The combined effect of static and dynamic loads on design of underground openings is currently receiving much attention. It is known that the time-dependent behavior of rock is considerably different when a dynamic load is superimposed on prestressed rock. The shape of an underground opening has a definite influence on the magnitude of the local stresses and these may be significantly larger than those due to the weight of the overlying rock alone. Under these conditions, even a small dynamic load in the form of vibrations caused by blasting, drilling and related operations may increase the stress beyond a critical value. Dynamic tests in the laboratory are much more involved than static tests, and it is very useful to determine the relationships that exist between the static and dynamic creep.
This dissertation presents the results of an investigation of the creep behavior of mortar under simultaneous static and dynamic stress. Two types of dynamic loads were applied in the form of a cyclic stress of low frequency: (1) an excitation load and (2) a cyclic load linearly additive to the static load.
Based on the static creep behavior of mortar, a mathematical analysis for the dynamic loading conditions is presented. A comparison is made between the experimental results and the mathematical predictions. Finally a mathematical model is presented which best fits the dynamic creep behavior of mortar --Abstract, page ii
