Mae Moh lignite mine is the largest open pit mine in Thailand. The current mine operation must deal with over 250 m depth. Currently, a new pattern of small cracks on a road link located on the top of the slope has been noticed. It is believed that these cracks would cause from the creep movement of claystone layers which are a main component of the cut slopes of Mae Moh mine. This would not currently affect to the main stability of the open-pit mine slope but it should be investigated to profound the long-term stability of the mine. Creep of a material is the time-dependent behavior of a material and it refers to the continued deformation under an effect of a constant stress condition. This study aims to investigate and model creep behavior of Mae Moh claystone based on specially designed laboratory creep tests. Such creep test facilities were designed and set up to address the muti-stage creep tests on the soft rock of the Mae Moh claystone. Claystone collected from the Mae Moh cut slope were subjected to three compressive stress levels with a 24-hr time interval for each stress level. A series of creep deformations corresponding to applied stress levels and time were observed. The elasto-plastic soft soil creep model was firstly studied to model the creep behavior of the Mae Moh claystone. Results shows that the designed creep test facility which can accommodate the 50 kN compressive load and the 5 MPa confining pressure, by which within a practical range of stress conditions of the Mae Moh open pit mine, creep behavior of claystone was well investigated. Furthermore, the elasto-plastic soft soil creep model can well describe the creep behavior of the Mae Moh claystone with a good agreement between test results and a modelled creep curve.

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