Slopes In Soils Presenting Special Problems.

a. Stiff-fissured clays and shales.   The shearing resistance of most stiff-fissured clays and shales may be far less than suggested by the results of shear tests on undisturbed samples.  This result is due, in part, to prior shearing displacements that are much larger than the displacement corresponding to peak strength.  Slope failures may occur progressively, and over a long period of time the shearing resistance may be reduced to the residual value-the minimum value that is reached only at extremely large shear displacements.  Temporary slopes in these materials may be stable at angles that are steeper than would be consistent with the mobilization of only residual shear strength.  The use of local experience and empirical correlations are the most reliable design procedures for these soils.

b. Loess.  Vertical networks of interconnected channels formed by decayed plant roots result in a high
vertical permeability in loess.  Water percolating downward destroys the weakly cemented bonds between
particles,  causing rapid erosion and slope failure.

Slopes in loess are frequently more stable when cut vertically to prevent infiltration.  Benches at intervals can
be used to reduce the effective slope angle.  Horizontal surfaces on benches and at the top and bottom of the
slope must be sloped slightly and paved or planted to prevent infiltration.  Ponding at the toe of a slope must be prevented.  Local experience and practice are the best guides for spacing benches and for protecting slopes against infiltration and erosion.

c. Residual soils.  Depending on rock type and climate, residual soils may present special problems with
respect to slope stability and erosion.  Such soils may contain pronounced structural features characteristic of
the parent rock or the weathering process, and their characteristics may vary significantly over short distances.  It may be difficult to determine design shear strength parameters from laboratory tests.

Representative shear strength parameters should be determined by back-analyzing slope failures and by using empirical design procedures based on local experience.

d. Highly sensitive clays.  Some marine clays exhibit dramatic loss of strength when disturbed and can
actually flow like syrup when completely remolded.

Because of disturbance during sampling, it may be difficult to obtain representative strengths for such soils from laboratory tests.  Local experience is the best guide to the reliability of laboratory shear strength values for such clays.

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