Geotechnical Engineering Online Library

The intrinsic relationships between energy dissipation, energy release, strength and abrupt structural failure are key to understanding the evolution of deformational processes in rocks. Theoretical and experimental studies confirm that energy plays an important role in rock deformation and failure. Dissipated energy from external forces produces damage and irreversible deformation within rock and decreases rock strength over time. Structural failure of rocks is caused by an abrupt release of strain energy that manifests as a catastrophic breakdown of the rock under certain conditions. The strain energy released in the rock volume plays a pivotal role in generating this abrupt structural failure in the rocks. In this paper, we propose criteria governing (1) the deterioration of rock strength based on energy dissipation and (2) the abrupt structural failure of rocks based on energy release. The critical stresses at the time of abrupt structural failure under various stress states can be determined by these criteria. As an example, the criteria have been used to analyze the failure conditions of surrounding rock of a circular tunnel.

An artificial water curtain system is composed of a network of underground galleries and horizontal boreholes drilled from these galleries. Pre-grouting measures are introduced to keep the bedrock saturated all the time. This system is deployed over an artificial or natural underground cavern used for the storage of gas (or some other fluids) to prevent the gas from escaping through leakage paths in the rock mass. An experimental physical modeling system has been constructed to evaluate the performance of artificial water curtain systems under various conditions. These conditions include different spacings of caverns and cavern radii located below the natural groundwater level. The principles of the experiment, devices, design of the physical model, calculation of gas leakage, and evaluation of the critical gas pressure are presented in this paper. Experimental result shows that gas leakage is strongly affected by the spacing of water curtain boreholes, the critical gas pressure, and the number and proximity of storage caverns. The hydraulic connection between boreholes is observed to vary with depth or location, which suggests that the distribution of water-conducting joint sets along the boreholes is also variable. When designing the drainage system for a cavern, drainage holes should be orientated to maximize the frequency at which they encounter major joint sets and permeable intervals studying in order to maintain the seal on the cavern through water pressure. Our experimental results provide a significant contribution to the theoretical controls on water curtains, and they can be used to guide the design and construction of practical storage caverns.

This paper studies the swelling of highly consolidated mudstones by theoretical considerations and laboratory experiments. A key assumption was made that saturated and uncemented clays behave as heavily dense colloid without direct contacts among solid particles. It leads to an important conclusion that the swelling pressure acting on adsorbed interparticle water-films is equivalent to the effective stress. This so-called clay-colloid concept is validated by various swelling experiments on two kinds of mudstones, the Callovo-Oxfordian argillite in France and the Opalinus clay in Switzerland. In the tests, water adsorption-desorption, swelling pressure and strain were measured on the samples at various suctions and load-controlled conditions. Results suggest that: (1) the mudstones can take up great amounts of water from the humid environment, much more than the water content in the natural and saturated states; (2) the swelling pressure increases with water uptake to high levels of the overburden stresses at the sampling depths of 230 to 500m, indicating that the adsorbed water-films are capable of carrying the lithostatic stress; and (3) the large amount of water uptake causes a significant expansion of mudstones even under the lithostatic stresses.

Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handling is an often-mentioned advantage. The paper describes the experimental program performed to characterize the hydro-mechanical H) behaviour of compacted pellet mixtures. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen’s dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range from 1.3 to 1.5Mg/m3. Pellets had a very high dry density, close to 2Mg/m3. The outstanding characteristic of these mixtures is their discontinuous porosity. Pore sizes of the compacted pellets varied around 10nm. However, the inter-pellet size of the pores was four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture. Performed tests include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction-controlled oedometer tests and swelling pressure tests. The interpretation of some performed tests required back analysis procedures using a hydro-mechanical HM) computer code. Material response was studied within the framework of the elastoplastc constitutive model proposed by Alonso et al. (1990) (Barcelona basic model, BBM). Parameters for the model were identified and also a set of hydraulic laws are necessary to perform coupled HM analysis. A large scale in-situ test (the “EB” test in Mont Terri, Switzerland) was described and analyzed. Rock barrier parameters were adjusted on the basis of available tests. The test excavation, barrier emplacement and forced hydration were simulated by means of the CODE_BRIGHT program. The comparison between measurements and computed results include data on relative humidity in the rock and the buffer, swelling pressures and displacements.

A few constitutive models for unsaturated soils have already been proposed, however, many classic models such as the Barcelona basic model can simulate neither complex volumetric soil behaviour (without forgetting its supreme merit of being the first consistently and rigorously formulated model) nor post-peak softening, and most advanced models generally comprise a large number of parameters making them more difficult to be applied to practical situations. In this paper, we present a new model for unsaturated soils based on an existing model developed originally for saturated soils. It comprises a minimum number of constitutive parameters. The extension to unsaturated state is achieved by following a general methodology previously developed in our laboratory. The capacities of this simple model are tested. With only 13 parameters, it can reproduce the basic behaviour of unsaturated soils such as rebound or collapse upon wetting, depending on the stress levels. It can also reproduce post-peak softening and transition from contractant to dilatant volumetric behaviour during undrained shear. Overall, the first tentative of validation gives a good correlation between simulations and experimental data, and shows encouraging signs for future developments.

This study addresses firstly the soil fabric variations of loose and dense compacted soil samples during a single wetting/drying cycle at suctions between 0 and 287.9MPa using mainly the mercury intrusion porosimetry (MIP) tests. Two suction techniques were employed to apply this wide suction range: the osmotic technique for suctions less than 8.5MPa, and the vapor equilibrium or salt solution technique for suctions higher than 8.5MPa. Secondly, the soil water retention curves (SWRCs) were predicted by the MIP test results for both loose and dense soil samples. A reasonable correspondence between MIP results and SWRCs was found on the wetting path at lower suctions close to saturation and on drying path at higher suctions.

For brittle materials, the tensile strength plays an important role in mechanical analyses and engineering applications. Although quasi-static direct and dynamic indirect tensile strength testing methods have already been developed for rocks, the dynamic direct pull test is still necessary to accurately determine the tensile strength of rocks. In this paper, a Kolsky tension bar system is developed for measuring the dynamic direct tensile strength of rocks. A dumbbell-shaped sample is adopted and attached to the bars using epoxy glue. The pulse shaping technique is utilized to eliminate the inertial effect of samples during test. The single pulse loading technique is developed for the effective microstructure analyses of tested samples. Two absorption devices are successfully utilized to reduce the reflection of waves in the incident bar and transmitted bar, respectively. Laurentian granite (LG) is tested to demonstrate the feasibility of the proposed method. The tensile strength of LG increases with the loading rate. Furthermore, the nominal surface energy of LG is measured, which also increases with the loading rate.

For a pipejacking, the jacking force is critical to balance the resistance force and to move the pipe string forwards. The driving mechanism of a curved pipejacking is more complicated than a straight-line pipejacking, and its jacking force is also more difficult to be determined. The paper theoretically studies the jacking force of a curved pipejacking by considering the static equilibrium of earth pressure, resistance at cutting face, friction at pipe surface, and the driving force behind the pipe string. The derived theoretical formula can be used to estimate the driving forces of a straight-line or a curved pipejacking. Case study was performed by applying the theoretical and empirical formulae. After calibration, the corrected formula is more accurate and more applicable.

Failure pressure is a key parameter in reservoir hydrofracturing operation. Existing analytical methods for calculating the failure pressure are based on the assumption that borehole fluid is under two extreme conditions: non-infiltration or complete infiltration. The assumption is not suitable for the actual infiltration process, and this will cause a great error in practical calculation. It shows that during the injection process, the dynamic variation in effective stress-dependent permeability has an influence on the infiltration, and the influence also brings about calculation errors. Based on the fluid-structure interaction and finite element method (FEM), considering partial infiltration during injection process, a numerical model for calculating rock failure pressure is established. According to the analysis of permeability test results and response-surface method, a new variation rule of rock permeability with the change of effective stress is presented, and the relationships among the permeability, confining pressure and pore pressure are proposed. There are some differences between the dynamic value of permeability-effective-stress coefficient observed herein and the one obtained by the classical theory. Combining with the numerical model and the dynamic permeability, a coupling method for calculating failure pressure is developed. Comparison of field data and calculated values obtained by various methods shows that accurate values can be obtained by the coupling method. The coupling method can be widely applied to the calculation of failure pressure of reservoirs and complex wells to achieve effective fracturing operation.

Stability of an ancient landslide in a reservoir area is analyzed by using centrifugal model tests, soil laboratory tests and numerical analysis. Special attention is paid to variation in water level, simulation of large-scale heterogeneous prototype slope, and strength reduction of sliding zone soils after slope sliding. The results of centrifugal model test show that reservoir impounding can reduce sliding resistance at the slope toe, followed by toe collapsing and front cracking of slope. Rapid drawdown can produce hydrodynamic pressure towards reservoir at the front of slope. Deformation is observed in the middle and upper slope, which reduces the slope stability further and forms the pull-typed landslide trend. Reinforcement of slope toe is effective for preventing the progressive failure. The results of laboratory test show that slope toe sliding will lead to the redistribution of soil density and moisture content, which will reduce the shear strength of soil in sliding zone, and the cohesion of immersed soil is reduced gradually and finally vanishes with time. The numerical results show that the strength reduction method used in finite element method (FEM) is very effective in capturing the progressive failure induced by reservoir water level fluctuations, and the evolution of failure surface derived from numerical simulation is very similar to that observed in centrifugal model test.

Roadways excavated in soft rocks at great depth are difficult to be maintained due to large deformation of surrounding rocks, which greatly influences the safety and efficiency of deep resources exploitation. During the excavation process of a deep soft rock tunnel, the rock wall may be compacted due to large deformation. In this paper, the technique to address this problem by a two-dimensional (2D) finite element software, large deformation engineering analyses software (LDEAS 1.0), is provided. By using the Lagrange multiplier method, the kinematic constraint of non-penetrating condition and static constraint of Coulomb friction are introduced to the governing equations in the form of incremental displacement. The numerical example demonstrates the efficiency of this technology. Deformations of a transportation tunnel in inclined soft rock strata at the depth of 1 000m in Qishan coal mine and a tunnel excavated to three different depths are analyzed by two models, i.e. the additive decomposition model and polar decomposition model. It can be found that the deformation of the transportation tunnel is asymmetrical due to the inclination of rock strata. For extremely soft rock, large deformation can converge only for the additive decomposition model. The deformation of surrounding rocks increases with the increase in the tunnel depth for both models. At the same depth, the deformation calculated by the additive decomposition model is smaller than that by the polar decomposition model.

In-situ stress measurement for deep reservoir formation is difficult in terms of security, reliability and technique. Acoustic velocity anisotropy test is a basic method for stress measurement of rock cores, which is based on the distribution of acoustic velocity in different directions around rock cores. The heterogeneity of core samples, such as fractures and gravel contained, can also lead to wave velocity anisotropy. Therefore, the corresponding reliability evaluation method is established to exclude some other anisotropy factors caused by non-tectonic stresses. In this paper, the reliability of testing results is evaluated from three aspects, i.e. phase difference, anisotropy index and waveform, to remove the factors caused by non-tectonic stresses.

Over the last decades, people from almost all over the world have realized that it is necessary to quickly develop strategies for the control and reduction of greenhouse gases (GHG) emissions. Among various GHGs, carbon dioxide (CO2) is the most abundant GHG. Its underground storage involves less risk and lower levels of dangerousness. The paper briefly describes the most effective technologies available in the market for background processes to storage (capture and transport) CO2, as well as the more secure solutions for its storage, in particular for the geological storage in carboniferous formations. This paper also outlines the methodologies for the risk assessment involved in storage of CO2, with a particular focus on cases where the injection is made into unminable coal seams and in abandoned coal mines. Methodologies used for risk analysis are described in detail with particular emphasis on Bayesian network (BN). Some applications regarding the risk assessment of CO2 injection processes and CO2 storage in carboniferous formations and contamination of aquifers are presented and analyzed. Finally, based on the applications of BN, several conclusions are drawn.

In this paper, simulations of real rockfall by discontinuous deformation analysis (DDA) are conducted. In the simulations, the energy losses of rockfall are categorized into three types, i.e. the loss by friction, the loss by collision, and the loss by vegetation. Modeling of the energy loss using absolute parameters is conducted by the DDA method. Moreover, in order to verify the applicability and validity of the proposed DDA, field tests on rockfall and corresponding simulations of rockfall tests by DDA are performed. The simulated results of rockfall velocity and rockfall jumping height agree well with those obtained from the field tests. Therefore, the new technique properly considers the energy-absorption ability of slope based on vegetation condition and shape of the rockfall, and provides a new method for the assessment and preventive design of rockfall.

This paper provides an overview of current seismic ground motion criteria for transportation infrastructures in USA. This is to facilitate an understanding of current seismic performance levels and design practices in USA for transportation professionals world-wide, especially those in Asian and Pacific regions. Seismic vulnerability of a transportation infrastructure is determined by the risk associated with the seismic ground motion and specified performance criteria. Determining an acceptable seismic risk is a very complex task that must consider both social and economic aspects. Obviously, the amount of risk that a railway bridge can accept may be different from that of a highway bridge. The economic tolerance in one country may be different from that in another country for the same type of infrastructure. In this paper, seismic performance levels and design criteria of ground motion for highway bridges, railway bridges and ports’ container wharves in USA are reviewed, and design examples are presented to demonstrate how to develop the code-based and site-specific design acceleration response spectra and time histories.