WOODEN POLES FOR POWER LINES HOLE DIGING AND POLE SETTING


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.

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