ROCK STRENGTH AND FAILURE MODES

ROCK STRENGTH AND  FAILURE MODES

(Rakesh Debnath)

The failure mode  is very  significant  to decide  upon true  strength of  rocks. Usually, hard  brittle  rocks  fails  in  longitudinal splitting  gives  the  maximum strength.  Rock  samples also  fail  in  simple  shear  or  multiple  shear  which  gives  relatively  lower  strength  compare  to longitudinal splitting.  The  stress-strain curves  for  brittle  rock material  under  uni-axial compression could be  divided into four  phases  namely  crack closure, linear  elasticity,  stable crack  growth  and unstable  crack  growth. Consequently, the  rock fails  with fractures developed  from  the coalescence of  several  micro  cracks. As  failure  modes  of  rocks  could provide  useful  information, the  examination  of  failed specimens  would  be  very  helpful  in design. The  relative  predominance  of  the  two failure  modes  depends  on the  strength, anisotropy,  brittleness  and  grain  size  of  the  crystalline  aggregates.  Common modes  of  failure in rock sample  under  compression are  shown in  Figure  2.23  (Szwedzicki, 2007).

The  failure  mode  of  a  brittle  rock changes  on the  application of  confining pressure  because usually  under  unconfined compression a  rock tends  to deform  elastically  until  failure occurs abruptly  (Figure  2.24a).  With moderate  amount  of  confining  pressure, longitudinal  fracturing is  suppressed  and  failure  occurs  along  a clearly  defined  plane of  fracture  (Figure  2.24b).  At very  high  confining  pressure  rock becomes  fully  ductile  (Figure  2.24c).

Rakesh Debnath

*Module  2:  PhysicoMechanical Properties  of  Rocks(NPTEL)#REF