Deadman at greater depth 3

[gojgo]

Can someone please explain this:
"Capacities of anchors at greater depth below the ground surface may be taken as the bearing capacity of the footing located at a depth equal to the midheight of the anchor (Terzaghi 1943)"
I have read this in many books but not seen any solved example! What is meant by greater depth? with different author, the ratio of h/H (h is the height from ground level to the top of deadman and H is the height of the deadman) varies from 2 to 5, which one is right? And have anyone tried using different material then concrete block or sheet pile as a deadman?

 

[GC_Hopi]

Your question is not clear and the Terzaghi quote appears to reference something in the previous/above text which you have not provided. Are you trying to determine the capacity of a deadman? There are a few methods. Caltrans keeps their shoring manual fairly up to date. You could use FRP sheet piles, or anything that is rigid/can be connected to using ties/can be designed.

 

[PEinc]

As a deadman gets deeper into the ground, the passive resistance increases. For a shallow deadman, the passive resistance in psf should be less than the allowable vertical bearing pressure. As the deadman gets placed deeper into the ground, the passive theoretical passive resistance starts getting higher and closer to the vertical allowable bearing pressure in psf.
Assume 120 pcf soil weight and Kp = 4.
Pp = gamma x Kp x D
At D = 2', Pp = 960 psf
At D = 5', Pp = 2400 psf
At D = 8', Pp = 3840 psf
At D = 10', Pp = 4800 psf
At D = 20', Pp = 9600 psf (probably greater than the vertical allowable bearing pressure at that depth)
At deeper deadman depths the passive resistance approaches or may exceed the vertical allowable bearing pressure.
At greater depth, it is reasonable to assume that a relatively small-sized deadman will not be able to push a full, very high, triangular wedge of soil and then bearing pressure may be a more accurate way to look at deadman capacity.
The problem with using deeper deadmen is you may need to install sheeting to install them unless the wall and deadmen are built in a fill situation (i.e., not top-down construction).

http://www.PeirceEngineering.com

 

[gojgo]

Thanks PEinc, that makes sense!
When you say, "At greater depth, it is reasonable to assume that a relatively small-sized deadman will not be able to push a full, very high, triangular wedge of soil and then bearing pressure may be a more accurate way to look at deadman capacity." you mean we need to check the overturning and sliding of the deadman if bearing pressure is enough to resist the load?

 

[PEinc]

No, the small deadman could fail by punching shear before it can mobilize a great passive resistance associated with a deep passive pressure diagram.

http://www.PeirceEngineering.com

 

[gojgo]

Hello PEinc,
Do you suggest any book or research paper to design deep deadman?

 

[PEinc]

Foundation Engineering Handbook, Hans F. Winterkorn & Hsai-Yang Fang, 1975, Van Nostrand Reinhold Company, Pages 436-439
Foundation Engineering Handbook, Hsai-Yang Fang, 2nd Edition, 1991, Pages 459-460
Foundation Design, Wayne C. Teng, 1962, Prentice-Hall, Pages 375-377
Foundation Analysis and Design, Joseph E. Bowles, 4th Edition, 1988, McGraw-Hill, Pages 636-637 (also try his other editions)
Foundations, Retaining and Earth Structures, Gregory P. Tschebotarioff, 2nd Edition, McGraw-Hill Kogakusha, LTD, 1975, Art. 14.4A, Pages 536-540

http://www.PeirceEngineering.com