The Third Terzaghi Lectures; Progressive Failure in Slopes of Overconsolidated Plastic Clay and Clay Shales

All evidence indicates that slides in slopes of overconsolidated clays and clay shales are preceded by the development of a continuous sliding surface by progressive failure. Requisite conditions for progressive failure are discussed and it is proposed that a progressive failure may result from a large content of recoverable strain energy of the clay resulting from its geological history. In clays with weak diagenetic bonds the strain energy is immediately recovered upon a reduction in load. In clay shales diagenetic bonds were formed when the clay carried its maximum consolidation pressure, with the result that the recoverable strain energy is locked-in and will only be liberated when the bonds are gradually destroyed by weathering near the surface. This is the Third Terzaghi Lecture presented at the ASCE Structural Engineering Conference, at Miami, Fla., February, 1966.