In Eq. 6.31, the term 2(B L) represents the entire perimeter of the spread footing. When this term is multiplied by T, it represents the total perimeter area that the footing must push through in order to reach the liquefied soil layer. For an assumed footing size and given loading condition, the only unknowns in
Eqs. 6.30 and 6.31 are the vertical distance from the bottom of the footing to the top of the liquefied soil layer T and the shear strength of the unliquefiable soil layer t
. The value of T would be based on the liquefaction analysis (Sec. 13.4) and the proposed depth of the footing. The shear strength of the unliquefiable soil layer tf can be calculated as follows