Effects of Organic Matter on Shrinkage and Water Retention Behaviour of Organic Dredged Sediments

Effects of Organic Matter on Shrinkage and Water Retention Behaviour of Organic Dredged Sediments

Chin A Moei, S.A.

van Paassen, L.A. (mentor)

Civil Engineering and Geosciences

Geoscience & Engineering

Geo-Engineering

2016-12-14

Peatlands in the Netherlands suffer from continued surface subsidence and accumulation of suspended sediments which leads to poor water quality. Excess suspended sediments are currently being deposited in depots on peatlands as a sustainable method for reusing sediment and to compensate for surface subsidence. The general consensus of researchers is to credit the majority of surface subsidence of peatlands to oxidation of organic matter. To assess this, the effect of organic matter and initial water content on the shrinkage and water retention behaviour was studied. Additionally, the effect of organic matter on gas formation was researched, along with the main mechanism for such an occurrence. From the peatland Wormer & Jisperveld, dredged sediments were collected from a depot and core samples were also taken over the course of three years. Soil characterisation tests were carried out and a novel approach of combining the extended evaporation method with computed tomography was used. Experiments were varied by differing the organic and initial water contents, through chemical removal (H2O2) of organic matter and fan drying, respectively. Results show that decreasing the organic content reduced liquid limit of the sediment. The effect of organic matter on the shrinkage characteristics is not significant for the range of water contents relevant during the desiccation process observed in the field. Only normal shrinkage was observed at near saturated conditions, where the loss of water volume is equal to the loss of bulk soil volume. Additionally, the effect of organic mat- ter on the water retention characteristic greatly reduces the water content at a given suction level. Varying the initial water content changes the onset of the water retention characteristic, while the curve’s shape was barely influenced. Slight variation of retention behaviour likely due to natural oxidation of organic matter. For the shrinkage characteristic only the onset was influenced. Computed tomography illustrated that the effect of organic matter stimulates gas formation within the sediment. Gradual movement of the gas, aggregation, bubble growth and ebullition events were also observed. The main mechanism of gas formation could not be determined with the applied test methods. However, it is considered to have been caused by either exsolution of dissolved methane, oxidation of organic matter or a combination of both. Further research would be needed for verification.

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Student theses

master thesis

(c) 2016 Chin A Moei, S.A.