135 * For correspondence. Journal of Environmental Protection and Ecology 20, No 1, 135–145 (2019) Risk assessment SOIL IMPROVEMENT TO COUNTER LIQUEFACTION USING COLLOIDAL SILICA GROUT INJECTION Y. MANAV a , S. TOPRAK b *, E. KARAKAPLAN a , M. INEL a a Department of Civil Engineering, Pamukkale University, Kinikli Campus, Denizli, Turkey b Civil Engineering Department, Gebze Technical University, 41 400 Gebze, Kocaeli, Turkey E-mail: stoprakss@gmail.com Abstract. Soil liquefaction due to earthquakes is a major reason of damage to buildings and other structures. This study deals with soil improvement against liquefaction by injection of a particular stabiliser, colloidal silica, which is nontoxic and stable. Laboratory experiments were performed to determine the effects of colloidal silica grout injection regarding soil strength and deformations. The experiments involved static and dynamic triaxial tests on untreated and treated soil samples. The variables used in the tests are the relative density (loose – 40%, medium – 60% and dense – 80%), the confining pressure (100 and 300 kPa), and the curing period of silica treated samples (7 and 28 days). The results clearly indicate the significant increase in strength of the soil with colloidal silica injection. Furthermore, the relative increase is the highest in the sand of the lowest relative density which is the most probable candidate for soil improvement. The observations that the increase in the strength of colloidal silica treated sands with curing time is gradual and continuous add to the advantage of this method for use in soil improvement works. By using the dynamic test results, the equivalent Young modulus (or shear modulus) and the hysteretic damping ratio of untreated and treated soils are compared. Keywords: colloidal silica, earthquake, soil improvement, liquefaction, triaxial tests. AIMS AND BACKGROUND Soils may not always have the required engineering properties suitable to support structures during their lifetime, especially under the effect of the natural hazards. For instance, a damaging phenomenon that can occur in the ground during an earthquake is liquefaction. Soil liquefaction causes significant effects on the ground surface and in structures. Damage by liquefaction has been proven by past earthquakes – especially those occurring near urban areas in recent years – such as Tokyo Bay area, Japan in 2011, Christchurch, New Zealand in 2011 and Kocaeli (Izmit), Turkey in 1999. These include the surface eruption of sand and water, large settlements, large amplitude ground movements, a reduction in the bearing capacity, damage to retaining walls, flows affecting houses, permanent horizontal