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# Friction between soil and r/f concrete2

## Friction between soil and r/f concrete

(OP)
Hi,

Could someone provide me information on friction coefficient between soil and reinforced concrete surfaces, please?

Or else, the method to calculate the friction coefficient between the two surfaces.

### RE: Friction between soil and r/f concrete

If the material is cohesive, I generally use half the bearing value (shear value via Mohr's) and a SF of 5.  For example a 3000 psf clay would yield 1500 for shear and 300 for shear at the pile soil interface.  This does not work for sensitive clays, etc.  Check with a local geotekkie for local soils.

### RE: Friction between soil and r/f concrete

The concrete/soil interface is pretty rough; the actual failure usually occurs some distance away within the soil itself.  As a result, the friction coefficient between soil and concrete is seldom of practical importance.

Talk to your geotechnical consultant -

### RE: Friction between soil and r/f concrete

(OP)

To check the stability of a retaining structure against sliding it is necessary to know the friction at the interface. I believe that depending on the surface texture of the concrete this value varies and I couldn't find any relevant material.

### RE: Friction between soil and r/f concrete

Hi dilukshi,

Check NAVFAC DM-7.2 pg.7.2-63, downloadable from the web at Vulcanhammer, I think.

Regards

VOD

### RE: Friction between soil and r/f concrete

If you are designing a cantilever retaining wall against sliding the CRSI handbook provides specific guidance and examples on the subject.

### RE: Friction between soil and r/f concrete

Generally, for sandy (frictional) soils a value of 3/4 friction angle of soil would be a good value.  Therefore, the friction coefficient would be tan(3/4 Fi).  In clayey soil o.5 undrained shear strength would be reasonable for medium to stiff clays.

### RE: Friction between soil and r/f concrete

Careful with using 0.5*cohesion - that is an upperbound value for weaker soils.  Use 0.3 for stronger soils.  And remember that you still need a factor of safety to get to allowable friction.

Why doesn't your geotech give you these parameters?

### RE: Friction between soil and r/f concrete

Daryoush seems to be right, in my practice, 2/3 fi is quite popular value for design purpose.

### RE: Friction between soil and r/f concrete

It also depends on the type of application.  We find that in the case of driven piles the NAVFAC values are overly conservative.  However, when the soil is exposed and the concrete foundation is poured against the soil, we would probably need to be more conservative in our estimation of the undrained shear strength of clayey soil as well as the friction angle of the sandy soil.  There is a fairly recent paper by Duncan which has a very useful discussion of safety factors in different categories of projects.  The safety factor is selected on the basis of the soil data as well as the project conditions.

### RE: Friction between soil and r/f concrete

Your reference to Duncan's paper was timely - I just sent the citation to BigH.

Duncan, J. Michael, "Factors of Safety and Reliability in Geotechnical Engineering", Journal of Geotechnical and Geoenvironmental Engineering Vol. 126, No. 4, April 2000.

It's very readable - and quite practical.  And it will change the way you look at risk -

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