Chance Technical Design Manual

DESIGN EXAMPLE 15: FOUNDATION EARTH PRESSURE RESISTANCE PROJECT A Chance® Type SS5 1-1/2” square shaft helical anchor is pro posed as part of a pier and beam foundation for a residen tial structure (see Figure 8-31). 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 capaci ty 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 P a P p

SOLUTION

EQUATION 8-84

= 0.5K a g H 2 = 0.5 x 0.2 x 95 x 42 = 152 lb/ft = 0.5K p g H 2 = 0.5 x 3 x 95 x 4 2 = 2280 lb/ft

DESIGN EXAMPLES

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 prob ably be required to fully mobilize the total lateral resistance. Partial mobilization requires less deflection.

COEFFICIENTS OF EARTH PRESSURE (DAS, 1987), TABLE 8-9

K 0 ' DRAINED K 0 ' TOTAL K a ' TOTAL K p ' TOTAL

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

SOIL

Clay, soft 1 Clay, hard 1

0.6

1

1

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.

Grade Beam

P p

P a

4'-0"

Soil: Loose Sand

EARTH PRESSURE ON A GRADE BEAM FIGURE 8-31

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