Transmission And Substation Foundations - Technical Design Manual

SECTION 7: DESIGN EXAMPLES Design Example 10

Foundation Earth Pressure Resistance

Solution

EQUATION 7-20

Project A Chance® Type SS5 1-1/2” square shaft helical anchor is proposed as part of a pier and beam foundation for a residential structure (see Figure 7-32). The top of the helical anchor is fixed in a concrete grade beam that extends 4’-0 below grade. The surface soils are loose sands. Determine the lateral capacity of the grade beam using the Rankine earth pressure method. Assumptions • The lateral capacity of the 1-1/2” square shaft helical anchor is limited based on shaft size. It is generally not assigned any contribution to the lateral capacity of a foundation • The effective length of the grade beam for lateral resistance is 25’-0 • Assume a unit weight of 95 pcf • The water table is well below the bottom of the grade beam

0.5K a g H 2

Pa

=

=

0.5 x 0.2 x 95 x 42

=

152 lb/ft

0.5K p g H 2

Pp

=

=

0.5 x 3 x 95 x 42

=

2280 lb/ft

Pp - Pa =

2280 - 152

= 2128 lb/ft Total lateral resistance = 2128 x 25’-0 = 53,200 lb NOTE: In this example, more than 1” of movement will probably be required to fully mobilize the total lateral resistance. Partial mobilization requires less deflection. Coefficients of Earth Pressure (Das, 1987), Table 7-1 SOIL K 0 ’ Drained K 0 ’ Total K a ’ TOTAL K p ’ TOTAL Clay, soft 1 0.6 1 1 1 Clay, hard 1 0.5 0.8 1 1 Sand, loose 0.6 0.53 0.2 3 Sand, dense 0.4 0.35 0.3 4.6 Note: 1 Assume saturated clays.

• There are no surcharge loads • From Table 8-9, K a = 0.2, K p = 3

Grade Beam Grade Beam

P p P p

P a

P a

4'-0" 4’-0”

Soil: Loose Sand Soil: Loose Sand

Earth Pressure on a Grade Beam Figure 7-33

www.hubbell.com/hubbellpowersystems | 7-35