Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Using the Rankine solution may be an over- simplification of the problem but tends to be conservative since the height of the projected area of the footing or pile cap is not large and the cohesion term will generally be small. Batters Lateral loads are commonly resolved with battered helical piles and anchors. Battered piles are used when passive lateral resistance of the surrounding soil is inadequate to transmit the lateral loads required by a structure, or the greater rigidity of the foundation is required.

Grade Beam

P p

P a

Soil: Loose Sand

CHANCE ® Helical Piles and Anchors have been supplied to the seismic prone areas of the west coast of the United States and Canada for over 30 years for projects. In tension applications, they have been in service for over 50 years. They have been subjected to many earthquakes and aftershocks with good experience. Our helical pre-engineered products have been used far more extensively than any other manufacturer’s helical product in these areas. To date, there have been no ill effects observed using battered helical piles and anchors in seismic areas. Due to the increased foundation stiffness design engineers must take in account the increased foundation stiffness of battered pile foundations when designing where dynamic horizontal loads are present. It is common practice to assume that all lateral loads are transferred to the lateral component of the piles axial capacity, but this assumption is not a complete model of how battered piles behave. A portion of the lateral load is transferred to the lateral component of the pile’s axial capacity but a load is also transferred passively to the surrounding soil. Allowable bending moment of the pile must be taken into account when designing a group. The bending moment induced typically will be less than that of a vertical pile under similar shear loads, but can excessive if a large enough shear load is applied to the pile group. Additional Comments The lateral capacity of round shaft (Type RS) helical piles and anchors is greater than the square shaft (Type SS) helical anchors and piles because of the larger section size. Typical pipe diameters of 2-7/8” (73mm), 3-1/2” (89 mm) and 4-1/2” (114 mm) OD are used for CHANCE helical piles. As shown in design example 8-13 in Section 8, enlarged shaft sections are used for certain applications. From a practical standpoint, the largest diameter helical pile available from Hubbell Power Systems, Inc. is 10-3/4” diameter, but larger shaft diameters are available on a project specific basis. As previously noted, there are several other methods used to analyze the lateral capacity of the shaft of piles. Murthy (2003) also presented a direct method for evaluating the lateral behavior of battered (inclined) piles. Figure 4-21. Earth Pressure on a Grade Beam

DESIGN METHODOLOGY

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