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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.

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