Pipeline floatation in liquefied soils under waves

Abstract

This paper summarizes the results of an experimental and theoretical investigation of (1) pipeline floatation in a soil (liquefied under waves) and (2) density of the liquefied soil. In the experiments, the soil was silt with d50 =0.078 mm. Pipeline models of 2 cm diameter were used. Waves (with 17 cm wave height and 1.6 s wave period, the water depth being 42 cm) were used to liquefy the soil. The pipes with specific gravity smaller than 1.85 (near the bed surface) 2.0 (near the impermeable base) floated when the soil was liquefied. 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 settling in the liquefied soil. Alphaline, pressure transducers were used in the pore-water pressure measurements. More information about the experimental set-up can be found in Sumer et al. (1999). In the tests, the process of liquefaction/compaction was videotaped from the side. 2-cm-diameter perspex pipes were used as pipeline models. The length of these pipeline models was 0.57 m, slightly smaller than the wall-to-wall width of the silt box. Small pieces of metals were placed in the pipes to obtain desired values of the pipe density that were to be tested. (These small pieces were distributed evenly along the length of the pipe to ensure a uniform distribution of the load on the soil). The two ends of the pipes were then ''sealed'' with rubber corks. The range of the specific gravity of pipeline tested in the experiments was sp = γp/γ = 1.47-2.05. The way in which the test set-up was prepared for a typical test is as follows. The silt box was filled with water. The pipeline models (three pipelines with different densities for each experimental run) were then placed in the water at a desired depth. They were 18 cm apart. (Three pipelines were used to reduce the number of runs). The pipelines were held in position by suspending them with nylon strings. The soil was placed in the water gently by hand, rubbing it between the fingers to get rid of air bubbles, and crushing soil lumps to ensure an even soil texture across the entire box. Then the flume was filled with water. The strings (which were holding the pipelines) were subsequently cut, and the waves were switched on. In this way the pipeline models were free to move when the soil was liquefied. The waves were stopped when the liquefaction/compaction process came to an end. (The latter lasted about 7 minutes). If the pipeline model was floated to the surface of the bed, it could be detected quite clearly. Otherwise, the soil was ''excavated'' carefully to determine the pipe's final burial depth. A total of 24 pipes with different specific gravities were tested in the experiments.