USE OF CYCLOIDAL ARCS FOR ESTIMATING DITCH SAFETY

In this closure to the discussion of his paper, the author observes that the suggestion that equations developed from the assumption of a cycloidal failure surface lend themselves to estimating the safe distances back from a ditch for surcharge loadings, have led to the development of a more generalized graph and equation which provide a solution for setback distance without computing the angle, theta. Although the comparison of the critical cyloidal shear failure surface with the Rankine-Terzaghi failure geometry wherein both start from the bottom of the trench at the angle of 45 + (phi/2) and rise to become vertical at the point of emergence is valid, it should not be construed as also being coincident at the surface. It is also shown that cycloidal surfaces emerge behind the tension cracks and that the tensions cracks actually intersect the cycloidal surfaces. The horizontal distance from the point of emergence of the cycloidal surface to the tension crack would be a function of the tensile strength that could be developed in the soil. Terzahi's observations indicate that the tension crack extends below the cycloidal surface and suggests the existence of tensile stresses being exerted and resisted across the surface and may explain how the prism of the soil can remain standing when the shear stresses exceed the mobilized shear strength. On the other hand, a surcharge over the area between the tension crack and the emergence of the cycloidal surface would depend upon the tensile strength of the soil for support, and is not recommended. The 33 percent greater depth indicated as safe by Rankine-Terzaghi formula is believed to depend upon tensile strength in soil as well as shear strength. Corrections to be made in the original paper are set forth.

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  • Accession Number: 00080527
  • Record Type: Publication
  • Report/Paper Numbers: Proc. Paper 10897
  • Files: TRIS
  • Created Date: Feb 27 1975 12:00AM