Passive Pressure on Anchored Wall w/ sloped bank

[jgeng]

I am designing a timber bulkhead. The grade in front of the toe of the bulkhead is sloping away and down at 4:1. I am trying to determine the appropriate embedment length. Can I still account for passive pressure gamma*kp since it is sloping down & away? I was thinking that instead I should be checking shear failure of the the soil wedge along the plane perpindicular to the bottom of the face pile (soil weight*tan phi?). Is there some way to determine minimum flat distance in front of the wall before it slopes to consider the passive pressure?

Also IBC has values for lateral active pressure, lateral bearing capacity, friction coefficent. Do these apply to retaining wall design or only foundation? If they apply I assume the friction value is for concrete against soil not soil shear (i.e. tan phi as mentioned above)? And regarding the lateral active pressure and lateral bearing capacity: what if you have a portion which is below the water table you would have to back calculate the ka & kp used based on an assumed soil weight and then recompute? this seems convoluted, would it be against code (ibc) to base your ka, kp off of an assumed phi & used assumed moist and dry soil weights?

Any help would be greatly appreciated, thanks

 

[InDepth]

Look at the navfac dm 7.2 document which contains charts for passive pressure values for soils sloping in front of the toe.

 

[jgeng]

thanks InDepth, I was looking in there but didn't see it at first, but I see it know...it looks like 7.2-64 gives me my Kp I was looking... not sure what 7.2-65 gives but anyways..

But how about my question about if and how to use those values in IBC table 1804.2, specifically the lateral bearing values...do you think they apply to passive pressure of earth retaining structures? my design seems unreasonably conversavtive using these values instead of basing on Kp and assumed soil weights??

 

[InDepth]

I don't have the code in front of me right now. I believe the IBC lists minimum values if a geotech report is not present. There are some basic assumptions behind the minimum passive that aren't stated. If you are comfortable with calculating your own passive pressures, then use your engineering judgement. (or higher a geotech) Take a look at the California trenching and shoring manual & and geotech circular 4, which has decent explanations of discrete and continous systems. The appendices of both lists a method for calculate passive pressures for discrete systems.

Can you describe your timber bulkead a little bit more. long term Deflection and preservative treatment are issues.

 

[jgeng]

InDepth - thanks for the response

Unfortunately it doesn't warrant a geotech. I was planning to calculate passive/active coeffiecients using the rankine method based on an assumed phi corresponding to the soil type, and using assumed moist and dry soil weights for pressure computations, what do you think?

Its a relatively small wall. 5' exposed face, with water level about half way, with tie rods to concrete deadman anchors. The slope running away from the wall is 4:1, SP-SM soils, & no significant surchage loading expected. I was using the following guide from the sp council as a starting point and backing it up with my numbers

http://newstore.southernpine.com/cgi-bin/newsopine/product?;26;.

I have another wall that has an 8' exposed face but this one is embedded in relatively level river muck. Any advice on passive pressures of muck?? I was thinking don't worry about the passive at all and provide two anchors one at top and one at the bottom to stabilize, any thoughts? I know in practice they don't go this overboard but I think alot of what is built cannot be backed up by calculations...