The lecture by A. W. Skempton, titled "Long-term Stability of Clay Slopes," focuses on the long-term behavior of clay slopes, particularly the changes in shear strength over time. Skempton highlights the importance of understanding the peak and residual strengths of clay, which are crucial for assessing the stability of slopes. He explains that while clay initially exhibits high peak strengths, these can decrease significantly over time due to factors such as strain-softening and the development of orientation domains within the clay particles. Skempton discusses the concept of the "residual factor," which measures the extent to which the average strength of the clay has fallen to its residual value. He presents case studies from various locations, including the Jackfield landslide, Selset dam, and failures in the London Clay, to illustrate how the use of peak or residual strengths affects the calculated factor of safety. The analysis shows that using peak strengths can lead to significant errors in stability assessments, emphasizing the need for accurate determination of residual strengths. The lecture also delves into the physical mechanisms behind the decrease in strength, such as the formation of orientation domains and the influence of fissures and joints. Skempton concludes by advocating for the use of residual strengths in stability analyses to ensure more reliable and conservative design outcomes.The lecture by A. W. Skempton, titled "Long-term Stability of Clay Slopes," focuses on the long-term behavior of clay slopes, particularly the changes in shear strength over time. Skempton highlights the importance of understanding the peak and residual strengths of clay, which are crucial for assessing the stability of slopes. He explains that while clay initially exhibits high peak strengths, these can decrease significantly over time due to factors such as strain-softening and the development of orientation domains within the clay particles. Skempton discusses the concept of the "residual factor," which measures the extent to which the average strength of the clay has fallen to its residual value. He presents case studies from various locations, including the Jackfield landslide, Selset dam, and failures in the London Clay, to illustrate how the use of peak or residual strengths affects the calculated factor of safety. The analysis shows that using peak strengths can lead to significant errors in stability assessments, emphasizing the need for accurate determination of residual strengths. The lecture also delves into the physical mechanisms behind the decrease in strength, such as the formation of orientation domains and the influence of fissures and joints. Skempton concludes by advocating for the use of residual strengths in stability analyses to ensure more reliable and conservative design outcomes.