Design of Geostructural Systems

Abstract

This paper begins with an extensive review of the literature covering the development of design rules for geostructural systems, beginning with traditional global safety factors and developing through partial factors for loads and resistances, and then considering the use of mobilization factors to limit soil strains. The paper then aims to distinguish two possible functions for geotechnical factors: to compensate for the uncertainty regarding soil strength, and to limit soil deformations that could compromise the associated structure before the soil strength can be fully mobilized, whatever it is. At present, design procedures generally conflate and confuse ultimate limit state (ULS) checks and serviceability limit state (SLS) deformation checks. Furthermore, most geotechnical engineers wrongly associate ULS with soil failure rather than with structural failure. The paper addresses this fundamental confusion by advocating mobilizable strength design (MSD), which is based on assumed soil-structure deformation mechanisms rather than soil failure mechanisms. It is argued that designs using MSD can guard against damaging structural deformations, either small deformations giving SLS or large structural deformations that must be regarded as ULS even though the associated soil strength may not yet be fully mobilized. This distinction effectively challenges much of the previous literature on limit state design principles for geotechnical applications, even where probabilistic approaches have been proposed. Nevertheless, the paper is informed by the concepts and techniques of decision making under uncertainty, and the paper concludes by considering whether MSD can also be placed in a reliability framework.

Variability of soil stress-strain non-linearity for use in MSD analyses evaluated using databases of triaxial tests on fine-grained soils

“…a simple constitutive model for soil behaviour which can also be incorporated into a reliability framework (cf. Vardanega & Bolton 2016a). Early papers on MSD-thinking for retaining structures include Powrie 1988 andBolton et al 1990).…”

Parameter variability of undrained shear strength and strain using a database of reconstituted soil tests

“…While much research effort has focused on understanding c u variability, less information is available to quantify the variability of shear strains. The MSF has been developed to incorporate undrained strength mobilization parameters in a framework suitable for reliability-based design style approaches (Vardanega and Bolton 2016a) by employing a simple power-law model. Equation 2 can be fitted to shear stress-strain data if the peak failure stress is known (Vardanega and Bolton 2011) and can be expressed as…”

Minding the geotechnical data gap: appraisal of the variability of key soil parameters for slope stability modelling in Saint Lucia

“…When data for a particular site or region are limited, a geotechnical database and derived probability distributions, can provide useful information. Vardanega and Bolton (2016) suggest that reliability-based design analysis is best used for assessing the performance of geo-structures, as opposed to failure analysis. For instance, analysis of various predictors of undrained shear strength using a database and probability density function has been recently reported Phoon, 2014a, 2014b).…”