Soils can rarely be described as ideally elastic or perfectly plastic and yet simple elastic and plastic models form the basis for the most traditional geotechnical engineering calculations. With the advent of cheap powerful computers the possibility of performing analyses based on more realistic models has become widely available. One of the aims of this book is to describe the basic ingredients of a family of simple elastic-plastic models of soil behaviour and to demonstrate how such models can be used in numerical analyses. Such numerical analyses are often regarded as mysterious black boxes but a proper appreciation of their worth requires an understanding of the numerical models on which they are based. Though the models on which this book concentrates are simple, understanding of these will indicate the ways in which more sophisticated models will perform.

Soil Behaviour and Critical State Soil Mechanics

The size and shape of soil particles reflect the formation history of the grains. In turn, the macroscale behavior of the soil mass results from particle level interactions which are affected by particle shape. Sphericity, roundness, and smoothness characterize different scales associated with particle shape. New experimental data and results from published studies are gathered into two databases to explore the effects of particle shape on packing density and on the small-to-large strain mechanical properties of sandy soils. In agreement with previous studies, these data confirm that increased angularity or eccentricity produces an increase in emax and emin. Furthermore, the data show that increasing particle irregularity causes a decrease in stiffness yet heightened sensitivity to the state of stress; an increase in compressibility under zero-lateral strain loading; an increase in the critical state friction angle cs; and an increase in the intercept of the critical state line there is a weak effect on the slope . Therefore, particle shape emerges as a significant soil index property that needs to be properly characterized and documented, particularly in clean sands and gravels. The systematic assessment of particle shape will lead to a better understanding of sand behavior.

Particle Shape Effects on Packing Density, Stiffness, and Strength: Natural and Crushed Sands

https://www.geo.uzh.ch/~chuggel/riskcourse/dai_landslide_risk_ass_enggeol02.pdf

Landslide risk assessment and management: an overview

▪ Abstract Field observations, laboratory experiments, and theoretical analyses indicate that landslides mobilize to form debris flows by three processes: (a) widespread Coulomb failure within a sloping soil, rock, or sediment mass, (b) partial or complete liquefaction of the mass by high pore-fluid pressures, and (c) conversion of landslide translational energy to internal vibrational energy (i.e. granular temperature). These processes can operate independently, but in many circumstances they appear to operate simultaneously and synergistically. Early work on debris-flow mobilization described a similar interplay of processes but relied on mechanical models in which debris behavior was assumed to be fixed and governed by a Bingham or Bagnold rheology. In contrast, this review emphasizes models in which debris behavior evolves in response to changing pore pressures and granular temperatures. One-dimensional infinite-slope models provide insight by quantifying how pore pressures and granular temperatures c...

Debris-flow mobilization from landslides

The understanding of soil behavior during the last 300 years has centered on mechanical principles, geological processes, and later on, mineralogy and the relevance of colloidal chemistry. More recently, research in biology and earth science has enabled important advances in understanding the crucial involvement of microorganisms in the evolution of the earth, their ubiquitous presence in near surface soils and rocks, and their participation in mediating and facilitating most geochemical reactions. Yet, the effect of biological activity on soil mechanical behavior remains largely underexplored in the geotechnical field. The purposes of this paper are to introduce microbiological concepts, identify and illustrate their potential roles in soils and rocks, and stimulate interest in seeking improved understanding of their importance and potential for advancing the states of knowledge and practice in geotechnical engineering. It is shown that microorganisms play an important part on the formation of many fine grained soils, can alter the behavior of coarse grained soils (including hydraulic conductivity, diffusion and strength), accelerate geochemical reactions by orders of magnitude, promote both weathering and aging, and alter the chemical and mechanical properties of specimens after sampling. While extensive research is needed to delineate the full impact of biomass and biomediated reactions on soil behavior, it is anticipated that a proper understanding of biological principles will lead to improved soil characterization, enhanced understanding of soil behavior, and even alternative geotechnical engineering solutions.

Biological Considerations in Geotechnical Engineering

The engineering properties of naturally occurring sedimentary and residual deposits which are usually treated in geotechnical engineering as ‘soils’ are reviewed, and it is shown that usually they have characteristics due to bonded structure which are similar to those of porous weak rock. While this structure can arise from many causes, its effects follow a simple general pattern that involves stiff behaviour followed by yield. This yield can be described in a similar way to that occurring due to overconsolidation, although it is a separate phenomenon. The effects of structure are as important in determining engineering behaviour as are the effects of initial porosity and stress-history, which are the basic concepts of soil mechanics. As it can be described in a general way, it is concluded that structure and its effects should be treated as a further basic concept of equal importance. L'article passe en revue les proprietes des depots sedimentaires et residuels naturels qui sont nor-malement traites comm...

The general and congruent effects of structure in natural soils and weak rocks

The drained residual strength of cohesive soils has been studied extensively during the last 20 years. Various correlations between residual friction angle and index properties have been proposed and these are reviewed. Residual strength is measured with least ambiguity in the ring shear apparatus. A large number of natural soils have been tested in the ring shear apparatus developed jointly by Imperial College and the Norwegian Geotechnical Institute, and the results of these tests are summarized. The mechanisms controlling residual shearing are considered. The results of three series of tests on different soil mixtures, for which the gradings of the soils could be varied artificially, are presented. The proportions of platy particles to rotund particles present in the soil and the coefficient of interparticle friction of the platy particles are confirmed as controlling the type of residual shearing mechanism which develops. Three modes of residual shear are demonstrated; a turbulent mode in soils with a...

The drained residual strength of cohesive soils

Delayed failure of railway cutting slopes in stiff clays excavated in the nineteenth and early twentieth centuries has been studied for some forty years. Known failures have generally been deep-seated. More recently failures have been a problem in the slopes of motorway cuttings and embankments, although generally these have been shallow. The average operational strength at failure in these slides has been significantly less than the peak strength, and progressive failure has been postulated as the probable cause of this. Progressive failure can now be analysed using advanced numerical techniques. A series of coupled finite element analyses have been conducted assuming strain-softening soil with properties based on the Brown London Clay, and the results are reported. They show that progressive failure is considerable, and fully explain the observed field behaviour. The delays experienced in the field are also recovered by the analyses. Progressive failure is generated primarily by the high lateral stresse...

Delayed collapse of cut slopes in stiff clay

The paper presents results from a laboratory investigation into the influence of fast rates of displacement on the residual strength of soil. This influence was studied in the ring shear apparatus. Shear zones were formed by slow drained shearing and then tested at alternately fast and slow rates of displacement. Results from a wide range of natural soils are presented. All soils show initially an increase in strength when resheared at fast rates of displacement. The strength then tends to decrease with fast displacement to a minimum value—the fast residual strength. Three types of rate effects on the residual strength are identified: a positive rate effect in soils showing a fast residual strength higher than the slow residual; a neutral rate effect in soils showing a constant fast residual strength, equal to the slow residual, irrespective of rate of displacement; and a negative rate effect in soils showing a significant drop in fast residual strength below the slow residual when sheared at rates higher...

Fast shearing of pre-existing shear zones in soil

Progressive failure occurs when brittle soils are loaded non-uniformly. The average shear stress at collapse is less than peak strength. The effect cannot be examined by conventional limit equilibrium stability analysis. The Carsington embankment slipped in 1984, just before it reached full height. The slide was 30 m deep and 500 m long, and instrumentation had been installed and read up to the moment of collapse. The strengths of the soils involved were carefully established, as was the exact geometry of the slip. Limit equilibrium analysis showed that, at collapse, the average shear stress mobilized was less than peak strength, with a safety factor based on peak strength of 1·2. Since the soils involved were brittle, progressive failure was a probable cause of the discrepancy. Finite element analyses were performed in which strain-softening soil properties were assumed. Some of these analyses are reported here. The pre-failure displacements, the collapse height and collapse mechanism were recovered by t...

P. R. Vaughan

Papers

The general and congruent effects of structure in natural soils and weak rocks

The drained residual strength of cohesive soils

Delayed collapse of cut slopes in stiff clay

Fast shearing of pre-existing shear zones in soil

Finite element analysis of progressive failure of Carsington embankment