ABSTRACT
Numerous uniaxial tests with stepwise variation of the relative air humidity ϕ (%RH) of the surrounding air show that steady state creep rates are increased in average by a factor of 90 between ϕ = 0 and 75 %RH in a nonlinear way. A number of triaxial tests with stepwise variation of the confining pressure p show that steady state creep rates are increased by an average factor of about 30 at p = 0.1 MPa compared to high values of p. It is explained with moisture assisted microscopic recovery at areas with strain hardening at sites of loaded contacts around open grain boundaries and microcracks. This humidity induced creep is observed only in the dilatant stress domain where the salt is permeable for moisture and it is strongly dependent on the distance from the dilatancy boundary. Therefore, it is active only in the damaged zone around underground openings. A reliable empirical function F has been formulated on the basis of 255 creep test phases with constant conditions describing the acceleration of steady state creep rates as a function of ϕ , p and of the distance from the dilatancy boundary.
