Subgrade liquefying during compaction of first lift

[Motto33]

Hi everyone, thanks in advance for your help.

We are inspecting a geogrid-reinforced ecology block wall being constructed. The subgrade soils are loose sands and sandy silt that are generally moist, but are wet right now in some areas due to surface water coming through the site. The surface water impacts about 10% of wall alignment. We excavated 18 inches below grade and placed a 6-inch layer of gravel below the first layer of blocks. However, when we compacted that lift, we saw the subgrade liquefying and pumping.

How concerned should I be about the stability of the wall (which will be 9 feet tall = 4 blocks high, and will have reinforcement)? Once pore pressures subside, will the subgrade be stable enough that pore pressure won't be an issue?

This is a temporary wall that will be used to support a temporary work platform - it will be removed in 4 months. There will be a perforated drain behind the wall.

 

[MTNClimber]

Talk to the geotechnical engineer of record and the wall designer. They have to provide guidance.

 

[EireChch]

I would not be overly concerned. I have worked with pumping silt before.

No when you say it liquefies, I think you mean that you just get a bit of water coming to the surface? Its not turning to completely mush?

Ill assume the former. The issue arises when you use vibration when compacting , a smooth drum roller and thinner lifts. or if its only trench width, you wont be able to use a roller. Placing good quality gravel directly on it with no compaction will likely be fine. How much can 6 inches of gravel settle really...

But not withstanding the above, you should still do your due diligence. I assume the wall designers have some criteria for the subgrade, "wall to be founded on material providing a CBR of x% or cu of xkPa. You still need to prove this. It probably wont meet requirements immediately after compaction but should do after a few days/week. To be extra save you could place your fill, compact it, wait and then test to confirm. You dont want to build a section of wall to find out subgrade doesnot meet spec.

 

[Ron]

Agree with EireChch.
You probably have normal dilation and pumping, not actual liquifaction.
You could also mix in a few percent portland cement and call it a day.

 

[howardoark]

Are you in earthquake country?