Numerical simulation of partial saturation in sands induced by flow and chemical reactivity

Abstract

Moderate to large earthquakes often cause ground failure due to soil liquefaction leading to major and sometimes catastrophic damages to the built environment. Liquefaction is the loss of bearing capacity and shearing resistance of fully saturated sand due to pore pressure buildup in an earthquake leading to geotechnical related failures of engineered structures. While measures have been developed to prevent the occurrence of liquefaction, often they are too costly and/or impractical at sites with existing structures. New liquefaction mitigation measures are being investigated by a number of researchers. "Induced Partial Saturation" (IPS) in sands is an innovative, practical, cost-effective and reliable liquefaction mitigation technique that was developed by researchers at Northeastern University. The IPS technique is based on hydraulic injection and transport of a solution of sodium percarbonate within liquefaction-susceptible sand. The reactivity of the solution results in a slow release of oxygen gas thus reducing the degree of water saturation and permeability of sand. The increase in effective compressibility limits buildup of pore water pressure during an earthquake and thus prevents liquefaction. To better understand, design and implement IPS, it will be necessary to develop a theoretical model for the technique. This research develops the theoretical basis of IPS and implements a numerical solution for modeling partial saturation in sand induced by transport and reactivity of a chemical solution within the pores of the sand. The model is based on coupled differential equations describing three-dimensional, transient, hydraulic flow and non-linear, advective-dispersive transport of the reactive chemical; coupled with algebraic equations describing the kinetics of the chemical, formation of oxygen gas, and associated decrease in saturation. To simulate partial saturation in sands through transport and reactivity, the computer program SUTRA, developed by the US Geological Survey, was adopted and modified. The new model "SUTRA-Bubble" is a modified version that simulates transient partial saturation created by IPS below the water table. SUTRA-Bubble models partial saturation in soil as a function of chemical reactivity of a solution which is transported through the pores of soil. SUTRA-Bubble also accounts for decrease in permeability of soil as degree of saturation in soil decreases.