Gravel for backfill

[Okiryu]

In my area, since we do not have flowable fill available, we are planning to use clean gravel (20mm~40mm) for backfill at depths where compaction cannot be attained. Close to the surface (GL-1.50m), the excavation is wider so compaction can be done. We are planning to use clay on top of the clean gravels compacted to 90% modified proctor.
The area then will be covered with grass. Surcharge loads are not expected. The excavation was done for placement of a utility manhole, so the gravel will be surrounding the manhole. Depth of excavation was approx. 2.5 m. No groudwater.
I think that clays due to their cohesion are more resistant to erosion so wash out of fines thru the gravel voids may not be an issue.

I have received different opinions from my reviewers, telling me that clean gravel is not adequate since it cannot be compacted and therefore can create future settlement problems. I think that with a slight vibration, gravels can "interlock" well and therefore I do not agree with the reviewer's opinion about future settlement.

Any comments about this? or any other options for backfill in areas where compaction cannot be attained?

 

[oldestguy]

The range between the high and low density figures for uniform sized materials is quite low. Thus the potential for settlement due to loadings is quite low. If there is substantial water flow into the open graded material, there could be loss of ground due to that erosion, but for the usual buried pipe, this is not a problem. Yours is not a pipe placed there for drainage. You appear to be well versed in the basics, so dismiss the negative comments.

 

[Okiryu]

OG, thank you for your positive comments. My reviewer's comments may implicate some cost impacts in my projects, so I just wanted to see other opinions. Unfortunately, the final decisions of the project are not made with technically qualified people... Thanks again.

 

[Ron]

Putting fine grained materials over gravel fill is not a good idea unless you use a geotextile that prevents the fines from migrating into the voids of the gravel.....if you don't do so, you will likely have settlement.....however for a grass area, who cares!

 

[Okiryu]

Thanks Ron. Yes, it is grass area, so I am just trying to make it easier for the field people. Also, significant water flow is not expected and the wide of the area where the gravel will be placed is only about 0.50 meters. Based on these conditions, I do not see important impacts regarding settlements. Thanks again.