Settlement of sand under fill
Settlement of sand under fill
(OP)
I have a site that will have 6 ft of fill placed over 6-feet of loose fine-grained sand with N value of 3 with water at the ground surface. I have looked at a schmertmann approach and came up with less than 1 inch of settlement but I'm not buying it. Any thoughts out there?





RE: Settlement of sand under fill
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RE: Settlement of sand under fill
RE: Settlement of sand under fill
RE: Settlement of sand under fill
RE: Settlement of sand under fill
RE: Settlement of sand under fill
RE: Settlement of sand under fill
RE: Settlement of sand under fill
RE: Settlement of sand under fill
RE: Settlement of sand under fill
Assuming you would be developing an area, while three ft of clay seems like a lot, what I would suggest, and again this depends on the nature of the job, is to remove the 3 ft of clay. You have indicated that the groundwater is at the surface of the ground - is this by standpipe or ??? If the water level is at the surface of the sand when the clay is removed, put in some strategically located sump pumps and pump. When the water level is drawn down, put a heavy vibratory steel-drum roller onto the sand which should densify the sand considerably.
Given the soft clay, I don't think that geogrids would be appropriate - they certainly have been used building roads in swamps, etc. but even so, the fills of the embankments above will have flexibility and settlements of the road surface of a few inches would not be detrimental especially with the ability to repave the road from time to time for smoothness.
RE: Settlement of sand under fill
You might consider surcharging or using a compacted sand column approach such as using a 30-36 inch diameter pipe vibrated about 10 feet into the soil on a 5 foot grid.
RE: Settlement of sand under fill
consider what the modulus value of the loose sand may be? Let's say, 40 tsf. Consider the unit weight of the 6 ft of new fill - let's say 125. Let's ignore Boussinesq and just claim the entire sand layer experiences a 750 psf stress increase.
Integrate the stress change with depth - 750x6=4,500 #/ft
convert units of modulus - 80,000 #/sf
Long division time!
4,500/80,000=0.056 ft - 0.67 in
The modulus could be a bit lower, but not by too much? So, is it 20 tsf? That'd then return 1.5 inches.
After such modulus-based compression, there is some likelihood of latent effects. These would be unlikely to exceed 30 percent in the two to three decades to follow.
I agree that the majority of compression occurs as fill is placed. Post placement settlement would then be the 30 percent component, which in my example would be 30 percent of 1.5 inches (my conservative assessment) or 0.3 inches.
Having fun with numbers. . .
f-d
ípapß gordo ainÆt no madre flaca!
RE: Settlement of sand under fill
Also, with saturated loose fine-grained sand and if the OP is in a seismic area, I would also be liquefaction concerns...