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
Figure 6 Compressive force (kN) and moment (kNm) in a post in the Geobrugg nets in
row II in Siglufjörður. Wiggles in the force curve near the beginning and end of the
record are due to differential heating of the post by the sun on clear days.
The measured tension in the upper anchors of the nets appears to be within the design
assumptions of the Swiss Guidelines (SLF, 1990). The moment load is, however, considerably
higher than assumed in the guidelines. The guidelines give a design moment load of
only 5.7 kNm when allowance has been made for the high density of the Icelandic snow. This
is less than one third of the measured maximum moment in Siglufjörður. The guidelines are
based on the assumption that the snow pressure on the post is given by the depth averaged
snow pressure on the construction applied over the width and length of the post (this is the
assumption η =1 in eq. (58) in the guidelines). In practice, the effective width of the post may
be expected to be substantially larger than this because the post will support more snow than
corresponds to its width.
4. CONCLUSIONS
The pilot experiment in Siglufjörður has provided many lessons for design of supporting
structures for Icelandic conditions after more than a decade of observations. The main conclusions
may be summarised as follows.
4.1 Snow properties
The gliding of the snow pack along the slope was found to be low, only several cm during the
winter. Reference values for snow density during maximum snowpack thickness
(400−450 kg m −3 ) and for spring loading with a higher density (500 kg m −3 ) were determined.
4.2 Loading
Measured loads on the structures were in general within the corresponding design loads of the
Swiss Guidelines from 1990, with the exception of the moment load on net posts which
turned out to be substantially larger than assumed. The maximum loading of the structures
148 Supporting structures under Icelandic conditions