Stabilizing Excavation Bottom at Water Table
Stabilizing Excavation Bottom at Water Table
(OP)
Hi:
I'm currently looking at some alternatives to address loose alluvial soils (silty sand: SM, poorly graded sand: SP) that extend to the water table (-10 bsg). In order to address static settlements above the water table, one approach proposed is to remove and reconstruct to the standards for engineered fill.
The obvious approach would be to dewater the excavation to provide a stable grade in order to build the fill. However, I suspect that given the size of the excavation (about 1 acre), dewatering will be very expensive, and may impact adjacent improvements.
I've also stabilized some soil excavations with cobble (walked into the unstable soil) and geogrid and coarse crushed rock 2 feet).
I'm curious to see if anyone out there might have some experience with this or some other non-structural ( rammed aggregate piers, etc.) approaches to this construction.
I'm currently looking at some alternatives to address loose alluvial soils (silty sand: SM, poorly graded sand: SP) that extend to the water table (-10 bsg). In order to address static settlements above the water table, one approach proposed is to remove and reconstruct to the standards for engineered fill.
The obvious approach would be to dewater the excavation to provide a stable grade in order to build the fill. However, I suspect that given the size of the excavation (about 1 acre), dewatering will be very expensive, and may impact adjacent improvements.
I've also stabilized some soil excavations with cobble (walked into the unstable soil) and geogrid and coarse crushed rock 2 feet).
I'm curious to see if anyone out there might have some experience with this or some other non-structural ( rammed aggregate piers, etc.) approaches to this construction.





RE: Stabilizing Excavation Bottom at Water Table
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RE: Stabilizing Excavation Bottom at Water Table
RE: Stabilizing Excavation Bottom at Water Table
We've reviewed structural alternative including geopiers, vibro-flotation, stone columns, grouting, and deep foundations w/ structural slabs, etc., and these approaches are viewed as more costly to implement. The over-ex and replacement approach is looked at as the least costly to implement - provided stabilizing the excavation bottom doesn't turn into a bottomless pit.
The sands extend to between 10 to 17 feet bsg, with groundwater at 9 to 10 feet bsg. The client understands that over-ex and replace above water table will not resolve potential seismic settlement of loose sands below groundwater.
RE: Stabilizing Excavation Bottom at Water Table
I have been on projects to improve bearing on shallow residual soils overlying very punky karst, but I know it has been used for a wide range of poor soil sites.
RE: Stabilizing Excavation Bottom at Water Table
otherwise, if you can use explosives there have been some loose soils densified with charges.
RE: Stabilizing Excavation Bottom at Water Table
RE: Stabilizing Excavation Bottom at Water Table
If the adjacent improvements are really adjacent, either DDC or explosives is likely to be trouble - even if you don't actually damage the structures, every pre-existing crack or blemish will become yours. I think you should be thinking of stone columns, displacement piles, etc. We've had good luck with jet grouting of silt, but my hunch is the displacement-vibration methods (stone columns, piles, etc.) would be cheaper. (We couldn't use them for the silt because of the low permeability.)
There is also a device that intrigues me for relatively shallow deposits, but I have never seen any hard data on its performance:
http://www.rapidimpact.ca/index.htm
It is small enough that you might need to excavate part way to the water table for the compaction to reach the bottom of the deposit. Don't know a lot of specifics about it, including whether it is easily available outside of Canada (or maybe you are in Canada so it doesn't matter).
DRG
RE: Stabilizing Excavation Bottom at Water Table
RE: Stabilizing Excavation Bottom at Water Table
Mickney - What soils were involved at your site? SP would consolidate about as fast as you can place the fill, as would many SMs.
RE: Stabilizing Excavation Bottom at Water Table
Whereas in dynamic compaction you have a 15 t weight dropped from 25 m once evry minute, for RIC you use a hydraulic hammer with a ram of 9 tons dropped from 1.2 m 50 times per minutes. The total energy given to the soil will be about the same but since the unit energy is lot less for RIC, the depth of influence will also be much less.
It will be from 3 to 5 meters ( the less fines, the dreater the depth of influence )
I can't imagine that soil replacement could cost less than mass ground improvement unless you completely underestimate difficulties coming along with the presence of groundwater.
Techniques related with dynamic compaction, RIC should prove the more economical.
RE: Stabilizing Excavation Bottom at Water Table
I agree that dynamic compaction would be the most cost effective to improve the soils and reduce the liquefaction potential. One concern is that the site surface will have to be improved - dynamic compaction needs a stiff/dense surface in order to transmit the shear waves to the deeper soils.
I've obtained some information on rapid impact compaction and it in general appears applicable to the site. Since the depth of improvement of this method appears to be up to 20 feet, it seems to fit this site. We do have some silty sands above the water table, and there is some concern as how well this will work (these soils fall between the general zones that respond and do not respond to dynamic compaction).
Fortunately, the sands generally become cleaner with depth, transitioning to poorly graded with silt and poorly graded at and beneath beneath the water table.
Blasting might work as well to densify the soils. Unfortunately, the regulatory aspects associated with this will make other, more conventional approaches seem simple.
RE: Stabilizing Excavation Bottom at Water Table
RE: Stabilizing Excavation Bottom at Water Table
On the subject of EPWP and blast densification, go to the following site and watch the video for a rather spectacular example in tailings.
http://www.rapidimpact.ca/inco_tailings_pond.htm
RE: Stabilizing Excavation Bottom at Water Table
Do not forget that limits of these techniques in terms of fine contents can be improved by combining them with vertical drains like shown in the video link provided by dgillette.