Drained Residual Shear Strength of Some Claystones from Front Range, Colorado

Drained residual shear strength data on claystone (shale) bedrock samples from seven sites along the Front Range in Colorado collected from various sources are summarized. The claystone bedrock specimens from these sites were from Dawson, Laramie, Arapahoe, and Denver formations. The testing procedures included reversal direct shear and torsional ring shear tests. Upper bound, lower bound, and average residual shear strength envelopes were developed using the data from the seven sites. The secant residual friction angle varied from an upper limit of about 30° to a lower limit of about 5.5° for effective normal stress ranging from 30 to 960 kPa. Selected published correlations relating the drained residual friction angle to the liquid limit, plasticity index, clay fraction, and effective normal stress were also compared to the data set. For this particular data set, the plasticity index-based correlations compared better than the liquid limit-based correlations. It is recommended that if any published correlation between residual strength and Atterberg limits and/or clay fraction is to be used, then the same sample preparation procedure should be used in the determination of these properties as were used in developing the particular correlation.