snowpack, showed that a layer of fine-grained,
structure of snow (see also Good 1975). In his
sintered snow deformed at a rate 10 times greater
experiments, snow samples were repeatedly de-
than that of an adjacent layer of depth hoar, al-
formed in uniaxial compression by rapid loading
though the densities of both layers were the same.
and unloading to determine values of the static
Finally, de Montmollin (1982) argued that break-
Young's modulus. Next the samples were allowed
ing and rapid redevelopment of bonds, even dur-
to creep at constant stress and the coefficient of
ing the course of an experiment, is important in
Newtonian viscosity was found from the rela-
snow deformation. These examples show that re-
tionship between stress and the "steady-state"
gardless of the specific mechanisms involved, the
strain rate (Kry 1975b). Each sample was deformed
bonding between snow grains (and not the den-
in stages until the strain reached about 30% and
sity) is the critical factor in determining the re-
observations of the bond structure were made
sponse of the snow to applied loads.
from samples collected at several stages. The re-
The importance of snow microstructure to def-
sults showed that the stress is transmitted through
ormational processes has been known for many
only a fraction of the grains, and that these are
years. Bader et al. (1939) made thin sections of
grouped into chains. Kry (1975b) hypothesized
snow after it was deformed in order to search for
that the chains should be regarded as the basic
changes in grain orientation that might have been
unit of snow structure, and used that concept to
attributed to deformation. Kragelski and Shakhov
interpret the variations in viscoelastic properties.
(1949) also recognized the importance of bond-
At the same time, Akitaya (1974) described the
ing. Yosida et al. (1956) referred to bonding in
"skeleton" structure of some types of depth hoar
their interpretations of test results, Bader (1962a)
in which grains were bonded primarily in the
discussed snow deformation in terms of bonding
vertical direction providing strength in vertical
in a general way, and Kinosita (1967) showed the
loading, but virtually none for lateral loads. This
difference in microstructural-scale process be-
skeleton structure is clearly similar to the "chains"
tween high- and low-rate tests in uniaxial com-
identified by Kry (1975b). Subsequently, Gubler
pression. In addition, several theories based on
(1978a,b) extended the idea of chains, and used it
assumptions about the processes that affect
to interpret data on the tensile strength of snow.
changes in bonding during deformation or over
St. Lawrence (1977, 1980), St. Lawrence and
time have been derived to describe snow consoli-
Bradley (1975) and St. Lawrence and Lang (1981)
dation (Feldt and Ballard 1966, Ebinuma and
have used acoustic emissions as indirect evidence
Maeno 1987, Alley 1987, Wilkinson 1988) and
of microstructural changes to develop constitu-
strength (Ballard and McGaw 1965, Ballard and
tive equations for snow. Similarly, Brown (1979,
Feldt 1966).
1980) derived a constitutive relationship based on
Keeler (1969a,b) was the first investigator to
a model of collapsing pore spaces to describe the
systematically study the relationship between mi-
volumetric compaction of snow. Alley (1987) used
crostructural changes in the fabric of snow dur-
a grain boundary sliding model to describe the
ing deformation and metamorphism. He credited
densification of highly porous firn. Wilkinson
Eugster (1952) with doing the first thorough fab-
(1988) used a density at which particle rear-
rangement can no longer act (about 600 kg m3)
ric study of snow in thin section, and Kinosita
and a multi-mechanism theory of pressure sinter-
(1960) with introducing the parameter of "joint
ing to describe the densification of polar firn to
order" (the number of intersections of lines halv-
ice. Hansen and Brown (1986, 1987) and Hansen
ing connecting grains), which is important in the
(1988) have also derived constitutive relationships
analysis of snow fabrics. However, it was Nakaya
for snow based on theoretical considerations of
(1961) who first tried to relate microstructure to
microscopic deformation mechanisms and mea-
mechanical properties when he experimentally
sured geometric parameters of the bonds. Edens
determined the relationship between the dynamic
and Brown (1991), and Brown and Edens (1991)
Young's modulus and the density of processed
have studied the deformation of the grain bonds
snow, and interpreted the results in terms of the
and produced a mathematical model to describe
degree of bonding between grains.
some of the processes involved. However, the
Kry (1975a,b) tried to determine how the mi-
general application of constitutive relationships
crostructure of snow changes with deformation,
based on models of the deformation on a micro-
and how the changes affect the mechanical prop-
structural scale is still considered to be some years
erties. He developed techniques and definitions
in the future (Weeks and Brown 1992).
to quantitatively describe the grain and bond
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