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
momentum and the assumption of a stationary flow (Salm and Sommerhalder 1964). In the
revised guideline the distribution of the deviation force was optimized (Fig. 5). The deviation
force qnU acts maximally over a distance of 6 times the flow height of an avalanche. Close to
the deviation point, meaning up to a distance of 1.5 times the flow height of an avalanche, the
deviation force becomes 4 times bigger.
Table 3 Formulas to calculate the actions on a snow shed (ASTRA/SBB. 2007)
Action Normal to the ground surface Parallel to the ground surface
Natural snow cover = γ ⋅ d ⋅ cosβ
= q ⋅ tan
Avalanche deposit β
Sliding avalanche β
Deviation force
(Fig. 4) g
qnS S
qpS nS β
(kN/m 2 )
qnA = γ ⋅ dA
⋅ cos
qpA = qnA
⋅ tanβ
(kN/m 2 )
qnL = γ ⋅ dL
⋅ cos
qpL = μ ⋅ qnL
(kN/m 2 )
2
γ ⋅ dL
⋅ vL
⋅ sinα
qnU
= ; g = 9.81 m/s
6 ⋅ dL
⋅
2 qpU = μ ⋅ qnU
(kN/m 2 )
3.5 Practical experience
The guidelines, which were applied since 1994, are considered by the practitioners to be very
useful. Especially the concept of the interface position allows a clear separation between the
competence of an avalanche expert and a design engineer. The more detailed specification of
the friction value μ and the adapted formula for the deviation force in the new guidelines
enable to calculate the actions more accurately. The experience shows that in a lot of locations
avalanche deposits are the determining loads. However the assessment of the height of
avalanche deposit is difficult because it is hardly possible to calculate it. Typical results for
design loads on snow sheds from real cases are compiled in Table 4.
Table 4 Typical loads on snow sheds (total loads due to avalanches and snow deposit).
Results from SLF consulting reports.
Location Site characteristics,
observations
Val Chasté,
Tschlin
Taverna,
Davos
Camp,
Vals
Seehorn,
Davos
Val Ota,
Susch
Cozz,
Mesocco
Lant,
Mesocco
Large avalanche, gully, return
period 2 y., deposit height 7
Load Variable action (30 y.) Accidental action (300 y.)
case
Normal qn Parallel qp Normal qn Parallel qp
3 43 kN/m 2 7 kN/m 2 73 kN/m 2 12 kN/m 2
m, no deflection. 4 50 kN/m 2 7 kN/m 2 74 kN/m 2 11 kN/m 2
3 38 kN/m 2 9 kN/m 2 60 kN/m 2 15 kN/m 2
Multiple avalanche events
from both valley sides, no
deflection. 4 54 kN/m 2 12 kN/m 2 88 kN/m 2 20 kN/m 2
2 11 kN/m 2 2 kN/m 2 23 kN/m 2 6 kN/m 2
No multiple avalanche
events, unconfined flow, 15°
deflection. 4 10 kN/m 2 2 kN/m 2 15 kN/m 2 3 kN/m 2
2 11 kN/m 2 2 kN/m 2 15 kN/m 2 3 kN/m 2
No multiple avalanche
events, unconfined flow, no
deflection. 4 20 kN/m 2 4 kN/m 2 27 kN/m 2 5 kN/m 2
2 24 kN/m 2 5 kN/m 2 36 kN/m 2 8 kN/m 2
Small avalanche, steep track,
deflection 20°,
avalanche deposit.
small
4 15 kN/m 2 4 kN/m 2 24 kN/m 2 7 kN/m 2
2 25 kN/m 2 3 kN/m 2 36 kN/m 2 5 kN/m 2
Small avalanche, deflection
20°, return period 10 y.
4 30 kN/m 2 3 kN/m 2 40 kN/m 2 4 kN/m 2
2 51 kN/m 2 7 kN/m 2 74 kN/m 2 9 kN/m 2
Large avalanche, multiple
avalanche events, canalized
flow. 4 50 kN/m 2 4 kN/m 2 80 kN/m 2 7 kN/m 2
36 New findings on the design of snow sheds