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Basement walls, sandstone

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Hi All,

I am trying to ascertain what type of basement wall is required
for holding back a highly weathered, highly fragmented sandstone 
hill, which has a shallow (2') top layer of non-expansive clay.
The soil is highly competent, and a freshly excavated sheer vertical
slope 18' high can stand on its own, without external shoring.

The extremely high cohesiveness (can cohesiveness even be applied 
to rock?) prevents the Coulomb or Rankine methods from yielding 
any meaningful figures for the soil to wall stresses.

I would prefer to use a cantilever wall for this application
without tiebacks (sewer easement behind wall).  It wouldn't seem
prudent to assume the soil (rock) cohesiveness to be zero as a 
worst case, as the resultant design would be massive overkill.
  
Assuming that the wall is backfilled with a light, yet structural
medium such as Elastizell (thanks Harold), is it reasonable to design
the wall to handle just the unit stress of the backfill plus an
appropriate safety factor?  My reasoning here is that even if an 
individual rock comes loose, it would be held motionless by the 
backfill and still be supported by the rock wall still below it.
Comments greatly appreciated.


Thanks,

Greg Leyh