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NAVFAC DM7 Method (SPT)

Settlement of spread footing using SPT tests according to NAVFAC DM7 method is based on the formula:

Where:

s

-

settlement of spread footing

C

-

footing width factor

σ

-

stress in the footing bottom

Kv

-

modulus of subgrade reaction

b

-

footing width

Cw

-

GWT influence factor

The formula is derived for imperial units [tsf, tcf, ft] - the program calculates automatically in the units used in the program.

Footing width factor C is determined as follows:

b < 20 ft -> C = 4

b > 40 ft -> C = 2

Intermediate values C are interpolated.

GWT influence factor Cw is determined as follows:

GWT is located between the terrain level and depth of 1,5.bef below the footing bottom:

GWT is located below the depth of 1,5.bef below the footing bottom:

Where:

hGWT

-

depth of ground water table from the terrain

d

-

depth of footing bottom

Modulus of subgrade reaction Kv depends on relative density factor ID:

Determination of modulus Kv (Navfac, 1982)

Relative density factor ID is determined as follows:

σzp 1.5 [ksf]:

σzp > 1.5 [ksf]:

Where:

N

-

uncorrected blowcount at a depth of b/2 under the footing bottom

σzp

-

geostatic stress at a depth of b/2 under the footing bottom

The formula is derived for imperial units [ksf] - the program calculates automatically in the units used in the program.

Literature:

Navfac, 1982

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