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Hydraulic fracturing is a potential cause of leakage of earth dams or loss of fluid in drilling and field permeability testing. The effect of hydraulic fracturing on soil grouting is also a major concern. Although hydraulic fracturing has been adopted for decades by the petroleum industry for oil recovery in rock formations, studies on fracturing in soils are relatively few and inconclusive. The aim of this study is to provide further insight into the mechanism of hydrofracturing in soils through a field grouting trial and numerical simulation. We observe hydraulic fracturing in soils during this field trial as predicted by generally accepted groutability requirements. The hydraulic fractures are found vertically developed up to the ground surface. Numerical simulations show the hydraulic fracturing is easier to be initiated in anisotropic stress conditions, where the minor principal stress is the key factor. Numerical simulations also demonstrate significant compressions and shears during injection, suggesting the mechanism of fracturing in soils would be a shearing type. Based on this study, we propose a punching and splitting mode for the hydrofracturing in soils. The equation associated with estimating fracturing pressure is verified, and the results are found to be in good agreement with the cases examined.

Generally, grout extent and ground deformation are of great concern in soil grouting practice. In this paper, the ground behaviour, injection mechanism and grout distribution relating to field grouting in soils are discussed. It is found that the mechanisms of injection comply with generally accepted groutability requirements based on particle size ratio, where suspension grouts fracture clayey soils and solution grout permeates sandy soils. Ground heaving is affected by the sealing effect of grouts in upper soil layers. This study shows ground heave increases with the depth of grouting. The final heave volume reaches about 22% of the total grout injected. Lateral ground expansion is influenced by the hydraulic fracturing of grouts. Hydraulic fractures, which are nearly vertically formed in soils, produce lateral movements of the ground. The final lateral expansion of the ground, within 1 m radial zone of the grout hole, is approximately 12% of the total grout injected. A three-dimensional grout distribution is postulated based on the mapped grout traces in soils. The observed vertical grout fractures, with apertures of less than 15 mm and lateral extents of 1~4 m, confirm that the site soil strata are normally consolidated.

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