Scientists have found that the shape of sand grains influences the liquefaction of sand, one of the major factors behind the collapse of structures during earthquakes.
Cyclic simple shear setup at IISc
Scientists have found that the shape of sand grains influences the liquefaction of sand, one of the major factors behind the collapse of structures during earthquakes. Liquefaction of sand is a phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading and leads to the collapse of structures resting on the liquefied ground.
As natural sand with regular shape liquefies easily, the scientists have concluded that natural sand used in structures like slopes and retaining walls can be replaced with irregular manufactured sand to improve the stability and sustainability.
Though the qualitative effects of grain size and grain shape on the resistance of sand to liquefaction are well established, quantitative correlations between them are elusive. Most of the studies in this direction used conventional methods to quantify the grains' size and shape, including sieve analysis and visual observations.
In a breakthrough study, researchers at the Indian Institute of Science (IISc) used digital image analysis for grain shape characterizations and related them to the liquefaction potential of the sands. They found a strong relation between the two. This is because the shear force (unaligned forces pushing one part of a body in one specific direction and another part of the body in the opposite direction) required to break the inter-particle locking is more for the grains with relatively irregular shapes.
Microscopic images of sand particles were analyzed through computational algorithms developed in MATLAB (MATrix LABoratory), which is a high-performance computing platform for analyzing data to determine their shape parameters. Cyclic simple shear tests in which specimens are subjected to simulated earthquake conditions of alternate cycles of tension and compression were carried out on sand samples to determine their potential to liquefy under specific earthquake conditions. For these tests, the scientists used the cyclic simple shear test setup (GCTS USA make) procured through Department of Science and Technology – Fund for Improvement of S&T Infrastructure in Universities and Higher Educational Institutions (FIST) funding. The study has been accepted for publication in Indian Geotechnical Journal, for carrying out cyclic simple shear tests.
The research team found that glass beads, which have regular shape with higher roundness and sphericity, liquefied first in the cyclic shear tests, while river sand, whose roundness and sphericity (how much of an even sphere it is) fall between glass beads and manufactured sand, liquefied next, followed by manufactured sand, whose shape is relatively irregular. These tests clearly highlighted the important effects of grain shape on the liquefaction potential of granular soils.
As the shape of the particles becomes irregular, with their overall form deviating from that of a sphere and their corners becoming sharper, they get interlocked with each other during shearing. Interlocking provides additional resistance to shear, and hence the tendency to get separated from each other to float in the fluid becomes lesser for particles with irregular shapes.
Further, tortuosity, or the deviation in the fluid path, increases with the irregular shape of the particles. Greater tortuosity decreases water flow through the pore network and decreases the chance for water to separate the particles.