of Tianjin City, China. This improved method consists of three main systems: loading devices, a vacuum preloading system, and a moisture separator technique. By using two loading devices, two lengths of the prefabricated vertical drains (PVDs), and a novel moisture separator, the shallow base and deep layer of the newly dredged fills can be simultaneously processed. The results of water discharge and pore water pressure were measured using three parallel laboratory model tests. The moisture content test and the mercury intrusion porosimetry (MIP) test were conducted along with the vane shear strength test. The testing data shows that the improved vacuum preloading method can substantially improve the bearing capacity of an ultrasoft ground. The multiple vacuum preloading method and the improved synchronous and alternate vacuum preloading method increased the water discharge amount by 8.4% and 27.9%, respectively, over the traditional method. The final dissipation of pore water pressure by the traditional method was one-fifth of that of the improved method because the motion directions of soil particles were changed in the improved method. In addition, the pore size distribution (PSD) of soil changed from a bimodal distribution to a unimodal distribution, as shown by the MIP test results. With similar values in total intrusion volume, porosity, and median pore diameter, the data show that by using this newly improved synchronous and alternate vacuum preloading method, the treatment of ultrasoft dredged fills can be more even, which successfully avoids the common problem of uneven settlement of the ground.

This study conducted finite-element (FE) modeling of uplift resistance from dense backfill sand. The prepeak hardening, postpeak softening, density, and confining-pressure-dependent soil behavior were implemented in FE analysis to simulate the progressive development of shear bands. The location of the shear bands was identified from soil failure mechanisms for a range of burial depths. For shallow buried pipelines, the inclination of the slip planes to the vertical was found to be approximately equal to the maximum dilation angle when the peak uplift resistance was mobilized and then decreased with an upward movement, resulting in a postpeak reduction of uplift resistance. For deeper conditions, in addition to two inclined slip planes, logarithmic spiral-type shear bands formed above the pipe. Based on mobilized shear strength parameters and inclination of slip planes, a method to calculate the peak and postpeak uplift resistances, using an equivalent angle of internal friction, is presented.

It is crucial to estimate the behavior of layered unsaturated soils with time-dependent loading for several reasons, including (1) construction of a building is a time-consuming process and loading on the soil can be altered over time, and (2) the soils found in nature are often unsaturated and layered. There are two different approaches for analyzing the soil consolidation, namely coupled and uncoupled approaches. The coupled approach is considered to be more difficult, but more valid because this approach involves an association between stress–strain analysis and the process of excess pore fluid pressure dissipation; whereas in the uncoupled approach, the analyses are conducted separately for these two phenomena. In this study, the theory of one-dimensional coupled consolidation for layered unsaturated soils is solved under different boundary conditions and general time-dependent loading through the incremental differential quadrature method (IDQM) by assuming both water and air phases are continuous. The research demonstrated that the IDQM is able to easily solve the coupled consolidation theory for layered soils under complex loading conditions.

In this study, using the kinematic approach of limit analysis, a new numerical model was developed to investigate the effect of the coupled flow deformation on tunnel face stability. By incorporating the innovative mesh-dividing optimization technology with the principle of generating a three-dimensional (3D) rotational failure mechanism into a customized program of linear interpolation, the pore-water distribution in the stage of the global failure ahead of the tunnel face determined by FLAC3D was interpolated on the 3D rotational failure mechanism. The support pressures predicted by the developed model improved the existing upper-bound solutions by at least 14.2% for the case of slight cohesion soils but made little difference with the existing upper-bound solutions for high cohesion soils. The support pressures predicted by the developed analytical method were in close agreement with those from numerical simulations and in situ data. The results showed that the critical face pressures increased linearly with increasing water table elevation. Several design charts are provided for parametric analysis, which can be directly used by tunnel engineers in the evaluation of face stability.

Cemented paste backfill (CPB) is a widely used porous medium to fill mined-out cavities in the subsurface due to its superior environmental and operational benefits. The stability of a retaining structure (barricade) is a crucial design factor in backfill operations as CPB is under pressure imposed by material conditions at an early age. Although field monitoring has revealed the impacts of mine-blast operations on the backfill pressure that acts on the retaining structure, current studies on stress distribution in backfilled stopes have not considered the effect of such dynamic loadings. Therefore, to analyze the redistribution of stress in backfill due to mine blasts, a total-stress viscoplastic cap model was used in this study to assess the dynamic response and generation of excess pore pressure of early-age backfill during blast loading. Next, a multiphysics model for CPB was used to assess the variations in the stress state after the blast impact in backfilled stopes during the binder hydration process of the material. The two adopted models were then validated against a series of laboratory and field experiments. Finally, they were integrated and applied to investigate the effect of drainage condition, stope size, barricade location, proximity of detonation, blast sequence, initial backfill temperature, and cement content on the redistribution of stress after blast loading in backfilled stopes. The insights obtained from the results of the study increase the understanding of the stability of retaining structures for backfills in the practical engineering conditions of mine fields.

Determining a suitable width of gob-side filling wall (GFW) is a major concern in the application of gob-side entry retaining (GER) technology in highly gassy coal mines. This paper presents a workflow for GFW width design based on a rigorous numerical modeling analysis. A meticulously validated numerical model was built to analyze the vertical stress, entry convergence, and plastic zone distribution across the GFW with various sizes. In order to ensure the reliability of the numerical model, a strain-softening model for GFW modeling and a double-yield model for gob modeling were implemented, and the relevant input parameters’ calibration was clearly illustrated. The results demonstrated that when the GFW’s width is 2.5 m, it possesses sufficient load-bearing capacity, and the entry convergence variation tends to stabilize. Consequently, the rational GFW width was estimated as 2.5 m. The field monitoring results demonstrate that a gob-side entry, with a 2.5-m-wide GFW, meets the cross section requirement for gas drainage. The proposed design workflow and modeling procedure can potentially be applied to other GER design under similar geological conditions.

In view of the postfailure simulation, the material point method (MPM) has higher computational efficiency and better computational stability compared with the other numerical methods. This paper aims to understand the entire failure mechanism of the rotational slide using MPM. The Mohr-Coulomb constitutive model is implemented in the modified update stress last (MUSL) scheme. The proposed MPM code is validated by a sand collapse experiment. The simulation result is in good agreement with the experiment result. We used the MPM with the strain softening model to simulate the entire failure of the rotational slide. Numerical results of the location of rupture surface, energy, and run-out distance are related to the parameters of the strain softening model, such as the value of the peak strength parameters and the residual strength parameters. In addition, the randomness of the landslide strength parameter is also one of the important factors that affect the landslide deformation and evolution of the rupture surface. The smaller value of the strength parameter in random MPM makes the landslide unstable.

An experimental investigation on the anisotropic permeability of coal under cyclic loading and unloading conditions is presented in this article. The results show that coal permeability decreases with increasing hydrostatic stress. The relationship between coal permeability and applied stress was quantified using an improved power model. The cyclic loading and unloading were found to induce an irreversible permeability reduction, which tended to be diminished with increasing loading/unloading cycles. The anisotropic permeability of the studied coal is notable, and it was found to nonlinearly decrease with an increase of the angle between the direction of flow and the direction of bedding planes (θ). The relationship between permeability and θ can be quantified using an exponential model with an anisotropic parameter (λ). Several factors (e.g., loading cycles, stress) influencing the anisotropic permeability are discussed in this article.

High ground temperature plays an important role in stability analysis when dealing with underground projects. To study the influence of ground temperature on the properties of surrounding rock, conventional triaxial compression tests were carried out at different temperatures and confining pressures with acoustic emission (AE) technology for the damage analysis and characteristic stress values. AE refers to the phenomenon that transient elastic waves are generated by a partial sudden release of energy in different materials. The results of this study show that many parameters increased at temperatures below 60°C such as the peak strength, maximum AE energy, and brittleness of granite while they all decreased at temperatures above 60°C. Good correlations were found between the damage analysis and characteristic stress values, which were determined on the basis of the volumetric strain method for high-temperature studies. It was concluded that thermal stress relates to the different thermal expansivity measures of mineral particles and that brittleness of granite is most obvious at 60°C. This finding promotes understanding of the mechanical behavior of underground caverns and the stability analysis of rock masses.

The uncertainties involved in the current design of footings supported by rammed aggregate piers (RAPs) have brought about the need to analyze the complete foundation system accurately using the three-dimensional finite-element (3D FE) method and define the settlement improvements used in the footing design. The postconstruction stage of RAPs is considered in the 3D FE modeling by accounting for the unique installation effect of RAPs, the geometrical variation with depth, and the nonlinear behavior of both aggregates and the surrounding soil matrix. An image-processing technique allowing 3D FE modeling of such a complex foundation system was used, and the initial numerical models were validated through back-analysis of the well-known experimental results reported in the literature for a trial RAP. In a parametric study, extension of the numerical analysis to the complete foundation system of a footing supported with a RAP group of reinforced clay was accomplished by considering the mutual interaction between the foundation elements. The effects of such factors as loads, material properties, footing dimensions, RAP spacing, and lengths on the settlement improvements were investigated by considering numerous design configurations and soil conditions. Multiple nonlinear regression was used to develop a settlement model based on the results from the presented 3D FE analysis. This article provides new practical insights into the 3D postconstruction FE modeling of RAPs considering the installation effects. It also demonstrates that the proposed statistical model within the considered limiting values of variables is desirably accurate for predicting the settlement improvements of the footings supported with the end-bearing RAP groups.

The Chicontepec Formation is a complex and heterogeneous calcitric-lithic sandstone reservoir with significant historical problems of borehole collapse. To predict and mitigate the subsurface issues, it is essential to characterize the formation rock strength. This paper presents two new correlations for predicting the unconfined compressive strength (UCS) of turbidite sandstones in the Chicontepec Formation based on a function of transit time, grain diameter, and Young’s modulus. Extensive laboratory tests, wireline and logging-while-drilling data (LWD), and borehole images from 24 offset wells were used to develop the new UCS correlations. A small dispersion of the data was identified using grain size and transit time, and this new model presented a median value (P₅₀) of 76 MPa, a P₁₀ of 55 MPa, and a P₉₀ of 89 MPa. This paper also demonstrates the use of an artificial neural network (ANN) to capture the nonlinear interaction of the UCS model with the complex lithological lateral variations of the Chicontepec Basin by using the new correlation of grain size and transit time, and a regression analysis to build a three-dimensional (3D) UCS model to understand the vertical and lateral heterogeneity. The prediction of the UCS using the ANN yielded a highly reliable prediction; the coefficient of determination (R²) was 0.95 compared with the multiple regression analysis (R²), which was 0.88. The results show that the prediction performance of the ANN models is higher than the multiple regression models. The results also indicate that the UCS varies between 27.57 until 89.63 MPa.

The deformation of twin tunnels was investigated under the influence of various physical conditions, namely, (1) mutual position and (2) change in the level of the water table, and the effect of using liners of different stiffness was assessed. The effects were defined in terms of settlement or deformation parameters suitable for tunnels. A detailed parametric study was conducted using the finite-element-based software Abaqus. The tunnels were assumed to be located within weathered granite rock, and the material properties were expressed in terms of equivalent cohesion and friction angle for rock. The liner was assumed to be elastic. A mathematical expression was proposed to quantify stiffness of the tunnel liner. All results are expressed in the form of nondimensional charts for better usability. Finally, two separate comparisons were shown to validate the modeling process. It is expected that the data presented in this note will help practitioners to understand and deal with the complex tunnel deformations associated with these field conditions.

Enhancing the efficiency and accuracy of the system reliability analysis of soil slopes is of great importance in engineering design. An advanced kriging metamodel integrated with the quasi–Monte Carlo simulation (AKQMCS) is proposed for such a purpose in this paper. The proposed advanced kriging model is used as a surrogate for the limit state function (LSF) of a slope, and it is established by a sequential design of experiments, which updates the kriging model step by step, with the help of a learning function based on the entropy theory. QMCS simulation is then performed on the advanced kriging metamodel to evaluate the system failure probability of the slope. Finally, three soil slope examples, including a two-layered cohesive slope, a three-layered c-φ slope, and a single-layered sand slope, are examined to verify the capability and validity of the proposed approach. The results indicate that the proposed approach can provide a reasonable and accurate estimation of the system failure probability of slope stability with a significantly reduced number of deterministic stability analyses compared with the ordinary kriging method and the direct Monte Carlo simulation (MCS). The QMCS well outperforms the MCS in efficiency due to the low-discrepancy sequences used in the QMCS. Hence, the proposed AKQMCS provides a new and efficient tool for the system reliability analysis of soil slopes.