Passive pressure coefficient 1

[BadgerPE]

Hey all. I am designing a concrete deadman to support an incorrectly built frost wall. The soil it will be built in is a medium stiff clay, but I have no soils report on the project. Is there a way to determine the passive pressure coefficient of the soil, i.e. assumed cohesion or something, that will give me an allowable value of Kp. If so, what code would it be found in. Thanks much!

 

[tai99]

First of all, you shall do a research on the previous soil report for the faulty frost wall.
If without success, I will assume sandy/gravel fill to come up a low passive coefficient for trail estimate use. If it works, great. Otherwise, you will need to order a soil testing to get the actual parameters.

 

[fattdad]

You should not design long-term strength based on cohesion. Using a friction angle of like 20 degrees or so, you get a coefficient of passive earth pressure of about 2.0. For deadmen you can also use the term Cp, which is about phi/2 (Cp adjusts for the fact that this is not plane strain loading). This would give you a Kp of about 4.0. Use this with the vertical earth pressure and you'll get the horizontal reaction acting on the deadman.

Hope this helps.

f-d

¡papá gordo ain’t no madre flaca!

 

[BadgerPE]

Fattdad

Thank you for the good explanation and information. Also, could you explain a little more about the Cp factor?

 

[BigH]

I know engineers who won't use more than 3 for Kp - regardless of the computations. Suggest you might want to look at Bowles 5th Edition on "Block Anchor" i.e., deadman. (Pages 778 to 781).

 

[tai99]

Pay attention to potential future excavation activities around the deadman, and in route the tie backs to be installed. You need to setup warning system similar to protection of underground utilities.

 

[fattdad]

Cp is roughly equal to phi/10 (not phi/2 as I posted above)
Brinch-Hanson has published on this value (1966)
Limiting Kp (refer to BigH's post) is a good thing, especially if there is concern on strain compatability.

f-d

¡papá gordo ain’t no madre flaca!

 

[mudman54]

Bowles textbook offers some specific advice for design of dead man anchors. theoretically, if your soil is cohesive and phi = 0 degrees, Kp is 1.0. you will need to use Bell's equation for the cohesive passive pressure.

 

[fattdad]

theoretically, all soils have a friction angle.

f-d

¡papá gordo ain’t no madre flaca!

 

[mudman54]

most practicing geotechnical engineers will use phi=0 and C= 500 to 1000 psf for a medium stiff clay; the equation you want to use for passive pressures is unit weight * height of soil * Kp + 2 * C * square root of Kp

 

[tai99]

As pointed out by fattdad, I would be hesitant to utilize cohesion for this type of application, especially when setup is at/near the ground surface. If it couldn't be avoid, the use of a greater safety factor should be considered.

 

[fattdad]

+1. For long term strength I use friction angle, even for clay. I'd also look at the phi=0, Su=C strength criteria for short term conditions.

I guess I just thought this was standard stuff covered in most graduate programs in geotechnical engineering. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[tai99]

-1, from another angle