Transmission And Substation Foundations - Technical Design Manual (TD06088E)

These two methods are illustrated in Figures 4-1a & c (Individual Plate Bearing) and 4-1b & d (Perimeter Shear). With Individual Plate Bearing, the helix capacity is determined by calculating the unit bearing capacity of the soil at each helix and then multiplying the result by the individual helix’s projected area. Friction along the central shaft is typically not used to determine capacity, but may be included when the central shaft is round, as will be discussed later in this section. The Individual Plate Bearing Method assumes that load capacity will be developed simultaneously and independently by each helix; i.e. no interaction between helix plates. The Perimeter Shear Method assumes that because of the close helix spacing, a prism of soil will develop between the helix plates and failure in this zone occurs along a plane as shown in Figure 4-1b & d. In reality, the Perimeter Shear Method includes both plate bearing and perimeter shear failure as illustrated. The following is Terzaghi’s general bearing capacity equation, which allows determination of the ultimate capacity of the soil. This equation and its use will be discussed in this section, as it forms the basis of determining helix capacity in soil. TABLE 4-2 TYPICAL DESIGN SITUATIONS FOR SINGLE-HELIX & MULTI-HELIX SCREW-PILES AND HELICAL ANCHORS Single-Helix Multi-Helix Failure Condition Failure Condition Shallow Deep Shallow Deep C T C T C T C T Clay Clay Clay Clay N/A N/A Clay Clay Sand Sand Sand Sand N/A N/A Sand Sand Mixed Soils Mixed Soils Mixed Soils Mixed Soils N/A N/A Mixed Soils Mixed Soils

= A h ( cN c + q’N q + 0.5 γ ’ BN γ ) = Ultimate capacity of the soil

Q ult Q ult

DESIGN METHODOLOGY

A h

= Projected helix area

c

= Soil cohesion

where

q’

= Effective overburden pressure = Footing width (base width)

B

γ ’ = Effective unit weight of the soil and Nc, Nq, and N γ are bearing capacity factors

Terzaghi’s Bearing Capacity Factors are shown in the Table 4-2.

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