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Shear Strength of Grouted Rip Rap Slope

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Hello group:

I have just joined the group with the idea that I will learn more about the
1997 UBC plywood diaphragm and shear wall requirements.  I'm of the old country
 and still look at them as flexible.  But that's not why I'm here now.

I was informed about a channel currently under construction (name of project
with-held) that our Civil Department designed.  Apparantly, at the site, the
weepholes at the bottom of the channel were placed at the wrong elevation.

The channel height varies from 15 to 25 feet.  The lining slopes at 1.5:1 and
the thickness is 2.53 feet with 0.83 feet exposed and ungrouted (1.7 feet of
grout).  The weepholes were supposed to be at 12" above the invert, and are
between 4 and 7 feet above invert elevation.

My initial comment was to provide new weepholes at the correct elevation with
less that called for spacing.  The permeable, fabric, and screen material won't
 be able to be placed unless the lining is demo'd and replaced.  This poses a
problem with adequate drainage of the bottom section.  Since the existing
weepholes will provide relief for the top portion, I've been asked to
investigate the hydrostatic effects on the lower 4 to 7 feet of rip rap.  I'm
used to using the standard Vc=2sqrt(f'c)bd equation for beams, slabs, and
footings.  Problem here.  Grouted rip rap is not your typical poured in place
concrete section.

With boulders as large as 12" to 24" in diameter and poor concrete placement,
is there an acceptable reduction of the "Vc" equation or better yet, a
different approach (and accurate one) to check the shear strength of the
section?

Respectfully,

Eric A. Johnson, P.E.
Anaheim, CA