Numerical Analysis Of Large Size Horizontal Strip Anchors

Abstract

Structures like transmission towers, tele-communication masts, dry-docks, tall chimneys, tunnels and burried pipelines under water etc are subjected to considerable uplift forces. The net effect of external loading on the foundations of these structures results in forces that try to pull the foundations out of the ground. Anchors are usually provided to resist such uplift forces.

Earlier theoretical research of anchor behavior has focused on elastic response and ultimate pullout capacity. Many investigators have proposed techniques for determining the collapse load of anchors. Essentially the approaches involve the use of limit equilibrium concepts, with some assumptions regarding the shape of the failure surface and/or the influence of the soil above the anchor. The possible effect of dilatency and initial stress state are not considered in these methods. A number of investigators have used the results of small size model anchors to understand the behavior and extrapolated the results for predicting the behavior of large sized anchors. This has lead to unsatisfactory results. It has been clearly shown by Dickin (1989) that the failure displacements and load displacement curve patterns are very different for small and large sized anchors, i.e. they are not just proportional to the size of the anchor. Critical pullout load and the load displacement behavior are required for the complete analysis of anchor foundations. Though, many theories have been proposed to predict the uplift capacity within the limits of accuracy required at engineering level, at present no simple rational method is available for computing deformations.

In the present investigation attempts have been made to analyze the load deformation behavior of large size strip anchors in sands, clays and layered soils using two-dimensional explicit finite difference program FLAG (Fast Lagrangian Analysis of Continua), well suited for geomaterials, by assuming soil to be a Mohr-Coulomb material in the case of sands and modified Cam-clay material in the case of clays.

It is now well understood that the shearing resistance of a granular soil mass is derived from two factors frictional resistance and the dilatency of the soil. So the peak friction angle can be divided in to two components critical friction angle Фcv and dilation angle Ψ. Critical friction angle is the true friction angle as a result of frictional resistance at interparticle level when the soil is shearing at constant volume. If Фcv for a given soil remains constant, the value of Ψ has to increase with the increase in initial density of soil packing. The dilatency of a soil mass gradually decreases with continued shearing from its initial high value to zero after very large shear strains, when the soil finally reaches a constant, steady volume at critical states. Correspondingly the observed friction angle Ф reduces from its peak value to Фcv at a very large strain.

In earlier days, clays used to be characterized by the strength parameters c and Ф. often, under undrained conditions, Ф would be even considered zero. But in the recent developments, it is understood that all the strength of clays is frictional. There is nothing like cohesion. The part of shear strength, which appears to be independent of normal stress, is shown to be the effect of over-consolidation and the resulting dilation. Thus although Cam-clay model uses zero cohesion for all clays, it reflects this component of strength through over-consolidation and in a more realistic way. Hence, it is appropriate to consider the pre-consolidation pressure as parameter in the analysis. More specifically, the various aspects covered in this investigation are as follows.

Chapter 1 provides the general introduction. In chapter 2, the existing literature for the analysis of anchors for both experimental and analytical investigations on the pullout capacity of anchors in homogeneous and layered soils and the load deformation behavior of anchors under pullout are briefly reviewed.

Chapter 3 deals with the features and the implementation of the two-dimensional explicit finite difference program, Fast Lagrangian Analysis of Continua (FLAC) and the constitutive modeling of soils. It discusses the background and implementation of Strain softening / hardening model. This model is based on the Mohr- Coulomb model with non-associated shear and associated tension flow rules. In this model the cohesion, friction, dilation and tensile strength may harden or soften after the onset of the plastic yield. Further the critical state concepts and implementation of the modified Cam-clay model have been discussed. Cam-clay model originally developed for clays reflects the hydrostatic pressure or density dependent hardening material response.

Chapter 4 focuses on the analysis of load deformation behavior of large size anchors in granular soils. Two-dimensional explicit finite difference program (FLAC) is used for the simulations and the soil is modeled as a Mohr-Coulomb strain softening/hardening material In this chapter a series of simulations have been carried out on large size anchor plates, with parametric variation. By analyzing these results, a generalized load deformation relationship for different sizes of anchors and different types of soil have been proposed. The results are presented in the form of influence/design charts which can be used in hand calculations to obtain an estimate of anchor capacity and deformation for a wide range of soil types and size of anchors.

Chapter 5 deals with the analysis of the drained and undrained behavior of large size horizontal strip anchors in clays using modified Cam-clay model. Earlier investigators have studied the undrained behavior of anchor plates in clays, but no studies are reported in literature for the drained behavior of anchors in clays. Further it is not clear whether, drained or undrained condition will be critical for an anchor. In this chapter the drained and undrained behavior of large size anchor plates in both normally consolidated and over-consolidated states have been made. It has been found that the undrained pullout capacity of an anchor in a soil of normally consolidated state will always be more than the drained capacity. This is contrast to the usual understanding that undrained behavior is more critical than the drained behavior.

In Chapter 6 an attempt has been made to analyze the behavior of large size anchors in two layered sands and in conditions where backfill material has a higher or lower strength than the native soil, for different shape of excavations. Soil is assumed to be a Mohr-coulomb strain softening/hardening material.

In Chapter 7 the entire investigation covered in earlier chapters has been synthesized and some specific conclusions have been highlighted.