VIII. SOME CLASSIC EXAMPLES
Photographs of avalanche defences
Photo No. 86
Staggered wall-dams designed to protect the entrance of a tunnel. The road blocked by an
avalanche is now protected by a tunnel.
Photo No. 87
Two wall-dams,slightly staggered, protecting part of a village. Between them a railway
line used only in summer wends its way. Notice the very slight angle between the avalanche
gulley and the walls. The church was the first building to be protected by a stone wedge.
The walls are a more modern addition to the defences and stop the avalanche from pouring
out in the direction of the village. A third stage of defences consisting of snow
stabilization in the upper part of the starting zone has been found to be necessary since
other dangerous gulleys also threaten the village.
Photo No. 88
A deviating wall-dam has protected this village perfectly. Note how the avalanche has
poured out in a fan shape. The avalanche was c. 8 m. thick in the area of its arrest.
Photo No. 89
Braking structure at the opening of a deep valley. The sections made of reinforced
concrete constitute imposing masses which are further weighed down by earth covered with
grass. By way of precaution, the two buildings below are individually protected.
Mixed defences. Functions simultaneously to combat erosion, channel torrents and
contain avalanches (Photo provided by Professor Cadenas)
Photo No. 91
Area of combined defences. Braking and retaining device in the ravine; walled terraces
on the left of the photograph. (Photo provided b Professor Cadenas)
Photo No. 92
Structure designed to contain an avalanche. It consists of a very simple earth barrier
which Completes an area of defences. In this particular case, natural braking structures
exist above this in the form Of large blocks of limestone. Given the permeability of the
subsoil (a karstic area) there is no problem over getting rid of water.
Photo No. 93
Photo No. 94
93 and 94: Tunnel through an alluvial cone used in winter by avalanches. Notice how the
flow Of the avalanche spreads out because of the change in gradient at the mouth of the
gulley and at the double deviating barrier. Because of the convexity of the ground it was
necessary to raise the height of the outer wall barrier with a metal structure.
Photo No. 95
Road tunnel below the tunnel in the photograph above. The same phenomenon has occurred
here and the avalanche has overflowed. It was later necessary to create a channel of earth
banks in order to control the flow of snow.
Photo No. 96
Detail of an old splitting-wedge behind a mountain pasture chalet. This structure,
which is slightly smaller than the building protected, does not provide adequate
shelter for the top of the building should there be a big avalanche. Furthermore,
notice how the roof is not joined to the wedge.
Photo No. 97
Modern wedge. A concrete wall reinforced with a former slightly higher than the building
and extending at the sides affords better protection to the sides of this house; the upper
side of the wall has been banked up. The roof of the building is also firmly joined to the
wall.
Photo No. 98
Avalanche snow bridge reinforced on its exposed side by a separating wall (Photo W.
Schwarz)
Photo No. 99
Effect of the separating wall (Photo W. Schwarz)
Photo No. 100
Braking structures with containing dam (Photo W. Schwarz)
Photo No. 101
Deviating wall which protects an area of snow bridges (Photo W. Schwarz)
Photo No. 102
Old area of banked-up walls with a modern extension high up on the left. This
extension was built with a discontinuous layout of steel snow bridges. These seem
to be too large since the area is covered with snow only with a southerly wind
and cleared progressively by snowfalls with westerly winds.
Photo No. 103
Avalanche defences must encompass a whole area of ground and rest their sides against
natural spurs.
Photo No. 104
Examples of snow bridges with either metal or prestressed concrete frames and
superstructures of squared timber.
Photo No. 105
Snow rake in winter. The beams hold all the strata of snow.
Photo No. 106
Photo No. 107
Photo No. 108
Photo No. 109
106, 107, 108 and 109:
Details of anchorage of a steel snow bridge to rock
We can distinguish the anchoring rod, the upper and lower joints which are sealed into
a lightly reinforced concrete base. Note the system for altering the length of the
supporting pillar.
In wooded areas one must never forget that avalanche defence works also have the aim of
allowing reforestation, which is the only real way of completing the defence.
Photo No. 110
In practice avalanches are often released below the lowest structures. Here old walls have
been supplemented by several rows of bridges. New forest protrudes slowly above the snow
cover, and it will take much more time before one has a protecting forest. The upper
structures will have to be kept permanently.
Photo No. 111
Example of a controlled area after 40 years. Even if the forest can theoretically reach
the ridge, one must in practice rely on the permanent structures in the upper part of the
slope. The advanced state of the forest shows the success of the reforestation lower down
the slope with the exception of the tracks left by a few avalanches which originated below
the lowest structures. The middle part is not yet wooded, for unlike the upper part it
does not enjoy the direct protection of defence structures.
Photo No. 112
Layout of a large area of wooden wind baffles in an area where it snows with a
variety of prevailing winds. Since they are made with planks with gaps between
them they are still well clear of the snow cover.
Photo No. 113
The remarkable efficacy of a wind baffle. Before it was placed there, snow drifts used to
build up and cover the first structures on the left of the photograph. An avalanche
released from this place caused considerable material damage in the valley below.
Photo No. 114
Detail of the deposition of snow produced by a wind baffle with vertical planks with very
little space between them
Photo No. 115
Detail of the deposition of snow caused by a wooden wind baffle with no gaps between the
planks
Photo No. 116
Wind baffle made of light metal to combat snow drifts
Photo No. 117
Light gallery, made of wood, to combat snow drifts
Photo No. 118
Light gallery made of metal to combat snow drifts, here combined with wind baffles
Photo No. 119
Effect on the snow of a complex of eddy panels
Photo No. 120
Use of eddy panels to unburden a complex of snow bridges (Photo Dr. Labelled )
Photo No. 121
Construction details of a wind baffle: post and steel raking shore. The planks are
suspended on loosely fixed cable.
Photo No. 122
Effects on the snow of a complex of jet-roofs (Photo provided by Dr. Benini)
Examples of damage caused by poor foundations:
Photo No. 123
The quality of the soil has been over-estimated because of an insufficient number
of compression tests. Had the foundation bases been joined by a tie-bar the structure
would probably have been more resistant.
Photo No. 124
Photo No. 125
Photo No. 126
124, 125 and 126: It is possible that in some places local stratification of the rock
is not suitable, or that the layers of rock have weak points; here the anchorage of a
series of bridges slackened on two occasions; first the prestressed concrete structures
were replaced by steel ones; eventually prefabricated foundations had to be sunk into the
rock.
Photo No. 127
Example of damage caused by bad workmanship. The anchoring rod of this net defence was not
sealed at a depth of one metre. Instead, someone made do with plating the linking line of
the anchor (20-30 em.) into a small base sunk into the rock. The base was torn out and,
behaving like a projectile, broke the supporting pillar below it cleanly. The other
anchorages, which were correctly placed, held.
Photo No. 128
Stays made of cable are not recommended when they are stretched in snow. It is much better
to use raking shores. Here the cable has not broken but the wood has given way.
Examples of damage caused by falling stones:
Photo No. 129
Prestressed concrete bridges are very susceptible to impacts.
Photo No. 130
Structures made of light metal are equally susceptible to knocks. In this case the alloy
used bent without breaking. They are metal sheets which have been pressed to give them an
appropriate static profile.
Photo No. 131
Here the alloy is more brittle and the stone falling from high up behaved like a veritable
projectile.
Photo No. 132
132 and 133: Examples of structures ill-adapted to the local pressure of snow. The
glide factor or the height of snow has simply been underestimated. In some cases such
damage is caused by erroneous spacing factor.
Photo No. 133
Photo No. 134
Deformation due to overloading on the exposed side.
Photo No. 135
Partial collapse of an old terrace. When such a structure within an area of defences
gives way, not only does it have to be repaired, but often all the modern structures below
it have to be replaced!
Photo No. 136
Only a clear separation between forest and grazing ground allows one to reconstitute
mountain forests: natural growth on the right of the fence.
