Hydrojacking of rock occurs when high pressure from a water conduit seeps into the cracks in the surrounding rock, causing those cracks to propagate (grow in width and length), actually jacking up the ground above. The consequences are serious: water typically floods into the underground facilities and the water reaching the turbines has markedly reduced discharge and head. The remediation – retrofitting the affected tunnel with a steel liner or Carpi (reinforced PVC liner) – is costly and time-consuming.
Concrete tunnel liners are porous and do not stop hydrofracture unless they are cast on top of a waterproof liner.
Hydrojacking occurs when the internal water pressure in the water conveyance in the rock around a water tunnel or shaft exceeds the minor principal stress in the rock. This can be understood in simple terms as follows:
Imagine there is a concrete-lined water tunnel like that shown below in cross section:
As long as the downward pressure from the rock, Prock, exceeds the uplift caused by the high-pressure water, Pwater, the small crack shown on the right side of the tunnel will not grow. This means:
Prock = Density of rock * height of rock above (𝛄r*h) must be greater than
Pwater = Density of water * hydraulic head (𝛄w*hw).
The factor of safety against hydrojacking is the ratio of these pressures:
FoS = (𝛄r*h) / (𝛄w*hw) … (Equation 1)
FoS is typically chosen as 1.25 to 1.5 in design.
In 1984, the Norwegian engineer, E. Broch published a technical paper that provides an adjustment for the slope of the rock surface. Consider a reservoir feeding a water conveyance that slopes down beneath a sloping ground surface as shown in the figure:
Broch found that the effective rock cover above a given point in the water conveyance can be closely approximated by the rock cover multiplied by the square of the cosine of the surface slope, i.e.:
h * cos² β
This effective rock cover height is used in place of h in equation 1.
Hydrofracturing (fracking) is different from hydrojacking; it occurs in massive (unjointed) rock when high-pressure slurry is injected into an oil well to recover shale oil or gas. In this case, the slurry pressure must exceed both the confining rock pressure and the tensile capacity of the rock.