Creep and Lateral Stress Behavior of Gulf of Guinea Miocene Mudrock Specimens

ABSTRACT We measured axial creep and lateral stress behavior in drained uniaxial strain experiments on intact and resedimented Gulf of Guinea Miocene kaolinite-rich mudrock specimens. At axial effective stress of ∼10 MPa (close to the in situ vertical effective stress of 7.9 MPa), the uniaxial lateral stress ratio (K0) is 0.75 for the resedimented material and 0.62 for the intact material. The intact material behaves over-consolidated because its in situ state (7.9 MPa) is less than the yield stress (∼20 MPa). The compressibility of the resedimented specimen is an order of magnitude higher than that of the intact specimen. The in situ void ratio is ∼0.15 void ratio units less than the void ratio of the resedimented specimen at the in situ vertical effective stress; this suggests that creep deformation has occurred over geological time. In addition, the creep rate of the resedimented material is an order of magnitude higher than the intact material. Overall, this paper illuminates the role of creep in the compression process. INTRODUCTION Intact specimens of in-situ clay deposits are usually impacted by sample disturbance (Lunne et al., 2006), especially for deep or offshore samples. The resedimentation technique has been used to prepare uniform test specimens that do not have sampling disturbance, and thereby study sediments for over half a century. It allows for control over stress history and produces specimens of desired porosity with complete saturation. It is commonly assumed that the laboratory resedimented material closely reproduces the mechanical behavior of intact specimens (Casey et al., 2016; Casey et al., 2019). However, recent studies have found that the resedimented specimens are usually less dense than the intact specimen at the in-situ vertical stress level (Finnegan, 2020), and the intact material (with no apparent geological unloading) shows measured yield stress markedly greater than the maximum past stress (Ewy et al., 2020; Finnegan, 2020). Such behaviors suggest that creep may have played a role in-situ. We systematically study intact and resedimented specimens to explore their behavior and understand mudrocks’ in-situ properties.