Power Transmission Tower - Helical Pile Foundation

A. PROJECT DESCRIPTION – FOUNDATION PILE CONFIGURATION

This article presents a pile foundation design for a monopole electricity transmission tower. This type of tower is commonly subjected to high overturning moments. The foundation is a 26’x12’ rectangular concrete cap supported on helical piles (Figure 1). Table 1 below presents the maximum ultimate loads on the pile cap on each direction. Table 2 presents the soil properties and stratigraphy that has been estimated and used within the DeepFND software.

Figure 1: Examined pole configuration & Cap Layout for Monopoles with High Overturning Moment

Table 1: Pile Cap Loads

Table 2: Soil properties & Stratigraphy

B. DEEPFND/HELIXPILE SOFTWARE – SOIL TYPES AND BORINGS

Figure 2: General soil properties (Soil S1)

Figure 3: Soil Layers

C. DEEPFND SOFTWARE – PILE SECTION PROPERTIES

An 8in diameter steel pipe (PP8x0.5) helical pile is selected with three 24’ inch diameter plates. Table 3 presents detailed pile properties. Figures 4 and 5 present the pile type and length, and the pile section properties respectively.

Table 3: Steel Pile Section Properties

Figure 4: Pile Type, Position and Dimensions Figure 5: Pile Section Properties

D. PILE GROUP – DIMENSIONS AND LOADS

The pile cap is defined a 26ft x 12ft concrete slab with a 3ft thickness, supported by 10 vertical helical piles (Figure 6). Both the transverse and the longitudinal external load combinations on the pile cap, as well as a case with 50% of the design loads on both directions are considered. Figure 7 presents the external loads applied on the cap centroid for each load combination.

Figure 7: Load Combinations on the Pile Cap

E. PILE GROUP – PILE LOCATIONS AND GENERATED MODEL

Finally, the pile configuration is illustrated in the figures bellow. Figure 8 presents the piles layout (pile center local x and y coordinates), while the 3d generated Model is illustrated in figure 9.

Figure 8: Pile Locations (Local X and Y Coordinates)

Figure 9: Generated 3D Model

F. PILE GROUP ANALYSIS – TRANSVERSE LOAD COMBINATION

Figure 10: Pile Check Ratios, Moment & Displacement Diagrams and Axial Forces, Pile Cap Moment (Section at Y=4’)

Figure 11: Pile Shear Diagrams, Pile Cap Settlements (Section at Y=4’)

G. PILE GROUP ANALYSIS – LONGITUDINAL LOAD COMBINATION

Figure 12: Pile Moment & Shear Diagrams and Axial Forces, Pile Cap Moment (Section at Y=0’)

Figure 13: Pile Cap Settlement Shadings, Finite Elements (Top View)

Figure 14: Pile Moment & Displacement Diagrams and Axial Forces, Pile Cap Settlements (Section at Y=0)

Figure 15: Pile Shear Diagrams, Pile Cap Shear Shadings (3D Model)

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