The long-term stability of clay slopes is a critical issue in geotechnical engineering. This lecture discusses the long-term condition of clay slopes, where pore pressures adjust to hydrostatic equilibrium with groundwater. In natural slopes, this condition is reached, while in cuttings, it takes months or years. The long-term stability of clay slopes is determined by the shear strength parameters, which are influenced by the clay's consolidation history. Over-consolidated clays, which have been subjected to higher pressures in the past, exhibit peak and residual strengths. The peak strength is the maximum resistance a clay can offer, while the residual strength is the strength remaining after significant deformation. The residual strength is often much lower than the peak strength and is influenced by the clay's structure and orientation of particles. The shear strength of clay can be described by the Coulomb-Terzaghi equation, which relates the shear strength to effective stress. The factor of safety (F) is a measure of the slope's stability, with F > 1 indicating stability. When a failure occurs, F = 1.0. The analysis of actual slips in clay has shown that the shear strength parameters determined by conventional tests may not reflect the actual field conditions at the time of failure. This discrepancy is due to the clay's behavior under long-term conditions, including progressive failure and the development of orientation domains in the clay. The lecture presents case studies of landslides in over-consolidated clays, including the Jackfield landslide, the Selset landslide, and the London Clay. These case studies highlight the importance of considering residual strength in stability analyses, as the actual strength of the clay at the time of failure may be much lower than the peak strength. The residual factor (R) is used to quantify the extent to which the clay's strength has decreased from the peak to the residual value. The results from these case studies show that the residual strength is often close to the actual strength at the time of failure, indicating that the residual strength should be used in stability analyses for over-consolidated clays. The lecture concludes that the presence of fissures and joints in clay slopes can lead to progressive failure, with the strength of the clay decreasing over time until it reaches the residual value. In contrast, clays without fissures or joints may not experience significant strength reduction. The analysis of natural slopes in London Clay shows that the residual strength is often close to the actual strength at the time of failure, indicating that the residual strength should be used in stability analyses for over-consolidated clays. The lecture emphasizes the importance of considering the long-term behavior of clay slopes and the need for accurate determination of shear strength parameters in stability analyses.The long-term stability of clay slopes is a critical issue in geotechnical engineering. This lecture discusses the long-term condition of clay slopes, where pore pressures adjust to hydrostatic equilibrium with groundwater. In natural slopes, this condition is reached, while in cuttings, it takes months or years. The long-term stability of clay slopes is determined by the shear strength parameters, which are influenced by the clay's consolidation history. Over-consolidated clays, which have been subjected to higher pressures in the past, exhibit peak and residual strengths. The peak strength is the maximum resistance a clay can offer, while the residual strength is the strength remaining after significant deformation. The residual strength is often much lower than the peak strength and is influenced by the clay's structure and orientation of particles. The shear strength of clay can be described by the Coulomb-Terzaghi equation, which relates the shear strength to effective stress. The factor of safety (F) is a measure of the slope's stability, with F > 1 indicating stability. When a failure occurs, F = 1.0. The analysis of actual slips in clay has shown that the shear strength parameters determined by conventional tests may not reflect the actual field conditions at the time of failure. This discrepancy is due to the clay's behavior under long-term conditions, including progressive failure and the development of orientation domains in the clay. The lecture presents case studies of landslides in over-consolidated clays, including the Jackfield landslide, the Selset landslide, and the London Clay. These case studies highlight the importance of considering residual strength in stability analyses, as the actual strength of the clay at the time of failure may be much lower than the peak strength. The residual factor (R) is used to quantify the extent to which the clay's strength has decreased from the peak to the residual value. The results from these case studies show that the residual strength is often close to the actual strength at the time of failure, indicating that the residual strength should be used in stability analyses for over-consolidated clays. The lecture concludes that the presence of fissures and joints in clay slopes can lead to progressive failure, with the strength of the clay decreasing over time until it reaches the residual value. In contrast, clays without fissures or joints may not experience significant strength reduction. The analysis of natural slopes in London Clay shows that the residual strength is often close to the actual strength at the time of failure, indicating that the residual strength should be used in stability analyses for over-consolidated clays. The lecture emphasizes the importance of considering the long-term behavior of clay slopes and the need for accurate determination of shear strength parameters in stability analyses.