Liquefaction - effect of liq. zone below 40 feet of dense fill 1

[VanNuysDave]

Ladies and gentlemen,

This is the site - 6 acres – flat area on an east/west trending, Southern California, alluvial filled canyon - soil profile transitions from bedrock on the south to dry loose sand to saturated loose sand in the bottom of the canyon (50 feet +-) in a northerly direction - bedrock below the alluvial zone has wide erosion gullies running southwest/north east to the axis of the canyon.

Proposed structures - several large 2-3 story tilt-up/masonry, rectangular, 300x100, industrial buildings that are parallel to the axis of canyon and transition across the areas of several of the above discussed erosion gullies.

The upper sands are prone to seismic settlement (4-6 inches differential) and the liquefiable sands at depth that fall within the erosion gullies at a depth of 40-60 feet are highly liquefiable (3-5 inches).

Not to go into great detail, but the client has determined that total ground modification methods are not cost effective but removal and recompaction of the upper 40 feet of loose sands is economically feasible.

Question: The grading takes care of the dynamic settlement of the dry sand - however there will still be the gullies below the dense fill that create two or three (quite wide) zones below the building footprint (40-60 feet below the surface) that contain saturated loose sand with the settlement potential described above. The gullies containing the liquefiable sand are not uniform in area relative to the building footprint area and thus create potential differential settlement of 3 to 5 inches over relatively short spans. Does anyone have information (papers etc.) that demonstrates the effect of a thick, very dense layer over such zones of highly liquefiable soil? Logically the fill provides an arching effect - but mathematically how can this be demonstrated?

Thanks ahead of time.

 

[DMcGrath]

Have you looked into blast densification? It has been used to reduce the liquification potential of deep bridge footings (>40 m) and rapid consolidation of tailings ponds. The first return on Google has a neat video (Inco).

 

[jdmm]

Practically, I think that you are right. Over 40 feet of dense soil, liquefaction at depth will be relatively insignificant at the surface. The only way I can think of justifying this mathematically is to using something like SIGMA (finite element for soils) to model the static condition of a stiff soil over a subgrade that displaces.