Foundations
The foundations for poles are just as
important as the structure above ground. The pole backfill should be
capable of withstanding structure reactions. Pole-setting equipment
should be moved clear of the structure site prior to backfilling.
Differences in ground elevation at each
pole location, and pole length tolerances permitted by ANSI O5.1-1987
[9] should be considered to ensure a level structure. The tops of
poles should not be cut. If cutting is necessary, the pole top should
be covered with a mastic-type cap.
Under no circumstances should the butt
of any pole be cut. The design engineer should specify a minimum hole
depth. The actual hole depths required to obtain a level structure
are the responsibility of the installing contractor.
Digging operations should not be too
far in advance of the setting operation. Holes open too long may
deteriorate due to ground water seepage and/or heavy rains and
increase the chance for accidents. Unattended pole holes should be
temporarily covered. All Local, State, and Federal safety regulations
must be met.
Structure Alignment
When the structure is set and the load
line completely released, the structure should remain plumb and
level.
If the structure is not plumb or the
crossarm is not level, additional material will have to be placed
under one
pole. The additional material should be
approved by the design engineer.
Pole Holes
All holes should be in the correct
locations and large enough to provide a minimum of 6 in of space for
tamping around the pole to the full depth of the hole. Pneumatic
tamping equipment is recommended to expedite the setting operation.
The poles should be placed to prevent
damage to the structure grounding materials. Poles not required to be
raked should be set plumb and in alignment. Unless otherwise
specified, structures at angles should be set to bisect the line
angle.
The holes may be backfilled with earth
excavated from the hole, provided this material can be properly
compacted. Frozen material for backfill should not be permitted. The
backfill should be compacted to a dry density not less than the
natural in-place dry density of the surrounding earth.
Since the measurement of the density
may not be practical, no more than one shoveler should be utilized
for three tampers. Front-end loaders are not recommended during
backfilling. Backfill should be banked and tamped around the poles to
a height of 12 in above the natural ground surface.
Excessive water should be pumped out,
leaving not more than 6 in of water in the bottom of the hole, and 6
in of granular material should be placed to firm up the bearing
surface. Care should be exercised where pumping will cause excessive
sluffing of the bottom of the hole. Casing should be used where
moving water and/or gravel is encountered, working the casing down as
the material and/or water is removed.
Backfill
Where unsuitable backfill material is
encountered, gravel or crushed rock backfill should be utilized.
Gravel backfill material should be thoroughly compacted using air
tamps in layers not more than 6 in thick. If necessary, sufficient
water should be added to the backfill to ensure adequate compaction.
Gravel backfill should be compacted to
70% relative density as determined by ASTM D4253-83 [14] and D4254-83
[15]. Immersion-type vibrators can be used in lieu of air tampers.
Vibrators should have sufficient operating length to permit uniform
compaction from the bottom of the hole to within 2 ft of the original
ground surface.
The gravel backfill material should be
vibrated as placed in the hole and the vibrators slowly withdrawn as
the hole becomes filled. The upper 2 ft of the hole should be
backfilled with excavated soil and compacted by tamping.
This material should be banked and
tamped around the pole to a height of 12 in above the natural ground
surface. When crushed rock is used as backfill, it should be
compacted using air tampers in layers not exceeding 6 in. The rock
backfill should be crusher run, a maximum of 2 1/2 inches in size,
and having a minimum of two faces fractured and 95% crushed.
Alternate Backfill
Polyurethane foam may be used as a
backfill material, in lieu of native or granular backfill, as it
develops excellent uplift and bearing resistance. Where polyurethane
foam is used, the hole should be sized to provide 1 1/2 to 2 in
around the pole, so as to minimize the amount of foam required and
provide better load transfer between the pole and the soil.
Care should be exercised to assure that
the polyurethane foam does not insulate the ground conductor from the
surrounding soil. After structures have been set and properly
aligned, polyurethane foam (water-insensitive type), should be
installed using the appropriate mixing and dispensing machine and
procedures that are in strict accordance with manufacturer’s
recommendations.
Sufficient polyurethane should be
sprayed on the pole from 12 in above to 12 in below the ground line
to coat the vertical surface of the pole. The operator should then
dispense sufficient polyurethane to completely fill all voids and
have expansion of foam to within 6 in of the ground line.
Structures should be held in a plumb
position for approximately 8 to 10 min (until the polyurethane has
hardened sufficiently to hold the structure). Polyurethane liquids in
the dispensing machine reservoir should be maintained at a minimum
temperature of 90 °F during normal operations. Chemicals should be
held at a temperature of at least 60 °F for 24 h prior to being
used. In cold weather, heated facilities should be provided.
Weak Soils
In soils with weak bearing and/or
lateral capacity, the design engineer should specify alternative
foundation designs and/or backfill to obtain required foundation
integrity. Some alternatives are increased imbedment, uplift plates
with crushed rock or concrete backfill, concrete backfill with or
without re-bar through the pole butt, or other designs that increase
bearing area, uplift capacity, or resistance loads as required. See
IEEE Std 691-1985 [30], for a more thorough discussion of foundation
design for direct embedded structures.
Rock Sockets
Where rock is encountered and cannot be
removed by the use of a rock auger, various types of explosives could
be considered. There are many different types of rock and the
procedure for dealing with each varies greatly. In order to obtain
the best results, the person in charge should be knowledgeable in
types and hardness of the various rocks encountered (i.e.,
homogenous, fractured, glacial till boulders, and bedrock) as this
will also determine the type of excavation to be used.
Conclusions
The methods mentioned above for setting
and backfilling of poles or structures are recommended for soils that
are encountered in most areas of the United States. It should be
recognized that conditions requiring other techniques may be
encountered. Techniques available for special soil conditions are
water and air jetting.
No comments:
Post a Comment