Page 259 - Civil Engineering Formulas

SOIL AND EARTHWORK FORMULAS 193

c 1
tan (8.30)
q u tan 2 dry b 2 K p (K p e 1)

2
2
where q ultimate bearing capacity of soil, lb/ft (kg/m )
u
2
2
c cohesion, lb/ft (kg/m )
angle of internal friction, degree
3
dry unit weight of dry soil, lb/ft (kg/m )
3
b width of footing, ft (m)
d depth of footing below surface, ft (m)
K coefficient of passive pressure
p
2
tan 45 2
e 2.718 . . .
For footings below the surface, the ultimate bearing capacity of the soil may
be modified by the factor 1 Cd/b. The coefficient C is about 2 for cohesion-
less soils and about 0.3 for cohesive soils. The increase in bearing capacity with
depth for cohesive soils is often neglected.

SETTLEMENT UNDER FOUNDATIONS

The approximate relationship between loads on foundations and settlement is
C 1 1 C 2 (8.31)
q 2d
P b b

2
2
where q load intensity, lb/ft (kg/m )
P settlement, in (mm)
d depth of foundation below ground surface, ft (m)
b width of foundation, ft (m)
C coefficient dependent on internal friction
1
C coefficient dependent on cohesion
2
The coefficients C and C are usually determined by bearing-plate loading tests.
1
2
SOIL COMPACTION TESTS

The sand-cone method is used to determine in the field the density of com-
pacted soils in earth embankments, road fill, and structure backfill, as well as
the density of natural soil deposits, aggregates, soil mixtures, or other similar
materials. It is not suitable, however, for soils that are saturated, soft, or friable
(crumble easily).