Consolidation of naturally gassy soft soil

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Biogenic gas is produced and contained in fine-grained soils in environments such as river estuaries and landfill sites. Planning dredging operations or predicting the volume capacity of a landfill requires information about the amount of gas that can be held in the soil, and about the effect that the gas has on the soil behaviour. The purpose of this paper is to present some experimental results and interpretation relevant to these and related topics. A series of experiments has been carried out at Oxford University with soil of estuarine origin from the Slufter disposal site in the Netherlands. The soils were introduced in settling columns, and the different stages of consolidation were documented by measuring height, densities, excess pore pressures and amount of gas. At the end of primary consolidation the gassy soil was consolidated to higher stress levels by applying a hydraulic gradient. The growth rates of bacteria producing gas within the soil were accelerated or reduced by controlling the temperature in the range 10–30°C. The experimental results show a sequence of events during consolidation. In the first phase, gas was produced and accumulated within the soil. The overall density of the gassy soil decreased and pore pressures fluctuated unpredictably. At the end of this phase the amount of gas within the soil reached a critical threshold value and thereafter began to escape through cracks and fissures. This marked the start of a second phase of events. Although the gas production continued to be high, the total amount of gas within the soil slowly decreased. The soil began a new phase of consolidation, with the settlement accelerating, as the cracks and fissures provided a quick route for pore water dissipation. The self-weight stresses in these experiments do not exceed 1 kPa, so that higher stress levels were achieved by the application of a hydraulic gradient. During this stage, the gas within the soil became less influential as the soil gained in strength.