Residual strength: Does BS 5930 help or Hinder?

BS 5930 offers little assistance to engineers wishing to use residual strength parameters in slope stability analysis. It wrongly suggests the ring shear gives lower parameters than the shear box. BS5930 does not mention the fact that the residual strength is stress dependent, hence the failure envelope is curved and the parameters must be assessed using an appropriate effective normal stress. For this reason the correlation charts relating ø ^′ _R to plasticity index or clay content need replacing with a series of charts in which these properties are plotted against ø ^′ _R values obtained at a number of effective normal stress loadings. Even then such correlations should be treated with caution.