Assessment of Rock Fracture Jacking During Grouting: Evaluation of an Analytical Solution Through Cases from Scandinavian Tunneling Projects-Paper

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The paper evaluates an analytical method for selecting grouting pressures in fractured rock tunnels while balancing grout spread, fracture dilation, and material use. The method estimates fracture opening and grout spread, then uses Deformation Limiting Curves (DLCs) to set a maximum acceptable dilation. Grouting can begin above in-situ stress to improve grout penetration, then pressure is reduced along the DLC to achieve the required spread without further excessive opening.<br /><br />Two cases from Sweden’s Haga tunnel illustrate the approach. In Case 1, a small pressure increase above the interpreted in-situ stress raised the estimated grout spread from about 4 m to 9 m over six minutes. Fracture dilation was modest (19 μm), consuming only about 1.5% of injected grout, suggesting higher pressure could have shortened grouting with little material penalty.<br /><br />In Case 2, low initial penetrability prompted a larger pressure increase. This improved spread, but opened the fracture beyond its critical aperture: estimated dilation reached 67 μm, and the resulting excess void consumed about 30% of the injected grout. Limiting dilation to the critical aperture would have reduced spread, but still improved it relative to grouting without dilation.<br /><br />The study concludes that controlled fracture jacking can improve penetrability and reduce grouting time, whereas excessive pressure wastes grout and may damage previously treated zones. Analytical pressure control and suitable stop criteria can help optimize performance. The analysis assumes grout flows through a single smooth fracture and does not account for fracture interaction.

Keywords

grouting pressure

fractured rock tunnels

grout spread

fracture dilation

Deformation Limiting Curves

in-situ stress

fracture jacking

penetrability

Haga tunnel

analytical method