Consolidation of naturally gassy soft soil

Biogenic gas is produced and contained in fine-grained soils in environment s 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 t he gas has on the soil behaviour. The purpose of this paper is to present s ome experimental results and interpretation relevant to these and related t opics. A series of experiments has been carried out at Oxford University wi th soil of estuarine origin from the Slufter disposal site in the Netherlan ds. The soils were introduced in settling columns, and the different stages of consolidation were documented by measuring height, densities, excess po re pressures and amount of gas. At the end of primary consolidation the gas sy soil was consolidated to higher stress levels by applying a hydraulic gr adient. The growth rates of bacteria producing gas within the soil were acc elerated or reduced by controlling the temperature in the range 10-30 degre esC. The experimental results show a sequence of events during consolidatio n. In the first phase, gas was produced and accumulated within the soil. Th e 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 c racks and fissures. This marked the start of a second phase of events. Alth ough the gas production continued to be high, the total amount of gas withi n the soil slowly decreased. The soil began a new phase of consolidation, w ith the settlement accelerating, as the cracks and fissures provided a quic k route for pore water dissipation. The self-weight stresses in these exper iments do not exceed 1 kPa, so that higher stress levels were achieved by t he application of a hydraulic gradient. During this stage, the gas within t he soil became less influential as the soil gained in strength.