In situ stress state and strength in mudrocks

Abstract

AbstractThe stress state of mudrocks buried under uniaxial strain conditions is defined through a large number of laboratory triaxial tests performed on water‐saturated resedimented mudrocks from a diverse set of geologic backgrounds. Unique relationships are found between the horizontal stresses that develop during normal uniaxial compression (given by K0NC), critical state friction angle, and shear strength during undrained loading. Tests were performed over the effective stress range of 0.1 to 100 MPa. Smectite‐rich mudrocks display a more rapid reduction in shear strength with increasing effective stress, which corresponds with a more rapid increase in horizontal stresses. The relationship between horizontal stresses and critical state friction angle found in this study compares favorably with the well‐known correlation of Jâky (1948), which was developed for very low stresses, even for friction angle values as low as 11°. Results for one mudrock suggest that this relationship also applies to mudrocks sheared from an unloaded (overconsolidated) state. The correlation between friction angle and K0NC is independent of the stress path applied during the compression phase of a test. This is not the case for shearing under undrained conditions, however, and the application of a stress path that produces uniaxial compression is necessary to measure a reliable shear strength. Systematic variations in K0NC and strength properties reflect an overall change in the shape and orientation of a mudrock's yield surface with effective stress level. The results of this study can aid in estimating the in situ stress state and strength properties of mudrocks, and this will have significant impact on a range of geoscience and engineering problems.