Density verification

[pelelo]

Engineers,

I have a case in which it is required to verify if a house is sitting on a competitive fill. The house has some cracks, so there is a concern about the foundation soils.

The engineer recommended to clear and grub the top 10 ft, and replace with fill and compact properly. The fill was placed and the house was built about 15 years ago. I have records of fill density and compaction, they all looked good.

We ran some SPTs around the house, and i got very low blowcounts (SPT < 10) (in some of them, weight of hammer), on this fill. The fill material is Sand (SP), clean, so was appropiate.

Anyone has any idea of what could be the reason of such low blow counts?. After compacting a sandy fill, i expect SPT > 20 at least (maybe 30 blows per foot). Underneath the "improved" area, there is loose to very loose sands and silty sands, (SPT-N < than 10).

The footings of this structure are less than 4 feet wide. Using Bousinessq chart, the "existing soils", even though, they are loose, they would not feel the pressures from the structure.

Anyone has any idea of the reason of this?

Please let me know,

 

[oldestguy]

Where is it? If in north country it may be that the placement of fill was done in winter and the soil froze before "compacting". A common problem is the low moisture content of sand and so there is not much water heat of fusion to release as it goes below 32 degrees and thus the freezing front can advance rapidly perhaps before placed or compacted. That moisture frozen on the grains makes them larger. The frozen soil does not have enough to freeze in chunks, but appears dry, a very misleading situation. Later when ice covering melts you have a loose condition.

Next comes what to do now. There are various methods to fix this, but injecting compaction grouting would appear to me to be best alternative.

 

[pelelo]

oldestguy,

It is in Florida, USA, so no soil freezing problems there. I just would like to know a reason of why to discuss.

I understand the best option is to underpin the property. I don;t think grout would be cheaper than underpining for this situation.

Thanks