The phenomenon of liquefaction can lead to significant damage to various structures, infrastructure, and superstructures situated on loose to medium dense saturated deposits during an earthquake. This occurs when the soil loses its resistance and transitions into a fluid-like state due to increased pore water pressure and decreased effective stress. Therefore, it is crucial to enhance the stability of such ground conditions before commencing any construction activities.
In recent years, the deep soil mixing (DSM) method utilizing a grid (lattice) pattern has emerged as one of the most widely adopted techniques for mitigating the effects of liquefiable soils. This method involves mechanically mixing stabilizing materials, such as cement and lime, with soil using either a dry method (in powder form) or a wet method (in slurry form) using hollow drills and mixing blades, depending on site conditions. This approach generates no vibrations in the soil or neighboring structures. The soil mixing process results in the formation of uniform soil-cement columns. Through the sequential and overlapping implementation of these columns before achieving full resistance, continuous walls and a grid structure are created beneath the ground surface, which are denser than the native soil. Consequently, these structures absorb maximum shear stresses induced by earthquakes, thereby preventing soil particle movement, reducing excess pore water pressure, and mitigating the effects of liquefaction.
Approximately half a century after the development of the DSM technique, its application has been limited in Iran due to technological constraints. The significance of employing this method is highlighted by the presence of coastal regions in both the northern and southern parts of the country. Given Iran's susceptibility to seismic activity and liquefaction in coastal zones, the implementation of deep soil mixing serves to mitigate liquefaction risks and avert associated challenges. However, very limited studies have been conducted on the performance of deep mixing column grid systems in the presence of structures during earthquakes. Most existing research has primarily focused on evaluating the performance of deep mixing grids in dealing with liquefaction without considering structural interactions. Consequently, there are currently no comprehensive guidelines for designing deep mixing columns with a grid pattern for liquefiable soils beneath structures.
This study investigates the most important influential factors in the design of deep mixing grids and their impact on the seismic response of structures situated on them. The research was conducted using Midas GTS NX, a three-dimensional finite element software, and the results were validated against centrifuge test data of Ishii et al. (2017). Liquefiable soil with a modified UBC sand behavioral model and deep mixing soil-cement columns was modeled with an elastic behavioral model. To investigate the seismic vulnerability of the structures, a conventional 10-story steel frame structure was designed and placed on the improved ground and then it was subjected to the Kobe (1995) and Northern California (1954) earthquakes..
The study reveals that increasing the diameter of soil-cement columns, the replacement area ratio, and the number of grids-while simultaneously decreasing grid dimensions-significantly reduces the generation of excess pore water pressure (r_u). This reduction is achieved by enhancing the system’s stiffness and minimizing shear stresses. These improvements result in an average 35% in the shear applied to the enclosed, as well as a fourfold decrease in the relative displacement of structural floors compared to unimproved ground conditions due to applied earthquakes.
Finally, the study proposes an optimized triple-grid design with dimensions of 10.7 × 13 meters, a replacement area ratio of 20.1%, and DSM columns with a diameter of 0.9 meters. This configuration has been identified as the most effective solution for mitigating liquefaction effects and reducing the seismic vulnerability of structures.