Anchoring geogrid to segmental blocks 1

[oldestguy]

There is a low bid here for a 14 foot high wall that is under the category of "segmental block wall MSE type". The blocks will be from a ready-mix plant where returned concrete is placed in forms with a rock-like face sides. There is a dimple in the bottom and a "bump" on the top. They measure 2x2x8 ft. The claim is the concrete strength is 3,500 psi. That is not the problem, even if it is weak concrete, the blocks do stack on top of each other OK and there will be a tilt back.

They work fine for small height walls, but this job will need geogrid reinforcing. Contractor's design engineer is struggling with how to secure the geogrid placed between each course of blocks. The geogrid type has yet to be selected. Backfill is clean fine sand.

Is simple friction enuff?

Any ideas for how to make the attachment will be helpful.
I'm the owner's engineer, but would like to suggest some ways to do this to get the job done right.

 

[concretemasonry]

All of the small(under 90#) SRW unit manufacturers have run extensive geogrid anchorage tests because they supply to 40' high walls. I would imagine any reputable, serious producer of the the large block would do the same to be approved for commercial and municipal/highway use. Just ask for documentation if they claim the block are suitable for the use.

Meeting unit strength and density requirements is no problem, even for "come-back" concrete. Unfortunately, some people buy some off-the-shelf forms for a product that cannot be used for engineering appications because the design of the unit shape has no documentation. Many units are purely designed to be used for gravity walls, and engineering applications are convenient and incidental.

Pure surface friction is not enough. You have to deal with rotation of the units, surface cleanliness and moisture. Usually, a great deal of distortion over a large area is the key to developing the strength of the geo-grid.

Dick

 

[msquared48]

Seems to me that the locking lugs at the top face of each block could be enough with the concrete block dead load, considering the distortion in the fabric mesh, to anchor the geogrid.

If further is needed, what about adding a piece of #3 bar threaded between the mesh on the far side of the lugs?

Mike McCann
MMC Engineering

 

[concretemasonry]

Continuous or relatively uniform attachement is desired for reliability.

Localized attachement by isolated pins or "knobs" lead to localiazed load concentrations and a "zipper" failure of the geo-grid fabric. Full scale tests or real life examples of actual installations are required to get away from the theoretic or microscopic analysis.

Individual locking lugs are not equal to a uniformly installed geo-grid connection. Poor setting of one 3-4' block can easily lead to rotation, shear or other load concentration.

You need documentation of the performance of the individual units used with geo-grid.

 

[PEinc]

Ask the concrete block producer if they can form a continuous, longitudinal keyway into the top (raised) and bottom (indented) surfaces of the blocks instead of their normal, small keys. I would think this could be easily done without much effort or form modification. Then, the geogrid would have continuous embedment into the key between block layers.

 

[concretemasonry]

If it is more than a low wall (<4'), then it must be enginnered.

Normally, these block are a secondary product made by a ready-mix producer to use the small amount of return concrete or to keep people busy when it rains. They are fine for low gravity walls, but they are really not made by a 'concrete block producer" (SRW mfgr.). Anything greater will require documentation of product performance regarding the ability to develop the real anchorage of the geo-grid to the wall face under the typical conditions (creep, moisture, friction reducing sand, etc.).

Any responsible material supplier should be able to provide an engineer with documentation of the product performnce in an engineered wall. If the contractor's engineer is "struggling" for a method to attach the block to the necessary geo-grid, that presents a question about the reliability of the system. If you make any changes in the ready-mix block producers product, you will be responsible/liable fo the results.

Since it is a relatively low(14') engineered wall, it may be prudent to seek out the smaller units (there are many SRW U.S. company products that are used both domestically and intenationally) that have a proven track record of walls over 40'. I am sure the local municipal engineering departments/DOT have some guidlelines for the smaller units since they are frequently use them for similar applications.

Dick

 

[BigH]

I always thought that the geogrid was laid "across" the female/male grooves - the weight of the overlying block would be more than sufficient to hold the geogrid in place. Of course the tolerances on the grooves must be enough to get the geogrid in. See link below - look at last page for a picture of similar kind of thing. In MSE walls, it was always the point that the wall (or facing) was just to keep the material at the face from falling out - see original Reinforced Earth details. Geogtrids have a tendency of creeping - metal grids are more rigid to lateral strains.

http://www.anchorwall.eu/ie/construction/PDF/HITEC_report.pdf