Multi-Method Strength Characterization for Soft Cretaceous Rocks in Texas

Modern methods for designing drilled shafts, ACIP piles and similar foundations in soft rock require knowledge of the compressive strength and modulus of the rock. However, jointing at many sites prohibits the recovery of samples of sufficient length and integrity to test rock cores in either unconfined or triaxial compression. Since rational design procedures usually require values of compressive strength, surrogate methods must be employed to estimate the compressive strength of the rock. The surrogate methods considered here are the splitting tension test, the point load index test and the TxDOT dynamic penetrometer test (in which a 76-mm-diameter solid steel cone is driven into rock at the bottom of a borehole in much the same way as a split spoon is driven during the performance of a standard penetration test in cohesionless soil). Soft rock formations typical of those for which such substitutions might be used are the upper Cretaceous formations of North Central Texas, including the Eagle Ford (clay shale) and Austin (limestone) formations. Correlations of the results of the tests listed above with compression strengths of soft rock cores from these formations are provided in the paper. The correlations are formation-dependent, most likely through the degree of cementation present in the geomaterial. The strongest and apparently most reliable correlation was between compressive strength and the TxDOT cone penetration test, although separate correlations were observed in limestone and in clay shale. A clear correlation, but with considerable variance, was found between point load index strength and compressive strength in clay shale. No clear correlations between either point load index or splitting tension strength and compressive strength appeared in the limestone.