Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY

Helicap Helical Capacity Design Software

HeliCAP software uses two empirical methods to calculate side resistance: The Gouvenot method and the US Department of the Navy method. The Gouvenot method is named after the French researcher who conducted tests on a variety of grouted-shaft micropiles including gravity-fed grout columns. The software uses the Gouvenot method to calculate side resistance for grouted columns only (Helical Pulldown® micropiles). The US Navy method uses the Department of the Navy Design Manual 7, Soil Mechanics, Foundations and Earth Structures (1974). The software uses the Navy method to calculate side resistance for both grouted columns and steel round shafts. 4.5.2.1 Gouvenot Method Gouvenot reported a range of values for unit side resistance of concrete/grout columns based on a number of field load tests. The soil conditions are divided into three categories based on friction angle ( ) and cohesion (c). The equations used to calculate f s are: • Type I: Sands and gravels with 35° < < 45° and c = 0:

EQUATION 4-36

20 ≤ N 60 ≤ 40

= 120 (lb/ft

3 )

EQUATION 4-37

41 ≤ N 60 < 50

= 120 + 2(N 60 - 40)(lb/ft 3 )

EQUATION 4-38

N 60 ≥ 50 3 ) These correlations were originally determined from Tables 3-2 and 3-3 in Bowles’ first edition of Foundation Analysis and Design. These relationships provide an approximation of the total unit weight. They have been modified slightly from how they were originally presented as experience has suggested. NOTE: The correlated total unit weight values determined by HeliCAP software can be overridden. This is encouraged when more reliable soil data are available. 4.5.1.3 Mixed Soils ( ’ > 0; c > 0) HeliCAP software determines the bearing capacity of a mixed soil, one that exhibits cohesion and friction properties, by use of Equation 4-25. This is straightforward when accurate values are available for the cohesion (undrained shear strength) and friction terms ( ’ & ’) of the equation. It is not possible to use ASTM D1586 SPT blow count correlations to determine all soil strength variables in the bearing capacity equation. Therefore, the designer must take another approach when accurate values are not available for both terms of the equation. One suggestion is to first consider the soil as fine grained (cohesive) only and determine capacity. Then consider the same soil as coarse grained (cohesionless) only and determine capacity. Finally, take the lower of the two results and use that as the soil bearing capacity and apply appropriate Factors of Safety, etc. = 140 (lb/ft

EQUATION 4-40

f s = σ o tan( ) where σ o = Mean normal stress for the grout column • Type II: Mixed soils; fine, loose silty sands with 20° < < 30° and sandy clays with 205 psf < c < 1024 psf (9.8 kPa < c < 49 kPa)

EQUATION 4-41

f s = σ o sin( ) + (c)cos( ) • Type III: Clays with 1024 psf < c < 4096 psf (49 kPa < c < 196 kPa)

EQUATION 4-42

f s = c for 1024 psf < c < 2048 pfs (49 kPa < c < 98 kPa) and:

4.5.2 HeliCAP Software Side Resistance Methodology

EQUATION 4-43

As discussed earlier in this section, the side resistance (Q f ) developed by round shaft or grouted-shaft helical piles is considered similarly to side resistance developed by driven piles. HeliCAP design software uses the traditional approach presented in most foundation design textbooks. The general equation is:

f s = 2048 psf (98 kPa) for 2048 psf ≤ c < 4096 psf (98 kPa ≤ c < 196 kPa) In HeliCAP® design software, this analysis assumes a uniform shaft diameter for each soil layer and, if required, the side resistance capacity of the pile near the surface can be omitted.

EQUATION 4-39

Q f = Σ [π(B)f s (∆L f )] where B = Diameter of steel or grout pile column f s = Unit side resistance (sum of friction and adhesion between soil and pile) ∆L f = Incremental pile length over which πB and f s are considered to be constant

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