Chance Technical Design Manual

The bearing capacity factor (N q ) is dependent on the angle of internal friction ( ϕ ’) of the soil. When a value is provided for the friction angle, HeliCAP software uses Figure 5-8 (N q vs. ϕ ’) and Equation 5-20 to determine the value for N q . The graph in Figure 5-8 allows the determination of N q for a specific angle of internal friction when measured in degrees. This curve was adapted from work by Meyerhof (1976). Equation 5-20 was written for the curve shown in Figure 5-8, which is Myerhof’s N q values divided by 2 for long-term applications. When the angle of internal friction is not known, the software estimates it (and N q ) by using blow counts obtained from the Standard Penetration Test per ASTM D1586. Equation 5-34 provides an estimate of the angle of internal friction from SPT blow count data. This equation is based on empirical data given by Bowles (1968) and its results should be used with caution. Note: The correlated φ ’ and N q values determined by HeliCAP software can be overridden. This is encouraged when more reliable soil data are available. EQUATION 5-33 Q h = A h q’N q = A h g ’DN q where A h = Projected helix area

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. 5.5.1.2 FINE-GRAIN COHESIVE SOILS ( φ ’ = 0; c > 0) HeliCAP® design software determines the ultimate bearing capacity of a helix (Q h ) in a cohesive or fine-grained soil with Equation 5-40, which is derived from Equation 5-32 with the coarse-grained (sand) term eliminated. Equation 40 is simi lar to Equation 5-18 with the overburden term neglected. The bearing capacity factor (N c ) is 9 provided the installa tion depth below grade is at least five times the diameter of the top-most helix. EQUATION 5-40 Q h = A h cN c = A h (s u )9 where A h = Projected helix area c = Cohesion; for φ ’ = 0, c = undrained shear strength = s u N c = Bearing capacity factor for deep failure = 9 (depth ≥ 5B) In the event that cohesion or undrained shear strength values are not available, HeliCAP software uses the following equation to estimate undrained shear strength values when SPT blow counts are available. This equation is based on empirical values and is offered only as a guide when undrained shear strength values are otherwise not available. Results obtained using estimated shear strength values should be used with caution. NOTE: The correlated undrained shear strength values determined by HeliCAP software can be overridden. This is encouraged when more reliable soil data are available. EQUATION 5-41 c (ksf) = N 60 /8 = 0.125N 60 c (kPa) = 6N 60 where c = Cohesion; for φ ’ = 0; c = undrained shear strength = s u N 60 = Blow count value per ASTM D1586 Standard Penetration Test If unit weight values are not available, the software uses the following equations to obtain estimated unit weight values when blow counts from ASTM D1586 Standard Penetration Tests are available. EQUATION 5-42 0 < N 60 ≤ 19 γ = 80 + 2N 60 (lb/ft 3 )

g ’ = Effective unit weight of the soil D = Vertical depth to helix plate N q = Bearing capacity factor for non-cohesive component of soil

EQUATION 5-34

φ ’ = 0.28N 60 + 27.4

where

φ ’ = Angle of internal friction N 60 = Blow count per ASTM D1586 Standard Penetration Test In the event unit weight values are not available, HeliCAP soft ware uses the following equations to obtain estimated unit weight values when blow counts from ASTM D1586 Standard Penetration Tests are available. EQUATION 5-35 N 60 = 0 γ = 65 (lb/ft 3 )

DESIGN METHODOLOGY

EQUATION 5-36

γ = 60 + 5N 60 (lb/ft 3 )

0 < N 60 ≤ 7

EQUATION 5-37

3 )

8 ≤ N 60 ≤ 10

γ = 100 (lb/ft

EQUATION 5-38

3 )

11 ≤ N 60 < 50

γ = 90 + N 60 (lb/ft

EQUATION 5-39

3 )

N 60 ≥ 50

γ = 140 (lb/ft

EQUATION 5-43

3 )

20 ≤ N 60 ≤ 40

γ = 120 (lb/ft

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