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
	K_v	-	modulus of subgrade reaction
	b	-	footing width
	C_w	-	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 C_w is determined as follows:

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

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

Where:	h_GWT	-	depth of groundwater table from the terrain
	d	-	depth of the footing bottom

Modulus of subgrade reaction K_v depends on relative density factor I_D:

Determination of modulus K_v (Navfac, 1982)

Relative density factor I_D is determined as follows:

σ_zp ≤ 1.5 [ksf]:

σ_zp > 1.5 [ksf]:

Where:	N	-	uncorrected blow count at a depth of b/2 under the footing bottom
	σ_zp	-	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

GEO5