The Effect of Non-Plastic Fines on Pore Pressure Generation Characteristics in Saturated Sands

Abstract: Liquefaction is one of the most important phenomena that occurs during earthquakes. Loose granular soils along with a high ground water level make soil deposits susceptible to liquefaction. Liquefaction occurs due to an increase in excess pore water pressure and decreases in effective stress in the soil deposit. Thus, pore water pressure generation has significant affects on the shear strength, stability, and settlement characteristics of a soil deposit, even if it does not cause full liquefaction in the soil. Most natural sands and artificial soil deposits like hydraulic fills contain some plastic and non-plastic fines, which make their behavior different from clean sands. The generation of pore water pressure in a saturated soil beneath level ground during an earthquake is generally assumed to be due to the cyclic shear strains. This study presents the results of strain-controlled cyclic triaxial tests on silty sands to evaluate pore pressure generation characteristics of these material sands under cyclic loading. Strain-controlled testing allows for a more fundamental assessment of pore pressure generation because of the strong relationship between shear deformation and pore pressure generation. The cyclic triaxial tests were performed on the specimens including mixtures of Monterey #0/30 sand and 0%, 10%, 20%, and 30% of Sil-Co-Sil 52 non-plastic silt. To prepare the specimens, wet tamping Undercompaction technique was used. In this method, each layer is compacted to a lower density than the final desired value by a predetermined amount, which is defined as the percent of undercompaction. The advantages of this method are minimizing particle segregation, preparing specimens in wide ranges of density, and making uniform specimens. In addition, all of the specimens were prepared at an initial relative density =50%. The specimens were subjected to 50 at cycles of axial strain at loading rate of 0.1 Hz. Axial strain was measured by a miniature LVDT and this value converted to shear strain using elasticity theory and a Poisson’s ratio of 0.5 for undrained condition ( =1.5. ). The specimens were subjected to various levels of shear strains ranging from 0.003% to 0.3%. The results show that overall void ratio decreases during the saturation and consolidation process, this effect increasing with increasing silt content. Also, excess pore water pressure decreases with increasing silt content up to 10%. At silt contents greater than 10% up to 30%, excess pore pressure increased because, the sand skeleton void ratio is greater than the maximum void ratio of the clean sand, thus the sand grain-to-grain contact is removed and the response is controlled by the silt matrix.