Stress-controlled direct shear tests were made on laboratory-desiccated specimens of kaolin clay (W sub D approximately - 1.5 percent). The standard apparatus was modified to permit progressive adsorption of water through the bottom porous plate. Various levels of constant shearing stress pi and a vertical stress of 24 kPa (3.5 lbf/sq in.) were applied, and shearing deformation measurements to failure were taken. When the deformation rate accelerated markedly, failure was defined for that particular pi and water content w. Failure surface w determinations were confirmed by an independent study of time versus wetting rate. Deceleration in wetting rate with increasing water content indicated decreasing negative pore pressure or soil water suction. Suction increases shearing resistance by its "reinforcement" of effective normal stress between soil solids; it is greater at lower w and reduces most at w greater than 23 percent--the point of contraflexure in the wetting rate versus w curve--which is followed by swelling. Higher constant stress levsls produced initial deformations at lower w and at higher deformation rates. The approximately linear relation between applied pi and w at initial deformation indicates that increases in w account for decreases in interparticle shearing resistance and corresponding losses of strength. This is confirmed by qualitative analysis of test results in relation to the Coulomb equation and the principle of effective stress. Because the maximum strength of specimens at W sub D was greater than maximum strengths between 1.5 and 23 percent, stress-controlled tests were replaced by strain-controlled tests; therefore, peak and residual strengths were obtained on desiccated specimens rewet to w in this range and held constant during stress application. These tests were extended to W sub E (equilibrium W approximately 35 percent) for comparison with stress-controlled test results. /Author/

Media Info

  • Media Type: Print
  • Features: Figures; References;
  • Pagination: pp 20-23
  • Monograph Title: Moisture and frost-related soil properties
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00193796
  • Record Type: Publication
  • Files: TRIS, TRB
  • Created Date: May 11 1979 12:00AM