Liquefaction | Grout Compaction

[bookowski]

There is a geotech on board for this project but I'd like to have a better understanding of what's going on here and what to expect:

This is a project where we will are doing a substantial vertical enlargement on an existing 6 story building. The enlargement is such that we are adding new foundations (and new columns) but preserving the envelope and some floors etc. Original building is early 1900's. After cutting through the basement slab on grade for test pits water immediately filled the pits (and slab had very thick membrane below).

Borings have shown that soil is subject to liquefaction. We have a report from a few years back for a job only a few doors down and that report showed similar results, i.e. potential for liquefaction.

The building enlargement has triggered a seismic upgrade to the building (original bldg has no distinct/designed lateral system).

I am new to the liquefaction issue, some questions:
- All of the foundation work will be in an existing cellar, maybe 9ft. clear head room. Piles would be tricky.
- From what I've found online compaction grouting sounds like a potential solution for limited access sites. When doing compaction grouting who specifies the process/performance and how is it verified?
- When compaction grouting how is the site classified (for seimsic site class A, B, C etc.)
- How is the bearing capacity determined after grouting?

 

[EireChch]

Hi,

Piles are most likely a non-runner given the limited head room. Notwithstanding this, is there potential bearing layer available? i.e a 4m thick non-liq layer of dense soil providing a N>25 or qc>15 (qc refers to CPT tip resistance).

Also who indicated the soils are liquefyable? Test pits are ok to find out what soil type is beneath the building however SPT data or CPT data would be needed to undertake a liquefaction analysis or assess an appropriate foundation upgrade options.

From my experience compaction grouting (or LMG columns) could be an option. There are portable rigs which could get below your 9ft limit. It is likely that the grout columns would need to extend for at least 4-5m to mitigate the effects of liquefaction induced settlement. This is subject to a liquefaction analysis.

A lot of companies offer a desibg and build solution. Verification can be done via CPT testing before and after however this would require more holes to be drilled in your slab on top of the grout holes (typically at 2m c/c). You may need to break out the old slab and recast which will be tricky in a basement below GW level!! I am unsure of the seismic classification however where I am from we do not change it after ground improvement has been done.

CPT data from your post treatment testing could be used for bearing capacity assessment. From my experience it will be at least 5OOkpa ULT. But you will need to confirm your own.

Hope this helps

 

[Mccoy]

Also who indicated the soils are liquefyable? Test pits are ok to find out what soil type is beneath the building however SPT data or CPT data would be needed to undertake a liquefaction analysis or assess an appropriate foundation upgrade options.

Since fine content is a fundamental variable in liquefaction analysis it is important that SPT samples be tested and CPT be sensitive to soil types. Mechanical CPT usually is not. If no reliable means to determine fine content is used, we should assume the most unfavourable condition of fine content=0

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