Stresses

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

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

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