Boulder Retaining Wall

[jjeng2]

I have a client who has an already installed retaining wall which is built in steps. It varies from 2'-10' in height. I have not seen it yet but he said the boulders are the size of cars. The town engineer wants calculations submitted. To me, this is just a pile of rocks and it would be tough to calculate anything. Are there any referneces or guidelines for this type of wall. My initial guess would be to take the weight of the boulders and see if the friction at their bottom is more than the soil lateral loads. There is no footing and no typical "retaining wall action" so overturning and bearing at the toe dont seem to come into play. Any advice or guidance would be appreciated.

 

[Geo69]

We do engineering on many of these boulder retaining walls. We analyze them using a slope stability analysis. We determine the height, slope of the face and size of the rocks so that the slope will have a suitable safety factor

 

[jdmm]

Depending upon the geometry and uniformity of the wall you can analize the wall as a gravity retaining wall. The contact between individual rocks should slope back into the slope so that failure cannot occur within the wall. If it is tiered it makes analysis more difficult. With respect to Geo69 that uses slope stability analysis, remember that the factor of safety for a Bishop simplified analysis for example will be less than that of a gravity retaining wall because of differences in the definition of the factor of safety.

 

[Geo69]

jdmm,

The concern that we have with analyzing boulder walls as a gravity wall is that you when you analyze overturning you cannot sum moments at the base of the wall. There is no rigid connection that will transfer forces from the top to the bottom of the wall. There is frictional resistance between the rocks so I guess there could be some transfer but how much depends on several variables (rocks, quality of construction, etc.)

I would be interested in your thoughts on this.

Thanks.

 

[BigH]

http://www.wsdot.wa.gov/EESC/Design...sh/1130-E.pdf#search='rubble retaining walls'

 

[Panars]

I think the type of wall you are describing are also referred to as a "rockeries". Seattle has a prescriptive design that you can find by searching for "rockeries design".

 

[jdmm]

Geo69

You are partly right. The overturning moment at every depth of the wall must meet the requirements of the design. Now, at the top of the wall, for overturning and sliding you may require a width of say 1 meter. As you progress down the wall the requirement increases so that at the base of the wall you need a width of 3 metres to maintain the stability against overturning and sliding. So, ok, now you need a wall that is 1 metres wide at the top and 3 metres wide at the base.

The point is that you must maintain stability at all levels of the wall. This is true for your case but also for Allen block or Keystone walls as well.

jim

 

[jjeng2]

Thank you for all the guidance. My take on this is that each piece needs to be analyzed independently for sliding, bearing on the rock below, and overturning about its front bottom. Also, the forces from the sum of the rocks above a certain level must be considered at each level.

 

[jdmm]

jjeng2,

you are quick! good luck!

jim