## 6 Summary and Conclusions  
Use of amorphous silica in modification of loose sands has received some attention. Although, limited understanding of the complex yield and post-yield responses of sand-NS mixtures continue to be barriers to more widespread application of the technique. The behavior of such binary mixtures is dependent on the base sand's particle size and shape and initial packing state, the fines content and its particle size and shape properties, as well as the conditions (e.g., confinement level and silica precipitation rate) under which the fines are placed into the skeletal structure of the base sand. The fines content proportion must be limited so that the sand skeleton maintains overall control of the behavior of the binary mixture. The presented bench-scale experimental work has added to the body of knowledge through a program of monotonic consolidated-undrained triaxial compression testing on medium-dense sub-angular fine sand in base form and after treatment with 1-15% NS solutions. With the main emphasis on post-yield response of the test materials, the findings from this work inform practitioners with an interest in using colloidal NS grout in remediation of distressed sandy earth systems/structures. Key findings based on the particular F161 sand investigated are:  
* The specimen preparation method employed successfully placed the NS grout into saturated metastable sand with marginal impact on the void ratio. As such, this demonstrated at bench scale the potential for proppant-less groundwork interventions. * Isotropic consolidation for \(p^{\prime}{}_{\text{c}}\leq 200\) kPa produced limited sensitivity of void ratio to NS content, with minimum impact occurring for 200 kPa. That is, the interparticle forces mobilized under isotropic consolidation reached their maximum possible level for \(p^{\prime}{}_{\text{c}}=200\) kPa. * Maximum failure and residual shear strengths mobilized for the NS-treated sand also occurred for \(p^{\prime}{}_{\text{c}}=200\) kPa. Both strength parameters increased in magnitude with increasing NS content, largely on account of resulting greater apparent cohesion. NS-treated sand behaved stiffer, underwent reduced strain softening and experienced a predominantly dilative response. The increase in residual shear strength was gentle for NS contents of up to 10% and more pronounced as NS increased to 15%. The mixtures' responses to undrained loading, and as such, are dependent on both confinement level and NS content, due to peculiar micro-mechanical events occurring at particle level. * Compared to silica silt, \(\phi^{\prime}{}_{\text{cs}}\) is sensitive to NS content (direct proportionality relationship). That is, the smaller-sized NS inclusions produced substantially higher sensitivity of \(\phi^{\prime}{}_{\text{cs}}\) to particle shape. Greater \(\phi^{\prime}{}_{\text{cs}}\) values are possible when adjustments are made for rapid precipitation of NS in sand. Quick precipitation produces rounder, more spherical-shaped NS flocs resulting in higher \(\phi^{\prime}{}_{\text{cs}}\) values. * Likely benefits of colloidal NS addition to metastable sand deposits include substantial preservation of soil drainage function for flood attenuation and in revitalizing soil life to balance their composition, thereby providing a much-improved ecosystem for biodiversity and opening up areas for green spaces where there may have been a lack of flora and fauna. This leads to improved wellbeing for all. Full-scale trails are required for field validation of the proposed approach. Conceptualization, S.G., A.A.-L., C.H., H.B. ; data curation, S.G., T.G. ; formal analysis, S.G.; funding acquisition, S.G., A.A.-L., C.H. ; supervision, A.A.-L., C.H., H.B. ; validation, S.G., B.C.O. ; writing--original draft, S.G., A.A.-L., C.H., B.C.O. ; writing--review and editing, S.G., A.A.-L., C.H., H.B., B.C.O. All authors have read and agreed to the published version of the manuscript.