Issue 30

E.T. Bowman, Frattura ed Integrità Strutturale, 30 (2014) 7-13; DOI: 10.3221/IGF-ESIS.30.02 8 F IELD OBSERVATIONS ield observations may help us to understand what additional processes are at work during rock avalanche propagation and arrest, beyond sliding, rolling, shearing and frictional dissipation, as observed for small scale experiments. Common observations of large rock avalanches are that: they attain high velocities (e.g. 75 m/s average was determined for the Huarscaran rock avalanche in Peru, 1970, with some boulders flung out at up to 280 m/s [6]); the deposits are very thin (e.g. averaging 5 m thick at Elm, Switzerland [7] in 1881; Fig. 1, left); there is a lack of sorting and mixing of debris, with geological layers being preserved intact; and, there is an extremely high degree of fragmentation of the rock within the deposit typically below a surface shell or “carapace” of intact blocks [5]. Recently, the spreading efficiency of rock avalanches has been found to correlate positively with the degree of fragmentation of the deposit [4] – i.e. the change in grain size distribution from the commencement to arrest – indicating that high mobility is linked to dynamic rock fracture. Figure 1 : Left: Mt Haast rock avalanche (also known as Mt Dixon rock avalanche) that occurred on 21 st January 2013 in the southern Alps of New Zealand (details in Hancox & Thomson, 2013 [8]). Right: Chalk cliffs of south Kent. Collapse deposits are generally rapidly eroded by wave action but leave characteristic rock shelf extensions. Photos: Author. The large size of high mobility rock avalanches is, in itself, interesting to note. Below 0.01 million m 3 , and potential energy of 10 14 J, rock falls, characterised by bouncing, rolling and breaking blocks, are common but long runout behaviour is almost unknown. The exception to this appears to be the long runout collapses of chalk cliffs that occur in parts of Europe (Fig. 1, right). Such collapses can behave very much like large rock avalanches, displaying low to high spreading efficiency (L/V 1/3 from 0.5 – 7) [9] at much smaller volumes (10 3 m 3 - 10 6 m 3 ) and with much lower initial potential energy (up to 10 10 J). As discussed by Bowman and Take (2014) [10], the reasons for the similarities with rock avalanches are likely to be due to the low strength of the chalk in comparison with more typical rocks, with weak chalk producing the greatest spreading efficiency. These observations point to two processes: a high degree of particle fragmentation via communition, and a predominance of collisional stress transfer between closely spaced (or even touching) fractured particles. This paper examines how the dynamic fragmentation of rock during avalanche propagation may lead to enhanced mobility. The paper focuses on comparisons between events involving two different rock types – i.e. limestone, which is a common source rock in long F