International Journal of Geomechanics

The effect of dynamic compaction forces is ignored in the analysis and design of underground structures. Several failure cases have been reported in which longitudinal cracks are observed during installation and compaction processes. Because, in the current specifications, compacting forces are not considered as design parameters, this study aims at determining the effect of dynamic compaction forces on RC pipes of different sizes. Thus, complete nonlinear three-dimensional finite-element models of pipe and soil are developed and verified against full-scale experimental testing. Particularly, crack initiation and propagation are modeled and predicted. Finally, the effects of dynamic forces are identified on the RC pipe.

Results obtained from numerical simulations of direct (or ring) shear tests on ASTM standard graded (Ottawa) sand using the discrete-element method with periodic boundary conditions in both two and three dimensions are reported. The data obtained from these simulations are quantitatively compared to experimental data for ASTM standard graded sand. The results show that the three-dimensional (3D) effects of nonplanar interparticle contact forces and particle motion are significant in the 3D discrete-element method simulations, even during nominally two-dimensional (2D) shear flow. Moreover, the 3D discrete-element method simulations accurately predict the peak and residual friction angles of ASTM standard graded sand. On the other hand, the 2D discrete-element method simulations fail to accurately predict the peak and residual friction angles. It is argued that the failure of the 2D discrete-element method simulations and the success of the 3D discrete-element method simulations in providing quantitatively accurate predictions of peak and residual friction angles are attributable largely to the respective absence or presence of 3D effects, including nonplanar interparticle contact forces and nonplanar particle motion, in these discrete-element method simulations.

With the advent of climate change, it is thought that desiccation cracking of soil will lead to increases in damage to buried infrastructure. Desiccation cracking is a complex phenomenon that historically has been very difficult to model either numerically or analytically. Therefore, there is a need for knowledge on such models to be advanced rapidly. This paper discusses the introduction of cohesive cracks to model fracture attributable to desiccation. The cohesive crack method does not have the limitations of linear elastic fracture mechanics, namely, that an initial notch must be present and that the bulk material must behave in a linear elastic manner. This paper describes how the cohesive crack method has been successfully used to model laboratory desiccation tests of a clay soil. It is shown that, in addition to the tensile strength, the magnitude of fracture energy specified for the material has an impact on crack propagation. The cohesive crack method is proposed as a tool for the numerical modeling of desiccation cracking in many diverse fields.

The paper pertains to the development of a methodology for the identification of optimal design parameters for a system of beams resting on a stone column–improved soft soil. The simulation optimization–based methodology combines a finite difference–based simulation model and an evolutionary multiobjective optimization model. Two different formulations subjected to stress constraints have been incorporated within the methodology; i.e., (1) minimization of settlement at the center of the beam and minimization of the maximum bending moment and (2) minimization of settlement at the center of the beam and minimization of the maximum shear force. Critical evaluation of an illustrative system shows that stiffness of the stone columns or modular ratio and flexural rigidity of the beam are the most important parameters for optimal design. The obtained results also show the potential applicability of the proposed simulation optimization–based methodology.

This paper discusses the development of a numerical model for a braced excavation to estimate the various design parameters that significantly influence the excavation's behavior. The results of the numerical model were compared with those of a reported case study of a braced excavation in sand, and close agreement between the results was observed. The developed model is used for parametric study to show the influence of different design parameters, such as strut stiffness, wall thickness, strut arrangement and the embedded depth of the wall on strut force, maximum moment developed in the wall, maximum lateral displacement of the wall, and maximum vertical displacement of ground surface. It was found that, among all the combinations studied, a particular type of strut arrangement for a particular ratio of embedded depth and excavation depth produces the best possible result. A design guideline is also presented based on the results of this numerical study.

An axial compression was generated in a stone column under vertical loads on its top, and was often accompanied by a radial expansion against the surrounding soil near the top portion of the column. Considering this deformation characteristic of the stone column, an analytical solution for the settlement of the composite foundations reinforced with stone columns was presented. The load sharing between the column and the soil and the distribution of column-soil interfacial shear stresses was also incorporated into the solution. From the present solution, the vertical settlement and lateral bulging of the column under any applied loads can be evaluated at any depth. The validity of the solution was verified through the comparison with the measurement data and other existing analytical solutions. The influences of stress-concentration ratio, internal friction angle and cohesion of the surrounding soil, and the elastic modulus of the column on the deformations of the stone column were discussed. The load acting on the top of the column had a great influence on its deformations. Thereby, the accurate determination of the load distribution between columns and the surrounding soil was vital for analyses of settlement of composite foundations reinforced with stone columns during the design. The increase of the internal friction angle of the soil, the cohesion of the soil, and the modulus of the column had reduction effects on column settlements and bulging.

The rock foundation of a gravity dam is usually cut into slide blocks by the structural surfaces. Its stability is controlled by the characteristics and the combining action of these structural surfaces. The failure of a gravity dam can be caused by the destruction or large deformation of local rocks under the condition of certain loads. The coupling of continuous-discontinuous deformation is a typical characteristic of the process. As a new design idea, the powerhouse and dam are combined into an integer with one structural pattern to improve the antisliding stability of the gravity dam. The external loads are undertaken together by the powerhouse and dam. The loads acting on the dam can be transferred partly to the powerhouse, and the powerhouse weight and tail water pressure can increase the dam’s stability. The optimal design of a gravity dam section can be implemented, decreasing the project quantity, and reducing construction costs. This design idea has been adopted by the Yangtze Three Gorges Project in China. The coupling methods of discontinuous deformation analysis (DDA) and FEM are used to implement the deep antisliding stability evaluation for a gravity dam under the combining action of both the powerhouse and dam. The blocks are subdivided into the finite element meshes; the displacement field and stress field in the blocks are solved by FEM, and the contacts between the deformable blocks are simulated and analyzed with the DDA method. The description ability for the block deformation can be increased, the calculation precision for the stress field in block can be improved, and the ability of DDA in solving the large deformation of a multiblock system can be inherited. Therefore, the destructive form determined by this coupling method is closer to the actual situation. As an example, the coupling methods of DDA and FEM are used to analyze and assess the deep antisliding stability of the No. 3 left powerhouse-dam section of one dam when the upstream loads are taken together by the high gravity dam and the combined powerhouse located behind dam. The effect of combining the action of the powerhouse and dam is appraised by the numerical analysis for the mechanism transferring force between the dam and powerhouse, the influence on the deformation and stress of dam and powerhouse under the combining action of powerhouse and dam, the interaction of structural planes, etc. The entire instability process of the dam is simulated and analyzed by means of reducing the mechanical parameters. Last, the qualitative and quantitative analyses are implemented to evaluate the local and the whole stability of the gravity dam under the combining action of the powerhouse and dam.

Finite-element analysis of many soil-structure interaction problems involves material zones of widely differing stiffnesses. The large relative differences in material stiffnesses usually result in an ill-conditioned system for which practical preconditioners become ineffective for the iterative solution of such systems. The study suggests that a block diagonal preconditioner can exploit such differences. However, the theoretical preconditioner is expensive for practical use. Less costly block diagonal preconditioners are numerically evaluated for various inexact forms of the blocks in conjunction with conjugate gradient solver. The study includes analysis of two representative soil-structure interaction problems and proposes two simplified block diagonal preconditioners that effectively mitigate such material ill conditionings.

This paper proposes an equivalent load transfer approach for simulating the response of passive piles owing to soil movement. The approach is elaborated for two commonly seen (normal and deep) sliding modes. In terms of compatibility conditions across sliding and stable layers, new coupled elastic (sliding layer)-elastic (stable layer) (E-E) solutions, and plastic (sliding layer)– elastic-plastic (stable layer) (P-EP) solutions are developed. The solutions are implemented into a program called GASMove operating in the mathematical software Mathcad. They are compared with available numerical analyses, and employed to the predict response of eight instrumented piles. The study reveals the proposed equivalent load–soil movement relationship works well along with the solutions; the E-E solution generally offers good prediction for piles with infinite lengths in both sliding and stable layers (deep sliding mode); the P-EP solution is good for piles rotating rigidly in a sliding layer (normal sliding mode); and similar predictions may be gained from different sets of $p_u$ and $k$ profiles, as with laterally loaded piles, but a linear $p_u$ should be used for the stable layer to gain the smallest pile resistance. Design charts are generated to facilitate the prediction of a nonlinear response of passive piles, for which example predictions are elaborated.

By using the axisymmetric quasi-lower bound finite-element limit analysis, the bearing capacity factors $N_c^p$ and $N_{\gamma q}^p$ have been computed for axially loaded piles, with the shaft embedded in a fully cohesive soil medium and the tip placed over cohesive frictional soil strata. The results were obtained for various combinations of $L/D$, ${\varphi }_l$, and $c_l/c_u$; the subscripts $l$ and $u$ refer to lower and upper soil strata, respectively. The factors $N_c^p$ and $N_{\gamma q}^p$ increase continuously with increases in $L/D$ and ${\varphi }_l$; the rate of increase of $N_c^p$ and $N_{\gamma q}^p$ with $L/D$, however, decreases with an increase in $L/D$. For $c_l/c_u>100$, the factor $N_c^p$ hardly depends on $L/D$.

Predicting when, where, and how pressure-induced borehole failures occur is of fundamental importance in borehole stability analysis and hydraulic fracture design. Numerical and statistical methods are not suited for system analysis, while existing analytical solutions depend on failure location, neglect shear stresses, or only consider vertical boreholes. This paper derives explicit analytical relations between far-field stresses and when, where, and how borehole failures occur in arbitrarily oriented boreholes. These relations reveal that when, where, and how failures occur varies characteristically with the borehole orientation. Further, the optimal drilling- and optimal hydraulic fracturing orientations are determined, together with two simple methods for determining the stress regime based on where or how failures occur. It was also found that failure prediction is more sensitive to noise when the formation is more anisotropic and/or closer to failure.