International Journal of Geomechanics

The creep behavior of soft clays is essential in evaluating the long-term deformation of foundations. Because this behavior of naturally deposited clays is very complex as a result of their structures, loading history, and physicochemical processes, in addition to the heavy workload and high cost for the sampling procedure of natural soft clays, oedometer creep tests were usually conducted on reconstituted clays, aiming to establish the fundamental work for the elastic-viscoplastic modeling (EVP) and the creep prediction of natural soft clays. In this study, based on a series of one-dimensional creep tests of reconstituted soft clays, the relationship between the creep coefficient (C_α) and the vertical effective stress (${\sigma \prime }_v$) and that between C_α and the ratio of vertical effective stress to preyielding stress (${\sigma \prime }_v$/${\sigma \prime }_y$) are presented in semilogarithmic planes. The results suggest that the C_α value increases with the vertical effective stress prior to reaching the threshold stress (∼2.17${\prime \sigma }_y^{}$), and then C_α gradually decreases after it. The results (C_α vs. compression index C_c) show that the statistical C_α/C_c value of reconstituted clays ranges from 0.015 to 0.05, which is more than that of natural soft clays (0.03–0.05). This suggests that the reconstitution process is beneficial to restrain the creep of soils. Furthermore, it was found that the C_α/C_c value of reconstituted clays linearly decreased with the void ratio when vertical effective stress exceeded the threshold stress (∼2.17${\sigma \prime }_y$). Based on the data of this study and data from previous studies, a prediction equation for C_α after the threshold stress is proposed, showing that C_α is a function of the void ratio and the void ratio at the liquid limit.

An improved approach is proposed here for searching the global noncircular critical slip surface in a slope with soil variability. To avoid the local minimum, the proposed approach combines a readily available optimization with the simulated annealing algorithm (SAA). The simplified Morgenstern-Price method was used to calculate the factor of safety (Fs) for the generated slip surface. The reliability of the searching strategy was evaluated through the effect of vertices number and the position of the initial slip surface. Subsequently, the proposed method was used to analyze two example problems involving slope profiles with different geometries and strength fields. The results showed that the location of the initial slip surface had a negligible influence on the global critical slip surface as well as the corresponding factor of safety, if the annealing time was sufficient. The location of the global critical slip surface diverged when the segment number was greater than 3. The proposed approach can seek the relative global factor of safety with a calculation accuracy of 1 × 10⁻⁴, which is suitable for engineering use. The noncircular slip surface assumption was superior to the circular slip surface assumption for both the simple multilayered soil slope and the spatially variable soil slope with a low level of strength variation. However, the circular slip surface assumption will also locate more critical slip surfaces if the variation of soil strength reaches a high level. The critical F_s value was determined from the given random strength field other than the assumption of a circular or noncircular slip surface. The noncircular slip surface searching strategy can provide a useful supplement for the limited-equilibrium method (LEM) searching for the critical slip surface in a spatially variable soil slope. Practitioners can select the smaller F_s from these two different assumptions.

This study incorporated the critical unstable condition into extended finite-element analysis to determine the safety factor of a slip surface that is regarded as a discontinuity embedded into the slope mesh. The displacement field and the safety factor can be obtained simultaneously by directly solving the final nonlinear equations. Moreover, the augmented Lagrange multiplier method and the vital vertex algorithm were used to improve the accuracy of the normal stress on the slip surface. Three examples were presented to demonstrate various aspects of the proposed method, including its efficiency in searching for the critical slip surface without remeshing and sensitivity to the finite-element mesh.

To estimate the factor of safety of soil slopes, the discretization technique is widely used to generate the critical slip surface under the linear Mohr-Coulomb failure criterion. However, the strength envelope of almost all soils has the nature of nonlinearity. This study develops a novel approach to calculate the factor of safety of soil slopes when the soil strength follows the nonlinear yield criterion. First, the numerical simulation is used to obtain the magnitude and the coordinate of minor principal stress. Then, the equivalent strength parameters are calculated by combining with the nonlinear failure criterion. Finally, slip surface is generated by the discretization technique and the factor of safety is derived on the basis of the upper bound theorem. The elastic stress analysis method of numerical simulation is used to obtain the stress distribution, and its validity is demonstrated by comparing the results derived from the elastic stress analysis and those from the elastic-plastic stress analysis. To further show the validity of the proposed method, the presented results are compared with those using GEOSLOPE software, which is a limit equilibrium method, and with those of previously published results. Based on the comparisons, the proposed method is an effective technique to calculate the factor of safety of soil slope under the condition of nonlinear failure criterion. The examples indicate that the proposed method has potential applicability in obtaining the stability of slope in layered soils.

Many elastoplastic models have been proposed to model the stress–strain behavior of granular soils. However, due to the dependence of stress-dilatancy on the material state, different model parameters are often needed to model the constitutive behavior of granular soils under different initial states. In this study, an attempt is made to develop a state-dependent fractional plasticity model by using the fractional-order Cambridge stress-dilatancy equation, in which detailed numerical approximations of the model are provided for further potential application. To validate the proposed model as well as the numerical solution, a series of triaxial test results on a number of different granular soils, including rockfill, sand, and ballast, is simulated. A good agreement between the model predictions and the corresponding test results is observed.

Soil nailing is a popular in situ reinforcement technique that is widely used to stabilize unstable slopes and vertical faces of excavations and tunnels. The pullout capacity of soil nails is a governing design parameter for the soil-nailing technique. Therefore, the determination of accurate pullout capacity is necessary to ensure adequate internal stability of the stabilized structures. The article presents a laboratory investigation on the pullout force–displacement behavior of the soil nails installed in a dry, cohesionless medium. The study describes the effects of surface roughness, surcharge pressure, and relative density on the pullout response of soil nails. The results show that with an increase in the roughness, there was an increase of more than twofold in the mobilized maximum pullout resistance of the soil nails. Furthermore, the relative density was found to have a significant influence on the pullout capacity and the mobilized peak interface friction angle of the soil nails. The pullout force–displacement behavior of the smooth surface soil nail was found to be elastic–perfectly plastic in nature, whereas a strain-softening behavior was observed for the soil nails with a rough surface. The experimental data were used to formulate an empirical model (substantiated by the previously published data) that can predict the peak and residual shear strength of the nail–soil interface at different combinations of surface roughness and surcharge pressure. The proposed empirical model was then incorporated into the existing constitutive models to reproduce the observed (experimentally) pre and postpeak pullout force–displacement response of the soil nails. The simulated results are in good agreement with the experimental observations.

The Lu Dila hydropower station has a complex geologic structure, given that it has a larger fault scale, exhibits large dam-site orientation changes, and has a more developed fissure in the dam foundation and riverbed. The present study established a three-dimensional seepage-field model of the Lu Dila hydropower station to analyze the seepage field and assess the reasonability and effectiveness of the seepage-control measures. The free surface was determined by an iterative algorithm, and the drainage hole was simplified as a line to simulate the function of the drainage hole. The present study analyzed the seepage field under different conditions, with focus placed on assessing and demonstrating the reasonability and effectiveness of the dam foundation’s impervious curtain and the drainage system. The results indicate a relationship between the large seepage discharge and the construction quality of the upstream left-bank curtain, the dam foundation fissure, and the fault connectivity. Reasonable references and suggestions are offered for project operation. In addition, the simulation method of the drainage hole was proven to be effective.

The hydromechanical coupling effect between soil skeleton and pore fluid phases, that is, dissipation of dynamically induced excess pore water pressure, can be a critical factor in determining the dynamic response of geotechnical structures in a certain permeability range. A theoretical study for investigating this certain permeability range is the propagation mechanism of compressional waves in geotechnical media, which has not been thoroughly investigated, especially for its dual behavior (i.e., the fast wave and slow wave) in the concerned frequency range for geotechnical engineering problems (0.1–20 Hz). Therefore, this paper numerically studies the propagation mechanism of compressional waves in saturated geotechnical media under soil dynamics loads. Investigations reveal the transition mechanism between the two kinds of compressional waves considering the effects of loading frequency, soil stiffness, and permeability. More importantly, based on the numerical results, this paper proposes empirical transition lines between the fast wave and slow wave, specified by loading frequency and material permeability. Practically, the proposed empirical lines can be used as the boundaries defining the applicable ranges of one-phase and two-phase coupled analysis for predicting the dynamic response of saturated geotechnical structures.

In the present study, numerical solutions have been developed to determine the internal compressive normal pressure required to be offered by the support system for maintaining the stability of long, circular tunnels located in clay with an overlay of sand. The compressive normal pressure is assumed to be uniform on the periphery of the tunnel, which can be offered by means of lining. The solutions were obtained on the basis of a finite-element lower-bound limit analysis and second-order cone programming. For different angles of internal friction of sand, the solutions have been obtained by varying the ratio of unit weight of the upper sand layer relative to that of the lower clay layer. The magnitude of required support pressure is found to be decreasing continuously with an increase in the internal friction angle of the overlying sandy soil for a given cover of the tunnel. The increase or decrease in the magnitude of support pressure required in the layered soil medium as compared to the case of the homogenous clay layer with properties same as that of the lower clay layer depends on the combined influence of the unit weight ratio, the thickness of both layers, and the angle of internal friction of the overlying sand layer. The parametric design charts are developed, which can be utilized by practicing engineers for determining the thickness and stiffness of lining for circular tunnels excavated in the clay underlying sand layer.

The lower- and upper-bound theorems of the limit analysis have been used in conjunction with finite elements and second-order cone programming (SOCP) for determining the bearing capacity of strip and circular skirted footings on sand. The analysis follows the Mohr-Coulomb’s yield criterion and the associated flow rule; sand is not usually considered to obey this rule, but the results of using it are discussed. The friction angle of sand was varied between 30 and 45°, and the depth (D_s) of the skirt increased from 0.25 to 2B; here B implies: (1) the width of a skirted strip footing, and (2) the diameter of a circular skirted footing. The results are expressed in terms of the bearing capacity ratio (BCR): the ratio of the bearing capacities of a skirted footing to that of the surface footing, with the same value of B but without any skirt element. The results reveal that the magnitude of the BCR increases quite extensively with an increase in the value of D_s/B. The skirted footing was found to be especially quite advantageous for loose sand. With the same D_s/B, the BCR for a circular skirted footing was found to be substantially greater than that for the strip skirted footing.