TM 5-852-4/AFM 88-19, Chap. 4
CHAPTER 5
SURVEY DATUM POINTS
solid enough at summer temperatures to keep water and
5-1. Permanent datum points.
soil from entering and accumulating in the annular
a. The establishment of reliable permanent
space. For a permanent datum point, this space should
(10 years) datum points for survey measurements in
extend at least 1 to 3 feet below the predicted annual
seasonal frost and permafrost areas often requires
19
frost zone depth after construction, depending on the
substantial effort and care .
In northern areas,
estimate confidence, to make sure that no frost heave
conventional types of benchmarks, satisfactory in warm
forces can act on the datum rod. The rod should extend
climates, are subject to frost heave and/or thaw
sufficient additional distance below this point so that the
settlement which may produce many inches of seasonal
rod will be stably supported, taking into account the
movement, often progressive. Trees, boulders and even
sometimes substantial length of unsupported rod within
foundations of structures may move seasonally. Even
the casing. For relatively shallow annual frost zones the
benchmarks placed in bedrock cannot necessarily be
rodembedment should be at least 5 feet; for longer
relied on as rock often contains mud seams which can
unsupported lengths it should be more.
produce frost heave and ice layers which will cause
f.
Permanent datum points in deep seasonal
settlement on thaw. The problem of frost heave is most
frost or borderline permafrost areas should desirably
difficult in those northern areas that have very deep
employ a standard weight pipe of minimum 2 inch
annual frost penetration because it is necessary to
nominal diameter for the datum point rod because of the
support the datum points at very substantial depths in the
relatively long unsupported length through the deep layer
ground to assure stability; also the magnitudes of frost
of seasonal freezing. The outer casing should be large
heave forces in these areas are potentially very large.
enough to allow at least 1 inch of space between the
b. Movements can result not only from simple
datum rod and the casing, for effective placement of oil,
frost heave but also from the soil volume changes
wax, or other substances and for positive clearance.
attending the horizontal and vertical moisture
Before placement of the viscous material the space
movements which accompany frost action, from the
between the datum rod and the outer casing should be
expansion and contraction caused by the annual
carefully checked to make sure that the space is free of
variation of temperatures within freezing or frozen soil
soil over 100 percent of the length of the outer casing,
and from solifluction and other types of downslope
that there is no water at the bottom and that the rod and
movement. Moisture movements during freezing may
casing are not in contact with each other. Otherwise, a
actually cause lowering of surfaces of some soils. The
plug of ice or frozen soil or friction might transmit drag
effects of contraction and expansion are not simply
from the outer casing to the datum rod. The top surface
lateral; because temperature gradients with depth cause
of the vicous material may be given additional protection
differential dimensional changes, bending tends to occur
against water or soil infiltration by a suitable light cover of
with complex combinations of vertical and horizontal
fabric, plywood or metal.
movement.
g. It is recommended that a standard floor
c. Permafrost areas containing patterned
flange or welded flat plate be fitted to the bottom of the
ground and ice wedges should be assumed especially
rod, the datum point hole being drilled large enough to
unfavorable. Areas showing evidences of solifluction or
permit this. This will help to stabilize the rod against
slope movement should be avoided whenever possible.
movement even if skin friction on the embedded portion
If datum points must be placed in such areas they should
of the rod should prove inadequate. Other types of
be tied in to more stable points and rechecked often
projections can be fitted to the length of rod below the
enough so that movements with time can be known.
casing for the same purposes. Holes or slots are also
d. Care should be taken to locate benchmarks away
sometimes cut in pipe-type datum point rods into which
from potential thawing influences such as buildings,
slurry will penetrate, helping to increase holding power.
roads and streams.
The casing should also have an external flange at its
e. In order to establish a reliable datum point
lower end to control the tendency for the casing to be
in a seasonal frost or permafrost area firm support must
jacked progressively out of the ground by frost heaving.
be provided in stable material below the annual frost
For a permanent benchmark, this flange should be
zone and the supporting rod must be isolated from the
designed to develop and withstand full passive forces
effects of frost action where it passes through the annual
equal to the
frost zone. As shown in figure 5-1, this isolation can be
provided by an outer casing, with the space between the
rod and casing filled with a viscous substance which has
very low relative shear strength at all temperatures yet is
5-1
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