Desiccation behaviour of colloidal silica grouted sand: A new material for the creation of near surface hydraulic barriers

This paper considers the mechanism of cracking in colloidal silica (CS) grout subjected to drying and wetting cycles, with the aim of testing its use for the creation of near-surface low hydraulic conductivity barriers for applications in nuclear decommissioning. The advantage in this context of CS over more traditional materials, such as clay liners or geotextiles, are its capability to permeate into in situ soils at low injection pressures and injectability within and beneath regions of existing contamination, thus reducing cost and removing any requirement for excavation and disposal of hazardous materials. In near-surface applications, hydraulic barriers are exposed to natural climatic variations, in the form of cycles of drying and wetting, which can result in cracking of the barrier material and a subsequent increase in the hydraulic conductivity. Ultimately, this reduces their ability to perform their end function. The aims of this paper are to study the mechanism of crack formation in colloidal silica grout barriers when exposed to severe drying and wetting cycles and to determine the effect on its hydraulic properties. To achieve these aims, grouted soil samples were created and exposed to severe drying and re-wetting. Samples were tested for hydraulic conductivity at each stage and 3D images of the pore structure were obtained from micro X-ray CT scanning. On drying, nanoscale cracks form within the CS matrix, which are 10s of nanometres in width, these have an associated air- entry value of ~20,000 kPa. Additional meso-scale cracks can also form in CS filled pores when surrounded by sand grains, due to conditions of restrained shrinkage. These cracks are typically hundreds of microns in width and have an associated air-entry value of ~200 kPa. X-ray CT analysis of the connectivity of this meso-scale pore space, filled by air after drying, indicates that although cracks form, a connected network does not, thus explaining the observation that even after severe drying the CS grouted sand retains a very low hydraulic conductivity. •The CS grouted sand exhibited a saturated hydraulic conductivity of 5·10−10 m/s.•Oven dried CS grouted sand showed a saturated hydraulic conductivity of 5·10−8 m/s.•Although cracks form in grouted sand upon drying, a connected network does not.•Nanoscale cracks with an associated AEV of 20 MPa form within CS upon drying.•Disconnected mesoscale cracks with an AEV of 200 kPa form within CS upon drying.