The post-peak drop in drained shear strength of an overconsolidated clay may be considered as taking place in two stages. First, at relatively small displacements, the strength decreases to the 'fully softened' or 'critical state' value, owing to an increase in water content (dilatancy). Second, after much larger displacements, the strength falls to the residual value, owing to reorientation of platy clay minerals parallel to the direction of shearing. If the clay fraction is less than about 25% the second stage scarcely comes into operation; the clay behaves much like a sand or silt with angles of residual shearing resistance typically greater than 20 degrees. Conversely, when the clay fraction is about 50%, residual strength is controlled almost entirely by sliding friction of the clay minerals, and further increase in clay fraction has little effect. The angles of residual shearing resistance of the three most commonly occurring clay minerals are approximately 15 degrees for kaolinite, 10 degrees for illite or clay mica and 5 degrees for montmorillonite. When the clay fraction lies between 25% and 50% there is a 'transitional' type of behaviour, residual strength being dependent on the percentage of clay particles as well as on their nature. The post-peak drop in strength of a normally consolidated clay is due only to particle reorientation. Measurements of strength on natural shear surfaces agree, within practical limits of variation, with values derived from back analysis of reactivated landslides. This 'field residual' strength can be recovered by multiple reversal shear box tests on cut-plane samples, but in high clay fraction materials it is typically somewhat higher than the strength measured in ring shear tests. Residual strength is little affected by variation in the slow rates of displacement encountered in reactivated landslides and in the usual laboratory tests, but at rates faster than about 100 mm/min qualitative changes take place in the pattern of behaviour. A substantial gain in strength is followed, with increasing displacement, by a fall to a minimum value. In clays and low clay fraction silts this minimum is not less than the 'slow' or 'static' residual, but in clayey silts (with clay fractions around 15-25% according to tests currently in progress) the minimum can be as low as one-half of the static value. (Author/TRRL)

  • Supplemental Notes:
    • This paper was delivered as a special lecture to the British Geotechnical Society on 6 June 1984.
  • Corporate Authors:

    Institution of Civil Engineers

    One Great George Street, Westminster
    London,   United Kingdom  SW1P 3AA
  • Authors:
  • Publication Date: 1985-3

Media Info

  • Features: Figures; References; Tables;
  • Pagination: p. 1-18
  • Serial:
    • Volume: 35
    • Issue Number: 1
    • Publisher: Thomas Telford Limited
    • ISSN: 0016-8505

Subject/Index Terms

Filing Info

  • Accession Number: 00451368
  • Record Type: Publication
  • Source Agency: Transport Research Laboratory
  • Files: ITRD, TRIS
  • Created Date: Aug 27 2004 9:57PM