increase in snow pressure with slope distance be-
detailed engineering primer for constructing ava-
tween the two containing walls because of the
lanche control structures and were first issued in
assumption that the downslope retaining struc-
1955. They have been revised when warranted by
ture supported the entire weight of the snow block
new findings.
less the basal frictional restraint (i.e., no frictional
The problem of determining creep pressures
resistance along the side walls) (Salm 1977).
on avalanche defense structures has attracted sig-
Snow deforms by viscous creep under the ac-
nificant attention beyond the Swiss effort to es-
tion of gravitational forces to increase the pres-
tablish engineering guidelines. Ziegler (1963, 1975)
sure acting on the avalanche defense structure.
applied plasticity theory to determine the pres-
The contributing factors to this pressure are the
sures acting on avalanche defense structures and
static load, the creep motion of snow particles
the resulting length of the backpressure zone.
downslope, the glide of the snow cover along the
Theories assuming Newtonian or non-Newtonian
ground, and the friction between the structure
viscosity have been used along with field mea-
and the snow. Static load is produced by the trans-
surements to develop estimates of the pressure
ference of the vertical stresses laterally (the mag-
and pressure distribution on the upright face of
nitude of the lateral stress is determined by the
avalanche defense structures and the backpressure
viscous analog to Poisson's ratio). The pressure
zone around such structures (McClung 1974, 1976,
due to creep and glide is caused by the retarda-
1982, 1984, McClung and Larson 1989, McClung
tion of downslope movement of the snow by a
et al. 1984, Brown and Evans 1975, Bader and
structure. The effect of this retardation is greatest
Salm 1989, Larson et al. 1985, Olagne and McClung
at the structure and decreases with slope distance
1990).
away from the structure (called the back pressure
When the Japanese tried to apply the Swiss
zone). Structure/snow friction retards the settle-
guidelines to construct avalanche control struc-
ment of the snow producing a force parallel to the
tures in their country, they found that the Swiss
upright face of the structure.
guidelines were not always adequate to prevent
Avalanche defense structures with a finite
snow creep damage to avalanche defense struc-
cross-slope length display three-dimensional flow
tures under Japanese conditions. Katakawa et al.
of snow around the structure and will produce
(1992) conducted a study to determine the appro-
end-effect forces as well. End effects increase the
priate design factors for Japan and found that
force acting on the structure compared to an infi-
glide factors and pressure distributions on struc-
nitely long structure (the force acting on an iso-
tures there were significantly higher (about 1.7
lated structure asymptotically approaches that of
times greater) than in Switzerland. These results
the infinite structure as the ratio of the structure
point to the empirical nature of avalanche struc-
length to snow depth increases). When multiple
ture design and the fact that the results of the
finite length avalanche defense structures are used
extensive Swiss efforts were fully applicable only
their force influences may overlap, allowing a
to snow very similar to that found in Switzerland.
reduction in the structural forces (the magnitude
of force reduction depends on the separation be-
Vehicle mobility in snow
tween structures).
Mobility of ground vehicles is defined as the
Initial theoretical work was done by Bader et
efficiency with which a vehicle travels between
al. (1939), Haefeli (1948, 1951) and Bucher (1948)
two points of interest. While this may include a
to obtain engineering formulae to estimate the
broad range of factors, the essence of mobility is
creep pressures and forces acting on infinite and
the balance of traction and motion resistance. Trac-
finite length avalanche defense structures. These
tion is the ability of the vehicle's running gear to
were modified as a result of later studies and
engage the terrain and the strength of the terrain
empirically adjusted to include the possible range
to resist horizontal shear deformation. These com-
of effects due to creep, snow density, and snow
bine to generate horizontal thrust from which a
depth that might occur throughout Switzerland
vehicle may move forward, accelerate, tow loads,
(Salm 1960, de Quervain and Salm 1963). The theo-
climb hills, or do other useful work.
retical developments and field measurements of
Many sources give rise to resistance and they
forces on avalanche defense structures (Kummerli
constitute a tax on the vehicle's available power.
1958) are the main basis for establishment of the
Resistance sources internal to the vehicle are var-
Swiss guidelines for avalanche control in the start-
ied and generally well known (e.g., drive train
ing zone (Switzerland 1990). The guidelines are a
gear losses, tire flexing or track bending resis-
7
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