MSE Wall with 2-1/2" Crushed Stone Backfill

[StructEng101]

An engineer has submitted drawings to my office for review and approval. They are proposing an MSE wall using 2'x2'x6' blocks and a geogrid reinforcing product extending back from the face of the wall into the reinforced soil zone. It is an approximately 10 foot tall wall at its tallest with the geogrid extending six feet from the back face of the wall. I take quite a few exceptions to his design, but I want to get gather some opinions on the proposed backfill. The engineer is proposing to backfill the entire reinforced soil zone with 2-1/2" crushed stone. This geogrid product doesn't seem to be tested with such a large stone. The RFid (installation damage reduction factor) for a 3/4" stone is 1.25, for sand is 1.10, and for silt/clay is 1.05. What would it be for 2-1/2" stone? I'm afraid backfilling this stone over the geogrid is going to tear it apart. Does anyone have any watch-its for using such a large stone in the reinforced soil zone? The engineer is assuming an angle of internal friction of 40 degrees. Is this valid? Is their compaction that you would need to achieve this?

 

[geotechguy1]

I can't answer the question definitively for you but my suggestion would be to call the manufacturer of the geogrid (and perhaps the manufacture of the blocks). They should be able to find a technical expert somewhere within the company that can answer the question about the RFid factor.

For the gravel, what is the gradation of the material and how many fracture faces. It's a high number in the sense that north american geotechnical engineers are extremely conservative but I think for a well graded, engineered crushed gravel it might be OK. They should have also provided a compaction specification with the wall design

 

[BridgeSmith]

The specified max size of aggregate is required for the proper interlock of the geogrid and backfill. There will be a reduction in pullout capacity with larger aggregate. The magnitude of the reduction may or may not be available from the geogrid manufacturer.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[MTNClimber]

Put it back on the engineer to submit the RFid lab data for 2-1/2" stone. Some manufacturers have the data but don't post it publicly. I think we have been able to get data up to 2" crushed stone from one manufacturer. 40 degrees is reasonable if its good quality stone. Can't test compaction though. You would need a spec of lift thickness, minimum passes using equipment of a minimum weight or force.