Boonchai Ukritchon

Bio: Boonchai Ukritchon is an academic researcher from Chulalongkorn University. The author has contributed to research in topics: Finite element limit analysis & Limit analysis. The author has an hindex of 23, co-authored 59 publications receiving 1158 citations.

Undrained Limit Analyses for Combined Loading of Strip Footings on Clay

Abstract: This paper applies numerical limit analyses to evaluate the undrained stability of surface footings on nonhomogeneous and layered clay deposits under the combined effects of vertical, horizontal, and moment loading. The analyses are able to resolve the true collapse loads to within ±5% by computing rigorous upper and lower bound solutions for these plane strain problems using linear programming methods and finite element discretization. The results focus on the geometry of the three-dimensional failure envelope and illustrate the effects of underbase suction and nonhomogeneous undrained strength profiles. The existing empirical bearing capacity factors for inclined, eccentric loading are conservative, often underestimating the exact collapse solutions for footings on homogeneous clay by more than 25%. However, the same correction factors can become completely unreliable when there is a significant undrained strength gradient. The numerically derived failure envelope for footings on homogeneous clay is described approximately by curve fitting techniques using relatively simple functions that can be used to update the existing bearing capacity factors.

Calculations of Bearing Capacity Factor Nγ Using Numerical Limit Analyses

Abstract: This paper presents numerical upper- and lower-bound solutions for the well-known bearing capacity factor Nγ of a surface strip footing on a frictional soil. The analyses use linear programming and...

Undrained Stability of Braced Excavations in Clay

Abstract: Short-term undrained stability often controls the design of braced excavations in soft clays. This paper summarizes the formulation of numerical limit analyses that compute rigorous upper and lower bounds on the exact stability number and include anisotropic yielding, typical of K0-consolidated clays and bending failure of the wall. Calculations for braced cuts bound the actual failure conditions within ±5%, and highlight limitations of existing basal stability equations. The analyses clarify how wall embedment and bending capacity improve the stability of well braced excavations. Careful selection of mobilized strengths at shear strains in the range 0.6–1.0% are necessary to match the predictions of anisotropic limit analyses with nonlinear finite-element predictions of failure for the embedded walls. Two example applications from recent projects in Boston highlight the practicality of the numerical limit analyses for modeling realistic soil profiles and lateral earth support systems, but also focus atte...

Three-dimensional undrained tunnel face stability in clay with a linearly increasing shear strength with depth

Abstract: The undrained tunnel face stability in clay with a linearly increasing shear strength with depth was investigated by three-dimensional finite element analysis. Three parametric studies were performed to study the effects of the cover depth ratio, overburden stress factor and linear strength gradient ratio on the load factor of the undrained tunnel face stability. The influence of the linear strength gradient ratio on the predicted failure mechanism of the undrained face stability was discussed and examined. An approximate closed-form solution was proposed for three-dimensional undrained tunnel face stability in clays with constant or linearly increasing shear strength profiles with depth.

Geotechnical stability analysis

Abstract: This paper describes recent advances in stability analysis that combine the limit theorems of classical plasticity with finite elements to give rigorous upper and lower bounds on the failure load. These methods, known as finite-element limit analysis, do not require assumptions to be made about the mode of failure, and use only simple strength parameters that are familiar to geotechnical engineers. The bounding properties of the solutions are invaluable in practice, and enable accurate limit loads to be obtained through the use of an exact error estimate and automatic adaptive meshing procedures. The methods are very general, and can deal with heterogeneous soil profiles, anisotropic strength characteristics, fissured soils, discontinuities, complicated boundary conditions, and complex loading in both two and three dimensions. A new development, which incorporates pore water pressures in finite-element limit analysis, is also described. Following a brief outline of the new techniques, stability solutions ...

Upper bound limit analysis using simplex strain elements and second-order cone programming

Abstract: In geomechanics, limit analysis provides a useful method for assessing the capacity of structures such as footings and retaining walls, and the stability of slopes and excavations. This paper presents a finite element implementation of the kinematic (or upper bound) theorem that is novel in two main respects. First, it is shown that conventional linear strain elements (6-node triangle, 10-node tetrahedron) are suitable for obtaining strict upper bounds even in the case of cohesive-frictional materials, provided that the element sides are straight (or the faces planar) such that the strain field varies as a simplex. This is important because until now, the only way to obtain rigorous upper bounds has been to use constant strain elements combined with a discontinuous displacement field. It is well known (and confirmed here) that the accuracy of the latter approach is highly dependent on the alignment of the discontinuities, such that it can perform poorly if an unstructured mesh is employed. Second, the optimization of the displacement field is formulated as a standard second-order cone programming (SOCP) problem. Using a state-of-the-art SOCP code developed by researchers in mathematical programming, very large example problems are solved with outstanding speed. The examples concern plane strain and the Mohr-Coulomb criterion, but the same approach can be used in 3D with the Drucker-Prager criterion, and can readily be extended to other yield criteria having a similar conic quadratic form.

Effect of strength non-homogeneity on the shape of failure envelopes for combined loading of strip and circular foundations on clay

Abstract: The capacity of surface foundations on clay under pure vertical (V), horizontal (H) or moment (M) loading may be expressed in non-dimensional form through the use of appropriate bearing capacity factors, with values that will be affected by the shape of the foundation and also any variation of undrained shear strength with depth. A common assumption has then been that the shape of the complete failure envelope in three-dimensional loading space (V, M, H) will be similar regardless of foundation shape and soil non-homogeneity, once scaled to the appropriate apex points. The appropriateness of this assumption has been explored by means of two- and three-dimensional finite element analyses of strip and circular footings, for a simple Tresca soil model where the shear strength varies linearly with depth. With a view to applications involving partially embedded foundations, such as offshore skirted foundations, full suction and ‘bonding’ with the underlying soil has been assumed. The paper documents the normal...

Pile Foundation Analysis And Design

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Stability of plate anchors in undrained clay

Abstract: Soil anchors are commonly used as foundation systems for structures requiring uplift resistance, such as transmission towers, or for structures requiring lateral resistance, such as sheet pile walls. To date, the design of these anchors has been largely based on empiricism. This paper applies numerical limit analysis to rigorously evaluate the stability of vertical and horizontal strip anchors in undrained clay. Rigorous bounds on the ultimate pull–out capacity are obtained by using two numerical procedures that are based on finite element formulations of the upper and lower bound theorems of limit analysis. These formulations follow standard procedure by assuming a rigid perfectly plastic clay model with a Tresca yield criterion, and generate large linear programming problems. By obtaining both upper and lower bound estimates of the pull–out capacity, the true pull–out resistance can be bracketed from above and below. Results are presented in the familiar form of break–out factors based on various soil s...