 However, although some of these modifications gained great popularity, they do not provide better estimation for formula_12.For example, Brooker and Ireland's formula_13 has been based on the laboratory determination of formula_12 of only five samples, whilst the effective angle of shearing resistance of the three of them was obtained from the literature, having no control on them.Moreover, refinements in the order of a few percentage points rather support the validity of the formula_8 expression than the superiority of the refined expression.