U. S. Army Corps of Engineers

Figure 6-3. Ultimate bearing capacity of shallow foundations under eccentric inclined loads.

than the width of the foundation, the factor of safety

where W is the total effective weight of soil and concrete

should be applied to the net load (total weight of

located within the prism bounded by vertical lines at the

structure minus weight of displaced soil).

base of the footing. Use total unit weights above the

c. Stratified subsoils. Where subsoils are

water table and the buoyant unit weight below. If the

shear resistance on the vertical sides of the prism

variable with depth, the average shear strength within a

defined above is considered, a minimum safety factor of

depth below the base equal to the width of the loaded

2 should be used. The lateral earth pressure on the

area controls the bearing capacity, provided the strength

vertical sides of the prism should not exceed earth

at a depth equal to the width of the loaded area or lower

pressure at rest and should be considered as active

is not less than one-third the average shear strength of

earth pressure if the soil is not well compacted.

the upper layer; otherwise, the bearing capacity is

governed by the weaker lower layer. For stratified

cohesive soils, calculate the ultimate bearing capacity

from the chart in figure 6-6. The bearing pressure on the

founded on rock, adequate exploration is necessary to

weaker lower layer can be calculated by distributing the

determine the number and extent of defects, such as

surface load to the lower layer at an angle of 30 degrees

joints, shear zones, and solution features. Estimates of

to the vertical.

the allowable bearing pressure can be obtained from

table 6-1. Conservative estimates of the allowable

Footings subjected to a

bearing pressure can be obtained from the following

sustained uplift force, Tu, should be designed with a

minimum factor of safety of 1.5 with respect to weight

qa = 0.2 qu

(6-2)

forces resisting pullout expressed as

Allowable bearing pressures for jointed rock can also be

W

>

1.5

(6-1)

estimated from RQD values using table 6-2. Local

Tu

experience should always be ascertained.

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