Dynamics of Rockslides and Rockfalls

In the wide field of scientific problems exists a marked difference between mainly academic questions and those bearing practical importance. It may be worthwhile, perhaps, to spend considerable work and money to find out that Mount Everest is two metres lower than assumed previously (and, of course, the value of precise measurements for the observation of tectonic processes is incontestable). But nobody except the involved scientists and their sponsors would consider it a catastrophe if a more accurate measurement would add another couple of centimetres to the actually accepted figure. On the other hand, the question of whether an expected rockslide will reach a certain village or not, involves, in the best case, emergency measures (like evacuation) and the loss of substantial material value, in the worst case the risk of inhabitants being killed.

It has already been pointed out (Sect. 1.2) that the purpose of this book differs from those of most studies in the field of gravitational mass movements. Therefore the most important chapters (3, 4, 5, and 6) deal in a physical, partly also a mathematical manner with various mechanisms, the lion’s share being devoted to the mechanisms of displacement (Chap. 5). This distribution of weights is clearly reflected by reduction of geomorphological information to the amount dictated by necessity. Nevertheless the fundamental importance of geomorphology remains untouched: every bit of physical reasoning must be derived from some geomorphological fact, and every mathematical equation must find its correlative in a geomorphologically expressed process.

In a book mainly dealing with the dynamic mechanisms of moving masses, the release, i.e. the initiation of motion, cannot be discussed in equal detail as motion proper. Nevertheless some comments on the subject, as presented in this chapter, will be useful for different reasons. The most important thereof are the following. In the present section, a general discussion of the conditions favouring release of large masses (Sect. 3.1) will pave the way for a quantitative treatment of various impending problems. Some light will be thrown upon the mechanisms connected with the breakdown of cohesion and pre-release creeping; thereby methods hitherto unusual in connection with rockslides and rockfalls will be presented (Sect. 3.2). Eventually details of several particular mechanisms will illustrate applied quantification in cases ranging from everyday occurrences to exotica in the true sense (Sect. 3.3).

Before being released, a mass may have assumed any possible state of coherence ranging from one single giant block to a heap of comparatively small particles. By contrast, in coming to a rest after displacement, the same mass is, as a rule, substantially more disintegrated. So disintegration obviously takes place in the course of the downhill ride. As, on the one hand, the discussion of pre-event disintegration does not fit into the frame of the present book, while, on the other hand, the degree of coherence (or of disintegration, if looking at things from the other end) can substantially influence the mechanisms of displacement, disintegration might with good reason be attributed to Chap. 5 where these mechanisms are treated. However, there is no doubt that the conversion of a more or less coherent large block into debris marks an important caesura in the descent of a rocky mass so that it cannot be insignificant where the process of disintegration is initiated, how it develops in the course of motion, and what are its consequences. Thus it is equally justified to reserve a short chapter of its own right to the questions of disintegration. In deciding to act according to this line of thought, two further facts were borne in mind: even so Chap. 5 remains one of the most extended, and — a more substantial argument — in the sections to follow it will be demonstrated that the fundamental differences in reach between coherent and disintegrated motion are not as dramatic as could be assumed at first sight. Continuity between the questions of disintegration and displacement will be established in the first section of Chap. 5 which will deal with the influence of disintegration upon displacement.

Before studying in more detail the role of energy dissipation in mechanisms like Coulumbian friction or lubrication, it is useful to look at coherent and disintegrated motion from a more general point of view, aiming at energetic differences depending on nothing but the degree of cohesion, irrespective of the mechanisms acting in a particular case. In doing so, inevitably three fundamental problems are encountered, and each of them can be formulated as a question.

So far, this book essentially bears an analytical character. It deals with mechanisms which, when considered individually, readily can be used as a more or less quantitative raw material for analytical statements. Their use in forecasting, however, besides the necessity to get at the working parameters, depends on the possibility of inserting them into a coherent line of thought intended to synthesise an event as a whole. Such a logical backbone is required from the moment when a mass is released somewhere high in the mountains to its final standstill (and possibly that of other masses mobilised by it) in the depth of a valley or a plain.

As an agent in the mechanisms governing the dynamics of descending rocky masses, water has been mentioned more than once in this book. And the roles it had to play in various contexts reach from the “anti-lubricant” postulated by Terzaghi (1960, 91) to Abele’s highly efficient pressurised lubricant (1997b, 2), both backed up by extensive field evidence. The role of water as the very destruction-bearing mass, however, though stressed for the catastrophes of Val Pola (Sect. 2.5) and Vaiont (Sect. 2.6), has, so far, not been discussed with respect to its physical background.