<p>The focus of this study is on potential problems resulting from construction on expansive soils in Kibaha region, Tanzania. For the fact that most of the affected structures are founded on expansive soils, a clear understanding of the soil behaviour and their interaction with structures, specifically as they relate to shallow foundations, has been of more interest to the study in order to evaluate properly the source of the problem.</p><p>The geotechnical behaviour of expansive clay soils is investigated by looking into the geomorphologic, geological and climatic conditions and mineralogical composition of the soils in the study area. The geotechnical results are linked with the performance of the foundation as well as structures.</p><p>Two sites, representative of known problem-areas in Kibaha were selected for geotechnical tests. Geotechnical site investigation consisted of open trial pits, profile description and the collection of both disturbed and undisturbed samples.</p><p>The collected samples were submitted to soil laboratories at KTH and DIT for mineralogical composition tests, natural water content, density, Atterberg limits and swell tests (free swell and swelling pressure). The results of this investigation indicate that soil in Kibaha contains clay (31%), have high liquid limit (59%) and plastic limit (37%) which indicate high potential swell.</p><p>Since swell potential and swell pressure are key properties of expansive soils, the swell parameters were measured by free swell tests and one-dimensional oedometer swell tests respectively. The free swell ranged from 100% to 150% and the swell pressure was in the region of 45 kPa.</p><p>The properties of expansive soils were confirmed by the x-ray diffraction test which showed the presence of montmorillonite in the soil. It is from this fact that the source of the problem is in the expansive soils coupled with poor building materials.</p><p>Physical conditions of the surveyed properties in the area confirmed the hypothesis of building damages due to poor building materials triggered by expansive soils. In support of the obtained data, the actual behaviour of the foundations is supplemented with prototypes of strip foundations whose performances are to be monitored over a long period. Finally, suggested are the ways forward to solve the problem of foundation on expansive soil.</p>

A technique that allows for nonparallel interfaces and lateral inhomogeneities in an irregular layered medium is described. The formulation combines a semidiscrete finite-element technique with a perturbation method, providing an approximate treatment of wave propagation in irregular layered media. The procedure treats the irregularities as perturbations with respect to a reference, horizontally-layered, laterally-homogeneous medium and produces approximations of the perturbed wave motion with little additional computation effort. Within the framework of the method, consistent transmitting boundaries and other semidiscrete hyperelements as well as Green’s functions, already available for regular layered media, can be reformulated. The method is relevant in problems of foundation dynamics, ground response to seismic waves and site characterization. Example problems are presented toward evaluation of the effectiveness of the method. / text

The research described in this dissertation undertakes a computational study of wave motion due to ground excitation in layered soil media. Adjustments of soil properties consistent with the level of deformation is applied using an equivalent linear approach. The finite element method provides the basis of the numerical procedure for soil-structure system response calculation in conjunction with a first-order perturbation scheme. Available experimental data are employed for shear-modulus and damping adjustments. The findings of the research are expected to lead to efficient calculation of structural response to earthquake ground motion. / text

No description available.

Pile foundations that support offshore structures are required to resist not only static loading, but also dynamic loading from waves, wind and earthquakes. The purpose of this study is to gain a better understanding of the behavior of offshore piles under cyclic or dynamic loading using the finite element approach. To achieve this goal, an appropriate constitutive model is required to simulate the behavior of soils and interfaces. The DSC constitutive model is developed for saturated interfaces to study the behavior under severe shear deformation at the soil-pile interface. Monotonic and cyclic simple shear experiments are conducted on Ottawa sand-steel interfaces under drained and undrained conditions using the Cyclic-Multi-Degree-of-Freedom shear device with porewater pressure measurement (CYMDOF-P). The effect of various parameters such as normal stress, surface roughness of steel, type of loading, and the amplitude and frequency of the applied displacement in two-way cyclic loading are investigated. The data from the simple shear tests on saturated interfaces are used to calculate the parameters in the DSC model. The resulting parameters are then used to verify the DSC model by back predicting tests from which parameters are determined and independent tests that are not used in parameters determination. The model predictions, in general, were found to provide a highly satisfactory correlation with the observations. In the context of DSC, the concept of critical disturbance is developed to identify initiation of liquefaction in saturated Ottawa sand-steel interfaces. This method is based on using microstructural changes in material as an indication of liquefaction identification. The finite element method, along with DSC constitutive model, is used to investigate the response of offshore piles to dynamic loading. These include cyclic loading of axially loaded instrumented pile in clay and full-scale laterally loaded pile in sand. The DSC model is used to model the nonlinear behavior of saturated soils and interfaces. A nonlinear dynamic finite element program DSC-DYN2D based on the DSC modeling approach and the theory of nonlinear porous media is used for this purpose. Results from numerical solutions are compared with field measurements. Strong agreement between numerical predictions and field measurements are an indication of the ability to solve challenging soil-structure interaction problems.Based on the results of this research, it can be stated that the finite element-DSC model simulation allows realistic prediction of complex dynamic offshore pile-soil interaction problems, and is capable of characterizing behavior of saturated soils and interfaces involving liquefaction.

Soil structure influences many chemical, biological and physical processes and it is well established that organic carbon acts as a soil binding agent. However, the precise location of organic matter and carbon in relation to structural features within intact samples is unknown. The sensitivity of organic carbon to decomposition is dependent not only upon its intrinsic chemical recalcitrance, but also its location within the soil structure. Soil structure provides organic carbon with chemical and physical protection, the extent of which varies between structural units. Furthermore soil structure is transient, and is sensitive to both environmental changes and physical disturbance, therefore it is difficult to determine and quantify the impact of this dynamic entity upon the storage of organic carbon. To date the majority of research that has advanced our understanding of the role soil structure plays in the storage of organic carbon, has relied upon some form of fractionation technique to separate aggregates from the bulk soil. However this approach has its disadvantages as much of the soil structure is destroyed; clearly when studying the impact of soil structure upon organic carbon-storage it is advantageous to implement any method that minimises disturbance to the soil structure. This study entails removing intact soil samples (through the use of kubiena tins) from long-term agricultural experimental fields at Rothamsted Research, (Hertfordshire, UK) with the aim of comparing and evaluating the location of organic matter and it’s associated organic carbon, in soils with contrasting organic carbon contents and a well documented land-use history. Thin sections will be analysed by integrating conventional micromorphology, image analysis and sub-microscopy combined with microscale chemical analysis scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). In doing so a new alternative method for analysing the distribution of organic matter and organic carbon is proposed. It was found that agricultural soils, which are the same in all aspects except total-OC content, differ in total organic matter, water release characteristics, aggregate stability and pore size distribution; therefore these differences could be attributed to the relationship between OC and soil structure. The water release curve, aggregate stability and pore size distribution also differed between soils with similar OC-contents but from different land-uses. The analysis of organic matter within intact soil samples provided evidence for the redistribution of organic matter as it is decomposed within the soil structure, for instance, less decomposed organ and tissue forms were located in or near to soil pores while more decomposed amorphous forms were located within the soil matrix. Since the same pattern of redistribution was observed in both agricultural and grassland soil this is likely to be directed by soil macro and micro fauna. It is concluded that since the location of different forms of organic matter is consistent across all soil, organic matter location is not responsible for creating differences in aggregate stability between treatments. Instead the results indicate that the amount and strength of organic carbon bonds and its hydrophobic properties are responsible. Micromorphology results demonstrated an absence of defined aggregation between treatments. Despite the difficulties in the interpretation of aggregation, the results contradict theories of aggregation, which state that aggregates are formed around “fresh” organic matter and it is argued that OM will undergo substantial decomposition before it acts as core for aggregation. Initial SEM-EDS analysis, has shown that in the soil matrix adjacent to organic matter (plant/organ) fragments there is a heightened concentration of C, indicating that these fragments are acting as a source of organic carbon. Interestingly BC, which represent one of the most recalcitrant C forms is also acting as a source of C, although these initial results suggest to a lesser extent than more labile C-sources. This source of organic carbon could stimulate microbial activity thereby enhancing soil structural stability. Alternatively, the release of liable carbon into soil pores may represent one route by which labile carbon enters sub-soil horizons.

A probabilistic framework is developed to assess the structural reliability of offshore wind turbines. Probabilistic models are developed to predict the deformation, shear force and bending moment demands on the support structure of wind turbines. The proposed probabilistic models are developed starting from a commonly accepted deterministic model and by adding correction terms and model errors to capture respectively, the inherent bias and the uncertainty in developed models. A Bayesian approach is then used to assess the model parameters incorporating the information from virtual experiment data. The database of virtual experiments is generated using detailed three-dimensional finite element analyses of a suite of typical offshore wind turbines. The finite element analyses properly account for the nonlinear soil-structure interaction. Separate probabilistic demand models are developed for three operational/load conditions including: (1) operating under day-to-day wind and wave loading; (2) operating throughout earthquake in presence of day-to-day loads; and (3) parked under extreme wind speeds and earthquake ground motions. The proposed approach gives special attention to the treatment of both aleatory and epistemic uncertainties in predicting the demands on the support structure of wind turbines. The developed demand models are then used to assess the reliability of the support structure of wind turbines based on the proposed damage states for typical wind turbines and their corresponding performance levels. A multi-hazard fragility surface of a given wind turbine support structure as well as the seismic and wind hazards at a specific site location are incorporated into a probabilistic framework to estimate the annual probability of failure of the support structure. Finally, a framework is proposed to investigate the performance of offshore wind turbines operating under day-to-day loads based on their availability for power production. To this end, probabilistic models are proposed to predict the mean and standard deviation of drift response of the tower. The results are used in a random vibration based framework to assess the fragility as the probability of exceeding certain drift thresholds given specific levels of wind speed.

The present work addresses a computational methodology to solve dynamic problems coupling time and Laplace domain discretizations within a domain decomposition approach. In particular, the proposed methodology aims at meeting the industrial need of performing more accurate seismic risk assessments by accounting for three-dimensional dynamic soil-structure interaction (DSSI) in nonlinear analysis. Two subdomains are considered in this problem. On the one hand, the linear and unbounded domain of soil which is modelled by an impedance operator computed in the Laplace domain using a Boundary Element (BE) method; and, on the other hand, the superstructure which refers not only to the structure and its foundations but also to a region of soil that possibly exhibits nonlinear behaviour. The latter subdomain is formulated in the time domain and discretized using a Finite Element (FE) method. In this framework, the DSSI forces are expressed as a time convolution integral whose kernel is the inverse Laplace transform of the soil impedance matrix. In order to evaluate this convolution in the time domain by means of the soil impedance matrix (available in the Laplace domain), a Convolution Quadrature-based approach called the Hybrid Laplace-Time domain Approach (HLTA), is thus introduced. Its numerical stability when coupled to Newmark time integration schemes is subsequently investigated through several numerical examples of DSSI applications in linear and nonlinear analyses. The HLTA is finally tested on a more complex numerical model, closer to that of an industrial seismic application, and good results are obtained when compared to the reference solutions.

The main objective of this research was to determine the short-term effects of cropping on the aggregation and the carbohydrate fraction of a marine clay soil. When compared to a fallow (bare soil) control, cropping to barley and alfalfa for two seasons resulted in significant increases in the size and stability of soil aggregates. The increase in water-stable aggregates $>$2.0 mm was at the expense of aggregates $<$1.0 mm. Under corn, soil aggregation was not different than under fallow. After two seasons, soil under barley or alfalfa contained 15 to 25% more carbohydrates than the fallow or corn treatments. The partial correlation (r = 0.63, P = 0.001) which was observed between carbohydrate content and mean weight diameter of water-stable aggregates, and periodate oxidation tests strongly suggested that carbohydrates were at least partly responsible for the increase in macro-aggregate stability. The remaining cropping effects were removed by tetraborate which suggest the participation of more-humified though ill-defined organic matter. The nature of the changes in organic matter was further investigated using particle size fractionation. Compared to the fallow control, cropping to barley and alfalfa resulted in an enrichment in carbon, nitrogen, and carbohydrates in the sand fraction. The carbohydrate composition of the soil and of its size fractions, determined by liquid chromatography, suggested that cropping treatments had only little effect on the origin of the soil carbohydrates.

No description available.