Some consequences of the qualitative analysis of the point-symmetric coupled consolidation models

The point-symmetric linear coupled consolidation models, known from the theory of the oedometric testing and from dissipation testing, can be summarized into a single mathematical model in the function of the embedding space dimension m ([1]). When a set of boundary conditions is specified equally for the 1, 2 and 3 dimensional models (i.e. oedometric, cylindrical and spherical models) then a family of related model: a “modelfamily” is obtained. Some inferences of the results of the qualitative analysis of two model-families are presented and discussed in this paper. These are (i) the similarity of the solution within a model-family, (ii) a direct proof that the uncoupled consolidation theories cannot be considered as a special case of the coupled consolidation theories and (iii) the interesting fact that an instantaneous dissipation may be predicted for the uniform initial pore water pressure distribution if the displacement is specified at both boundaries

The influence of geologic structure on design and construction in a moderately deformed paleozoic sequence in Eastern Australia

Abstract This paper presents a case study to illustrate the role played by geologic structures in the design and construction of major transportation infrastructure, in a setting of moderately deformed Paleozoic sedimentary rocks in eastern Australia. It describes a complex development of folding, faulting and jointing that has resulted in significant inclination of beds, juxtaposition of strata and affected the weathering characteristics of a wide range of rock types. The sequence, which displays an upward transition from marine to terrestrial sediments, comprises an interbedded succession of conglomerate, sandstone, shale and erratic volcanics and crystal tuff. Unfavorable relationships between major excavation faces, inclined beds and jointing have resulted in problems with the stability of road cuttings. Also, the presence of faults and dykes at various scales has had a significant effect on weathering and rock strength. The paper demonstrates the importance of the choice of alignment at design stage, and how a basis of good structural interpretation and geologic mapping can be used to avoid problems during construction and issues with ongoing maintenance

Prehistoric landslides: significance, recognition, examples

Prehistoric landslides with a wide range of ages and sizes exist worldwide in both rock and soil. Many are thought to have occurred during Pleistocene time when climates in some areas were harsher and wetter. Subsequent weathering and erosion have subdued topography and other features of prehistoric landslides, often making them difficult to recognize. Recognition is the key to dealing with prehistoric and other old landslides. Old slide masses are usually only marginally stable because past movements reduced available shear strength on their failure surfaces to residual levels. These masses are susceptible to reactivation by construction activities, heavy precipitation, and earthquakes. If prehistoric landslides are recognized, they can be avoided or steps taken to minimize interference with them. Where they cannot be avoided, they must be stabilized or lived with. Stabilization typically involves robust retaining structures, large buttress fills, or excavation of most of the slide mass - all of which are expensive. Living with old landslides may involve continuing maintenance for distress caused by creep or other movements; this can also be expensive. Problems arise when prehistoric and other old landslides are unrecognized, then reactivated during or after construction. Then, unexpected ground movements cause damage, increase costs, set back construction schedules, and disrupt partially completed or completed facilities and operations. Geologic considerations, features of prehistoric and old landslides, and guidance for recognizing them are presented. Then examples of prehistoric landslides in the United States, Papua New Guinea, and Australia are given

Impact of grading on steady-state strength

International audienceIn the mining industry, waste dumps are earthen structures typically built by loose waste tipping. They may reach heights of hundreds of metres and undergo large deformations. For this reason, their stability design is based on the steady-state shear strength of the waste material. Waste materials are widely graded and may contain particles of up to metric order. Particle shape depends on the pattern of dissecting discontinuities at the source rock mass and the relation between the size of the fragments and discontinuity spacing. The shear strength of this material is determined in the laboratory using scaled samples with altered particle-size distribution (PSD). However, altering the PSD is known to impact shear strength, and this impact is poorly studied. The representativeness of laboratory parameters obtained from scaled samples is thus arguable. Discrete-element simulations are used here to investigate steady-state shear strength changes with the alteration of the PSD when particle size and shape are correlated. It is observed that shear strength changes result from the variation of the particle shapes induced by the alteration of the PSD. Consequently, identifying size−shape correlations and their potential impact on shear strength is of paramount importance when scaling materials for laboratory testing

Some Comments on the Entropy-Based Criteria for Piping

This paper is an extension of previous work which characterises soil behaviours using the grading entropy diagram. The present work looks at the piping process in granular soils, by considering some new data from flood-protection dikes. The piping process is divided into three parts here: particle movement at the micro scale to segregate free water; sand boil development (which is the initiation of the pipe), and pipe growth. In the first part of the process, which occurs during the rising flood, the increase in shear stress along the dike base may cause segregation of water into micro pipes if the subsoil in the dike base is relatively loose. This occurs at the maximum dike base shear stress level (ratio of shear stress and strength) zone which is close to the toe. In the second part of the process, the shear strain increment causes a sudden, asymmetric slide and cracking of the dike leading to the localized excess pore pressure, liquefaction and the formation of a sand boil. In the third part of the process, the soil erosion initiated through the sand boil continues, and the pipe grows. The piping in the Hungarian dikes often occurs in a two-layer system; where the base layer is coarser with higher permeability and the cover layer is finer with lower permeability. The new data presented here show that the soils ejected from the sand boils are generally silty sands and sands, which are prone to both erosion (on the basis of the entropy criterion) and liquefaction. They originate from the cover layer which is basically identical to the soil used in the Dutch backward erosion experiments

Case Studies and Benchmark Examples for the Use of Grading Entropy in Geotechnics

The grading entropy concept can be adapted to the field of geotechnics, to establish criteria for phenomena such as particle packing, particle migration and filtering, through a quantified expression of the order/disorder in the grain size distribution, in terms of two entropy-based parameters. In this paper, the grading entropy theory is applied in some geotechnical case studies, which serve as benchmark examples to illustrate its application to the characterisation of piping, softening and dispersive soils, and to filtering problems in the context of a leachate collection system for a landfill site. Further, since unstable cohesive (dispersive) soils are generally improved by lime, the effect of lime addition is also considered, on the basis of some measurements and a further application of the grading entropy concept, which allows evolutions in the entropy of a soil to be considered as its grading is modified. The examples described support the hypothesis that the potential for soil erosion and particle migration can be reliably identified using grading entropy parameters derived from grading curve data, and applied through an established soil structure stability criteria and a filtering rule. It is shown that lime modification is not necessarily helpful in stabilizing against particle migration

Some Notes on Granular Mixtures with Finite, Discrete Fractal Distribution

Why fractal distribution is so frequent? It is true that fractal dimension is always less than 3? Why fractal dimension of 2.5 to 2.9 seems to be steady-state or stable? Why the fractal distributions are the limit distributions of the degradation path? Is there an ultimate distribution? It is shown that the finite fractal grain size distributions occurring in the nature are identical to the optimal grading curves of the grading entropy theory and, the fractal dimension n varies between-¥ and ¥. It is shown that the fractal dimensions 2.2-2.9 may be situated in the transitional stability zone, verifying the internal stability criterion of the grading entropy theory. Micro computed tomography (μCT) images and DEM (distinct element method) studies are presented to show the link between stable microstructure and internal stability. On the other hand, it is shown that the optimal grading curves are mean position grading curves that can be used to represent all possible grading curves

Issues related to stability design of very high spoil dumps

As spoil dumps get higher, particularly in strip mining where most overburden is placed in-pit, consequences of slope failure become disproportionately greater. Current understanding of the shearing behaviour of spoil for stability design has involved a combination of laboratory-scale diagnostic testing and engineering judgment. This is a relatively empirical approach that provides a linear shear strength envelope for materials known to exhibit non-linear behaviour, particularly under high confining stresses. A shortcoming to the diagnostic testing is that oversize particles are usually scalped to accommodate the device capacity. The influence of prototype-size particles on the geomechanical behaviour of mine spoil is not truly captured. In response to concerns about overestimating the shear strength and stability of high spoil dumps, and current plans for coal mine dumps to exceed 400 m in height, there is a need to rationally define the stress-strain behaviour of more characteristic spoil masses under representative compressive and shearing loads. A Large Direct Shear Machine (LDSM) has been designed at The University of Newcastle to generate reliable stress-strain data on large samples of coal measures spoil (0.72 m x 0.72 m x 0.6 m) subjected to loads representative of very high dumps (~3.5 MPa). This paper reviews current methods for predicting shear strength parameters in the context of very high spoil dumps, and presents an overview of the design considerations of the DSM

Calibration of a TDR probe in an expansive soil

The measurement of soil water content in situ is not straightforward. Capacitance methods such as Time Domain Reflectometry (TDR) can be used to estimate the volumetric water content in situ using a passive “probe” which is embedded in a soil and connected via a cable to a signal processor/recorder. In this study, a systematic series of laboratory measurements was undertaken to determine the relationship between true volumetric water content, gravimetric water content and TDR measured water content in uncracked Maryland clay. The results show not only that the TDR measured water contents differ from the true volumetric water contents, but that the TDR is potentially relatively insensitive over a significant interval of mid-range water contents in uncracked clay. Tests to estimate the effect on measurements from the presence of cracks of different width found that errors increase steadily as crack width increases. The study also determined that the measurements are relatively insensitive to misalignment of the waveguide prongs

The significance of relative density for particle damage in loaded and sheared gravels

For granular assemblages of strong particles, an increase in the relative density usually leads to a significant increase in shear strength, which is evident as a peak strength, accompanied by significant dilation as the peak strength is attained. This paper describes an experimental study of shearing in assemblages of weak particles, where particle breakage offsets dilation for all but the lowest of confining stresses. In such materials, prone to particle breakage, the shear strengths of loose and dense assemblages rapidly converge to similar values as confining stress increases, and any benefit of greater relative density is lost. This is attributed to the densification effect associated with the loading under a high stress prior to shearing, which is characterised by widespread particle breakage and the formation of smaller particles to occupy space between coarser ones. Interestingly, under both low and high stresses, there was a tendency for greater particle breakage in the loose samples, as a result of both shearing and compression. This result suggests that, despite the denser assemblage having its particles more rigidly constrained and less able to rearrange to avoid direct loading, the influence of greater load-spreading capacity afforded by an increased number of particle contacts in a denser sample, is more dominant in controlling breakage