Soil densification due to static sand pile installation for liquefaction remediation

Abstract

Among various soil densification techniques, the sand compaction pile (SCP) has been one of the most frequently used methods to improve loose deposits of sandy soils encountered in Holocene or reclaimed lands. In this method, columns of densely compacted sand are created in the ground by imparting vibration to the sand at the bottom of a pipe which is lifted stepwise while supplying sand from the ground surface. Because of noise and vibration produced during its installation, the SCP is losing its popularity and an alternative technique employing a static driving force is being exploited to install columns of dense sand. To study the degree of soil densification due to such static sand pile installation, multiple series of large-scale hollow cylindrical torsional shear tests were conducted in the laboratory on clean fine sand, simulating stress changes conceived to be occurring in a soil element in the vicinity of the pile being penetrated. To determine the stress changes in the field during the pile penetration, analysis was conducted based on the classical theory of elasticity. The sequence of stress changes thus established was applied to saturated sand specimens prepared in a torsional hollow cylindrical shear test apparatus. This process allows complex stress paths to be reproduced in the specimens, including the rotation of a principal stress direction. In the course of the tests, shear stresses were applied first undrained on loose and medium dense fine sands and induced pore water pressure was dissipated by opening the valve of the drainage system, thereby monitoring the volume decrease of saturated samples. Particular attention was drawn to the influence of the amount of shear strains imposed undrained on the soil specimens on the subsequent drained volume changes. It was found that a volume change of 5-10% was observed in the test samples, which is considered sufficiently great to bring about substantial densification in the sand. The experimental results of the tests were shown to provide a basis for the assessment of soil densification due to static sand pile penetration. On this basis, a diagram was provided to facilitate the evaluation of the degree of soil densification. In addition, case studies were carried out by taking advantage of soil improvement projects which have recently been implemented at three sites in Japan. These are considered to provide field verification on the effectiveness of soil densification due to static sand pile installation. Finally the degree of in-situ densification as evidenced by increased SPT N-value was interpreted in the framework of the conception established through the laboratory tests.