Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY

Application Guidelines for Chance Helical Piles/Anchors The uppermost helix should be installed at least three diameters below the depth of seasonal variation in soil Grouted Piles in Sand [Navy Manual DM-7 (1974)], Table 4-13

properties. Therefore, it is important to check the frost depth or “mud” line at the project site. Seasonal variation in soil properties may require the minimum vertical depth to exceed five helix diameters. The influence of the structure’s existing foundation (if any) on the helical pile/anchor should also be considered. Hubbell Power Systems, Inc., recommends helical piles/anchors be located at least five diameters below or away from existing foundation elements.

Effective Angle of Internal Friction ( ’) (degrees) 20 25 30 35 40 S = Average Side Resistance on Pile Surface (psf)

P o (psf)

500 182

233

289 350 420

• The uppermost helix should be installed at least three helix diameters into competent load-bearing soil. It is best if all helix plates are installed into the same soil stratum. • For a given shaft length, use fewer longer extensions rather than many shorter extensions. This will result in fewer connections and better load/deflection response. • Check the relative economic feasibility of helical pile/anchor options if more than one combination of helix configuration and overall length can be used. 700 839 1500 546 699 866 1050 1259 2000 728 933 1155 1400 1678 2500 910 1166 1443 1751 2098 3000 1092 1399 1732 2100 2517 3500 1274 1632 2021 2451 2937 4000 1456 1865 2309 2801 3356 1000 364 466 577

Recommended Adhesion Values in Clay [Navy Manual DM-7 (1974)], Table 4-12

Cohesion (c) (psf)

Adhesion (C a ) (psf)

Pile Type Soil Consistency

Very soft

0-250

0-250

Soft

250-500

250-480

Medium stiff

500-1000 480-750 1000-2000 750-950 2000-4000 950-1300

Grout

Stiff

Very stiff Very soft

0-250

0-250

Soft

250-500

250-460

Medium stiff

500-1000 460-700 1000-2000 700-720 2000-4000 720-750

Steel

Stiff

Very stiff

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