Buoyancy During Liquefaction

[moe333]

Hello all,

I have a project where a 4 level basement garage with 4-8 stories above grade will be constructed. The two lower basement levels will be below groundwater and the soil below the basement is liquefiable.

Excess pore pressures during liquefaction should be approximately twice the hydrostatic value if the Factor of safety against liquefaction is 1.0. But if the FS is lower than 1.0, I think the pore pressure may be higher than twice the hydrostatic. This may be important in terms of uplift due to buoyancy.

The building may be on piles, or stone columns, not yet decided. If it is on stone columns it may not be as much of an issue since pore pressures would not be as high, but piles is another story. Does anyone know of methods or have experience reducing pore pressures due to liquefaction in terms of building/slab uplift and buoyancy?

Thanks

 

[howardoark]

The soils under your basement are pretty deep to be liquefiable. It would seem that the bouyant force would be the volume of the basement in the liquefiable strata times the total unit weight of the soil (a fluid with a unit weight of 130 pcf).

On another issue, do you have a feeling regarding the specification of differential settlements due to earthquakes. SP 117 says use 2/3s of the total. SCEC guidance for implementing SP 117 says use 1/2 of the total. Neither provides the distance over which the differential settlements are to be applied.

 

[dgillette]

Worst-case scenario is to have the uplift pressure approximately equal to the total pressure just outside of the footprint, equivalent to 100% pore-pressure ratio there (assuming that it's greater than the bearing pressure of the structure).

Why not just put in stone columns as a means of densifying the foundation? They are pretty good for that if the soil is clean enough. That avoids the whole issue of liquefaction under the floor, and would probably reduce settlements substantially without needing end-bearing piles.

Don't count on your stone columns for much drainage of pore pressure. They tend to get clogged up with the native material during installation, as shown in test pits at several different sites.

 

[moe333]

Thanks for the responses.

I am aware of the lack of guidance the the SP 117 provides regarding the distance over which the differential settlement applies. I guess it's one of those things where you have to use judgement. If the settlement is very similar at all boring locations, this would indicate that differential settlement may not be very large. That said, it is difficult to accurately estimate the effects the dynamic condition of an earthquake may have in terms of soil-structure interaction. I generally use an initial distance of 40 feet, then look at the sensitivity of different distances.

Stone columns are one of the options I am presenting. End bearing piles is the other option and would probably be more economical but there will be additional issues such as settlement and uplift.