## 1 Introduction
This is even the case with ultra-fine cement, where the mean grain size decreases to 3-5 \(\upmu\)m, which has been proposed to be used in pressure grouting in recent years. Even this cement is only appropriate for use in fine to medium sand. However, liquefaction is more likely to occur when the stratum is mainly composed of silty sand or fine sand [16,17], so there are some limitations when using these traditional materials to mitigate liquefaction risk. With the development of nanotechnology, nanomaterials have been proposed by many researchers to be introduced into ground treatment to improve mechanical properties and mitigate liquefaction risk [18, 19, 20, 21, 22, 23, 24, 25]. The concepts and inspirations of nanotechnology originate from a famous talk hold by [PERSON] entitled \"There's Plenty of Room at the Bottom\" at Caltech, in Pasadena, California, in December of 1959 [26]. Since then, a new field, exploring and manipulating nanoscale microscopic particle has begun to develop at a breathtaking pace. At present, the economic benefit created by nanomaterials and relevant products annually increases by 20%. Meanwhile, the cost of nanoproduction decreases significantly with the promoting effect of booming market development [15]. Under these circumstances, nanomaterials, such as carbon nanotubes, nanoalumina, colloidal silica, nanobentonite, laponite, and so on, have an expansive application space in civil engineering, especially as kinds of additives to improve the properties of cement, concrete and soils [27, 28, 29]. Among these advanced nanomaterials, the cheapest and most widely used one in soil treatment is colloidal silica (CS) [30,31]. CS is a kind of powerful material that can be used for passive site stabilization, which is a relatively new technique that has been proposed by [PERSON] (2002) as a nondisruptive treatment to mitigate liquefaction risk [32]. The passive site stabilization technique involves three critical procedures, namely, the injection of grouting materials at the upgradient edge of the site, the delivery of the stabilizing materials to the targeted location, and gelation into a rigid gel (Figure 1). The most critical point in passive site stabilization is the smooth delivery of the grouting materials, with the condition of low-gradient terrain. Here, it is required that the given material in use has a low initial viscosity and excellent permeability characteristics, which it needs to sustain for sufficient time such that the modifier could be transported to the desired location [32; 33]. Since CS has an extremely low initial viscosity of around 2 cP (where, as a comparison, pure water has an initial viscosity of 1 cP), after mixing with a salt solution, it can easily flow through the voids of liquefiable soil and permeate across the soil stratum, along with the underground water flow induced by the extraction wells [32]. Once CS reaches the target location, extraction is eliminated, waiting for the CS grouting gel to change into a firm solid. CS has been investigated by many scholars as a kind of grouting material for soil improvement and liquefaction mitigation. The mechanical behavior of the soil stabilized with CS has been evaluated in conventional geotechnical tests, bench-scale model tests and field tests [17; 18; 19; 32; 33; 34; 35; 36; 37]. It has been proven that CS can decrease the axial strain and restrain pore pressure development in the soil subjected to cyclic loading. Therefore, it is potential material to be adopted in ground treatment, especially for strata mainly composed of liquefiable soils. The paper reviews the main experimental studies and scientific findings in the field of the applied research of colloidal silica for soil improvement and liquefaction mitigation. The paper is organized as follows: The first section describes the physical, gelling, curing, and mechanical properties of CS. The effect of ionic strength and pH on the gelation process are both discussed in detail. The paper then focuses on the mechanism of CS transport.