International Symposium on Mitigative Measures against Snow ...

<strong>International</strong> <strong>Symposium</strong> on Mitigative Measures against Snow Avalanches
Egilsstaðir, Iceland, March 11–14, 2008
gliding and should be such that irrelevant variations with height above sea level and aspect of
the slope are not imposed.
In addition to the different conditions with regard to snow density and gliding, extreme snow
depths in many starting zones in Iceland may be expected to pose serious problems for
supporting structures under Icelandic conditions. As a consequence of frequent snow drift in
the windy Icelandic climate, snow depth in starting zones is often quite variable. The snow
preferentially accumulates in depressions and gullies, where vertical snow heights in excess of
6 m are common, even in average winters, whereas the snow depth on ridges and concave
parts of the starting zones remains low throughout the winter. One may expect that supporting
structures are impractical due to extreme snow depths in many important starting zones above
inhabited areas in Iceland due to this reason. This problem is not unique to Iceland, as similar
problems are also encountered in high altitude avalanche starting zones in Alpine countries.
In spite of these problems, it is clear that supporting structures are a viable avalanche
protection for several avalanche prone areas in Icelandic villages, especially where conditions
are unfavourable for other protection methods and where extreme snow depths in depressions
and gullies are not expected. The goal of the pilot experiment was to expose several types of
commercially available structures from Alpine countries to Icelandic conditions, measure key
quantities related to the loading of the structures, investigate corrosion of the structures under
the highly corrosive maritime conditions typical for Icelandic starting areas and test
traditional drilling and anchoring methods in Icelandic bedrock and loose materials, which are
of volcanic origin and have different properties compared with hillsides in typical Alpine
starting areas.
2. THE TEST AREA: LAYOUT AND GENERAL OBSERVATIONS
The supporting structures are located at 490−530 m a.s.l. in Grindagil in the mountain Hafnarfjall
west of the village of Siglufjörður. They have Dk in the range 3 to 5 m and are arranged
in four rows labelled I, II, III and IV from above (Figure 1, Table 1).
Table 1: Rows of supporting structures in Grindagil in Siglufjörður.
Row Type Producer Length
(m)
Number
of posts
Height
Dk (m)
Cost
(kIKR/m)
I and IV bridges J. Martin 110 38=24+14 3−5 161
II nets Geobrugg 50 14 3−4 156
III nets EI 41.5 15 3−5 158
The structures were installed during a five week period in the autumn of 1996 (Hopf, 1996).
A separate account was kept of the cost of the structures and installation work for each type of
structures. The average cost per m for each type of structures was similar as tabulated in the
last column in Table 1 in Icelandic kronas at the 1996 price level. All the supporting
structures were hot-dip galvanised, as is generally the rule for out-door steel structures in
Iceland. This led to a relative increase in the price of the steel bridges compared with Alpine
prices as steel bridges are typically not galvanised in Alpine countries.
144 Supporting structures under Icelandic conditions