Friction resistance of no fines gravity wall

[tristrium]

Does anyone have any information on the coefficient of friction of a segmental retaining wall with gravel or no fines backfill on the concrete base?

It seems to be general practise to interchange either a concrete or crushed rock base, but I am concerned that the concrete base will provide a lower frictional resistance.

Keystone suggest a value of 205+0.3N (lbs/lf).(effectively a coefficient of 0.3). I have also seen values of 0.8 for concrete on concrete (PCI design handbook). This seems like quite a range.

 

[BridgeSmith]

Is the COF value from PCI for interfaces between 2 precast concrete members? I might expect that high for CIP against precast, but not precast on precast.

In AASHTO, Table C3.11.5.3-1 (taken from US Dept. of the Navy) give COF values for precast concrete against various materials ranging from .40 to .49 (clean gravel, gravel-sand mix, etc.) down to .25 (silt, fine sandy silt). The 0.3 may be conservative, based on what I'm seeing, 0.5 might be overestimating the friction.

 

[EireChch]

0.3 is definitely too conservative. 2/3 phi is more appropriate.

 

[tristrium]

EireCHch

Yes I would normally use 2/3 phi which would be appropriate for a crushed rock base. I am concerned that if the crushed rock is substituted with concrete then the sliding resistance will actually be reduced. But I see concrete being substituted for crushed rock on a regular basis.

 

[BridgeSmith]

For crushed rock with a phi of 34 degrees, the typical COF value of TAN(2/3 phi) is 0.42. 2/3 TAN(phi) is .45. Either way, it seems fairly similar to what we're expecting for concrete, at least unfinished or rough-finished concrete. If the concrete was finished smooth, there could be a significant reduction in the COF.

 

[MTNClimber]

Always use what the manufacturer recommends as they perform lab tests on their products to develop design criteria. Are you using Keystone or a different product?

 

[NickWarrier]

@tristrium.

I Have been advised by Redirock - Another large Big Block supplier to reduce friction by a factor of 0.8 when concrete is used - But I usually put broom finish on concrete in my notes to discourage smooth surfaces and still use a reduction factor although I am tended to use 0.9 over sites where I have more control.