International Journal of Geomechanics

Numerical simulation of staged-construction modeling of large-diameter steel pressure pipes is accompanied by several complexities. Finite-element (FE) analysis was performed to simulate the behavior of buried steel pipes during staged-construction installation. The model deflections during the staged-construction process were verified with four different experimental soil box test results. The FE model and its associated analysis algorithm considered large deformation using total Lagrangian formulation. The material and contact nonlinear algorithms were also included in the analysis for both soil and steel pipe materials. The contact between each soil layer and soil-to-pipe was carefully implemented. Uniform thermal loading was applied to simulate the stresses induced by compaction forces on the pipe and trench walls. Finally, the vertical and lateral load-deformation plots obtained from the FE analysis results were compared with the full-scale experimental test results during the staged construction process and after the application of surcharge load. The results of FE analysis are shown to accurately model and simulate the test results.

Knowledge of fracture stiffness, in situ stresses, and elastic parameters is essential to the development of efficient well patterns and enhanced geothermal systems. In this paper, an artificial neural network (ANN)–genetic algorithm (GA)-based displacement back analysis is presented for estimation of these parameters. Firstly, the ANN model is developed to map the nonlinear relationship between the fracture stiffness, in situ stresses, elastic parameters, and borehole displacements. A two-dimensional discrete element model is used to conduct borehole stability analysis and provide training samples for the ANN model. The GA is used to estimate the fracture stiffness ($k_n$, $K_s$), horizontal in situ stresses (${\sigma }_H$, ${\sigma }_h$), and elastic parameters ($E$, $v$) based on the objective function that is established by combining the ANN model with monitoring displacements. Preliminary results of a numerical experiment show that the ANN-GA-based displacement back analysis method can effectively estimate the fracture stiffness, horizontal in situ stresses, and elastic parameters from borehole displacements during drilling in naturally fractured geothermal reservoirs.

A method is presented for determining the ultimate bearing capacity of a circular footing reinforced with a horizontal circular sheet of reinforcement placed over granular and cohesive-frictional soils. It was assumed that the reinforcement sheet could bear axial tension but not the bending moment. The analysis was performed based on the lower-bound theorem of the limit analysis in combination with finite elements and linear optimization. The present research is an extension of recent work with strip foundations reinforced with different layers of reinforcement. To incorporate the effect of the reinforcement, the efficiency factors ${\eta }_{\gamma }$ and ${\eta }_c$, which need to be multiplied by the bearing capacity factors $N_{\gamma }$ and $N_c$, were established. Results were obtained for different values of the soil internal friction angle ($\varphi$). The optimal positions of the reinforcements, which would lead to a maximum improvement in the bearing capacity, were also determined. The variations of the axial tensile force in the reinforcement sheet at different radial distances from the center were also studied. The results of the analysis were compared with those available from literature.

By using the lower-bound theorem of the limit analysis in conjunction with finite elements and nonlinear optimization, bearing-capacity factors, $N_c$ and $N_{\gamma q}$, with an inclusion of pseudostatic horizontal seismic body forces, have been determined for a shallow embedded horizontal strip footing placed on sloping ground surface. The variation of $N_c$ and $N_{\gamma q}$ with changes in slope angle ($\beta$) for different values of seismic acceleration coefficient ($k_h$) has been obtained. The analysis reveals that irrespective of ground inclination and the embedment depth of the footing, the factors $N_c$ and $N_{\gamma q}$ decrease quite considerably with an increase in $k_h$. As compared with $N_c$, the factor $N_{\gamma q}$ is affected more extensively with changes in $k_h$ and $\beta$. Unlike most of the results reported in literature for the seismic case, the present computational results take into account the shear resistance of soil mass above the footing level. An increase in the depth of the embedment leads to an increase in the magnitudes of both $N_c$ and $N_{\gamma q}$.

Nonlinear power-type failure envelopes of the form $\tau ={\left(a+b\sigma \right)}^n$ were examined in this paper. It is shown that equations for which $0<n<1\text{/}2$ are legitimate failure envelopes provided that $a$ is greater than some function of $b$, contrary to earlier assertions. The principal stress ${\sigma }_1\text{-}{\sigma }_3$ relations corresponding to these laws have been derived explicitly for the quadratic law ($n=1\text{/}2$) and implicitly for $n=1\text{/}3,2\text{/}3,\text{and}\hspace{0.17em}3\text{/}4$. For other $n$ values, a numerical algorithm for deducing the principal stress relations has been given. The procedure for evaluating the parameters $a$ and $b$ from triaxial test data for a specified $n$ value is presented in detail, and it parallels Baker's earlier effort. Almost all previous studies on nonlinearity have concentrated on its effect on the factors of safety of slopes. This study provides a numerical method for evaluating the earth pressures on smooth retaining walls, under plane-strain conditions, for the case $n\ne 1\text{/}2$. When $n=1\text{/}2$, closed-form equations, which are nonexistent in the literature, were derived for both the earth pressures and the slip surfaces in two-dimensional plane-strain active and passive stress states. A new explicit formula is presented for the depth of tension cracks in plastic soils for $n=1\text{/}2$, whereas new implicit formulas are developed for $n=1\text{/}3,2\text{/}3,\text{and}\hspace{0.17em}3\text{/}4$. The assumed value of this depth has a profound influence on the calculated factor of safety of a slope. Existing Rankine, Bell, and Coulomb formulas overestimate the passive resistance of geomaterial, and this study shows that the use of a nonlinear law predicts more realistic reduced values of passive resistance. Therefore, the factor of safety of 2 or more hitherto applied to passive resistance in the design of embedded walls can now be reduced to a lower value. A computer program was included for automatically determining the best $n$ value that matches the triaxial test data together with the associated $a$ and $b$ and also for doing the rest of the calculations rapidly. As a consequence, a best-fit nonlinear power-type envelope can now be fitted effortlessly to the Hoek-Brown criterion.

The paper describes the results and lessons learned using a FEM model to simulate quantitative performance features of the Minnow Creek steel-strip reinforced soil wall structure located in the United States. The Minnow Creek Wall structure was constructed and instrumented in 1999. It is a unique case study because of the comprehensive measurements that were taken to record a wide range of wall performance features. Two different constitutive models for the soil were used (a linear-elastic Mohr-Coulomb model and hardening soil model with a Mohr-Coulomb failure criterion), and numerical outcomes were compared with physical measurements. The numerical results were shown to be sensitive to boundary conditions assumed at the toe of the wall. The generally encouraging agreement between physical and numerically predicted results gives confidence that commercial FEM software packages can be useful for the analysis and design of these types of structures, provided that care is taken in the selection of input parameters.

A thermomechanical constitutive model for predicting the isothermal behavior of saturated clays in different temperatures is presented in this paper. This model is developed based on the general framework of critical state soil mechanics and modified Cam-clay formulation. Most of the characteristics of saturated clays in temperatures lower than water’s boiling temperature have been taken into account. An attempt has been made to use the lowest possible number of extra parameters compared with the original Cam-clay model and to ensure that these new parameters have clear physical interpretations. An important feature in the model is thermal dependency of the critical state line in the deviatoric stress plane. The predictions have been compared with five sets of laboratory data available in the literature.

This paper presents a simplified solution for consolidation of plastic clays under cyclic loading in disturbed state concept (DSC) framework. When normally consolidated (NC) clay undergoes cyclic loading, at the beginning of the loading, it is in a normally consolidated state. In each cycle of loading until the average ratio of consolidation is less than its maximum value in the last cycle of loading, the clay is in an overconsolidated (OC) state and then becomes normally consolidated. The process continues until a certain number of cycles have occurred and the clay reaches the OC state during all the loading cycles, which is called the steady-state condition. There is a gradual and continuous change from NC to OC state in successive loading half-cycles. The changing properties of the clay during cyclic loading mean that it is impossible to find a closed-form solution for this problem. Because the state of the clay changes gradually from a known NC state at the beginning to a known OC state at the final steady state, DSC can be adapted to this kind of problem using propitious state functions describing the changes in clay state. An analytical solution of a consolidation partial differential equation is used to obtain a general state function. The state function is used to describe the gradual change in state of the clay from OC to NC during successive cycles. Then the solutions of the problem in two elastic conditions of NC and OC states are interpolated using a derived state function to obtain the solution of the plastic problem. The validity of this method is ensured by a series of numerical and experimental tests.

The plate-load test (PLT) is widely performed at large dam sites to determine the rock mass deformability. Analytical relations based on the theory of elasticity, as suggested by the International Society for Rock Mechanics and the American Society for Testing and Materials test standards, are commonly used to interpret the test results and calculate the rock mass deformation modulus. However, because of inconsistency of the test gallery geometry with the half-space medium assumption embedded in the analytical relations, a systematic error occurs in the calculation of the deformation modulus. Furthermore, when the tests are conducted on jointed rock masses, the test results show a considerable scatter, mainly due to the anisotropy of the deformability of jointed rock masses. In this paper, numerical modeling is used to interpret the results of a series of PLTs conducted at the Bakhtiary dam site in southwest Iran. An equivalent continuum model, called JointedRock, is used to back-calculate the stiffness of bedding planes from the test results. Finally, the equivalent continuum deformation moduli of the rock mass are determined numerically for various directions of loading relative to the orientation of major joint sets.

Long-term stability of geotechnical structures is critical to any engineering design. Design of underground structures is initially planned using empirical approaches based on the rock mass–classification system. This design is mainly based on limited data obtained from boreholes and surface geological mapping. However, this design needs to be continuously updated based on the information gathered as the excavation work progresses. Over the last few years, geotechnical monitoring has emerged as a major engineering tool for engineers to review and update their existing design configuration based on the actual behavior of excavated rock mass. This paper presents a case study of the interaction of two large unlined rock caverns that were excavated using a drill-and-blast technique with a heading-down benching approach. The mechanical response of the two caverns was numerically modeled and studied in terms of the stress-deformation behavior of the excavated caverns and rock support installed, taking into account the actual sequence of heading and benching followed during parallel excavation activity at one of the construction sites. It was observed from the analysis that the crown/wall junction is sensitive to displacements under both good and fair rock-mass conditions. Limited comparative study was also carried out between the measured and calculated displacements; this showed encouraging results. Progressive buildup of forces in the rock bolts and development of the stresses in pillars were also studied as part of the interaction response of two caverns.

Shallow foundations are designed to limit settlements within tolerable limits. Additional settlements produced by the rise of the water table due to rain or floods can jeopardize the integrity of the foundation. This paper proposes a rational method, based on strain influence factors, to predict the additional settlement produced by the rise of the water table on a footing resting on sands. The proposed method was validated using extensive laboratory test data, where model footings of five different shapes were loaded in sand placed at two relative densities, and the water level was raised from the bottom while the additional settlements were measured. The clean sands used in the tests were such that there were negligible capillary effects. Separate tests were carried out to demonstrate that the capillary effects were insignificant. The proposed method can be used as the basis for further studies that incorporate the effect of grain size distribution or the effects of water table fluctuations on additional settlement.

Reinforcing soil with fibers is a useful method for improving the strength and settlement response of soil. The soil and fiber characteristics and their interaction are some of the major factors affecting the strength of reinforced soil. The fibers are usually randomly distributed in the soil, and their orientation has a significant effect on the behavior of the reinforced soil. In the paper, a study of the effect of anisotropic distribution of fibers on the stress-strain response is presented. Based on the concept of the modified Cam clay model, an analytical model was formulated for the fiber-reinforced soil, and the effect of fiber orientation on the stress-strain behavior of soil was studied in detail. The results show that, as the inclination of fibers with the horizontal plane increased, the contribution of fibers in improving the strength of fiber-reinforced soil decreased. The effect of fibers is maximum when they are in the direction of extension, and vice versa.