Reduction in Ground Motion with Soil Replacement
Author:Richard T Hastings
Mentor:Dr. Binod Tiwari, Associate Professor of Civil & Environmental Engineering, California State University Fullerton
Loose sands are susceptible to the magnification of ground motion. Due to this effect, the base seismic acceleration at the focus of an earthquake, sometimes, magnifies by a factor of 4 when it reaches the ground surface. This requires the superstructures to be designed for a high seismic coefficient, increasing the construction cost significantly. One approach to reduce the design seismic coefficient is to replace certain portion of soil with soil-cement mixture.
In this study, a portion of loose sand was replaced by soil-cement mixture and the reduction in ground acceleration at different frequencies and cycles of loading was examined. The first stage of the study involved testing different composition of soil-cement mixture to determine the strongest mixture with the most proper curing time. Then a Plexiglas box filled with loose sand was shaken for different seismic accelerations with a) no soil replacement, b) 10% soil-cement mixture replacement of the surface area, c) 25% soil-cement mixture replacement of the surface area. Accelerations were measured at different depths, using triaxial accelerometers. The recorded data showed around 10% and 30% reduction in ground motion amplification factor for 10% and 25% soil-cement panel replacement ratio, respectively. The reduction of the max peak ground acceleration is sufficient to improve the International Building Code (IBC) site class by one level in terms of seismic force to be included in the design thereby reducing the amount of building elements required to counteract the force applied by seismic waves. A preliminary study shows that more than 20% of construction cost can be saved by improving the IBC site class from F to E.