Dynamic properties of colloidal silica soils using centrifuge model tests and a full-scale field test

Traditional ground improvement methods to mitigate the effects associated with liquefaction damage are often not feasible in developed areas. Commonly used soil improvement methods can have adverse affects on the surrounding infrastructure and less invasive methods are therefore required. Passive site stabilization is a non-invasive grouting technique where a stabilizing material can be injected at the edge of a site and delivered to target locations through the groundwater. As the stabilizer flows through the subsurface, it displaces the pore water and subsequently forms a permanent gel that binds to soil particles, resulting in a stronger soil formation. Based on its unique characteristics, colloidal silica has been selected as an ideal material for passive site stabilization. For purposes of liquefaction mitigation, the dynamic behavior ofcolloidal silica soils was studied through centrifuge model tests and a complementary, full-scale field test. The centrifuge tests provided comparisons of the response for untreated sands and sands treated with 4%, 5%, and 9% colloidal silica concentrations (by weight) subjected to a sequence of dynamic shaking events. To complement the model tests, a full-scale field test was conducted to compare the response of a liquefiable soil formation to a soil grouted with colloidal silica. Permeation grouting techniques and field procedures were developed in order to treat an approximately 1.5 m (5 ft) thick liquefiable soil layer. The centrifuge model tests and field test both show that colloidal silica soils reduce settlement, lateral spreading, and shear strains induced when subjected to large dynamic loads. For purposes of developing soil models, shear modulus degradation curves were developed and relationships that govern unloading-reloading behavior were identified in centrifuge model tests. Amplification in the acceleration response and increases in excess pore pressure ratios were determined to be direct indications of treatment levels. Large transient changes observed in pore pressure response were shown to describe the behavior of stress transmittal between the soil and gel during cyclic loading. Additionally, the hysteretic response of colloidal silica soils exhibited greater hysteretic damping and cyclic mobility consistent with dense sands. The response also revealed a lower degree of cyclic degradation for higher concentrations of colloidal silica.