International Journal of Geomechanics

Seismic pounding occurs when the separation gap between buildings and structures is not wide enough, particularly during major earthquake events; this can cause them to collide, causing local damage or, in extreme cases, collapse. This study investigated the impact that this separation gap has on the seismic response of midrise buildings supported on piles while considering seismic soil–pile–structure interaction (SSPSI). To achieve this aim, three 15-story reinforced concrete buildings sitting on pile foundations and with five different separation gaps under excitations from the 1994 Northridge and 1995 Kobe earthquakes were numerically simulated. This study used three-dimensional numerical modeling to simultaneously capture the effects of seismic pounding and SSPSI. Because the considered structure, pile foundation, and soil deposit are three-dimensional in nature, the adopted three-dimensional numerical modeling can provide a more realistic simulation to capture the seismic behavior of the system. The nonlinear behavior of structural elements was included, and the dynamic soil properties were obtained from field data and backbone curves. A contact pair interface with small-sliding surface-to-surface formulation between buildings was used to capture possible seismic pounding, and contact interfaces with a finite-sliding formulation were used to simulate the interaction between the piles and the soil. The results, including lateral building deflections, interstory drifts, structural shear forces, foundation rocking, lateral pile deflections, and the distributions of bending moments and shear forces of the piles, are presented and discussed. The findings of this study will give engineers a better insight into the possible effects of seismic pounding on the seismic performance of buildings, and the response of end-bearing piles in soft soils.

Research on the evaluation of microfractures in tight sandstone reservoirs is a frontier topic worldwide. A marine tight sandstone reservoir is present in the Silurian strata in the northern Tazhong area of the Tarim Basin, Northwest China, at a depth of more than 5,000 m. The study of the regular opening and closing of microfractures can provide guidance for the exploration and development of tight sandstone reservoirs. In this paper, a triaxial acoustic experiment was designed, and a new method for assessing the microfracture stress sensitivity of tight sandstone reservoirs was proposed by combining the Biot-consistent and the differential equivalent medium (DEM) theory models. By using the coupled model, the rock microfracture density ε and the pore aspect ratio were calculated. The pore-fracture morphological evolution and regular distribution within the rocks were discussed, and the microfracture stress sensitivity was evaluated. This method could effectively overcome the drawback of using a single method. The results showed that the microfracture density ε decreases and the pore aspect ratio increases with an increase in the confining pressure, which is mainly due to the closure of microfractures. According to the decreasing magnitude of microfracture density values under different confining pressures, the proportion of microfractures that stayed open or had closed could be determined. The stress sensitivity of the samples that had greater microfracture density under higher confining pressure was weak. In contrast, the stress sensitivities of the other samples were strong. According to the change rule of the rock pore aspect ratio, a critical pressure was defined. It was found that the rock microfracture density values corresponding to the critical pressures for all samples were relatively close. This ε value could be treated as a common feature of tight sandstone reservoir microfractures, and it could also be used as a contrast indicator to quantitatively characterize the degree of development of various tight reservoir microfractures. The correlations between the microfracture density ε under a confining pressure of 65 MPa, the defined critical pressure, and the rock porosity indicated that the advantageous types of microfractures were mainly influenced by the rock physical properties. There was a close correlation between the microfracture development degree and the rock physical properties. For the tight sandstone reservoir of this study, the degree of microfracture development with weak stress sensitivity was significantly higher in feldspar than in quartz. This method could be used for the quantitative evaluation of microfracture properties.

The evaluation of the state of stress in a backfilled stope is necessary for assessing the stability of the sides and assisting in barricade design. To date, a few analytical models have been proposed in the literature for the estimation of the earth pressures of fill material; however, these models do not consider the three-dimensional (3D) inclined-stope geometry or consolidation of the material in a passive state. In this paper, an analytical model is proposed for the estimation of earth pressures by considering pore-water pressure and the consolidation effect of the fill material for different state conditions in an inclined 3D stope. The proposed model was validated with experimental works and another analytical model. The geotechnical properties of cemented backfill material were determined as the key inputs for this model. The model was applied to estimate the vertical and horizontal stresses that may develop in a filled stope with length, width, and height of 40, 20, and 60 m, respectively, and is inclined at 75° from the horizontal. This information is useful for designing the thickness of a barricade during a filling operation.

This study presents a novel smoothed particle FEM (SPFEM) for large-deformation problems in geomechanics. Within the framework of the particle FEM (PFEM), a strain smoothing technique for nodal integration is incorporated. The problem domain is divided into strain smoothing cells associated with particles, and the equilibrium of the continuum medium is achieved at these strain smoothing cells. The corresponding computational formulations and numerical procedure are given. Compared with the original PFEM, the SPFEM possesses the following advantages: (1) all the field variables are calculated and stored at the particles, and the frequent information transfer between Gauss points and particles, which inevitably introduces error and adds considerable complexity to solution procedures, is avoided; (2) the SPFEM possesses the upper bound property, providing a conservative estimation for problems in geomechanics; and (3) linear elements can be used directly without suffering from the volumetric locking, so special treatment to bypass the volumetric locking is not required. By solving two benchmark examples, the SPFEM has been verified to be a promising numerical method for analyzing large-deformation problems in geomechanics.

In this study, laboratory model tests and three-dimensional (3D) numerical analysis were carried out on stone column–improved ground under embankment loading. In the experimental study, the time-dependent change in stress on the stone column and clay (at the embankment–ground interface) was measured for different spacing-to-diameter ratios. Finite-difference software was used for further investigation to measure the change in settlements, stresses, and pore pressures with time. In the numerical analysis, the stone column and the sand embankment were represented with the Mohr-Coulomb model, whereas the modified Cam-clay model was used for the soft clay. It was observed that to ensure no differential settlement on the embankment surface, a minimum embankment height of 2 times the clear spacing between the stone columns is necessary. The maximum lateral deformation of the stone column was observed at approximately 2.5 times the diameter of the column from the top.

This work aimed to gain insight into the influence of the dynamic component of vehicle load (caused by pavement roughness) on the dynamic stress responses in saturated subsoil. The analytical solutions for the three-dimensional dynamic stresses in the saturated poroelastic half-space, subjected to a harmonic rectangular moving load, were derived by using the Fourier integral transform, along with the propagator matrix method. The numerical results clearly show the advantages of the layered poroelastic half-space computational model, as well as the significant influences of the subsoil stiffness on the dynamic stress responses. Moreover, the dynamic stresses in the subsoil may be dramatically affected by the dynamic component of the moving load (caused by pavement roughness). It was also found that the influences of the dynamic component on the resulting dynamic stresses become much more profound when a stiffer subsoil and/or lower vehicle speed are encountered.

The polynomial chaos expansion method was used to perform probabilistic analysis on a tunnel face driven in rock masses. The generalized Hoek-Brown (HB) failure criterion was used to characterize the strength of rock masses. Deterministic calculations of required face pressures were based on the upper bound limit analysis theory. Four HB input parameters, the geological strength index (GSI), the uniaxial compressive strength σ_c of the intact rock, the constant m_i, and the disturbance factor D_i, were considered as random variables. The influence of HB input parameter uncertainties on probability density functions of the required face pressures, sensitivity analysis, and failure probabilities were discussed with respect to different coefficients of variance (COVs), distribution types, and correlation coefficients. A reliability-based design was performed to compute probabilistic face pressures under different COVs at a target safety level. The obtained results show uncertainties in the input random variables have an important influence on the required face pressures, and it is necessary to analyze the problem in the probabilistic framework.

The three-dimensional (3D) stress-strain response of uniform sands has been the focus of extensive laboratory investigations, resulting in well-established understanding of their 3D stress-strain-strength behavior. However, the applicability of stress-dilatancy theories and 3D stiffness, strength, and volumetric responses have not been sufficiently evaluated for well-graded fill soils commonly used in design. A series of drained true-triaxial tests at three levels of confining stress on specimens of well-graded Kanaskat gravel were conducted to address pertinent questions regarding this behavior. Constant mean effective stress paths during shearing corresponded to triaxial compression (TC), simple shear (SS), and triaxial extension (TE). Previously reported results from plane strain quasi-K₀ consolidated (PSK₀) stress paths on the same material are incorporated and the general behavior suggest that 3D stress-dilatancy and frictional responses differ from those of uniform soils. The secant shear modulus at an initial stage of shearing was found to be stress path dependent with TC specimens being the highest. However, the stiffness at a more advanced stage of shearing was found to be relatively independent of the stress path and fitted power laws adequately capture pressure-dependent stiffness. Friction and dilation angles at failure are interpreted with respect to experimental boundary conditions and are significantly higher than those measured in the TC stress path. However, properly calibrated stress-dilatancy theories and 3D failure criterion are sufficiently capable of capturing this behavior. Fitting parameters are provided for a commonly used stress-dilatancy approximation and two 3D failure criteria for the soil investigated here. The data presented here should help those seeking to estimate the 3D response of a well-graded gravelly soil.

This study examined a composite foundation that was improved by the joint usage of two types of columns: soil-cement and stone columns. The composite foundation that consists of soil-cement columns, stone columns, and natural soils is herein referred to as a multicolumn composite foundation (MCF). Explicit expressions of the equivalent modulus of the MCF under elastic and/or plastic conditions were considered. The spatial variability in the material properties of natural soils and soil-cement columns was taken into account during the calculation of the equivalent moduli. Through considerations of variability, the MCF settlement can be predicted in a statistical manner, thus enabling a reliability-based design for the composite foundation under the serviceability limit state. Finally, the reliability-based design methodology was shown to achieve a desirable balance between safety and economic considerations when designing the area replacement ratios of both types of columns in a MCF.

This article puts forward a quantified characterization method of micro- and macrocontacts of granular materials. The microcontact is defined as the contact surface with two voids in its two ends. The macrocontact is defined as a sequence that is composed of microcontacts and satisfies the condition of a one-dimensional manifold. The two dual geometrical systems can be extracted from the granular assembly. The edges and paths in the solid-cell system can realize the quantified topological characterization of the micro- and macrocontacts. For cemented granular materials, each crack can be treated as a gaping contact. The contact characterization can be used to visualize the cracks in the simulation results of the discrete-element method (DEM). The shortest-path algorithm can help judge if there is a macrocrack in the numerical specimen and acquire the quantified failure route. The contact force of an arbitrary macrocontact can be calculated by the vector addition of the force on the particle contacts that compose it.

The support design for rock tunnels involves a complex problem because of the various types of uncertainties present in rock mass properties; therefore, probabilistic approaches are used to consider these uncertainties systematically in the presented analysis. In earlier studies, uncertainties in peak strength parameters and deformation modulus were considered while uncertainty in residual strength parameters was neglected. Another important factor is the assumption regarding the postpeak behavior of the rock mass, which depends on the rock mass quality in the field. An average quality rock mass generally shows strain-softening behavior in a range from peak to residual strength, which generally has been neglected because of the assumption that the rock mass is elastic-perfectly plastic. However, it was observed that the yield zone depth and displacements around the tunnel are highly sensitive to residual strength parameters, and residual strength parameters can considerably influence the estimation of the support requirements of a tunnel. In this paper, a new computational approach, based on the geological strength index (GSI) and the use of deterministic and probabilistic methods, is described for the reinforcement design of a tunnel in an average quality rock mass. This approach considers the variability in residual strength parameters along with peak strength parameters and the deformation modulus. Moreover, the strength drop in the stress-strain behavior of the rock mass is also considered in the analysis. An underground powerhouse cavern from the Himalayan region of India is taken as the case study to show the methodology used in the research. The study brings out the advantages of the probabilistic over the deterministic approach for the support design of a tunnel. Another important conclusion from the study is that the support requirements for the tunnel are greatly influenced by residual strength parameters, and therefore, uncertainty and strength drop in the residual strength parameters should be properly considered when designing support for tunnels.

Analytical solutions are proposed to evaluate the average settlement under an embedded footing in an elastic soil. The rigid foundation was simplified as a uniformly loaded area. Four factors, the Gibson, trench, sidewall friction, and retaining wall effects, were considered to reduce the settlement of the footing due to embedment. Immediate settlement of a footing can be calculated using either conventional influence factors or by a finite-layer-based consolidation analysis with a short time interval. The effectiveness of the proposed techniques was evaluated by comparing results obtained from different approaches, and a maximum difference of 10% was observed. The method was also used to analyze the settlements of the base slab of an excavation, which were compared with field measurements of the West Extension Line of the Nanjing Metro Line 10. On the basis of the analysis, the reason why a compensated foundation could experience excessive settlement is discussed. Overexcavation induced by basal heave results in a very loose state of the soil underneath the footing where swelling could occur. Reloading after backfilling easily causes large settlements. Therefore, an arched beam foundation is proposed to reduce the magnitude of further settlement of the footing.

Probabilistic analyses are conducted for seepage through a rock-fill dam having two different core-filter configurations: one sloping and the other a central symmetrical core-filter arrangement. Uncertainties in core and filter are considered, assuming their hydraulic conductivities as random variables. For this purpose, finite-element software used for groundwater flow and seepage analyses is coupled with a random-number-generation algorithm. Monte Carlo simulations are performed for probabilistic seepage analyses. The suggested method is applied to a 30-m-high rock-fill dam. Results showed that the uncertainty in core and filter causes a decrease in the seepage rates of both configurations. It is concluded that in case of uncertainty, the central symmetrical core-filter configuration is more successful than the sloping configuration in decreasing seepage.