Question

(a) Schematically explain how size of a rock sample affects the strength of the rock in a triaxial experiment. (4.0 mark) (b) Using a schematic graph explain how different loading rates affect the elastic properties and strength of the rock. (4.0 mark)   

Answer 1

For the triaxial test we are taking a sample having size Lx D, length of specimen = L and diameter of specimen = D.

For the better result we have to kept the L/D ratio 1.5 to 2.5 as per international rock mechanics guidelines. Now the reason behind is if we kepth length is small and diameter is more means L/D <1.5 then rock will fail in comoression and if L/D >2.5 then rock will fail in buckling. Generally rock has high comoressive strength and it will fail in either in shear or in tension.

It can be conclude that triaxial test result depends on size of rock sample i.e. L/D ratio.

B

loading rate of 0.02 mm/sec for sandstone

Loading rate of 0.002 mm sec for sandstone

From above two graphs we can say as loading rate increases peak stregnth of failure will increases because when loading rate is very small then cracks has sufficient time to propogate and we get stress strain curve like as cyclic loading. When loading rate is high then cracks has not time to propogate and we get smooth curve of stress vs strain.

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