Transmission And Substation Foundations - Technical Design Manual

SECTION 4: DESIGN METHODOLOGY

Lateral Capacity of Helical Piles

P=fx9xCuxd

SHEAR AT DEPTH ((1.5Xd)+f)=0 f x 9 x Cu x d = P f = P/9 x Cu x d AREA 1 = (PX(e + 1.5d)) + 1/2 x f x P = P x (e + 1.5d + 1/2 x f) AREA 2 = 1/2 x g x g/2 x 9 x Cu x d = (g л 2/4) x 9 x Cu x d = 2.25 x d x g л 2 x Cu g = ((Px(e + 1.5d + .5f))/(2.25 x d x Cu)) л 5 M MAX = AREA 1 or AREA 2 M MAX = P x (e + 1.5d + .5f) L = REQUIRED DEPTH INTO SOIL WITH COHESION OF Cu

1.5Xd

AREA 1

P

AREA 1 = AREA 2

g/2

AREA 2

L=1.5d+f+g

g/2

M MAX = AREA 1 M MAX = AREA 2

9xCuxd

L = 1.5d + f + g

g/2x9xCuxd

M MAX

PILE SHEAR (KIPS)

PILE MOMENT (FT. KIPS)

SOIL REACTION (KIPS/FT.) RECOMMENDED UNITS d

ENERGY STRUCTURES INCORPORATED

TITLE BROM’S METHOD - COHESIVE SOILS DWG NO. 921102

PILE DIAMETER (FT.) SOIL COHESION (KSF)

Cu

P

LATERAL LOAD

f

FT. FT.

g

MAXIMUM PILE BENDING MOMENT (FT KPS) MAXIMUM PILE BENDING STRESS (KSI)

M MAX FB MAX

Broms’ Method for Short Piles in Clay (Energy Structures, Inc., 1994) Figure 4-19

and battered, have been subjected to several earthquakes of magnitude 7+ on the Richter scale with no adverse effects. Anecdotal evidence indicates that the structures on helical piles experienced less earthquake-induced distress than their adjacent structures on other types of foundations. Their performances were documented anecdotally in technical literature, including the Engineering News Record. Full scale seismic tests of helical piles were performed in 2016 at the University of California - San Diego shake table. Several different simulations were performed, including the Northridge and Takatori seismic events. Various helical pile configurations were installed in the test box, including square and round shaft piles of varying diameters. The reader is referred to numerous documents published about these studies (Cerato et al., 2017, Elsawy et al., 2017 and 2019. 4.7.3 Additional Comments The lateral capacity of round shaft (RS) helical piles and anchors is greater than that of square shaft (SS) because of the larger section sizes. Typical pipe diameters of 2-7/8” (73mm), 3-1/2” (89 mm), and 4-1/2” (114 mm) OD are used for Chance® round shaft helical piles. As shown in Design Example 7-13 in Section 7, 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 pile shafts. Murthy (2003) also presented a direct method for evaluating the lateral behavior of battered (inclined) piles.

GRADE BEAM Grade Beam

P p

p

P a

a

Soil : Loose Sand

SOIL: LOOSE SAND

FIGURE 5-21 EARTH PRESSURE ON A GRADE BEAM

Earth Pressure on a Grade Beam Figure 4-20

4-30 | www.hubbell.com/hubbellpowersystems