This study investigated the influence of Micro-Silica (MS) on the compressibility of poorly graded sand, with the aim of determining coefficient of consolidation ( \({c}_{v}\) ). Tests were conducted using different MS contents:0%, 1%, 3%, and 5% under various saturation conditions: Sr = 1, 0.9, 0.7 and 0.3. The results showed, as expected for untreated specimens, that a higher normal stress value led to higher immediate settlement ( \({\text{S}}_{\text{e}}\) ) for Sr = 1; however, the inclusion of MS as an additive significantly reduced \({\text{S}}_{\text{e}}\) in both unsaturated and, unexpectedly, saturated states. For example, at Sr = 1 and σ = 200 kPa, the addition of 5% MS resulted in a 62.5% reduction in \({\text{S}}_{\text{e}}\) which was not in line with the case of Sr = 1 and MS = 0%. The sole effect of Sr was also found to be significant. A 7.79-fold reduction in \({\text{S}}_{\text{e}}\) was obtained between Sr = 1 and 0.7 at MS = 0% for σ = 200 kPa. More significantly, the combined effect of MS and Sr resulted in a 92.2% reduction in \({\text{S}}_{\text{e}}\) , 12.8-fold maximum. The optimal MS% depended on several factors such as particle size, suction stress and soil fabric. Experimental \({c}_{v}\) values were obtained using the Taylor (1942) method, then compared with \({c}_{v}\) values obtained from a reformulated equation that took the slope of soil water retention curve and the unsaturated coefficient of permeability into consideration. The unsaturated coefficient of permeability was obtained using different equations, which took into account suction, effective degree of saturation and residual degree of saturation. The comparison showed reasonable agreement between the experimental results and some of the equations used.
