Retaining Wall Friction Angle in 'At Rest' Condition

[JordanPyle]

Hi,

I am designing a timber pole propped cantilever retaining wall for the 'At Rest' condition. For a static/long-term analysis, what wall friction would we expect? For an active case I would usually use 1/3 to 2/3 of the soil internal angle of friction, but for the at rest case there is (notionally) no relative movement between the wall and the backfill. Does this prevent wall friction from being mobilised?

Thanks in advance for the help.

 

[DaveAtkins]

I agree with you--if the wall does not move, then the wedge behind the wall cannot move downward. Therefore, there is no friction against the back of the wall to help against lateral movement.

DaveAtkins

 

[PEinc]

How can the wall be propped (braced) and cantilevered (unbraced)?

http://www.PeirceEngineering.com

 

[ATSE]

Friction between the wall and backfill will exist, not only because both materials are frictional (almost always), but also due to the construction activities (wall backfill or pile installation) to create the earth retaining structure. Very small movements are necessary to mobilize friction.

 

[JordanPyle]

Interesting to hear the different opinions. I agree that in reality we will get some small movements. I've also considered the fact that friction doesn't necessarily require movement - overcoming the initial/static friction to slide an item across a desk for example. The question remains what value to use for wall friction angle, are the small movements expected enough to mobilise wall friction to the same extent as an active case?

All thoughts appreciated.

@PEinc - By propped cantilever I am referring to a cantilever wall that has been tied back at the top (e.g. by bending the reinforcement in a block wall into the topping slab over the backfill).

 

[jdgengineer]

Wouldn't the compaction of the backfill prior to the slab placement likely cause deflection of the wall and engage active pressure?

I would check the wall as cantilever as a temporary condition during construction. It may be a propped cantilever after the slab is in place for seismic loads and other loads which may occur after the slab is in place, but seems soil loads should be checked in cantilever condition due to construction sequencing.

 

[jdgengineer]

The slab would also need to continue well past the failure wedge in order to effectively anchor the wall. Slab reinforcing would be in tension as well. May need a dowbturned edge in the slab to act as a shear key to resist the load. Seems like cantilevered wall design may be easier to handle.

 

[fattdad]

if you design the wall under at-rest conditions, there'd be no movement and wall friction would be moot.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Doctormo]

What are you planning to do with the friction value in this analysis? A simple Rankine analysis would set it equal to zero for a level back slope condition and Ko is a simple earth pressure condition value such as Ko = 1-sin(phi) for no deflection.

Order of construction would play into this calculation also as noted above.

 

[JordanPyle]

@Doctormo - I am following some published guidance which uses Coulomb method in overturning calculations. This guidance doesn't mention the Ko case. The Coulomb method doesn't make the same assumption of smooth walls, and including wall friction can have a noticeable effect on overturning as it changes the direction of the resultant driving force on the wall.

 

[Doctormo]

By definition, Coulomb has to have some wall movement to develop the active wedge which moves downward as a result of the lateral wall movement. The Ko condition is defined as a "no movement" condition (not Coulomb active wedge) so as f-d noted above, the wall friction would be moot and therefore the thrust would be lateral. Your first inclination was correct I believe.