Design and Stability of Direct-embedded Structures
Foundation design, as we all know, is important to any overhead structure.
Deepa Akula, PE
Foundation design, as we all know, is important to any overhead structure. Most of our distribution poles are direct-embedded structures, so the structural integrity of our distribution system depends on reliable structure design and foundation design. Structure class is selected through extensive calculations (that includes line angles, cable type, wind spans and framing type), but the rule of thumb of 10 percent of total length of pole plus two feet is usually used for foundation embedment. Rule of thumb is a good start for estimating the project cost at the inception of the project, but we need to understand the limitations of the rule of thumb and use it only when applicable. After all, our power grid is only as strong as its weakest component.
According to Rural Utilities Service (RUS) Bulletin 1724E-200, rule of thumb of “10 percent +2 ft.” is adequate for most wood pole structures in good soils and not subjected to heavy loadings. This means the rule of thumb is applicable only if the pole is embedded in good soils and is a tangent structure with small wind spans (not heavily loaded). Lateral loads a foundation needs to resist is generated by wind load on the wires and the pole and wire tensions due to a change in cable direction, so a tangent structure with small wind spans is not heavily loaded. Structures that qualify for the two requirements of the rule of thumb is a small percent of the overhead electric system.
What constitutes good soils? And what should we do if we encounter poor soils or need to design a direct-embed foundation for heavily loaded structures? Per the RUS bulletin, very dense, well graded sands and gravel; hard clays; and dense, well graded, fine and coarse sands are considered good soils. When we encounter poor soils or need to design a direct embed foundation for heavily loaded pole, we should design the embedment increase. Custom embedment depth can be calculated by considering soil properties, bearing area of the structure, and the design loads applied to the structure. Bearing area and the design loads applied to the structure can be calculated by the designer, but the soil properties might not be readily available.
Geotechnical investigation can provide the required soil information but could be cost prohibitive for small distribution projects. Geotechnical investigations are usually conducted for new building and bridge construction if there is a building or bridge near the proposed pole site; geotechnical information can be obtained by requesting the geotechnical report and soil borings from the owner, and a competent design engineer can estimate the soil properties for calculating the direct-embedment depth for the pole. If a geotechnical report is not available for the surrounding area and a geotechnical investigation is not a viable option, a field survey can be conducted. Per RUS bulletin 1724E-200, “The engineer may use a hand auger, light penetrometer, or torque probe” and designate the soil as “good,” “average” or “poor.”
Research conducted by Sivapalan Gajan and others show that the rule of thumb overestimates the embedment depths in good soils and underestimated the embedment depths in poor soils. Good soils should also be native and undisturbed to qualify for rule of thumb. If good soils are disturbed, compaction of the native soil could be compromised and bearing capacity of the disturbed soil could be very small compared to native soil - resulting in leaning or kick out of the structure. If disturbed soils are encountered, a field survey should be conducted by a competent design engineer and the embedment length should be increased to spread the forces to avoid failure.
Additional embedment depths should be considered for H-Frames with cross braces because they can walk out of the foundations; increasing the embedment depth increases the contact area of the pole with the backfill material, increasing the friction forces that resist uplift. Additional embedment should also be considered if the pole installation site is located near rivers, lakes or other bodies of water to account for the saturated state of the soil. Embedment depth should be increased for poles that require a higher factor of safety - for example, structures supporting cables spanning interstates, highways, roads, rivers and railway crossings.
When poor soils are encountered, installing poles in caissons should be considered to provide a stable foundation to the pole. Caissons increase the bearing area and spread the forces from the pole over larger areas, avoiding local failure in the soils. Caissons can be backfilled with crushed rock or geotechnical foam. When crushed rock is used to backfill a caisson or augured hole in the ground, it should include particles of varying size down to the rock dust, installed in small lifts (less than 6 inches), and tamped until the tamp makes a solid sound - indicating the crushed rock is compacted. Rocks of varying size interlock, compact better, provide superior resistance, and avoid pole leaning and kick out. Concrete backfill can be used for steel and concrete structures but is not recommended for wood structures because it can trap moisture. Stability of a direct embed structure depends on the quality of the backfill material, so it should be carefully selected.
Overhead structure foundations are not held to the same standards as building or bridge foundations, but the embedment depths should be selected with utmost care using the rule of thumb if the structure is not heavily loaded and is embedded in good soils. Embedment depth should be custom designed if the rule of thumb is not applicable to account for the soil conditions, bearing area of the structure, and design loads applied to the structure.
About the author: Deepa Akula, PE, is the engineering manager at McFarland Cascade. She has nine years of experience designing overhead utility and telecommunication structures. She holds a Master of Science degree in Engineering from the University of Missouri and is a registered civil/structural professional engineer.