Rubble Stone Wall 2

[DaveHolder]

Hi,

I was wondering how can determine the capacity Of existing wall made of rubble stone 300mm thick and 3 meter high! I have not got much details about the wall! I managed to work out the proposal load of new construction which will be applied on the existing wall, but as I said lack of info, I’m not able to assess if the wall will carry the load from new structure? Is there any assumptions for the minimum strength of stone wall? What is the size of stone unit?


Thanks

 

[STrctPono]

Is this "rubble stone wall" mortared or unmortared. If unmortared, I wouldn't even use it to support my kids playset. Is this a retaining wall or just a wall?

What kind of condition is it currently in? Are there loose rocks dislodging themselves from the mortar?

If it's a retaining wall, does the wall protrude enough above the ground that you can estimate what the batter is on the backface? If so, you can at least get a decent idea of what the geometry of the wall is which is important since it is a gravity wall.

Is the new load being applied to the top of the wall or surcharging the wall on the backside with additional horizontal earth pressure? The check for gravity retaining walls is essentially one of global stability... ensuring the wall will not overturn, slide, or exceed the allowable bearing capacity of the soil. If not a retaining wall, I would check to see if the load from the framing members on top do not exceed a reasonable compressive strength of the stone and mortar. Also check the allowable bearing pressure of the soil. I would however, try to detail it in such a way that you avoid dumping any out of plane lateral load into the wall.

I am EXTREMELY cautious when I consider justifying an existing rubble stone retaining wall to carry additional load especially when it is being surcharged by something important (i.e. the foundation of a building or bridge). I will try with all efforts to avoid this and will usually succumb kicking and screaming. If not a retaining wall, then I would feel more comfortable with the proposition so long as the wall appears to be in sound condition.

 

[STrctPono]

As an additional note... and take this with a very large grain of salt because I am not entirely sure who decided this or where they come from, but our State DOT allows rubble masonry walls (we call them "grouted CRM" walls) to be used as retaining structures but are not allowed to support any bridge. They allow the following stresses within the "grouted" wall:

Compressive Stress (Under any loading condition) = 100 psi
Tensile Stress (Under sustained loads only) = 0 psi
Tensile Stress (With extreme event loads) = 10 psi

People in my State love these type of walls and they are very ubiquitous. What worries me about them is that they are almost 95% of the time not designed by an Engineer but rather designed by "rule of thumb" by the local stonemasons. I've seen many of these with leaning issues and have even seen catastrophic collapse of these crushing cars since they are very brittle with little ductility. I was even personally involved as legal counsel for some rich folk who were suing their neighbor over a 12ft tall one that was leaning into his property. Most all of the problems with them stem from drainage issues. Still, I've designed lots of them as they are cheap, pretty durable, and very aesthetic.

 

[Settingsun]

I have two standard drawings for ungrouted rock retaining walls from the good old days in my library. STrctPono, how do these stack up with rules of thumb in your parts? Moonknight, might be helpful for comparison, though it sounds like yours isn't a retaining wall?

Standard design #1:
- 1:4 front face batter.
- 0.75m minimum embedment at toe.
- Base width = 0.4*H + 0.6m. H = overall height including buried part.
- Top width = 0.6m minimum.

Standard design #2:
- 1:2 front face batter.
- 0.5m minimum embedment at toe.
- Base width = 0.5*H for H>2m.
- Top width = 0.5m for H<4m; increasing to 1.0m at H=7m.


For design calculations, what porosity of wall would you usually assume for ungrouted walls?

 

[STrctPono]

steveh49,

I have never designed an unmortared rock wall before so my parameters would be different for a mortared rock wall.

Out here I normally size the wall with the following general parameters then check to see if anything needs to be modified. This obviously depends on wall height.

2:12 front face batter
4:12 to 6:12 back face batter
1.5 ft to 2 ft minimum embedment
12" to 16" minimum top width. (This dimension is typically what I adjust as the wall gets taller. However, this may become uneconomical at some point and switching to a multi tiered or multi battered back face may be more economical.)