Determination of rockfall design blocks in Upper Triassic limestones and dolomites (Dachstein Formation, Northern Calcareous Alps)

Abstract

Design block size is of vital importance for rockfall countermeasure design. According to the Austrian normative document ONR 24810, the design block is specified as fractile (V95-V98) of the block size distribution depending on rockfall frequency. For rock formations, which form very large rockfall blocks (> 10 m3), the use of fractiles V95–V98 appears too high with regard to economically justifiable protective measures. The paper deals with the determination of design blocks for the Dachstein Formation on the basis of data from three Austrian test regions. The formation’s bedded limestone represents a key rock of the eastern part of the Northern Calcareous Alps and is very common along some major transport routes. A range of design blocks is derived from a synopsis of block size distributions, magnitude-frequency relations, and considerations about the rockfall barriers’ service life. In the process, the knowledge of the return period of the determinated design block is considered more important than the overall rockfall frequency. Analyzing available rockfall event data and block size distributions, it is possible to identify a return period of 21–26 years for a rockfall event ≥ 1 m3/1000 m route section. Thus, such events are rare. Events that affect the V97–V100 fractiles of the block size distribution are even rarer; they yield very large design block volumes, which show return periods > 100 years. For the bedded Dachstein limestone of the test areas, the application of the fractiles V95–V96 (0.15–2.25 m3) is useful for adapting the return period of the design block to the working life of the rock fall barrier (25–50 years).