Mortarless Rock Retaining Wall

[pcronin]

I am in the process of gathering information for the design of a mortarless rock gravity wall. I was wondering if anyone has a defined procedure for this type of wall design. I do have the required geotech information and have designed various types of retaining walls in the past (cantilever, soldier pile, sheet pile, modular block, gabion basket, etc.). The wall in question has a max retained height of 5 feet and portions with sloped backfill and moderate surcharge loading. My main concern is the correct checks for global stability as well as internal stability for both sliding of the stones at the joints and possible localized failures. If anyone has prior experience with these walls, any information will be helpful. I have already reviewed the previous threads on this site and still need more information. Thank you.

 

[ishvaaag]

Mortarless masonry can be thought just a special class of masonry, and so des piet 70 code, a very valuable gathering of information for masonry design published in Spain in the 70's. Of course tension at the joints is not allowed since dry masonry.

The values for the limit compressive stress to be taken in the calculation using the brute sections when compared to the factored forces are

Granite, Sienite, Basalt 7 kgf/cm2
Sandstone, Hard limestone, Marble 6 kgf/cm2
Soft Sandstone or Limestone 5 kgf/cm2

When the stones are worked to "perfect" box shape (by say roman or middle ages' standard of workmanship, better not to polished faces) the respective limit compressive stresses for the dry masonry are

Granite, Sienite, Basalt 80 kgf/cm2
Sandstone, Hard limestone, Marble 40 kgf/cm2
Soft Sandstone or Limestone 20 kgf/cm2

Of course the effects of slenderness need be taken unto account, and the general procedure on how for a column or bearing wall is portraited in the code and other derived codes still extant here. You may freely download worksheets in Mathcad to this purposes, but not directly for retention wall design.

For a wall application one likely would terrace the face of the wall against the ground, and use a high coefficient of friction backfill in order to produce downwards resultants helping to stabilize the wall.

For foundation one may use somewhat wider slabs of sound hard stone, able to take the footing effects.

Interlock is foremost to the integral work of masonry, and more if dry, that is, joints must fall "sewed" by faces adjacent stone blocks.

 

[BigH]

A 5 ft wall is not that high. I have seen Chinese and Indians to this using about 40% of the height for the width - I'd suspect this is okay. I understand the desire for doing the whole 9 yards, but don't lose perspective. For higher walls that would be a different story.

 

[GeoPaveTraffic]

As to global stability, the type of wall has not effect on the global stability calculation.

 

[cdh61]

pcronin,

Have a look at the Design Manual for Segmental Retaining Walls published by the National Concrete Masonry Association. ph (703) 713-1900

Outlined in the manual is both gravity and geosynthetic reinforced walls.

 

[PacificSteve]

I would have to agree that intense analysis for a 5 foot wall is probably more of an academic exercise than a real pratical necessity. Hyrdostatic pressure will be nil, but I would personally backfill the near side with gravel or other readily available small loose stone, especially if your soil is clayey.

When properly selected stones are placed "commonsensically" for geometry and sloped backwards, I think a 1 in 5 or 1 in 4 run to rise would work fine.

Key the larger bottom stone to sit on undisturbed soil, ie. dont dig too deep, just enough. Dig 1/3 the depth of the height of your bottom boulders. Use gravity to create force to drive topper rocks into the rocks below, creating friction while effectively tyring to drive the rocks to the side (which they wont move to the side). It will lock them in. ie. they don't have to "look" orhtogonal. Some setting in a diaganol fashion is OK, and even good.

Most important is the character of the stone facing the viewpoint. The "chi" of the stone, and its particular placement will establish a long lasting structure and pleasant view.

The expertise and gumption of the rock setters will be more important in determining the finished quality and integrity, than any amount of structural calculations. A beautiful wall will receive the minimal care required for long life. An ugly wall will be ignored and more subject to long term weathering effects and neglect related to the same.

A large purchase of boulders and rocks at one time will allow the masons the best opportunity to take the old Bostonian approach to letting the "work find its own way". The less desireable rocks can be used as backer stone behind the main wall, to increase drainage and provide better support against the inclined nature of the wall.

Backfill and tamp the soil as you build the wall up. Too mcuh loose soil behind the rockwork may eventually destabilize your structure, but of course, you know this already.

If you need more for permit purposes, you can create some calcs that put the nil hyrdostatic against the sloped gravity force related to the back slope. If you are going 7 to 12 feet, I would take calcs seriously, and perhaps even consider a concrete footing with the first course set into the concrete. Less than that, use common sense and dont fret too much.

Steve

 

[jheidt2543]

PacificSteve,

Your comments show how far the engnineering profession has come from the days of cold, hard calculating facts regarding retaining walls. Your comments might fall into the catagory of "The Zen of Wall Design & Construction". Engineering solutions should not only solve a problem, they should be aesetically pleasing too.

 

[CSEllc]

I have designed several rubble stone walls. My criteria has always been to reduce or eliminate the tension stress, since there is no mortar to transfer this force. I use about 110pcf for the weight of the stone.

 

[g7mann]

g7mann [geotechnical]

It is unclear to me specifically what sort of "stone" you intend to use for this wall. We often "engineer" stacked-in-place rock walls of heights up to thirty (30) feet. In order to determine the appropriate mass of rock in-place to resist lateral sliding and overturning [toppling] we conduct a wedge analysis. Typically,m we use a unit weight of 155 pcf for the rock and the rocks themselves are described in man-sizes. There is a document available for determining sound rock wal construction practice, it's the Associated Rockery Contractors Rockery Construction Guidelines. This contains information regarding the appropriate testing of rock size and quality, and described good construction practice. Most local public agfencies refuse to believe that a rock wall can act as a retaining wall, but if sufficient mass is generated it will act as a mass gravity wall. For your five foot high structure it would likely require only one or two stacked rocks. The rocks used [from the quarry] are typically rectangular, cubical, or "square" in shape and fit together relatively well.

Hope this helps.

 

[DRC1]

I found a copy of the "AIA Graphical Standards" (an older edition) at my library. You would be surprised at the amount of information it has. They have a section on rock walls - how wide for a given height, sugested batters, how to handle transitions, etc. No calculations (its for architects remember) But for a 5 foot wall, unless you have something unusual, you probably don't need them. Anyway, if you are doing a rock wall check it out.
Good luck!