TM 5-818-1 / AFM 88-3, Chap. 7
CHAPTER 6
BEARING-CAPACITY ANALYSIS
6-1.
Bearing capacity of soils.
Stresses
6-3.
Methods of analysis.
a. Shallow foundations.
transmitted by a foundation to underlying soils must not
(1) Groundwater level (GWL).
cause bearing-capacity failure or excessive foundation
The
settlement. The design bearing pressure equals the
ultimate bearing capacity of shallow foundations
ultimate bearing capacity divided by a suitable factor of
subjected to vertical, eccentric loads can be computed
safety. The ultimate bearing capacity is the loading
by means of theformulas shown in figure 6-1. For a
intensity that causes failure and lateral displacement of
groundwater level well below the bottom of the footing,
foundation materials and rapid settlement. The ultimate
use a moist unit soil weight in the equations given in
bearing capacity depends on the size and shape of the
figure 6-1. If the groundwater level is at ground surface,
loaded area, the depth of the loaded area below the
use a submerged unit soil weight in the equations.
ground surface, groundwater conditions, the type and
(2) Intermediate
groundwater
levels.
strength of foundation materials, and the manner in
Where the groundwater level is neither at the surface nor
which the load is applied. Allowable bearing pressures
so deep as not to influence the ultimate bearing capacity,
may be estimated from table 6-1 on the basis of a
use graphs and equations given in figure 6-2.
description of foundation materials. Bearing-capacity
(3) Eccentric or inclined footing loads. In
analyses are summarized below.
practice, many structure foundations are subjected to
horizontal thrust and bending moment in addition to
6-2. Shear strength parameters.
vertical loading.
The effect of these loadings is
a. Appropriate analyses.
Bearing-capacity
accounted for by substituting equivalent eccentric andlor
inclined loads.
Bearing capacity formulas for this
calculations assume that strength parameters for
condition are shown in figure 6-3. An example of the
foundation soils are accurately known within the depth of
method for computing the ultimate bearing capacity for
influence of the footing. The depth is generally about 2
an eccentric inclined load on a footing is shown in figure
to 4 times the footing width but is deeper if subsoils are
6-4.
highly compressible.
(1) Cohesionless soils. Estimate φ' from
(4) Loading combinations and safety
factors.
The ultimate bearing capacity should be
the Standard Penetration Test (table 4-5) or the cone
penetration resistance. For conservative values, use φ' =
determined for all combinations of simultaneous
loadings. A distinction is made between normal and
30 degrees.
maximum live load in bearing capacity computations.
(2) Cohesive soils.
For a short-term
The normal live load is that part of the total live load that
analysis, estimate su, from the Standard Penetration
acts on the foundation at least once a year; the
Test (table 4-5) or the vane shear resistance. For long-
maximum live load acts only during the simultaneous
term loadings, estimate φ' from correlations with index
occurrence of several exceptional events during the
properties for normally consolidated soils.
design life of the structure. A minimum factor of safety of
b. Detailed analyses.
2.0 to 3.0 is required for dead load plus normal live load,
(1) Cohesionless soils. Determine φ' from
and 1.5 for dead load plus maximum live load. Safety
drained (S) triaxial tests on undisturbed samples from
factos selected should be based on the extent of
test pits or borings.
subsurface investigations, reliability of estimated
(2) Cohesive soils.
For a short-term
loadings, and consequences of failure. Also, high safety
analysis, determine s,
from Q triaxial tests on
factors should be selected if settlement estimates are not
undisturbed samples with 03 equal to overburden
made. In general, separate settlement analysis should
pressure. For a long-term analysis, obtain φ' from
be made.
drained direct shear (S) tests on undisturbed samples.
b. Deep foundations. Methods for computing
For transient loadings after consolidation, obtain φ and c
the ultimate bearing capacity of deep foundations are
parameters from consolidated-undrained (R) triaxial tests
summarized in figure 6-5.
These analyses are
with pore pressure measurements on undisturbed
applicable to the design of deep piers and pile
samples. If the soil is dilative, the strength should be
foundations, as subsequently described. When the base
determined from drained S tests.
of the foundation is located below the ground surface at
a depth greater
6-1
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