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# Coefficient of Friction b/t clay and concrete base

## Coefficient of Friction b/t clay and concrete base

(OP)
Anyone have a table or a reference?  I have a sand reference but nothing on clay?  Also, is this dependent on the adhesion like a pile?

Thanks.

### RE: Coefficient of Friction b/t clay and concrete base

Table 11-6 in my 3rd edition of Bowles Foundation Analysis and Design gives a friction angle of 17 - 19 for a medium stiff & stiff clay & silty clay to mass concrete; and a value of 22 - 26 for very stiff & hard residual or preconsolidated clay to mass concrete.

As for adhesion, I have always used 0.6 times the cohesion for cast in place concrete.

Of course any resistance value should be limited by the less of the adhesion and friction values.

### RE: Coefficient of Friction b/t clay and concrete base

tocoa,

NAVFAC DM 7.2 Table 1 p63 shows friction angle of 17-19 degrees for concrete/cohesive soils. It's common practice to take the coefficient as 0.3.

Jeff

Jeffrey T. Donville, PE
TTL Associates, Inc.
www.ttlassoc.com

The views or opinions expressed by me are my own and do not necessarily reflect the views or opinions of my employer.

### RE: Coefficient of Friction b/t clay and concrete base

Disregard the last line of my last post, it is not always the case.  Whether you use the friction or adhesion value will depend on the type of loading and soil type.  Just remember that the soil/concrete boundery cannot exceed the values in the soil imeadiately below the boundry.

### RE: Coefficient of Friction b/t clay and concrete base

GeoPaveTraffic: I would like to see your 2 cents regarding when either friction or adhesion values should be used.

### RE: Coefficient of Friction b/t clay and concrete base

I will first state that I believe that most, not all, clays have some amount of long term cohesion; i.e. I do not believe that all clays by default have a long term c=0.

That said, for most retaining wall situations, I would use the friction value between the wall and the soil.  If I had actual lab testing consisting of drained test results for the soil at the bottom of the wall, then I would use 2/3 of the drained friction angle plus 0.3 or so times the cohesion.

My experience with clay soils and retaining structures, indicates that most wall designs are not controlled by base sliding, but are controlled by global stability or overturning.  Where it gets ify is when you have a clayey silt kind of soil with a relatively high drained strength.  In those cases sliding of the retaining wall can controll and you have to make a judgement on how much, if any, of the adhesion to include.

I would also point out that it is relatively simple to add a shear key to the bottom of footing in clay or silty clays.  This pretty much gets rid of the question of how much of the adhesion to use since a good portion of the potential failure plan passes throught the soil and not at the boundry between the materials.

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