International Journal of Geomechanics

Mechanical behavior of organic-rich shale strongly influences the stimulation performance of hydraulic fracturing. This article presents an approach for the characterization and evaluation of mechanical properties of shale at different scales. Nanoindentation and modulus mapping were performed to measure the Young’s modulus and stiffness coefficients of the main constituents in shale at the mineral scale. The contact area of the nanoindentation on quartz is approximately 3.79 × 10⁵ nm², whereas the contact areas of modulus mapping are almost below 7.85 × 10³ nm². This comparison indicates that modulus mapping has higher resolution and is more suitable for the characterization of clay minerals. At the core scale, microindentation was applied to investigate the macroscopic properties. From the statistical analysis, we find that the quantity of the successful microindentation should be more than 70 to obtain effective results. In addition, an upscaling method was proposed to predict the macroscopic properties. Both of the predictions from the Mori-Tanaka (MT) model and the self-consistent method agree well with the microindentation results, but the results from the MT model show stronger anisotropy. Finally, we provided a new brittleness index (BI) and an example to estimate the mechanical properties along the well with the mineralogical logging data.

In the present paper, the interference effects on bearing capacity and settlement behavior of two closely spaced identical footings resting on the surface of granular fill (sand layer) of limited thickness underlain by soft clay have been studied by laboratory model tests. Three pairs of model footings with different shapes are used to study the effect of the aspect ratio of footing on footing interference. The width (B) and length (L) of the footings are selected in such a way that the aspect ratio or L:B ratio of the footings is varied as 1.0 (square), 1.5 (rectangular), and 2.0 (rectangular). Results show that the maximum interference factor for closely spaced footings on a single-layer sand bed is 40% higher in magnitude compared with the sand-clay layered soil bed or homogeneous clay bed. Furthermore, the interference factors increase with the increase in the L:B ratio of the footings. The optimum spacing ratio is independent of the L:B ratio of the footings, and it is 1.5 times the width of the footings. Based on the load-spread mechanism and the deformation of sand-clay interface, an analytical solution has been proposed to determine the bearing capacity of two closely spaced rectangular footings.

This paper proposes a practical method of determining embankment loads-induced vertical superimposed stress in subsoil by incorporating the effects of contact stress and soil elastoplastic behavior. The particulate–probabilistic theory is introduced to establish a new way of determining contact stress for overcoming the assumptions in the traditional trapezoidal stress distribution approach with instantaneous loading and perfectly flexible loaded area along the base. Then, the Flamant solution is introduced to propose a practical method of determining the vertical superimposed stress in subsoil. A reduction coefficient of 0.85 is suggested for considering the effect of elastoplastic of subsoil. The measured values of contact stress and field settlements are compiled from literature available to validate the proposed equations. It is found that the suggested methods in this study can improve significantly the degree of accuracy of the Osterberg method (Osterberg 1957) in calculating embankment loads induced settlement.

This study focuses on the settlement of soft soil improved by floating soil cement columns in a small-scale physical test. The effect of area improvement ratio (a_p) and column height (H_c) on the improved ground under design load (W_d) were investigated via small-scale physical modeling tests. The area improvement ratios of 21.7, 32.5, and 43.4% and column heights of 50 and 100 mm were examined. The models were instrumented to measure displacements and soil pressures. Two loading scenarios were applied on treated and untreated soils. The first series was conducted under strain controlled mode to identify the failure mechanism. The second series was conducted under design load to evaluate the stress distribution, settlement, and failure pattern. The settlements also were measured using particle image velocimetry (PIV) technique. The PIV showed that the final settlement of the improved ground decreases as the area improvement ratio (a_p) increases and the column height (H_c) increases. For controlling the distribution of stresses, an intermediate range of area improvement ratio is recommended. In addition, this study proved that the PIV technique is an effective optical method to simulate ground deformations at the lowest strain without performing a full-scale test.

Uncertainty is a notable feature of the geotechnical field. Reliability analysis methods provide a more rational solution than deterministic ones in the presence of the spatial variability of soil properties. Two types of reliability analysis methods were employed in the face stability analysis of pressurized tunnels in this study: the random variable method (RVM), which ignores spatial autocorrelation of soil parameters, and the random finite-difference method (RFDM), which considers spatial autocorrelation. In the RVM, the first-order second-moment (FOSM) method was adopted to calculate the reliability index with less computational effort. Then, the computational efficiency and accuracy of the two types of reliability analysis methods were systematically compared through a typical tunnel problem. Further sensitivity studies using the RFDM were performed to explore the effects of the scale of the fluctuation and autocorrelation functions. The authors found the probability of failure predicted by the RVM to be significantly overestimated because the spatial structure of soil parameters were ignored. Finally, using equivalent soil parameters instead of realistic ones, the corresponding results predicted by the equivalent FOSM method were found to be consistent with those of the RFDM, but they required far less computational effort. Thus, the equivalent FOSM method can be used to efficiently predict the stability of the tunnel face in variable soils, and it provides a practical tool for designers.

Coring operations in the petroleum industry are generally performed to study geological, petrophysical, geomechanical, and drilling properties of reservoir formations and characteristics of reservoir fluids. Some of these studies need intact cores for performing mechanical tests or for measuring reservoir fluid properties. Therefore, retrieval of intact cores is crucial to coring jobs. During the coring operation and tripping a core out of a well, it experiences a sudden or gradual decrease in confining stress, pore pressure, and temperature (time-dependent boundary conditions) depending on the mud pressure, in situ stress, reservoir temperature, and reservoir pore pressure. Some porothermoelastic (PTE) properties of the retrieved core also change due to the change in reservoir conditions at or near the surface. Specifically, changes in the core properties depend on the in situ conditions, mud pressure, and the tripping velocity. These conditions or phenomena, which occur simultaneously, induce mechanical, poroelastic (PE), and thermal stresses in the core, which may cause failure. In this paper, the PTE behavior of unconventional tripping of cores is studied with the goal of identifying the most influential variables and the risk of core damage. First, the PTE and in situ properties of six different shale plays located in the US are collected from the literature. The Marcellus shale data are then used to identify the following three most important controlling factors and their effect on the core damage: use of PE and/or PTE approaches, use of constant or variable properties, and change in tripping velocity. Second, the gradual change in stress, pore pressure, and temperature during core tripping is examined in the Marcellus shale. Third, PTE behavior and damage potential of the selected six shale plays during tripping are investigated, and the results among these reservoirs are compared. Fourth, a comprehensive sensitivity analysis is performed for each of these shale plays via changing the input variables by selected percentages and observing the change in the effective stress field. Finally, some guidelines are presented, along with an optimization procedure for tripping velocity, for efficient coring operations. A commonly used software, FLAC^3D, is used for all numerical analyses because of its capability and features of simulating coupled problems as a PTE medium with variable properties and time-dependent boundary conditions.

This work discusses the results from tests performed to investigate the damage and microcracking behaviors of granites. The samples, previously loaded to varying predefined stress levels and then unloaded to zero, were tested using a nuclear magnetic resonance (NMR) technique. The variations in the micro-NMR parameters, including the NMR-derived porosity, transverse relaxation time (T₂) spectra distribution, and T₂ area of the samples, were quantitively analyzed prior to and after the uniaxial loading/unloading tests. The results show that the induced damage in the samples increases with the loading ratio, and the rate of increase is significantly accelerated when the loading ratio is greater than 0.7. An NMR-based damage parameter was introduced to evaluate the degradation process under unconfined compression. Three stages of microcracking evolution were distinguished under the unconfined loading condition according to the variations in the proportion of the micropores and macropores. The NMR technique offers an alternative method to characterize the mesodamage and microcracking of rock under compression and has the advantages of being accurate and nondestructive.

High variability in the results of laboratory tests with peat poses difficulties in the application of the classic consolidation theory to the prediction of settlement in the field. This article presents a formulation involving a three-parameter function that can define a wide range of strain history in peat. This formulation can estimate the maximum possible strain under a specific stress and the entire strain history as a function of time. The parameters necessary for the formulation can be derived from a simple short-term compression test. The formulation is based on the results of strain-history measurement in laboratory one-dimensional, triaxial, and isotropic triaxial consolidation tests of fibrous peat specimens and field measurement of embankment settlement. The formulation is extended to estimate the strain under multiple stress increments based on the measured strain history corresponding to the first stress increment. Based on this formulation, settlement time charts can be developed for an individual peat site, and the strain history and maximum possible strain under different loads can be directly estimated based on the measured initial strain history.

The coastal areas in Japan suffered devastating damage due to the great East Japan earthquake and tsunami in 2011. Breakwaters collapsed mainly because of foundation failures during the earthquake and tsunami. Due to the breakwater failures, the tsunami entered the coastal zones and imposed deep devastation. This study focused on the development of reinforcing countermeasures for a breakwater foundation that can produce a resilient breakwater against earthquakes and tsunamis, such as foundations reinforced with sheet piles and gabions. Physical model tests were carried out for scaled-down breakwater models to examine the performance of the reinforcing countermeasures under an earthquake and tsunami. During the tests, the developed reinforced model was found to be effective in mitigating the damage of the breakwater created by the earthquake and tsunami. Numerical simulations were performed to further clarify the mechanism.

Large underground voids created by removal of ore are usually backfilled with mill tailings. The stress state in the backfill must be estimated to ensure safety. Most of the existing analytical solutions for estimating stress at any depth within mine fills had adopted the assumption of uniform vertical stress. However, this assumption does not make for a valid hypothesis. This paper presents an analytical solution to estimate the stress state in vertical backfilled stopes with stress patterns of distribution in the fill considered. The trajectory of minor principal stress in the fill from the centerline to the wall was assumed to be either a circular-arc, parabola, or catenary. The direction of the minor principal stress at the centerline was vertical to the centerline, and the direction along the rock mass was determined using the geometrical relationship of Mohr’s stress circle. The stress states of the differential flat elements in the backfill were analyzed through equilibrium considerations. Then, the analytical solution for estimating the stress state in the backfill was obtained. A series of examples are presented and compared with existing analytical solutions and numerical results carried out in ABAQUS (version 6.13). A nonuniform stress distribution across the backfill width is also presented. The vertical normal stress component at the centerline was greater than that along the wall. The stress state in the backfill was affected by the friction angle and cohesive strength of the backfill soils, friction angle at the wall interface, and trajectory of the minor principal stress. A discussion is presented after the comparison of numerical and analytical results and addresses the limitations of the proposed analytical solutions.

A method is developed in the paper for the design of retaining walls to calculate three types of lateral earth pressures using the plastic limited equilibrium approach by refining the existing trial wedge method. Use of the developed method makes the calculation of lateral earth pressures contributed by surcharge more logically and cogently consistent with the Rankine and Coulomb theories compared with the current practice of using the combined Rankine and Coulomb method and Boussinesq elastic solution. A specific procedure based on the proposed trial wedge method is developed to more precisely calculate at-rest lateral earth pressure for Coulomb’s retaining wall conditions. The procedure presents advantages over the current practice of using either the Danish Geotechnical Code method or the Army’s strength mobilization factor (SMF) method.