International Journal of Geomechanics

The changes in subgrade moisture conditions play an important role in the in-service performance of a pavement. These conditions are sensitive to environmental conditions, such as a rise in the water table, precipitation, freeze-thaw cycles, and wet-dry cycles, among others. Several studies have been conducted to develop relationships between the mechanical properties, namely, the resilient modulus ($M_R$) of subgrade soils and moisture conditions. No studies, to the authors’ knowledge, have directly addressed the hysteretic behavior of $M_R$ with moisture conditions. This study evaluates the effect of moisture hysteresis on $M_R$ values of a subgrade soil following two different paths: (1) drying-wetting-drying path and (2) wetting-drying path. Initially, cylindrical specimens were prepared at approximately optimum moisture content and maximum dry unit weight. The moisture content in each compacted specimen was altered to achieve the target moisture content. Specimens were then tested for $M_R$ values. The laboratory results revealed that the relationships between $M_R$ and the moisture content exhibit a hysteretic behavior similar to the soil-water characteristic curve; specimens subjected to drying exhibited higher $M_R$ values than specimens subjected to wetting. $M_R$ values on the drying-wetting-drying path are different than the corresponding values on the wetting-drying path. Finally, an experimental-empirical approach is proposed to predict the hysteretic behavior of $M_R$ with moisture content. The study demonstrated that the model was capable of predicting the $M_R$ hysteretic behavior, which would be helpful in predicting the response of pavements under seasonal conditions.

Expansive soils pose a major nightmare for practicing geotechnical engineers in the semiarid and arid zones of the world. The introduction of rigid substructure is a costly approach but can be made practical and financially tolerable for soils where expansion is characterized within medium to low problematic classes. This paper outlines justifications and design concepts for a rigid substructure foundation of a two-story concrete frame structure. The project was constructed in a district in Saudi Arabia where many buildings experienced serious damage because of expansive soil problems. Upheaval forces likely to act against the proposed structure were determined and used in a finite-element analysis to design an appropriate rigid substructure. Plots of moments and shear forces were determined, and critical sections were pointed out. The main reason for choosing the rigid alternative is to reduce the differential settlement, which is likely to be exaggerated in flexible designs. The settlement measured on site indicated that the rigid system design gives a reduced total settlement and differential settlement.

In this study, a series of model tests has been carried out to develop an understanding of the influence of the geocell material on the load-carrying mechanism of the geocell-reinforced sand foundations under strip loading. Geocells of different types were prepared using geogrids of different types. The parameters studied are as follows: the footing load-settlement response, deformation on the fill surface, strain in the geocell, pressure transmitted to the subgrade soil underlying the geocell mattress, and load dispersion in the geocell mattress. The test results indicate that the strength, stiffness, aperture opening size, and orientation of the rib of the geocell material influence the performance of the reinforced-sand foundation bed. Geocells made of geogrids of higher strength, relatively smaller size aperture opening, and ribs of orthogonal orientation give better performance improvement.

This paper addresses the hydromechanical postclosure behavior of an underground long cylindrical deep tunnel drilled into a massif (host medium) and backfilled at the end of its service life. It is concerned with the development of quasi-analytical Laplace transform solutions based on a simplified life cycle of the tunnel, idealized in four stages: (1) excavation, (2) pore-pressure dissipation before lining installation, (3) lining installation and tunnel exploitation, and (4) postclosure. The solutions proposed in this paper therefore extend the previous work of the authors, in this case by explicitly taking into consideration the excavation, waiting for lining installation, and tunnel exploitation stages of the life cycle. Results from a parametric study are also presented to highlight the hydromechanical responses of the tunnel over periods of its life cycle. The accuracy and correctness of the solutions from a finite-element model have been verified with the quasi-analytical results. The present solutions will be used later for benchmarking finite-element–based solutions currently under development, taking into account more complex material behaviors.

A purely numerical parametric study of 24 flow-through rockfill dam geometries was conducted. The nonlinear nature of the p-LaPlacian–like partial differential equation was dealt with using a finite-difference scheme that directly incorporated the exponent of a power law that replaced Darcy’s law. Convergence, use of specialty nodes, nodal density, and boundary condition effects were quantitatively investigated. The flow-field angle of the toe was found to be a useful starting point in studying the potential for unraveling failure. Factors of safety (FS) against this type of failure are then presented for a range of downstream slopes, thus showing which combinations of slope and particle diameter are unsafe. It is shown that the FS tends to drop below unity under the seepage face primarily because of the strength of the exit gradient near the toe of the structure and secondarily because of the overflow velocity. It is hoped that the techniques and results presented will facilitate the design and assessment of flow-through rockfill structures.

Since original publication in 1997, the CS2 numerical model has been widely used for consolidation analysis and has formed the basis for several more advanced consolidation models. In this paper, an enhanced version of the CS2 model is presented along with simulations to illustrate its new capabilities. The algorithm includes features of the original model with the addition of time-dependent loading, an external hydraulic gradient, and unload/reload effects. Verification checks show excellent agreement with analytical solutions for time-dependent loading under small strain conditions. A variety of numerical examples indicates that the new capabilities describe important considerations for consolidation analysis. The simulations also indicate that failure to correctly account for compressibility and hydraulic conductivity constitutive relationships and self-weight of the soil in particular can lead to significant errors.

Conventional methods used for the design of anchored sheet pile walls are based on the lateral force and moment equilibrium of active and passive earth pressures and anchor force. Although it has been known for decades that the stress concentration occurs around the anchor level because of the restricted wall movements, this stress concentration is not considered in determining the lateral earth pressures. A parametric study using conventional and numerical methods was performed to investigate the behavior of single-level anchored sheet pile walls, and the lateral earth pressures, wall bending moments, and anchor forces were analyzed. The study results indicate that the conventional methods for the conditions considered and cases studied overestimate the wall bending moments, whereas the anchor forces are underestimated. New lateral earth pressure coefficients that consider the stress concentration around the anchor level were developed and proposed to be used in the design of single-level anchored sheet pile walls to provide more realistic earth pressure distributions acting on the wall and to obtain more accurate wall designs.

Biot's symmetric indefinite linear systems of equations are commonly encountered in finite-element computations of geotechnical problems. The development of efficient solution methods for Biot's linear systems of equations is of practical importance to geotechnical software packages. In conjunction with the Krylov-subspace iterative method symmetric quasi-minimal residual (SQMR), some zero-level fill-in incomplete factorization preconditioning techniques including a symmetric successive overrelaxation (SSOR) type method and several zero-level incomplete LU [ILU(0)] methods are investigated and compared for Biot's symmetric indefinite linear systems of equations. Numerical experiments are carried out based on three practical geotechnical problems. Numerical results indicate that ILU(0) preconditioners are classical and generally efficient when adequately stabilized. However, the tunnel problem provides a counterexample demonstrating that ILU(0) preconditioners cannot be fully stabilized by preliminary scaling, reordering, making use of perturbed matrices, or dynamically selecting pivots. Compared with the investigated ILU(0) preconditioners, the recently proposed modified SSOR preconditioner is less efficient but is robust over the range of problems studied.

This paper describes the application of artificial neural network (ANN) and support vector machine (SVM) methods for prediction of field hydraulic conductivity of clay liners based on in situ test results such as compaction characteristics, lift thickness, number of lift, and soil classification tests like Atterberg’s limits and grain size. Statistical performances criteria, root mean square error, correlation coefficient, coefficient of determination, and overfitting ratio are used to compare different ANN and SVM models. Different algorithms are discussed for identification of important soil parameters affecting the hydraulic conductivity of clay liners. A model equation based on the parameters obtained using SVM is also discussed.

The method of fragments is a simple technique for determining the flow rate, pore water pressure, and exit gradient in two-dimensional confined flow problems. This method is extended to a three-dimensional hydraulic fill stope. Expressions were developed for computing the flow rate and the maximum pore water pressure within the stope, which are two important design parameters for analyzing the drainage characteristics of the stope during filling. These expressions were validated using laboratory tests on model stopes and a numerical model using an explicit finite difference code, and good agreement was observed. The equations developed in this research note enable quick estimates of flow rate and the maximum pore water pressure within the hydraulic fill stope, without resorting to sophisticated simulation models. It is shown that the method of fragments can be applied to three-dimensional seepage problems, provided that appropriate form factors are developed.