(e.g. rockfill) shoulders. Mobilization of core shear strength and cracking
is associated with consolidation of a clay core supported by relatively
incompressible granular shoulders. Assuming that the full undrained shear
strength of the core is mobilized in load transfer, it can be shown that the
nominal vertical total stress vat depth zbelow crest in a core of width 2a
is given byvz (^) . (2.27)
The corresponding horizontal total stress, h, is given by
hK 0 (v uw)uw (2.28)
whereK 0 is the ‘at rest’ coefficient of lateral earth pressure for zero lateral
strain.
Most of the many recorded instances of hydraulic fracturing have
occurred on first impounding or shortly thereafter. The phenomenon has
been discussed by Vaughan et al.(1970), Sherard (1973, 1985) and Lun
(1985). The risk of fracturing is moderated if a wide and relatively plastic
core zone of low to intermediate shear strength is employed, and if core
and shoulder are separated by a transition or filter zone. Evidence as to
the influence of core water content and other factors on susceptibility to
fracturing is contradictory. A laboratory simulation of fracturing and study
of the influence of certain parameters for a specific soil type is presented in
Medeiros and Moffat (1995).
(c) Cracking
Cracking other than by hydraulic fracturing is generally associated with
strain incompatibilities, e.g. at interfaces. Potentially dangerous transverse
or longitudinal cracking modes can develop from a number of causes:
- shear displacements relative to very steep rock abutments or to badly
detailed culverts; - differential strains and/or local arching across irregular foundations;
- vertical steps along the axis of a deep cut-off trench;
- progressive slope deformation and distress;
- strain incompatibility at zone interfaces within the dam.
It has been suggested that the risk of serious internal cracking due to 5
will be of increased significance for earthfill embankments if the ratio
of maximum post-construction settlement to Hcot, where His the height
of embankment and is the slope angle, exceeds 0.003–0.005 (Justo,
1973).cu
a