International Journal of Geomechanics

The soil ground treated by deep cement mixing (DCM) in the field normally consists of cement–soil mixed columns and untreated soils. Although many attempts have been made, research on the consolidation behavior of the treated soil ground has been limited. To better understand the consolidation process of the DCM treated ground, in this study, an axisymmetric physical model test with full instrumentation was carried out. The physical model ground consisted of a central cement–soil column and surrounding soft soil. Excess pore water pressures in the soil and vertical pressures carried by the DCM column and the untreated soil were recorded throughout the test. Responses of excess pore pressure under loading and unloading stages are highlighted. Based on the data analysis, it is revealed that the improved ground consolidates faster than the pure soil ground. The major reason is considered to be that the DCM column reduces the vertical stress increment in the soil and results in a lower value of excess pore pressure. The decrease of excess pore water pressure in the middle of soil seems to be controlled by the reduction of total stress in the soil. Besides, a delayed pore water pressure increase was observed in the early period of the loading stage. During the unloading stages, the stress on the DCM column was found to be reduced by approximately the same magnitude as the decrease in the vertical pressure on the model ground. In addition, a small residual pore pressure in the soil was also found at the end of the unloading stages.

Soil-structure interaction is traditionally simplified to prescribed boundary conditions or modeled by joint elements. Both of these approaches are limited to small and continuous relative displacements at the interface. The use of contact constraints opens up a fresh range of possibilities for geotechnical analysis, especially for cases involving large interfacial deformation. This paper demonstrates the application of computational contact mechanics in geotechnical engineering. It first outlines a general description of kinematic constraints for frictional contact and the associated numerical algorithms. A number of classical geotechnical problems are then analyzed using finite-element contact methods. These problems include a strip footing under eccentric and inclined loads and a cone penetration test. It is shown that the finite-element method with frictional contact is indeed very useful in geotechnical analysis, and can provide solutions to problems that are otherwise very difficult to analyze.

A new equation for effective stress is derived from a physically based approach of force balance on submerged granular particles. The Euler cut principle and the Euler’s first law of mechanics are used to analyze the interparticle contact stress in a fluid saturated porous media. The analysis starts from basic principles and provides a detailed derivation of the formulas. Tensor algebra together with Coulomb’s friction law, the mean value theorem and area weighted averaging techniques are used to simplify the mathematical formulation leading to effective stress. A new mathematical constraint, requiring spatially averaged interparticle contact stress to be orders of magnitude larger than spatially averaged pore pressure, is presented for the widely used Terzaghi’s effective stress equation.

There is no established procedure for the calculation of bearing capacity of a shallow foundation system comprising cojoined footings. Ad hoc approaches are relied on and may simply involve summing the ultimate limit states of the individual footings as if they acted independently; neglecting additional capacity of the system available from the kinematic constraint provided by the structural connection between the footings. In this study, the undrained capacity under general loading of rigidly connected two-footing systems at various separations has been investigated with finite-element analyses. Results are presented in terms of ultimate limit states under pure vertical $\left(V\right)$ , horizontal $\left(H\right)$ , and moment $\left(M\right)$ loading, and failure envelopes defining limiting load states under combined $VH$ , $VM$ , $HM$ , and $VHM$ loads. Kinematic failure mechanisms observed in the finite-element analyses are presented and in cases used to provide the basis for upper bound solutions.

Engineers often model pipe/soil interaction events based on the concept of subgrade reactions originally proposed by Winkler. Engineering models often utilize beam and nonlinear/plastic spring elements to represent pipelines and the surrounding soil medium, respectively. The spring formulations, defining soil resistance to deformations in three-dimensional space, are usually assumed to be independent and the responses are discrete between adjacent soil zones. However, this idealization does not truly replicate a soil medium behavior. This study presents coupled numerical analyses of pipeline for the specific problem of subgouge deformations due to ice gouge events. Three dimensional continuum analyses of coupled pipe/soil/ice keel interaction using an explicit arbitrary Lagrangian finite- element approach were performed. The study compares the continuum finite-element results with Winkler-type analysis for the specific analyzed problem. A Lagrangian adaptive meshing technique was employed to model very large movement and achieves a steady-state condition; and reasonable ice/soil and soil/pipe interaction interfaces are employed. The numerical analysis shows the potential for continuum modeling of pipe/soil interaction events and develops a better understanding of ice gouging and pipe/soil/ice keel interaction.

The low-pressure grouting with cement mixtures is one of the techniques adopted for improving the mechanical characteristics of granular soils prior to the excavation of underground openings. When applied to shallow subway tunnels, this procedure often induces heaves of the ground surface, which are followed by settlements during the excavation phase. In urban areas, these displacements may lead to appreciable damages of existing buildings. Here, a finite element analysis is presented of the surface displacements recorded during the construction of a subway tunnel in the city of Milan (northern Italy). The critical comparison between numerical and experimental data singles out the limits of an analysis approach used in tunneling practice that simulates the injections through a nonelastic volume strain. To overcome them, an alternative analysis scheme is adopted, based on the application of a neutral pressure within the grouted zone, which seems preferable for estimating the surface displacement caused by the field works.

Wetting/drying tests including cycling effects were performed on an argillaceous deep rock coming from a Callovo–Oxfordian deposit in eastern France. Bulk volume and water content were measured in order to estimate the importance of physical changes. Then, mercury intrusion porosimetry (MIP) was performed on dry and moist samples in order to quantify the influence of hydraulic history on fracture. Results showed that shrinkage and swelling caused microcracks, which led to volume variations thus altering the structures of the rock. MIP tests performed on moist samples equilibrated with different hygrometric saline solutions allow unsaturated pores and classic water retention curves to be described so as to explain changes in the microstructure.

Over the last $25\text{years}$ , the time domain boundary element formulations for the linear consolidation theory of Biot involving fully coupled governing differential equations of flow through porous media and those of elastic deformation of porous skeleton have been fully developed and implemented for both single-region and multiregion two-dimensional plane strain, axisymmetric and three-dimensional problems. However, this storage-based convolution method used in those developments was not found to be suitable for solving large practical problems because of the substantial computer disk space requirements. In order to find a better alternative, an accurate integration-based scheme was developed by the present writers and co-workers, in which, the storage was eliminated by accurately recalculating the summation (involved in the time convolution) of the right-hand side at each step during the time marching process. Although this work was not published in any literature, by using this type of approach, solving large scale problems became possible in an accurate manner, but the computational cost was significantly high, and there was a further need to develop a more practical and efficient time stepping algorithm. In the present work, an efficient and simplified integration-based fast convolution algorithm for two- and three-dimensional soil consolidation analysis has been subsequently developed. In this new algorithm, all of the time convolution steps have been calculated by assuming an equivalent spatial and temporal averaged value of the variables over each element to represent the total effect. The number of Gauss points used has been calculated in an efficient manner based on time-embedded distance criteria to accurately capture the past effects. The efficiency and accuracy of this newly developed fast convolution algorithm are compared with the accurate integration-based convolution approach and also with the analytical and other available solutions. Examples of applications involving two- and three-dimensional practical soil consolidation problems are presented to demonstrate the usefulness of the developed algorithm.