Quantcast Improving Electrical Grounding in Frozen Soils

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MIL-HDBK-419A
2.11.2 Improving Electrical Grounding in Frozen Soils. High electrical resistance of grounding sites is
common in areas where the ground freezes. The performance of grounding installations can, however, often be
increased through site selection and various electrode installation schemes. The degree of improvement will
depend on the local existence and accessibility of conductive soils. The most common conductive sites are
associated with thaw zones or clay-rich soils. The greatest grounding problems usually occur where bedrock,
coarse-grained soil, or cold, ice-rich soil is found near the surface.
In temperate regions, small field installations can usually be adequately grounded by driving a simple vertical
electrode into the soil. This technique has been unsuccessful in areas of frozen ground because: (1) driving
electrodes is difficult, (2) frozen materials tend to be electrically resistive, and (3) high contact potentials can
develop between a rod and the frozen soil because a thin ice layer can form around the cold rod.
Installation procedures can be modified in some frozen ground settings to eliminate some of these problems,
permitting order-of-magnitude reductions in the resistance to ground. However, in many regions of the Arctic,
electrical resistivity of the frozen ground is extremely high, and grounding may not be significantly improved by
local modification or treatment of the soil surrounding the electrode. Achieving "low" resistance grounds of
less than several ohms will often require that the site be selected in a zone of conductive material and is
described in paragraph 2.11.1.
Other factors such as accessibility to water, power, roads, real estate, siting requirements, electromagnetic
compatibility, etc, may however require that a site be located in an area of low soil conductivity.  T h i s
establishes the rather high probability of not being able to attain a low resistance to ground without
considerable cost and effort.  Studies (2-17) conducted to determine methods to obtain low or acceptable
resistances in areas of low soil conductivity in turn raised additional questions:
a .  What is the influence of ground temperature, material type and associated variations in unfrozen
water content on the performance of an installation?
b .  What is the influence of material type and associated differences in permeability and saturation on
salt solutions added to the soil surrounding an electrode?
c.
What is the effectiveness of using more than one electrode for lowering resistance to ground?
d .  What is the long-term influence of conductive backfills and what is the suitability of various
materials for backfill around electrodes placed in holes of larger diameter than the electrodes?
The main procedure which can be used to reduce resistances to ground is to place the ground rod or electrode in
open holes having diameters greater than the electrodes thereby making emplacement easier and permitting the
use of conductive backfill. The holes can be made by drilling or blasting with shaped charges. Another
procedure which may be used in limited situations is to lay or drive an array of horizontal rods into an active
layer.
2-70





 


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