Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY

Chance Helical Pile/Anchor Ultimate Bearing Capacity

Q ultU

EQUATION 4-10

Q ultU = W s + {π K 0 tan( ’)cos D 3 tan( ’/2)/3]} + cA c where A c = Surface area of truncated cone

2 ( ’/2) [B(D) 2 /2 +

The surface area of a truncated cone can be obtained from:

D

EQUATION 4-11

A c = π{(R

2 + r 2 ) + [(R 2 – r 2 ) + (D(R + r)) 2 ] 0.5 }

’/2

’/2

where r = Radius of helical plate = B/2

R = Radius of cone failure surface at the ground surface = B/2 + (D)tan( ’/2)

B

The additional component of uplift resistance resulting from soil cohesion is sometimes ignored since soil cohesion is often lost due to water infiltration or a rising water table.

Proposed Failure Mechanism for Shallow Single-Helix Anchors in Dense Sand Figure 4-3

4.2.1.2.b Sands ( ’ > 0; c = 0) In sands, uplift loading of shallow (generally D/B < 5) single-helix anchors develops a failure zone that looks similar to an inverted, truncated cone. The failure is assumed to take place by perimeter shear acting along this failure surface, which is inclined from the vertical at an angle of about ’/2 as shown in Figure 4-3. The uplift force must also lift the mass of the soil within the truncated cone. The ultimate uplift capacity (Q ultU ) is calculated from: EQUATION 4-8 Q ultU = W s + {π K 0 tan( ’)cos 2 ( ’/2) [B(D) 2 /2 + D 3 tan( ’/2)/3]} where W s = Mass of soil in truncated cone = V = Total (wet) unit weight V = Volume of truncated cone K 0 = At-rest lateral earth pressure coefficient B = Helix diameter D = Vertical plate depth

4.2.2 Single-Helix Helical Piles and Anchors—Deep Installation Deep installations of helical piles and anchors are generally more common than shallow installations provided there is sufficient soil depth to perform the installation. The reason is simply that higher load capacities are generally developed from a deeper installation in the same soil, so it makes more sense economically to utilize a deep installation when possible. Figure 4-4 illustrates the single-helix plate capacity model, wherein the soil failure mechanism follows the theory of general plate bearing capacity. Compression capacity is mobilized from soil below the helix plate and tension capacity from soil above the helix plate. 4.2.2.1 Compression Loading (Deep Single Helix) A deep installation, like a deep foundation, is one in which the ratio of depth (D) of the helix to diameter (B) of the helix is greater than or equal to 5, i.e., D/B ≥ 5. In this case, the design is analogous to compression loading of a deep end-bearing foundation. 4.2.2.1.a Saturated Clays ( ’ = 0; c > 0) Under compression loading, the ultimate capacity of a single helix helical pile in clay is calculated from Equation 4-10 as: Q ult = A h (S u N c + ’D) where N c = Bearing capacity factor for deep failure = 9 which yields

The volume of the truncated cone is determined from:

EQUATION 4-9

V = πD/3 {2(B) 2 + [B + 2Dtan( ’/2)] 2 + 2BDtan( ’/2)}

The value of the at-rest lateral earth pressure coefficient for sands can reasonably be calculated as: K 0 = 1 – sin( ’) 4.2.1.2.c Mixed Soils ( ’ > 0; c > 0) For shallow installations in mixed soils with frictional and cohesive components of shear strength, there is another component of the resisting force in uplift added to the components included in equation 4-8. This added component results from cohesion acting along the surface of the truncated cone failure zone between the helical plate and the ground surface. Adding a new term to equation 4-8 to account for the cohesion effect yields:

EQUATION 4-12

Q ult = A h (9S u + ’D)

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