A newly permeation grouting technique using a colloidal silica has been developed to prevent the liquefaction of sandy ground beneath existing structures. The aim of the present paper is to study the deformation and the strength characteristics of a treated sand with the colloidal silica. A series of laboratory tests are performed on the treated sand such as monotonic and cyclic torsional shear tests. In the case of cyclic torsional shear test with the treated sand, a relatively large strain is developed in the early stage of loading; however, both the development of shear strain and the decrease of mean effective stress resulted without any collapse and liquefaction. As the results, the remarkable improvement of cyclic shear strength by the colloidal silica treatment can be exhibited. This phenomenon leads to the expansion of dilation region across which it faced the phase transformation line and the failure line.
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