Boulder rubble wall design 4

[mad2323]

I'm looking for some guidance on evaluating an existing boulder/rock rubble wall. What are the critical components and requirements in terms of height vs. thickness, slope, toe conditions, backfill type, and geofabric/drains?

 

[jayrod12]

It's a gravity wall. There's negligible tensile capacity and therefore you need to analyze it and ensure that no net tension occurs across the section.

In order to get that to work, generally speaking it can't be expected to retain any water, therefore the backfill should be relatively free draining with an appropriate drainage system. That being said, there's rarely a proper drainage system on the old walls, but it explains why they all leak so badly, or are still in use as root cellars only.

 

[EireChch]

Is this a mortared or un-mortared wall?

Either way, I dont think they can be relied upon. Are you going to confidently assess it and say this wall is good for another 25 years.

Boulder / rock walls (or any significant height) are not constructed anymore for a reason.

But thats just my opinion, share some photos, retained height, block size, ground conditions etc and you will get a far better response.

 

[bridgebuster]

This manual might help

Link

I've looked at a number of rubble walls in my time. These are walls that were built in the late 19th/early 20th Century. They have to be looked at carefully. Their appearance can be deceiving. Since the stones are rough, the wall could appear to be bulging. The wall may have been built that way or the semblance of a bulge could be due to temperature. How and what they were back filled with is anyone's guess. Another consideration is that these were typically constructed with open joints to allow for drainage. Over time the fill behind the stones ay have been washed out leaving voids behind the wall. Conversely, at some walls, the open joints have been mortared without any provisions for drainage. This can result in a build up of hydrostatic pressure creating stability issues. On some projects, we called for weep holes to be installed vegetation in a rubble wall can also be detrimental to stability. Another consideration for any stone wall is the type of stone used. Granite is very durable, but stones such as schist and slate can deteriorate due to weathering.

 

[EireChch]

Good reference BB. Interested to know the size of the walls you assessed. When I have encountered rubble/stone retaining walls then have almost always been thin walls, approx. 0.5m, made of stones that can be lifted by hand, and less than 2.5m id say. Often times it was two layers of stone to make up the 0.5m. We have never attempted to assess these walls as we do not consider them a long term solution.

I can see from the link you sent, most walls are being constructed with minimum rock widths of 4ft, 1.2m at the base, and reducing in size vertically. These are massive rocks so I can see how these can be stable.

Again, more info from the OP will be helpful, for him/her and us.

 

[bridgebuster]

EireChch - the tall ones were on the order of 5 to 7 m, there were a number of short ones 1 to 2m. When I have some time, I'll look through the old reports and find some photos. One wall we inspected that was replaced was ~76m long and varied in height from ~1.5 m to ~11m. It needed work; deteriorated stones (mostly schist), missing stones; trees growing out of it. We would monitor it periodically, it was stable but visually it frightened people. A few years earlier there was a major wall collapse on the upper west side of Manhattan; the City didn't want a repeat.

Another wall we looked at, ~350m long, upwards of 7m. We had a surveyor scan different areas to see how much the bulging would change. That one wasn't replaced. The rehab design (by others)consisted of installing a grid of micropiles behind the wall. 10 years later still standing. This one was behind the collapsed wall mentioned above.

 

[mad2323]

Thanks everyone. I actually have to design the wall. I looked at the site the other day. Max 8' height all lean clay. The rocks are 30" in diameter. I'm thinking 45 degrees max with a layer of dimple board and a 4" base drain to grade. Maybe two rocks in thickness (60") for the base (first 4') and then 1 rock thickness for the top 4'? Maybe just 1ea rock thickness the entire height?

 

[STrctPono]

mad2323, I think everyone believed that you were going to assess an existing wall... not design a new one. I for one have no issues with grouted rock walls. I have designed many before and they are commonly built in my location. We are not allowed to use them to support bridge structures (for good reason) but in the residential market they are still very much alive.

As jayrod mentioned, it is a gravity wall so you not only need to check for all the typical stability issues (overturning, sliding, bearing, etc.) but you also need to check tension on the back face of the wall and make sure it is zero. I typically start with a back batter of 4H:12V and a front batter of 2H:12V. You can adjust from there depending on whether the design works or not. Some people want no front face batter, which is understandable. I like to put a 4" concrete laying bed beneath the bottom course of rocks.

Yes, you want a perforated drain board on the backside of the wall connected to through drain or a layer of crushed gravel wrapped in filter fabric for drainage. Pretty standard stuff.

Non-grouted rock walls are a different story and I wouldn't touch those with a 10 foot pole.

 

[BigH]

Just to provide information - you might wish to check out these publications

https://www.fhwa.dot.gov/clas/pdfs/RockeryDesignandConstructionGuidelines013007.pdf

(talks about rockery walls from 8 to 12 ft high as typical heights)

http://www.nbmg.unr.edu/nesc/NESC_Seismic_Building_Guidelines/Rockery_Wall_NV_Guidelines_2005.pdf

 

[EireChch]

I am honestly amazed that someone would there to construct one 7m high. How wide are the bases?

Would love to see the calcs. My experience has been somehat similar to SPs. And similar levels of fright when talking about unmortared walls.

Re the discussion on tension in the back face. How do you assess this, there most be some interesting assumptions on transfer of stress through one stone, mortar and then to the other!

 

[STrctPono]

BigH. Good technical article. I was aware that FHWA/CFL was very keen on using rockery design in Colorado.

EireChch, I always assume that the wall is one rigid body with composite action between the rocks and mortar. From there it's essentially a stress check based on the thickness of the wall at the point of interest and the externally applied soil pressures.