Gravity Wall design that incorporates drainage stone?

[dmband]

Hell All,

I'm reviewing a set of gravity wall calculations for a segmental block retaining wall. Where the wall height begins to get around two feet, the designer seems to change methodology to incorporate the drainage stone behind the segmental block facing units. When analyzing sliding and overturning, the base width of the wall is equal to the segmental block facing unit depth plus the width of the drainage stone - essentially doubling the width of the wall. The designer also (conservatively?) only uses the unit weight of the segmental block for the entire width of the "modified" wall instead of using the unit weight of the drainage stone (which is heavier).

Has anyone come across this before, and if so, are there any publications that promote its use?

Thank you!

 

[concretemasonry]

dmband -

You are right about the conservative situation, but it is designed for D0-It-Yourselfers in many situations, so that is necessary. Obviously, there is a weight added to the soil being slightly higher that a more fluid soil not being quite as heavy as backfill that has more defined specifications.

The main reason for the drainage stone is mainly for reduction of the moisture directly behind the soil and the lessening of amount of material that can be washed through the non-sealed joints. This is especially true with the curved walls that are very common in landscaping applications.

Each type of unit has a specification for the unit and the gravity wall properties, especially since the "batter" of different units can vary.

The major 4 products have extensive libraries of tests and designs based on getting engineering requirements globally since all are made and used globally and for different association and government requirements.- Just do not ask for all possible test and designs unless you have a very large truck. The other brands of units are usually copies without all the features and the amount of back-up and support varies wildly.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[dmband]

Dick, thanks for your response. I think perhaps I explained the situation in a confusing manner. I understand why the drainage stone is necessary regarding alleviation of hydrostatic pressure. What my question is actually about is the validity of including the drainage stone into the resisting mass. Basically, the designer is analyzing sliding and overturning as if the segmental facing units are twice as wide, or as if the drainage stone is actually a column of segmental facing units. Specifically, my question is whether it is acceptable to analyze the gravity with the drainage stone resisting lateral earth pressure from the retained fill.

 

[moe333]

I would not count the drain rock as wall. It would be unconservative.

 

[oldestguy]

If the designer apparently is assuming the stone is part of the wall (assumed to be solidified, glued together and glued to the wall), then why not assume the retained earth also is one glued mass, also glued to the wall. Hell, no need for a wall then. Might as well figure the stone will stand by itself with no wall in front and a glued mass of soil behind it. You can see how stupid the idea is.

 

[concretemasonry]

Most of the landscape walls are not designed by engineers and based on performance of what has worked for certain types of walls and applications. Somewhere a person coming up with standards for the installation and tries to use some sort of engineering, even if not possible in many cases.

Normally, codes do not require engineering until a wall until it is over 4' or 5' tall, but even then the engineering approach is not realistic, but history and safety have shown this is a rational limit.

Trying to finely engineer a 24" high wall using 8" or 12" wide units defies any engineering logic since to many assumptions are made. Next people, will want to come up with a standard for the coefficient of sliding friction between two rough surfaced units that may or may not have moisture of sand between them.

Segmental retaining walls are not adhered, grouted or placed on concrete footings since they are an engineered flexible wall system

For real engineering concerns for engineered walls (5 to 20'), I have seen instrumented tests of the soil, stresses in the geogrid, interaction of the connections to different type of units and the connection of the grid to the units (friction or pins) performed by several systems groups. - I don't think that engineering applies to 24" high walls, but does begin to apply as wall heights and construction methods change since there is better controls over specifications and materials.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[RFreund]

I'm a bit late here but yes I have seen this. It is a way to 'justify' a 2' tall wall. Basically it is a check for sliding at the top of the leveling pad. I don't necessarily like it but you don't see many 2' tall wall failures (atleast for 12" deep blocks). I think a better way (maybe) to justify this is to use vertical soil forces, use a slightly higher friction angle as the failure plane must pass through the drainage aggregate which tends to have a slightly higher friction angle than the retained soil, or to say that the friction reduction factor between the base unit and the leveling pad is higher than 0.7.

I'm not exactly sure where the method you described comes from but I have seen it more than you'd think.

EIT

http://www.HowToEngineer.com