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The case study consists of a piled embankment built over a very soft, organic compressible clay layer 10 m deep. The height of the embankment supported on piles was about 1·3 m, and the piles, which measured 0·18 m × 0·18 m, were spaced 2·5 m apart in a square pattern, with 0·8 m wide square pile caps covered with a bidirectional 200 kN/m geogrid polyester reinforcement. A region of the piled supported embankment was instrumented to provide a better understanding of the overall embankment-reinforcement-pile cap system. Three-dimensional and two-dimensional pile layouts were adopted. An excavation of 1 m under the geogrid was carried out to induce rapid load transference to the geogrid. The settlement in between pile caps was 0·10–0·4 m, whereas settlements in the region under similar embankment heights were much higher, thus demonstrating the efficacy of pile-supported embankments with geogrids in controlling settlements. Strains at the reinforcement were also measured, and showed values in the range 0·25–2·05%.

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The oxidation of iron sulphides in dredged sediments can be a source of long-term contamination of surface and underground water. The adverse effects are related to a decrease in the ability of soil to retain heavy metals, which is caused by acidification. Laboratory tests were conducted, and pH variations and consequent release of heavy metals, especially Zn, Cu, and Pb, into the solution were monitored over time. The rate of pyrite oxidation was determined based on the sulphate production rate, through the stoichiometry of the oxidation reaction. Samples of sediments from the Jacarepaguá lagoon complex, State of Rio de Janeiro, were dried and oxidized over a period of 10 months. During this period the pH fell from 6.9 to 3.3, with oxidation of approximately 20% of the sulphur present in the form of iron sulphide, and 45% of the Zn, 15% of the Pb, and 5% of the Cu contained in the material were released. The oxidation rate at the initial stages was of the order of 1.0 × 10–4 g(O2)·h–1, decreasing to 1.0 × 10–5 g(O2)·h–1 during the test. This study presents the laboratory methodology developed for determining the oxidation rate in a system where the oxygen consumption is governed not only by the pyrite oxidation, but also by the organic matter. In these experiments, a key measure of intrinsic oxidation rate (IOR) is the oxidation rate where oxygen is freely available at normal atmospheric concentration. By comparing the values obtained for the dredged material with the results of the IOR of mining waste materials reported in the literature, it was observed that the oxidation rates of the iron sulphide formed in the Jacarepaguá lagoon environment were one to two orders of magnitude higher than those of mineral iron sulphides from coal wastes. This behavior can be explained considering that the iron sulphides in dredged materials are freshly formed and have more specific surface area exposed to air than the iron sulphide mineral in mining wastes.Key words: sediments, dredging, sulphides, oxidation, acidification, heavy metals.

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