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# RE: friction coefficient on rock

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: friction coefficient on rock
• From: "Joe Grill" <jgrill(--nospam--at)swiaz.com>
• Date: Thu, 28 Oct 2004 10:56:42 -0700

```Lloyd,
Yes, in my second post I said I was going to show some dowels into the rock
as that is what the geotech. suggested.  We have a lot of excavations around
here that end up on rock and generally the contractors prefer to dowel
rather than cut the rock for a key.  Epoxy dowel values given are generally
for embedment into concrete or in some cases CMU, but of course not rock.
As an old engineer friend of mine told me once, "check for belts and
suspenders if you can".
Joseph Grill, PE

-----Original Message-----
From: L. Pack [mailto:Lloyd(--nospam--at)pecid.com]
Sent: Thursday, October 28, 2004 10:38 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: friction coefficient on rock

On 28 Oct 2004, at 9:29, Joe Grill wrote:

> After excavation it is found that a retaining wall will be founded on
solid
> rock (sandstone). Can anyone give me a friction coefficient for this
> condition if any?  I don't have a reference that addresses this situation.
>
> Thanks!
>
> Joe Grill
> Joseph R. Grill, P.E.
> jgrill(--nospam--at)swiaz.com

Joe,

I've found a couple of references for frictional resistance to sliding,
with respect to foundations.  This is the inertial resistance and not
the resistance once sliding has begun.  In my foundations book,
by John Cernica, he states that for concrete on soil ( I know that
you said rock) the use of a friction coefficient of 0.5 is typical.
(Foundation Design, copywrite 1995, p. 324)

In Foundation Engineering, 4th ed., Braja M. Das, on pp 398-400,
He uses the an adjustment factor of 1/2 to 2/3 of the friction angle
of the material that the foundation bears on, and takes the tangent
of that product to arrive at the friction coefficient. So, if the internal
angle of friction of the rock is 45 degrees and we take half of that,
22.5, and take the tangent of that; we get .41 for a coefficient
of friction.  I expect that the internal angle of friction for rock may
be somewhat higher, but you'd be conservative with the above
friction coefficient.

Since sandstone is easy to cut a keyway in, why not just key
the retaining wall into the rock?  Or you could but dowels into
the rock and provide shear resistance that way, too.

HTH,
Lloyd Pack, PE

Thank you,
Lloyd Pack, P.E.
Project Engineering Consultants, LTD.
(208)466-7190
(208)466-7168 Fax
(208)250-2992 Cell

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