Critical Flotation Density of Pipelines in Soils Liquefied by Waves and Density of Liquefied Soils

This paper summarizes the results of an experimental and theoretical investigation of: (1) pipeline flotation in a soil (liquefied under waves); and (2) density of the liquefied soil. In the experiments, the soil was silt with $d_{50}=0.078\mathrm{mm}$ . Pipeline models of $2\mathrm{cm}$ diameter were used. They were buried in the soil at different depths in the range $3–15.5\mathrm{cm}$ . The total depth of the silt layer was $17.5\mathrm{cm}$ . Waves (with $17\mathrm{cm}$ wave height and $1.6\mathrm{s}$ wave period, the water depth being $42\mathrm{cm}$ ) were used to liquefy the soil. The pipes with specific gravity smaller than 1.85–2.0 floated when the soil was liquefied, the critical specific gravity for pipe flotation. The lower bound of the above range corresponds to the initial pipe position near the surface of the bed, and the upper bound to that near the impermeable base. Furthermore, the pipe floated (or sank) to a depth where the pipe specific gravity was equal to the previously mentioned critical specific gravity for flotation. The density of liquefied soil was determined in two different ways in the experiments: (1) from the aforementioned flotation tests where the pipe acted as a “hydrometer”, the instrument to measure fluid density; and (2) from the force balance equation for the pipe, in the vertical direction, corresponding to the critical condition for the pipe flotation. The results from these two methods agree well. The density of liquefied soil varied with depth; it was 1.85 near the surface of the bed and 2.0 near the impermeable base. A hydrodynamic model has been developed to predict the density of liquefied soil. The model is based on the force balance (in the vertical direction) for a soil grain of the liquefied soil. The model indicates that the density of liquefied soil is influenced by the soil category; the soil “class”; the initial soil specific gravity; the specific gravity of soil grains; and the coefficient of lateral earth pressure.