Moving on Sand - Dune Worlds: How Windblown Sand Shapes Planetary Landscapes - page 245

22

Moving on Sand

The challenges of moving people or equipment across sand
can be a significant impediment to the researcher, and
indeed have been important in shaping the history of
commerce and warfare in desert regions. We describe in this
section some of these issues—which are now no longer
confined to the Earth, since humanity can now claim getting
vehicles stuck on Mars among its achievements.
We hope the following may be of direct practical utility
in the field. If not, since getting stuck is an inevitable part of
fieldwork and often leads to extended periods of waiting,
then at least the elaboration of the mechanics involved may
serve as points of discussion while your colleagues dig
out…

how it was emplaced, etc.) lies between these values, then
running will cause the toes to slip whereas slow walking
may not. The phenomenon is discussed, incidentally, in the
context of walking on snow in Miss Smilla's Feeling for
Snow (Hoeg 1992).
A casual inspection will readily show what is perhaps not
intuitively obvious—that the sand fails on a usually-curved
surface. This surface (see Fig. 22.1 ) forms the basis of soil
stability evaluation. Soil mechanics, the scientific study and
engineering prediction of the failure of loaded soils, is of
course a whole field unto itself (beyond the scope of this
book, but upon which many texts exist; among free
resources, the US Army Corps of Engineers has a variety of
manuals that clearly describe analysis techniques). Specific
measurement techniques of the strength and friction exist
(we touch on them in Part III) and elaborate analyses can
refine the estimation of when and how a soil will give way.
But to an order of magnitude, at least, simple consideration
of loading pressure and simple strength or friction estimates
can give a good sense of what works and why.
Usually the stoss slopes are rather safe, with modest
slopes and the sand surface somewhat packed by the sal-
tation process. The presence of sand ripples in this regard is
usually a good indicator of a somewhat stiff surface. The lee
slopes are a different matter: not only are the slopes steeper,
but avalanched sand is usually soft.
One quickly learns, then, to move along dunes either on
the stoss slopes, or better yet, clear interdunes if they are
present. It may be that an adjusted gait (walking steadily,
and keeping the soles of the feet as flat as possible) will
reduce the stresses applied and thus probability of soil
failure, but maintaining such an unnatural gait may prove
more fatiguing than accepting the occasional slip while
walking normally.
Of course, if the route requires one to traverse dunes,
then there is little choice but to tackle the slipface. One can
choose to slowly struggle up a lee slope, making one step

22.1 Walking on Dunes

We cannot tell anyone how to walk on a dune any more
than a set of written instructions can tell anyone how to play
tennis or execute some other motor function. However, we
can offer a few observations for the reader to keep in mind
once they get to a site of interest.
The challenge is that, above a certain soil loading or
stress, the material fails and flows. When this happens, the
foot or tire may move in an undesired way, and the person
or vehicle may be obliged to perform work on the sand, thus
expending energy that they would not need to on a rigid
surface. That threshold stress can be rather small for a loose
slip face of dry sand.
Some brief numbers are appropriate to consider. A 90 kg
person with their weight on a single foot (say 200 cm 2 )
exerts a pressure of about 0.5 N/cm 2 , or 0.05 bar. If, how-
ever, they run (perhaps doubling the instantaneous force on
the leg, and the pronation of the foot means most of that
force is exerted on the toes), then the pressure on the sand
may increase to 10 times that, or 5 N/cm 2 . If the failure
stress of the sand (which will depend on its moisture level,