Geotechnical Engineering Online Library

The paper focusses on the use of physical modelling in ground movements (induced by underground cavity collapse or mining/tunnelling) and associated soil-structure interaction issues. The paper presents first an overview of using 1g physical models to solve geotechnical problems and soil-structure interactions related to vertical ground movements. Then the 1g physical modelling application is illustrated to study the development of damage in masonry structure due to subsidence and cavity collapse. A large-scale 1g physical model with a 6 m3 container and 15 electric jacks is presented with the use of a three-dimensional (3D) image correlation technique. The influence of structure position on the subsidence trough is analysed in terms of crack density and damage level. The obtained results can improve the methodology and practice for evaluation of damage in masonry structures. Nevertheless, ideal physical model is difficult to achieve. Thus, future improvement of physical models (analogue materials and instrumentation) could provide new opportunities for using 1g physical models in geotechnical and soil-structure applications and research projects.

In the context of the Cigéo project, the French National Radioactive Waste Management Agency (Andra) is studying the behaviour of a deep geological facility for radioactive waste deposit in the Callovo-Oxfordian (COx) claystone. The assessment of durability of this project requires the prediction of irreversible strain over a large time scale. The mechanical interaction of the host rock and the concrete support of tunnels must be investigated to ensure the long-term sustainability of the structure. The instantaneous and time-dependent behaviour of the claystone-concrete interface is experimentally investigated with direct shear tests and long-duration shear tests of a few months. The mechanical and structural state of the claystone which is affected after interaction with concrete reflects to the response of the claystone-concrete interface, and thus different types of COx claystone-concrete interfaces are tested. The delayed deformation of the interface is found to be linked to the level of the normal loading and the loading history, while a different response of the interface was observed from the short- and long-duration tests, indicating a possible progressive modification of interface under long-duration loadings.

Realising the importance of pore fluid salinity on the dewatering behaviour of fine-grained porous systems, the present study systematically investigated such impacts on temporal moisture dynamics of kaolin subjected to evaporative dewatering. A detailed discussion is provided pertaining to the background processes dictating evaporative dewatering response and corresponding alterations in the dielectric behaviour of kaolin. Frequency dependent dielectric spectra of soil, which can be considered as the fingerprint of the transient changes in the condition of water phase within the pore system of the soil and associated densification, are monitored in real time during dewatering using an open-ended coaxial probe with a vector network analyser. The spatial sensitivity of the coaxial probe has been quantified through layered media approach. Combining the results of volume change behaviour of the material along with its moisture loss response, the study characterised the hydro-mechanical response of the material from the windows of frequency dependent dielectric spectroscopy.

In this work, a multi-scale pore network with fractures is developed against experimental data in a wide range of degrees of water saturation. The pore network is constructed based on the measured microstructure information at several length scales. The gas transport is predicted by different gas transport equations (e.g. Javadpour, dusty gas model (DGM), Civan and Klinkenberg), which can consider the fundamental physics mechanisms in tight porous media, such as Knudsen diffusion and viscous flow. Then, the model is applied to simulating the gas permeability of the Callovo-Oxfordian (COx) claystone. The predicted gas permeability is basically in good agreement with the experimental data under different degrees of water saturation. Then the effects of micro-fissures are studied. The results suggest that this model can predict the gas flow in other tight porous media as well and can be applied to other fields such as carbon capture and storage.

The uniaxial compressive strength (UCS) of rock is an important parameter required for design and analysis of rock structures, and rock mass classification. Uniaxial compression test is the direct method to obtain the UCS values. However, these tests are generally tedious, time-consuming, expensive, and sometimes impossible to perform due to difficult rock conditions. Therefore, several empirical equations have been developed to estimate the UCS from results of index and physical tests of rock. Nevertheless, numerous empirical models available in the literature often make it difficult for mining engineers to decide which empirical equation provides the most reliable estimate of UCS. This study evaluates estimation of UCS of rocks from several empirical equations. The study uses data of point load strength (Is(50)), Schmidt rebound hardness (SRH), block punch index (BPI), effective porosity (n) and density (ρ) as inputs to empirically estimate the UCS. The estimated UCS values from empirical equations are compared with experimentally obtained or measured UCS values, using statistical analyses. It shows that the reliability of UCS estimated from empirical equations depends on the quality of data used to develop the equations, type of input data used in the equations, and the quality of input data from index or physical tests. The results show that the point load strength (Is(50)) is the most reliable index for estimating UCS among the five types of tests evaluated. Because of type-specific nature of rock, restricting the use of empirical equations to the similar rock types for which they are developed is one of the measures to ensure satisfactory prediction performance of empirical equations.

There are a large number of abandoned coalmines in China, and most of them are located around major coal-fired power stations, which are the largest emission sources of carbon dioxide (CO2). Considering the injection of CO2 into abandoned coalmines, which are usually in the flooded condition, it is necessary to investigate the effect of CO2–water–coal interaction on minerals and pore structures at different pressures, temperatures and times. It reveals that the CO2–water–coal interaction can significantly improve the solubility of Ca, S, Mg, K, Si, Al, Fe and Na. By comparing the mineral content and pore structure before and after CO2–water–coal interaction, quartz and kaolinite were found to be the main secondary minerals, which increased in all samples. The structures of micropores and mesopores in the range of 1.5–8 nm were changed obviously. Specific surface areas and pore volumes first increased and then decreased with pressure and time, while both increased with temperature. By using the Frenkel–Halsey–Hill model, the fractal dimensions of all samples were analyzed based on Ds1 and Ds2, which reflected the complexities of the pore surface and pore volume, respectively. The results show that fractal dimensions had very weak positive correlations with the carbon content. Ds1 had a positive correlation with the quartz and kaolinite contents, while Ds2 had a negative correlation with the quartz and kaolinite contents.

The mechanical properties and fracturing mechanism of shale containing beddings are critically important in shale gas exploitation and wellbore stability. To investigate the effects of shale bedding on crack behavior and fracturing mechanism, scanning electron microscope (SEM) with a loading system was employed to carry out three-point bending tests on Longmaxi outcrop shale. The crack initiation and propagation of Longmaxi shale were observed and recorded by taking photos during loading. The cracking paths were extracted to calculate the crack length through a MATLAB program. The peak load, fracture toughness and fracture energy all increase with the bedding angle from 0° to 90°. The crack length and energy were also found to increase with the bedding angle in the range of 0°–60° and then drop slightly. The fracturing mechanism of shale includes the main crack affected by the bedding angle and disturbed by randomly distributed particles. The main cracking path was accompanied by several microcrack branches which could form an interconnected crack system. When the main crack encounters larger sedimentary particles, it will deflect around the particles and then restore to the initial direction. A numerical technique using extended finite element method (XFEM) coupled with anisotropic cohesive damage criteria was developed, which is able to capture the dependence of crack propagations on bedding angle and sedimentary particles. This study sheds light on understanding and predicting mesoscale fracture behavior of shale with different bedding angles.

Gaomiaozi (GMZ) bentonite is a potential buffer/backfill material for a deep geological disposal of high-level radioactive waste. It has a wide pore size distribution (PSD) with sizes ranging from several nanometers to more than one hundred microns. Thus, properly characterizing the pore structures of GMZ bentonite is a challenging issue. In this study, pressure-controlled porosimetry (PCP), rate-controlled porosimetry (RCP), and scanning electron microscopy (SEM) were used to investigate the PSD of GMZ bentonite. The results indicate that each method has its limitation, and a combined use of PCP and RCP is suitable to obtain the full-scale PSD of GMZ bentonite. Moreover, we also compared the full-scale PSD with nuclear magnetic resonance (NMR) result. It is found that there is no significant difference in the range of PSD characterization between NMR and mercury intrusion method (PCP and RCP). However, in a certain range, the detection accuracy of NMR is higher than that of mercury injection method. Finally, permeability prediction based on PCP and SEM data was conducted, and both of the two methods were found to be able to predict the permeability. The combined method is effective to obtain the full-scale PSD of GMZ bentonite, which is the key to estimation of the sealing ability of bentonite buffer.

This study attempted to investigate the potential of sugarcane press mud (PM) as a secondary additive in conjunction with lime for the stabilization of an expansive soil. The physico-mechanical properties of an expansive soil, such as plasticity, shrink-swell behavior, unconfined compressive strength (UCS), mineralogical and microstructural characteristics were investigated. The expansive soil was stabilized at its optimum lime content (7%) for producing maximum strength, and was modified with four different quantities of PM in small dosages (0.25%–2%). Cylindrical soil samples, 38 mm in diameter and 76 mm in height, were cast and cured for varying periods to evaluate the strength of the amended soil. The spent samples after strength tests were further used for determination of other properties. The test results revealed that PM modification led to a substantial improvement in 7-d strength and noticeable increase in 28-d strength of the lime-stabilized soil (LSS). The addition of PM does not cause any detrimental changes to the shrink–swell properties as well as plasticity nature of the stabilized soil, despite being a material of organic origin. Mineralogical investigation revealed that the formation of calcium silicate hydrate (CSH) minerals, similar to that of pure lime stabilization with only the type of mineral varying due to the modification of PM addition, does not significantly alter the microstructure of the LSS except for superficial changes being noticed.

The realisation of microwave-induced fracturing of hard rocks has potential significance for microwave-assisted mechanical rock fracturing and stress release in deep rock masses. In this context, compact basalts were treated by microwave heating in a multi-mode cavity at a frequency of 2450 MHz, and then, we investigated the mechanical behaviour of basalt samples after microwave treatment under uniaxial compression and conventional triaxial compression (CTC) tests. After microwave exposure, cracks appeared on the surface and inside of the rock sample, and the temperature of the sample's surface was unevenly distributed. The results show that the conventional triaxial compressive strength (CTCS) of basalt samples decreased linearly with microwave exposure time, and the higher the confining pressure, the smaller the reduction in the strength of basalt samples after microwave treatment. Under uniaxial compression, microwave exposure greatly affected the axial deformation, suggesting that deformation resistance of the samples gradually decreases with increasing microwave exposure time. Under triaxial compression, some microcracks induced by microwave exposure closed due to the effect of confining pressure, resulting in the confining pressure inhibiting any rightward shift of the axial deformation curve. Furthermore, under uniaxial compression, the elastic modulus and Poisson's ratio of basalts also decreased in a quasi-linear manner with elapsed microwave exposure time. Under triaxial compression, microwave exposure has slight influence on elastic modulus and Poisson's ratio. After microwave treatment, the changes in rock strength and deformation mainly result from changes in between the mineral structures. Confining pressure results in the closure of microcracks produced by microwave exposure, so that effects of microwave treatment on strength and deformation decrease, thus reducing the influence on elastic constants. The cohesion decreases with increasing microwave exposure time and shows an approximately linear decrease over time. In the basalt samples, new microcracks in various directions generated by microwave exposure can increase the discreteness of test results, while the discreteness of test results caused by microcracks gradually reduces with increasing confining pressure.

Shearing behavior and failure mechanism of bolt-grout interface are of great significance for load transfer capacity and design of rock bolting system. In this paper, direct shear tests on bolt-grout interfaces under constant normal load (CNL) conditions were conducted to investigate the effects of bolt profile (i.e. rib spacing and rib height) and grout mixture on the bolt-grout interface in terms of mechanical behaviors and failure modes. Test results showed that the peak shear strength and the deformation capacity of the bolt-grout interface are highly dependent on the bolt profile and grout mixture, suggesting that bolt performances can be optimized, which were unfortunately ignored in the previous studies. A new interface failure mode, i.e. ‘sheared-crush’ mode, was proposed, which was characterized by progressive crush failure of the grout asperities between steel ribs during shearing. It was shown that the interface failure mode mainly depends on the normal stress level and rib spacing, compared with the rib height and grout mixture for the range of tested parameters in this study.

Layered rock mass of significant strength changes for adjacent layers is frequently observed in underground excavation, and dynamic loading is a prevalent scenario generated during excavation. In order to improve the driving efficiency and reduce engineering accidents, dynamic compression characteristics of this kind of rock mass should be understood. The dynamic properties of a layered composite rock mass are investigated through a series of rock tests and numerical simulations. The rock mass is artificially made of various proportions of sand, cement and water to control the distinct strength variations at various composite layers separated by parallel bedding planes. All rock specimens are prefabricated in a specially designed mould and then cut into 50 mm in diameter and 50 mm in height for split Hopkinson pressure bar (SHPB) dynamic compression testing. The test results reveal that increasing strain rate causes the increases of peak strength, σp, and the corresponding failure strain, εp, while the dynamic elastic modulus, Ed, remains almost unchanged. Interestingly, under the same strain rates, Ed of the composite rock specimen is found to decline first and then increase as the dip angle of bedding plane increases. The obtained rock failure patterns due to various dip angles lead to failure modes that could be classified into four categories from our dynamic tests. Also, a series of counterpart numerical simulations has been undertaken, showing that dynamic responses are in good agreement with those obtained from the SHPB tests. The numerical analysis enables us to look into the dynamic characteristics of the composite rock mass subjected to a broader range of strain rates and dip angles than these being tested.

Partially submerged deposit slopes are often encountered in practical engineering applications. However, studies on evaluating their stability under seismic loading are still rare. In order to understand the seismic behavior of partially submerged deposit slopes, centrifuge shaking table model tests (50g) were employed. The responses of horizontal accelerations, accumulated excess pore pressures, deformation mode, and failure mode of the partially submerged deposit slope model were analyzed. In dynamic centrifuge model tests, EQ5 shaking event was applied numerically. The results indicated that in the saturated zone of the deposit slope, liquefaction did not occur, and the measured horizontal accelerations near the water table were amplified as a layer-magnification effect. It was also shown that the liquefaction-resistance of the deposit slope increased under multiple sequential ground motions, and the deformation depth of the deposit slope induced by earthquake increased gradually with increasing dynamic load amplitude. Except for the excessive crest settlement generated by strong shaking, an additional vertical permanent displacement was initiated at the slope crest due to the dissipation of excess pore pressure under seismic loading. The result of particle image velocimetry (PIV) analysis showed that an obvious internal arc-slip was generated around the water table of the partially submerged deposit slope under seismic loading.

Accurate seismic assessment and proper aseismic design of underground structures require a comprehensive understanding of seismic performance and response of underground structures under earthquake force. In order to understand the seismic behavior of tunnels during an earthquake, a wide collection of case histories has been reviewed from the available literature with respect to damage classification, to discuss the possible causes of damage, such as earthquake parameters, structural form and geological conditions. In addition, a case of Tawarayama tunnel subjected to the 2016 Kumamoto earthquake is studied. Discussion on the possible influence factors aims at improving the performance-based aseismic design of tunnels. Finally, restoration design criterion and methods are presented taking Tawarayama tunnel as an example.

Rockburst is a typical rock failure which frequently threatens both human life and construction equipment during highly stressed underground excavation. Rock lithology is a control factor of rockburst. In this paper, rockburst tests were conducted on rectangular prismatic specimens of six types of intact hard brittle rocks, i.e. granodiorite, granite, marble, basalt, sandstone and limestone, under one-free-face true triaxial loading conditions. With the use of high-speed cameras, an acoustic emission (AE) system and a scanning electron microscope (SEM), rockburst of different rocks was investigated. The results show that the strainbursts of granodiorite, granite and marble were accompanied by tensile splitting near the free face, and consequently were relatively strong with a large amount of fragment ejection and kinetic energy release. For basalt, sandstone and limestone, failure was primarily dominated by shear rupture. The strainbursts of basalt and sandstone were relatively small with minor fragment ejection and kinetic energy release; while no burst failure occurred on limestone due to its relatively low peak strength. Rock strength, fracturing and fragmentation characteristics, and failure modes of different rocks can significantly affect rockburst proneness and magnitude. The AE evolution coupled with SEM analysis reveals that the differences in the inherent microstructures and fracture evolution under loading are the primary factors accounting for different rockbursts in various rock types.