liquefaction Boundary

[655321]

How would you explain, conceptually, to a "non-geotechnically" trained civil engineer what occurs in saturated soils at the boundary between dense (non-cohesive AND non-liquefaction prone) soils; over, looser (non-cohesive) soils that are susceptible to liquefaction?

For example:
-what keeps a dense cohesion-less layer from simply collapsing into the liquefaction prone layer during a major seismic event?

-are there degrees of liquefaction? Will the soils near the water table be "less" liquid like and support the mass of soils above it?

Any comments or thoughts would be appreciated.

No buildings involved, just a mental exercise.


Thanks

 

[YannisChasiotis]

I will try to explain in simple terms...

Assume the ground consists of sand only and that the water table is at a depth of 5m.

1. In order to have liquefaction, soil must be under the water table. So, the top 5m are OK (be careful of seasonal variation of the water table...)

2. During shearing, loose sands tend to contract while dense sands tend to dilate.

3. Loose sand: During seismic shaking, the sand particles rearrange. If the soil is dry, after shaking it will be denser. You can try that at home with a bucket full of sand. But if the soil is saturated, the rearrangement of the particles is accompanied by an increase of the pore water pressure. This increase of the pore water pressure, may lead to a total loss of strength, which is defined as liquefaciton.

4. Dense sand: Dense sands tend to present a decrease of the pore water pressure due to dilation (increase in volume), so liquefaction is not an issue.

5. Now, it should be clear that there is no clear boundary between dense and loose, liquefiable and not soils. Whether a particular soil will liquefy depends on many factors (earthquake and soil characteristics, depth, etc)

...Hope that 's helpful...