Soil meeting compaction and moisture requirements but still showing significant pumping 5

[jbray241]

We are placing fill for a dam project upstream side, fairly high up. It will likely only see water for a ~25 year event. No structure on it.

The fill material is a silty sand (SM) (~18% fines) being placed in 12in loose lifts. It has very little cohesion. The contractor is doing a great job with moisture control (usually within 1% of optimum), and every lift is coming in at greater than 95% compaction (meeting spec). Despite this we are seeing significant pumping during and after compaction (even just a pickup truck can cause some pumping). We run tests side by side with the contractor on every lift and both indicate the same thing.

Is this just the nature of the material? How concerned should I be? We could have them reduce the moisture but then meeting compaction may start to be an issue. I understand compaction does not equal stability, but what is the long term affect of instability during compaction?

 

[Ron]

That happens in fine sands and slightly clayey or slightly silty sands. It is due to a pore pressure buildup in the material. One way to mitigate this condition is to compact in thinner lifts and use lighter compaction equipment. Keep the moisture content low of optimum.
You can also mix in some larger particle material such as gravel or stone tailings to help.

 

[Ron]

Also, if your application is static (such as a slab on grade for a building), then the pore pressure will dissipate. If your application is dynamic (pavement) you will need to stabilize the material.

 

[GeoEnvGuy]

I am assuming you are using a smooth drum vibratory roller. Literature will tell you to use rubber tire or sheepsfoot when compacting SM materials. For a dam job SM material is erodible so you will likely need to cover with compacted filter gravel and compacted rockfill so it shouldn't cause issues.

 

[oldestguy]

Welcome to the practical problem of compacting this material. The best I can recommend is don't do repeated loadings in one place in short time intervals. Spread the work out over wide areas and avoid closely spaced in time applications.

 

[geomane]

If your earth-moving equipment is constantly driving over this material, it can overcompact the material. The void space will reduce, thus increasing the saturation of the soil, which results in pumping.

 

[cvg]

recommend no more than 8 inch lifts.
suggest keeping MC on the dry side -3% to +1%
pad foot compactor should be used

 

[r13]

Many sound advices here. Wish to see OP's feedback on the result and the method used to resolve this problem.

 

[geotechguy1]

Do you have any gradation testing + hydrometers on the sand?