Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Mixed Soils φ ’ > 0; c’ > 0 The determination of the bearing capacity of a mixed soil, one that exhibits both cohesion and friction properties, is accomplished by use of Equation 4-31. This is fairly uncomplicated when accurate values are available for both the cohesion (undrained shear strength) and friction terms ( φ ’ & γ ’) of the equation. It is not possible to use ASTM D 1586 Blow Count correlations to determine all soil strength variables in the bearing capacity equation. Therefore, unless the designer is quite familiar with the project soil conditions, it is recommended that another approach be taken 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. Helicap ® Shaft Resistance Methodology As discussed earlier in this section, the shaft resistance developed by pipe shaft or grouted shaft screw- piles is considered in much the same way that shaft resistance in a driven pile develops. HeliCAP ® uses this traditional approach that is available in most foundation design textbooks.

DESIGN METHODOLOGY

The general equation is:

Qf = Σ [ π (D)fs( ∆ Lf)]

Equation 4-40

where:

D = Diameter of steel or concrete pile column f s = Sum of friction and adhesion between soil and pile ∆ L f = incremental pile length over which π D and fs are taken as constant

HeliCAP ® uses two empirical methods to calculate shaft resistance - the Gouvenot Method and the US Department of 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. HeliCAP ® uses the Gouvenot method to calculate shaft resistance for grouted columns only (Helical Pulldown ® micropiles). The US Navy method uses the Dept. of Navy Design Manual 7, Soil Mechanics, Foundations and Earth Structures (1974). HeliCAP uses the Navy method to calculate shaft resistance for both grouted columns and straight steel pipe shafts. • Gouvenot reported a range of values for skin friction of concrete/grout columns based on a number of field load tests. The soil conditions are divided into three categories based on friction angle (f) and cohesion (c). The equations used to calculate f s are:

Type I: Sands and gravels with 35° < φ < 45° and c’ = 0:

f s = σ o tan φ

Equation 4-41

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

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