Recent research now indicates that, rather than slope failure being the eventual result of the strength required to maintain equilibrium being greater than the residual strength of the soil (i.e. the soil's shear strength after large deformation), the strength mobilized in first time failures is significantly higher than the residual strength and corresponds more closely with the fully softened or "critical-state" strength of the clay. First time failures are preceded by movements too small to reduce the strength of the soil to its residual value. The apparent mobilization of only residual strength in old, natural slopes is probably due to a history of successive slides in the material resulting in substantial displacements. It is also felt that the use of residual strength as the limiting strength is too conservative in many cases. Another important phenomenon which should be taken into account is the substantial increase in water content in a thin zone containing the failure surface. This increase must be associated with dilation of the soil in the failure zone and concomitant migration of pore water into the failure zone. It has been known for many years that the strength of a clay is a function of its water content. Migration of pore water into the failure zone will cause a continued decrease in strength until the fully softened condition is reached. The rate at which this softening occurs therefore depends to a large extent on rate of water movement in the clay and thus is strongly affected by the permeability of the clay.

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  • Supplemental Notes:
    • Discussion of Proceeding Paper 13342 by John D. Nelson and Erik G. Thompson, November, 1977.
  • Corporate Authors:

    American Society of Civil Engineers

    345 East 47th Street
    New York, NY  United States  10017-2398
  • Discussers:
    • Cheney, J A
  • Publication Date: 1978-9

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Filing Info

  • Accession Number: 00183218
  • Record Type: Publication
  • Report/Paper Numbers: ASCE 13999
  • Files: TRIS
  • Created Date: Nov 14 1978 12:00AM