Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY Lateral Capacity of Helical Piles

LATERAL LOAD

Total Moment

74919.737 67427.763 59935.790 52443.816 44951.812 37459.869 29967.895 22475.921 14983.947 7491.974 0.000E+00

COMPRESSION PILE

TENSION PILE

VERTICAL PILE

Figure 4-21.Group Box Pile Deflection

The tension pile sees large passive resistance than the compression pile due to the force pushing the pile downward (Ft). The compression piles transfers a greater portion of the lateral load axially. The pile head amount of fixity is a critical component when designing battered piles. For lateral deflection a fixed head creates the stiffest load response, but will induce the highest bending moments within the piles. Fixed head conditions are typical of concrete or moment resistant caps and grillages. A pinned head condition will induce lower bending moments, but will allow much higher deflections. This typically results in a less efficient pile cap that can withstand less shear load. Pinned head conditions are typical of non-moment resistant caps and grillages.

Figure 4-21 shows 30 times the deflection of an RS3500 pile group within allowable bending moment. The actual deflection of the pile group is approximatly 5/16.” The tension and compression battered piles both resist the shear axially and passively while the vertical pile is only resisting lateral load passively. The lateral load causes the tension pile to be pushed downward into the soil and compression pile is pushed upward. This behavior is shown in Figure 4-22.

Ft

Fc

Rt

Vt

Vc

Rc

TENSION PILE

COMPRESSION PILE

VERTICAL PILE

Figure 4-22 Behavior of Battered Piles

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