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This thesis describes a numerical investigation of shallow circular foundations resting on various types of soil, mainly siliceous and calcareous sands. An elasto-plastic constitutive model, namely the MIT-S1 model (Pestana, 1994), which can predict the rate independent behaviour of different types of soils ranging through uncemented sands, silts and clays, is used to simulating the compression, drained triaxial shear and shallow circular foundation responses. It is found that this model provides a reasonable fit to measured behaviour, particularly for highly compressible calcareous sands, because of the superior modelling of the volumetric compression. The features of the MIT-S1 model have been used to investigate the effects of density, stress level (or foundation size), inherent anisotropy and material type on the response of shallow foundations. It was found that the MIT-S1 model is able to distinguish responses on dilatant siliceous and compressible calcareous sands by relatively minor adjustment of the model parameters. Kinematic mechanisms extracted from finite element calculations show different deformation patterns typical for these sands, with a bulb of compressed material and punching shear for calcareous sand, and a classical rupture failure pattern accompanied by surface heave for siliceous sand. Moreover, it was observed that the classical failure pattern transforms gradually to a punching shear failure pattern as the foundation size increases. From this evidence, a dimensional transition between these failure mechanisms can be defined, referred to as the critical size. The critical size is also the limiting foundation size to apply conventional bearing capacity analyses. Alternative approaches are needed, focusing mainly on the soil compressibility, for shallow foundations greater than the critical size. Two approaches, 1-D compression and bearing modulus analyses, have been proposed for those foundation conditions. From the validations, the former is applicable for extremely large foundations, very loose soil conditions and highly compressible calcareous materials, while the latter is suitable for moderate levels of compressibility or foundation size. It is suggested that appropriate assessment of compression features is of great importance for shallow foundation analysis on sand.

En un siècle troublé, né du séisme de 1789, le bagne, lieu de fracture, peut se comprendre comme le modèle spatial du doute, de la contrainte et de la souffrance, des fins et des commencements. Surtout, il est le fondateur du forçat, personnage de tous les paradoxes, repoussant les limites, animé de la fièvre des résurrections. Dans un roman cherchant à affirmer son identité, au milieu des récriminations d’une Histoire en mouvement, que signifie le surgissement en littérature, comme une invitation à imaginer l’inimaginable, de cet homme déchu, de ce coupable révélé, qui ne se satisfait d’aucune finitude et représente pourtant la finitude elle-même ? A la frontière de la réalité et du mythe, le bagnard, cet être du dehors, éclairé de la lumière étrange que projette sur lui le lieu extrême du bagne, apparaît comme un personnage nécessaire à la mise en place du roman, pêle-mêle vindicatif d’observations et d’inventions, composé du silence et de la parole. Il réunit en effet les cheminements de la différence et de la quête propres aux drames romanesques. Cet être de l’opposition – chez Victor Hugo, Balzac, Paul Féval ou encore George Sand – s’impose donc ainsi qu’un héros. L’imagination des romanciers le transfigure et lui offre le pouvoir du symbole. Comme le Titan révolté de l’Antiquité, le forçat romanesque du XIXe siècle suggère la vérité mythique d’une humanité à la recherche du sens. / In a troubled century, ensuing from the upheaval of 1789, the penal colony, place of divide, can be interpreted as a spatial model of doubt, constraint and suffering, of endings and beginnings. Above all, it is the founder of the convict, character of multiple paradoxes, pushing back the limits, motivated by the heat of resurrections of a History in motion, what is the meaning of the emergence, in literature, as an invitation to imagine the unimaginable, of that fallen man, that uncovered culprit, who is not satisfied with any finiteness yet stands for finiteness himself? Halfway through reality and myth, the convict, that man from outside, lit up by the weird light that the extreme place that is the penal colony sheds on him, appears as a necessary character for the setting up of the novel, vindictive hodge-podge of observations and inventions, made of silence and speech. It links, indeed, the development of the notions of difference and quest that are inherent to fictional drama. This being of opposition – in the works of Victor Hugo, Balzac, Paul Féval or else George Sand – imposes himself as a hero. The imagination of the novelist transfigures him and gives him the power of the symbol. As the rebellious Titan of Antiquity, the convict of the 19th century suggests the mythic truth of mankind in search of meaning.

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No description available.

A study of the dynamics and functions of a coastal dunefield at the Bushman’s River Mouth in Kenton-on-Sea was conducted to provide essential information for the formulation of a management strategy based on an ecosystem approach. Bi-monthly tacheometric surveys were undertaken at the Westbourne Road Car Park dunefield to investigate the sedimentological processes. Changes in the topography, the amount and rate of sand accretion were monitored. Accretion and progradation were the dominant long-term sedimentological processes at the site. Long-term spatial variation in these processes corresponded to the temporal pattern of sedimentation in the river mouth while short-term variation resulted from dynamics inherent to the types of dunes present. Factors affecting aeolian accretion at the site are topography, river mouth configuration, dune types and the presence of vegetation as well as the seasonal wind regime. A vegetation study, recording species present, cover and abundance of the plants along the surveyed transects, was also conducted to examine the progressive establishment of vegetation. Five plant communities were discerned namely, saltmarsh, pioneer, enriched pioneer, open scrub, closed scrub and scrub-thicket communities. The spatial distribution of these communities coincided with the north-westerly development of the sandbank. A successional trend in community change occurred along a gradient of increasing diversity, complexity and age. Initially, autogenic changes wrought by pioneer species facilitated the recruitment and survival of other species and commenced succession at the study site. Multiple successional pathways were discussed in relation to subsequent species establishment that occurred after facilitation. The amalgamation of ecological data with a social study in the form of public perception surveys allowed for a greater understanding of the present interactions between the ecosystem and the demands placed on it. The area primarily functions as a recreational site that offered numerous beach and water-related activities. A lack of information regarding the ecology and management programmes influenced public perception of the environment and management issues in general. Different management perspectives and the use of the information presented in this study were also discussed.

Ammophila arenaria (L.) Link is a European sand binding plant which was introduced to South Africa in the 1870's for the purpose of dune stabilisation. Because of its known invasiveness along the west coast of North America, and the problems South African ecosystems experience with alien invader plants, it was deemed necessary to study the biology and ecology of this species in South Africa. The aim of this thesis is to establish the potential invasiveness of A. arenaria on Cape coastal dunes and assess whether its use for dune stabilisation is still justifiable. A. arenaria occurs nowadays between the Langebaan area on the west coast and Gonubie in the Eastern Cape. Although widespread, the grass appears to occur only in areas where it has been planted. Its unaided spread may be prevented by adverse climatic conditions. Studies on the community biology of South African A. arenaria communities as compared to indigenous dune plant communities and natural A. arenaria communities in Europe cannot confirm the aggressive behaviour that A. arenaria shows in California and Oregon. In South Africa, A. arenaria does not exert strong floristic control over other species or outcompete and replace them, neither does it alter the topography of South African beaches and dunes. It forms weaker species associations and tends to develop communities of little species variability along the coast, thereby proving its alienness in South Africa, but this does not imply its invasiveness. Studies on succession of A. arenaria stabilisation areas show that monospecific A. arenaria plantings can be succeeded by a species-rich indigenous dune scrub or dune fynbos within a few decades. Plant-parasitic nematodes have been recorded, which may play an important role in the succession of A. arenaria stands in South Africa as was observed in Europe. Monitoring of A. arenaria communities and indigenous communities over nearly three years shows that A. arenaria is not spreading and replacing indigenous plants but in fact rather being replaced by the latter. A. arenaria profits from a superior sand burial tolerance but is affected by adverse climatic factors, mostly the lack of rainfall and strong radiation. In comparison to the indigenous dune grasses Thinopyrum distichum and Ehrharta villosa, it does not show any superior demographic traits such as an unusually high growth rate or large aboveground biomass production. Although A. arenaria produces viable seed in South Africa, the indigenous grasses show better germination and seedling establishment in the field. This study indicates that A. arenaria is not invasive in South Africa, nor likely to become an invader species in the near future. However, more research is required to confirm these results and more caution recommended regarding the further use of this alien grass for dune stabilisation.

Seagrass meadows offshore Ras Ghanada, as elsewhere, are an important component to the ecosystem providing numerous benefits to both aquatic and human life. This work focused on mapping the spatial and temporal distribution of seagrass meadows offshore Ras Ghanada using aerial photography acquired in 1996 and high-resolution satellite images captured in 2006 and 2012. The movements of sand shoals were also tracked, so as to further explain the dynamics of this ecosystem, as it is the area between the shoal crests that hosts the best developed seagrass meadows. The natural limiting factor for seagrass on the Ras Ghanada coastal shelf seems to be the fact that they cannot inhabit the (mobile) crests of the sand shoals, but rather, are restricted to the (more stable) sands of the shoal troughs. In the considered time period, both sand shoals and seagrass meadows migrated predominantly in a southeastern direction. The changes of seagrass that occurred in this study occurred on a fairly rapid timescale, in such that they were able to come back when there was disturbance as long as they had available habitat to move into. Furthermore, although seagrass cover declined by 3.4% from 1996 – 2012, there was a greater increase than decline in the areal coverage of seagrass post-Khalifa port construction in 2010. If sediments offshore Ras Ghanada can remain stable and the waters are not polluted by further construction, seagrasses should continue to thrive in the future.

Limit analysis based upon the theory of plasticity is one of the very useful numerical techniques to determine the failure loads of different civil and mechanical engineering structures for a material following an associated flow rule. The limiting values of the collapse loads, namely, lower and upper bounds, can be bracketed quite accurately with the application of the lower and upper bound theorems of the limit analysis. With the advancement of the finite elements and different robust optimization techniques, the numerical limit analysis approach in association with finite elements is becoming very popular to assess the stability of various complicated structures. Although two different optimization methods, namely, linear programming and nonlinear programming, have been both successfully implemented by various researchers for solving different stability problems in geomechanics, the linear programming method is employed in the present thesis due to its inherent advantage in implementation and ease in achieving the convergence. The objectives of the present thesis are (i) to improve upon the existing lower bound limit analysis method, in combination with finite elements and linear programming, with an intention of reducing the computational time and the associated memory requirement, and (ii) to apply the existing lower bound finite element limit analysis to various important planar stability problems in geotechnical engineering. With reference to the first objective of the thesis, two new methods have been introduced in this thesis to improve upon the existing computational procedure while solving the geomechanics stability problem with the usage of the limit analysis, finite elements and linear programming. In the first method, namely, the method-I, the order of the yield polygon within the chosen domain is varied, based on the proximity of the stress state to the yield, such that a higher order polygon needs not to be used everywhere in the problem domain. In the second method, the method-II, it has been intended to use only a few selected sides, but not all, of the higher order yield polygon which are being used to linearize the Mohr-Coulomb yield function. The proposed two methods have been applied to compute the ultimate bearing capacity of smooth as well as rough strip footings for various soil frictional angles. It has been noticed that both the proposed new methods reduce the CPU time and the total number of inequality constraints required as compared to the existing lower bound linear programming method used in literature. With reference to the second objective, a few important planar stability problems in geomechanics associated with interference of footings and vertical anchors have been solved in the present thesis. Footings are essentially used to transfer the compressive loads of the super structures to underlying soil media. On the other hand, vertical anchors are used for generating passive supports to retaining walls, sheet piles and bulkheads. A large number of research investigations have been reported in literature to compute the collapse load for a single isolated strip footing and a single vertical anchor. It is a common practice to estimate the bearing capacity of footings or pullout capacity of anchors without considering the effect of interference. There are, however, clear evidences from the available literature that (i) the ultimate bearing capacity of footings, and (ii) the ultimate pullout capacity of anchors, are significantly affected by their interference effect. Based on different available methods, the interference of footings, in a group of two footings as well as an infinite number of multiple footings, has been examined by different researchers in order to compute the ultimate bearing capacity considering the group effect. However, there is no research study to find the ultimate bearing capacity of interfering footings with the usage of the lower bound limit analysis. In the present thesis, the ultimate bearing capacity of two and an infinite number of multiple strip footings placed on sandy soil with horizontal ground surface, has been determined. The analysis has been performed for smooth as well as rough footings. The failure loads for interfering footings are found to be always greater than the single isolated footing. The effect of the footings' interference is expressed in terms of an efficiency factor ( ξγ); where, ξγ is defined as the ratio of the magnitude of failure load for a footing of width B in presence of the other footing to the magnitude of failure load of an isolated strip footing having the same width. The effect of the interference on the failure load (i) for rough footings becomes always greater than smooth footings, (ii) increases with an increase in soil frictional angle φ, and (iii) becomes almost negligible beyond the spacing, S > 3B. It is observed that the failure load for a footing in a group of an infinite number of multiple strip footings becomes always greater than that for two interfering footings. Attempts have been made in this thesis to investigate the group effect of two vertical anchors on their horizontal pullout resistance (PuT). The anchors are considered to be embedded at a certain clear spacing (S) along the same vertical plane. The group effect has been studied separately for anchors embedded in (i) sandy soil, and (ii) undrained clay, respectively. For anchors embedded in clays, an increase of soil cohesion with depth, in a linear fashion, has also been taken into consideration. The magnitude of PuT has been obtained in terms of a group efficiency factor, ηγ for sand and ηc for clay, with respect to the failure load for a single isolated vertical plate with the same H/B. The pullout capacity of a group of two anchors either in sand or in undrained clay becomes quite extensive as compared to a single isolated anchor. The magnitudes of ηγ and ηc become maximum corresponding to a certain critical value of S/B, which has been found to lie generally between 0.5 and 1. The value of ηγ for a given S/B has been found to become larger for greater values of H/B, φ, and δ. For greater values of H/B, the group effect becomes more significant in contributing the pullout resistance. The horizontal pullout capacity of a single isolated vertical anchor embedded in sand in the presence of pseudo static horizontal earthquake body forces has also been determined by using the lower bound finite element limit analysis. The variation of the pullout factor Fγ with changes in the embedment ratio of the smooth and rough anchor plates for different values of horizontal earthquake acceleration coefficient ( αh) has been investigated. The analysis clearly reveals that the pullout resistance decreases quite significantly with an increase in the magnitude of the earthquake acceleration coefficient. For the various problems selected in the present thesis, the failure patterns have also been exclusively drawn in order to understand the development of the plastic zones within the chosen domain for solving a given problem. The results obtained from the analysis, for the various problems taken up in this thesis, have been thoroughly compared with those reported in literature.

The soil strata is often subjected to various kinds of vibrations such as that caused by earthquakes, water waves, traffic loads, wind power plants, construction related equipments, pile driving and vibratory machines. The strains induced in a soil mass due to the vibrations generated by these different sources often lie in a range of 0.0001% - 0.1%. The estimation of the shear modulus (G) and damping (D) of soils in this strain range becomes an important aspect for performing the analysis and design of various geotechnical structures subjected to different kinds of vibrations. Strain amplitude, effective confining stress, void ratio/relative density, number of vibration cycles and cyclic strain history are some of the key parameters that influence the modulus and damping characteristics of sands. Although, the effects of strain amplitude, confining pressure and relative density have been studied quite extensively in literature, only limited studies seem to have been reported in literature to examine the effects of the cyclic strain history and the vibration cycles on these dynamic properties. The objective of this thesis is to study the effects of the cyclic strain history and the number of vibration cycles on the shear modulus and damping ratio of dry sands in a strain range of 0.0001% to 0.1%. A number of resonant column tests have been performed on dry sand specimens to examine the effect of the cyclic shear strain history, by including both increasing and decreasing strain paths, on the shear modulus and damping ratio for different combinations of relative densities (Dr) and confining pressures (σ3); an increasing strain path intends to simulate a situation when a vibratory machine is just started before reaching a steady state of vibration, and on the other hand, the decreasing strain path matches a condition when the machine is shut down after running continuously in a steady state for some time. The specimen has been subjected to a series of cycles of increasing and decreasing shear strain paths approximately in a shear strain range of 0.0006% - 0.1%. For chosen values of relative density and confining pressure, two different series of tests beginning with either (i) an increasing strain path or (ii) a decreasing strain path, were performed. In addition, the influence of the numbers of the vibration cycles which are used to measure the resonant frequency of the specimen, referred to as the cycle constant, on the values of shear modulus has also been analyzed. Irrespective of the strain path adopted to commence the test or the cycle constant used to perform a resonant column test, for a given strain amplitude, the shear modulus along the increasing strain path has been found to be always greater than the corresponding modulus value along the decreasing strain path. For the series of tests which were commenced with the increasing strain path, the shear modulus corresponding to the first increasing strain path becomes always the highest as compared to the subsequent strain paths. For a given strain cycle, irrespective of relative density of sand, the difference between the values of G associated with the increasing and decreasing strain paths becomes always the maximum corresponding to a certain shear strain level. The maximum reduction in the shear modulus, due to the cyclic variation of the shear strain, was noted to be approximately one fourth of the maximum shear modulus (G0). This reduction in the shear modulus, on account of the cyclic variation of the shear strain, increases generally with decrease in the values of both relative density and confining pressure. The damping ratio for a given shear strain for the increasing strain path was noted to be lower than the corresponding value for the decreasing strain path except for the first increasing strain path. For a particular strain level, the series of tests started with the decreasing strain path resulted in a lower value of shear modulus for all the cyclic strain paths as compared to the tests which were commenced with the increasing strain path. The modulus reduction curve for the first increasing strain path was noted to be more or less the same irrespective of the value of the chosen cycle constant. For the subsequent strain paths, an increment in the cycle constant value caused a reduction in the shear modulus at a particular shear strain level. In order to match a situation when the machine is running continuously in a steady state of vibration, resonant column tests were conducted in a torsional mode by inducing a large number of the vibration cycles with the shear strain amplitude in a range of 0.0005%-0.05%. Corresponding to a given input voltage of the drive mechanism, the specimens were subjected to a number of vibration cycles ranging from 1,000 to 50,000. The values of shear modulus and damping ratio, before and after the application of vibration cycles, were determined for several input voltages ranging from 0.001 V (minimum) to 0.3 V (maximum). The tests were carried out for different combinations of relative densities and confining pressures. For the chosen relative densities, hardly any influence of vibration cycles on the values of G and D were noted for the strain amplitude below the threshold strain level (0.0024% - 0.0044%). Beyond the threshold strain level, an induction of the vibration cycles leads to a continuous increment in the shear strain which eventually causes (i) a decrease in the shear modulus, and (ii) an increase in the damping ratio. This effect was found to become especially more significant for lower values of relative densities as well as confining pressures. The percentage changes in the values of (i) shear strain, (ii) shear modulus, and (iii) damping ratios after the introduction of vibration cycles were noted to increase with an increment in the number of vibration cycles. However, for a given increment of the vibration cycles, the changes in the values of shear modulus and damping ratio were generally noted to subside with an increase in the number of the vibration cycles. At various strain levels, the magnitude of the shear modulus was observed to increase continuously with an increase in the values of both relative density and confining pressure. For the shear strain greater than the threshold strain (0.0024% - 0.0044%), a reduction in the damping ratio values was also noted with an increase in the magnitudes of the confining pressure. On the other hand, the influence of relative density on the damping ratio was found to be relatively negligible. The shear modulus reduction curves from the present tests' data were found to compare reasonably well with the empirical curves proposed in the literature, especially for low values of the confining pressure. A deviation of the present modulus reduction curves from the empirical curves was observed generally at large shearing strains. However, the damping values obtained from the present study were noted to be lower than the values predicted by the existing empirical correlations, particularly for low values of the confining pressure. An attempt has also been made to improve the accuracy of the measurement of the arrival times of both primary (P) waves and shear (S) waves while conducting bender/extender element tests. For this purpose, a series of laboratory tests were performed on dry sand at different frequencies, varying between 1 kHz and 10 kHz, for medium dense and very dense sands with different values of the confining pressures. While determining the times of arrival of both P and S waves, two corrections have been proposed to incorporate (i) the presence of an initial offset in the input signal, and (ii) the time lag due to an existence of peripheral electronics between the input and received signals when the source and receiver elements are kept in direct contact with each other. The absolute magnitude of the resultant of these two corrections was found to reduce with an increase in the frequency of the input signal. The determination of the P-wave arrival time does not pose much difficulty. It has been noted that it becomes equally accurate to measure the arrival times of the S-wave provided the proposed corrections are incorporated. The maximum shear modulus values measured from the resonant column tests and the bender element tests by incorporating these two corrections were found to compare reasonably well with each other. The thesis brings out the effects of the cyclic strain history and the vibration cycles on the shear modulus and damping ratio of dry sand. The results obtained are expected to be useful while doing the analysis and design of geotechnical structures subjected to different kinds of vibrations.