NEW CONSIDERATIONS ON THE VAIONT SLIDE

THE PAPER TRIES TO GIVE A SYNOPSIS OF ALL OPINIONS CONCERNING THE ROCK SLIDE OF MOUNT TOC PUBLISHED SINCE THE CATASTROPHE. AFTER OUTLINING THE DOCUMENTS AVAILABLE ON THE SLIDE, THE DATA WHICH ARE IMPORTANT FROM A GEOMECHANICAL POINT OF VIEW ARE DERIVED FROM THE GEOLOGICAL SITUATION CLEARED UP IN THESE DOCUMENTS. PARTICULAR ATTENTION IS GIVEN TO THE SHAPE AND CONDITION OF THE SLIP SURFACE, AND IT IS CONCLUDED THAT IN THE DECISIVE AREAS THE SLIP SURFACE HAD A CHAIR-LIKE SHAPE, AND THAT IT WAS ESSENTIALLY HORIZONTAL IN THE LOWER WESTERN PART. CAREFUL INVESTIGATIONS HAVE PROVEN THAT, CONTRARY TO SEVERAL PUBLICATIONS, NO CLAY EXISTED ON THE SLIP SURFACE. THE PRACTICE USED SO FAR OF MAKING A-POSTERIORI CALCULATIONS OF THE SLIDE ON THE BASIS OF STATIC CONSIDERATIONS IS CRITICALLY EXAMINED. SEVERAL FACTORS OF EQUILIBRIUM ARE NOT TAKEN INTO ACCOUNT IN THESE CALCULATIONS UP TO NOW, SUCH AS THE STIFFNESS OF THE ROCK LAYERS, THE FORMATION OF SECONDARY FAILURES AS WELL AS THE SHEAR STRENGTH PRIOR TO SHEARING THE AREAS OF THE LATER SLIP SURFACE AND THE EXACT SHAPE OF THE SLIP SURFACE. THESE FACTORS HAVE BEEN CONSIDERED IN A DETAILED ELECTRONIC RECOMPUTATION. THE COMPUTATION SHOWS THAT THE FRICTION VALUES ON THE SAME PLANE NECESSARY TO MAINTAIN THE LIMIT EQUILIBRIUM IMMEDIATELY BEFORE THE ONSET OF THE MOVEMENT ARE RIDICULOUSLY SMALL. IT IS ALSO INDICATED THAT THE VALIDITY OF STATIC RECALCULATIONS IN THIS CASE IS OPEN TO QUESTION, ESPECIALLY SINCE SUCH CALCULATIONS CANNOT EXPLAIN DIFFERENT PHENOMENA WHICH OCCURRED DURING THE VARIATION OF THE RESERVOIR LEVEL. THEREFORE, AN ATTEMPT HAS BEEN MADE TO CONSIDER THE SLIDE AS A CREEP PHENOMENON. RECENT INVESTIGATIONS SUGGEST THAT THE FINAL HIGH SLIDE VELOCITY MUST BE ATTRIBUTED TO THE ACTION OF WATER THRUST IN THE JOINTS RESULTING IN A SPONTANEOUS LOSS OF FRICTION. THE PRESENT KNOWLEDGE LEADS TO THE CONCLUSION THAT "MASS THIXOTROPY" MAY HAVE PLAYED ONLY A MINOR ROLE IN THE CASE OF MOUNT TOC, BUT IT IS MAINTAINED THAT MASS THIXOTROPY OCCURS FREQUENTLY IN JOINTED MEDIA. /AUTHOR/