As a result of actually applied technologies of excavation drifting and natural structure of strata the obtained contour is not regular.

This results in a concentration of stresses on the surface of the excavation and thus in its lower stability.

The dependence between tangential stress on the contour excavation and the amount and amplitude of irregularity, the ratio of semi-axis of the eliptical excavation and its depth has been determined.

The obtained dependences have been illustrated by means of diagrams and isochromatic patterns obtained from photo-elastic model investigations.

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