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Re: Basement walls, sandstone

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Earthpressure theories are based on semi-infinite soil mass.  Properties
of material in the limited amount of backfill you will have behind the
wall wil not induce significant pressure on the wall.  In short,
pressure on the wall is more a function of the native soil behind the
backfill.  Since the native "soil" is rock in your project, its "phi"
value is high and consequently, active pressure from it to the wall can
be ignored.  By the way, what is the height of the wall? I noticed your
statement that 18' high cut can remain without shoring, but is that the
height of the wall?

Though earthpressure can be ignored, you will have to assume hydrostatic
pressure behind the wall, because the rock is highly weathered and

Therefore design the wall for hydrostatic pressure.  Also, provide weep
holes to drain water from backfill.  By the way, you can use the
excavated material for your backfill.


Greg Leyh wrote:
> 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