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

The types of extrusion:

The types of extrusion:

the study of extrusion phenomena was  perfected by analysing maps of  displacement at given points measured  topographically on a large number of  faces in different types of ground and under a very wide variety of stress strain conditions.

• It was established from these studies  that  the  advance core extrudes from  the  face  according to tree basic types  of deformation depending on the  type  of material  involved and the stress it is subject to.

• cylindrical extrusion: the face presents  translational movement parallel to the  alignment of  the  tunnel with intensity  increasing from top to bottom;

•spherical dome-like extrusion:  maximum extrusion at the Centre of the  tunnel or immediately below it;

 •combined  cylindrical  domelike  extrusion:  the  wall  of  the  face presents  movement  which  is  a  combination  of  the  two  previous types  of  movement  described.  It  is  the  type  of  extrusion  which  is encountered  most  frequently  in  practice.

•if, however the stress state is high,  then on the contrary, it will be necessary to  act  above all on the core  reinforcing it with longitudinal  intervention  and  calibrating  radial  measures  after  the  passage of the face appropriately. If  a  medium  is  stressed  in  the  failure  range,  it  is  of  primary  importance  to  stiffen  the  advance  core  with  preconfinement  of  the  cavity, which  may  be  supplemented  with  appropriate  action  to  confine  the cavity  after  the  passage  of  the  face.  In  these  cases,  experience  (and that  described  in  the  previous  paragraphs  is  particularly  significant) advises

• working  ahead  of  the  face  on  the  form  and  volume  of  the  advance core by creating a protective crown of improved ground around it. This  method  was  used  effectively  on  the  construction  of  the  Vasto tunnel to advance through particularly difficult ground.

Ref:From the research to ADECO-RS
Author:
Pietro Lunardi
Italy

EDIT -Rakesh Debnath

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The reaction at support A of the beam shown in below figure, is

[A]. zero [B]. 5 T [C]. 10 T [D]. 1 T [E]. 4 T. Answer: Option A

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