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 groundwater table from the terrain |
d | - | depth of the 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 blow count 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