ENGINEERING PROPERTIES OF YIELDING ROCK.

Rock masses may exhibit long term creep when under stress or subjected to change of stress; in other cases mass movement may occur due to high shear stress at critical surfaces within the mass. The geological factors which give rise to these effects are described. Case histories are quoted which indicate the effects on engineering works and, particularly, the time scale of these movements which, according to circumstances, may have continued for several weeks - or for several years - or for several thousand years. The scope of site investigations necessary in such cases is described; the conclusions of the paper put the case for further research.