Rapid population increase and economic growth in eastern China has lead to the degradation of many water bodies in the region, such as Lake Taihu, the third largest freshwater lake in China. Using data from recent investigations, the correlations between algae (measured as chlorophyll-a) and water quality indices in Lake Taihu were described by multivariate statistical analyses, and the key driving factors for the lake eutrophication were identified by principal component analysis. Results revealed strong spatiotemporal variation in the correlations between algae and water quality indices, suggesting that the limiting factor for the dominant algae growth depends on seasonality and location and it is necessary to reduce both nitrogen and phosphorus inputs for a long-term eutrophication control in this hyper-eutrophic system. Water temperature was another important controlling factor for algal growth in the lake. Using principal component analysis, nutrient contaminations from anthropogenic and natural inputs were identified as the key driving factor for the water quality problems of the lake. Moreover, five principal components were extracted and characterized with high spatial and seasonal variations in Lake Taihu. The key driving factors were believed to influence spatial variations including heavily polluted areas located in the northern and northwestern parts of the lake, where many manufacturing factories were built and wastewater from domestic and industrial plants was discharged. Based on this analysis, attention should be paid to effective land management, industrial wastewater treatment, and macrophytic vegetation restoration to reduce the pollutant loads and improve water quality. Principal component analysis was found to be a useful and effective method to reduce the number of analytical parameters without notably impairing the quality of information in this study.
This paper examines the effect of the lattice leg on the cavities of deeply embedded spudcans in clays using centrifuge model tests and finite-element analyses. The lattice legs are characterised by two parameters, namely the opening ratio and area ratio. Both centrifuge and numerical results show that, in the presence of lattice legs, the cavities consist of two zones: a deep inner cavity within the cross-sections of the lattice legs and a much shallower outer cavity beyond the periphery of the lattice legs. The latter can be attributed to cavity wall collapse around the rim. The volume contributed by the outer cavity is much smaller than that contributed by the inner cavity. Hence, the latter is the focus of this study. The findings show that the lattice leg affects the cavity in two ways. First, it restricts soil back-flow to the region outside the lattice leg. Second, it enhances the stability of the soil on the walls of the cavity, thereby limiting the wall collapse zone to the topmost portion of the cavity. Both of these effects contribute to an increase in the cavity depth. Based on the centrifuge model test results and finite-element analyses, relations are proposed to predict the inner cavity depth which incorporates the opening and area ratios of the lattice leg, spudcan diameter and undrained shear strength of the soil. The effect of the lattice leg on bearing capacity is also illustrated using centrifuge results. Finally, the possibility of cavity wall collapse and spudcan settlement due to consolidation of soil and cyclic loading are briefly discussed using centrifuge data.
Punch-through failure of mobile jack-up rigs is still a main concern for spudcan foundations. Using centrifuge model tests and numerical modelling, this paper examines the mitigating effect of top-mounted sleeves and a downward skirt on spudcan punch-through in medium dense sand overlying clay. The findings show that the top-mounted sleeves and downward skirt are both useful separately for reducing punch-through distance. The top-mounted sleeve functions by interfering with soil backfilling above the spudcan and maintaining confinement to the sides. This results in an earlier recovery in penetration resistance and reduces the peak-to-trough drop in resistance. The downward skirt appears to mobilise the soil further ahead of the spudcan, which reduces the peak penetration resistance and increases the rate of post-trough recovery in penetration resistance. A combination of a top-mounted sleeve with a downward skirt appears to confer a smaller peak-to-trough drop in resistance and higher rate of post-peak recovery, and thereby a much smaller punch-through distance. The results would suggest that, compared with the downward skirt, the top-mounted square sleeve with a similar area ratio to the existing lattice leg is supposed to be a more effective option in mitigating spudcan punch-through failure.
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