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Grouting in vertical fissures in rock

Grouting in vertical fissures in rock

Isn't the resultant fluid force on a vertical boundary limited by the volume of fluid available to exert the force?

I am going to use an example of a fissure in rock but the same could be said of a retaining wall with a finite amount of flowable material behind it, for example a wall cut into rock with a narrow zone of structural backfill behind the wall face.

Let's say one has a vertical fissure in impermeable, unsaturated rock and one wants to grout the fissure to stabilize the column of rock between the fissure and a nearby free surface. While the grout is still fluid, the fissure walls will theoretically be subjected to a hydrostatic pressure per unit of wall length equal to 0.5*(height^2)*fluid unit weight. For example let's say (for simplicity) the grout has the density of water and the height is 10 ft; then the theoretical horizontal fluid pressure on the fissure walls is 0.5*62.4*100 = 3120 lb/LF of fissure. Now let's say the fissure is only 1 inch wide. It contains only 1/12*10*62.4 = 52 lb/LF of fluid. As I said the fissure contains fluid, but the walls are dry and impermeable, so there is no hydraulic communication between the fluid in the fissure and any other fluid level.

I see these analyses all the time where the hydrostatic pressure is used, even though it exceeds the weight of the water available to exert it?

Am I missing something here?

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