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Lab. No.
184847
Lateral Soil Pressure Design For
Masonry
Block Residential Basement
Walls
Lateral pressure which acts
against typical masonry block
basement walls of 10 feet or less
in height is related to the
backfill material type, backfill
compaction, and height of water
adjacent to the wall. Various
backfill materials develop
different lateral pressures due
to the individual backfill unit
weight, internal friction angle
and cohesive characteristics. The
degree of compaction alters the
unit weight of the material and
the depth of water in the
backfill adjacent to the wall
adds to backfill pressures
against the wall. The design
lateral pressure against a wall
is primarily a function of the
unit weight and angle of internal
friction as the cohesive
characteristics of the material
tending to reduce the backfill
pressure against a wall, are
generally discounted in backfill
since cohesion cannot be
considered dependable under all
climatic conditions.
Lateral pressures against a
wall are typically presented as a
unit pressure per foot of wall
height. The following Table lists
unit lateral pressures for
various types of backfill
materials within usual ranges of
unit weights and angles of
internal friction. Portions of
this table have been taken from
"Soil Engineering", Third
Edition, by M. G. Spangler and R.
L. Handy, 1973.
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USUAL VALUES OF UNIT
WEIGHT, FRICTION
ANGLE,
AND LATERAL PRESSURE FOR
SOIL BACKFILL
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Backfill
Material
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Unit Weight
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Internal
Friction
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Unit Lateral
Pressure Per Foot Of
Height
psf/ft
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Soft Plastic Clay
Wet, Fine Silty Sand
Dry Sand
Gravel
Loose Loam
Compact Loam
Compact Clay
Cinders
Compact Sand-Clay
Water
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105-120
110-120
90-110
120-135
75-90
90-100
90-100
40
115-125
62.4
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0-10
15-30
25-40
30-40
30-45
30-45
25-45
25-45
40-50
0
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75-120
40-70
20-45
25-45
15-30
15-35
15-45
10-15
15-30
62.4
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As can be noted from the above
Table, the unit lateral pressure
varies extensively, however, a
unit lateral pressure of not less
than 30 pounds per square foot
per foot of wall height is
generally used for design
purposes, assuming a drained
condition.
The lateral pressure against a
wall increases linearly with
depth; the maximum pressure
against the wall acting at the
base of the wall. For example,
the design maximum pressure at
the base of a wall with 10 feet
of backfill adjacent to the wall
would be the respective
material's unit lateral pressure
presented in the above table
multiplied by 10 feet. Since the
shape of the pressure
distribution against the wall is
triangular, the resultant force
against the wall acts at a point
above the base of the wall equal
to one-third the backfill height.
It should be recognized that
in addition to backfill loads
against a wall, other forces may
contribute to loading, such as
swelling soils or landslide
conditions which require
individual design considerations.
Wall heights greater than 10 feet
also typically require special
consideration.
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