Subgrade Stabilization

[trey25624]

I am looking to find a way to stabilize a soft subgrade for a large retail store parking lot. The site soil consists of saturated silty clay. Proof rolling indicates rut depths of about 4" in most places. What would be an appropriate fix for this? I was thinking several options.

1. Undercut 1 foot, backfill with 12" of sand. I figure I might need to go to something more like 16" of undercut and sand backfill with underdrains.

2. Stabilize the upper 12" with lime.

Any input would be much appreciated. Thank you.

 

[civilperson]

Remove a foot of soft soil, place geofabric, proof roll to produce ruts and then use crushed rock for sub-grade and compact to tension geofabric.

 

[dirtsqueezer]

We always excavated at least 2' for a subgrade fix. Try taking a t-probe and seeing how deep you get. If it's more than 2', you definitely need to take the extra soft stuff out. Mifafi would be required, of course.

 

[DMcGrath]

I'll add to the chorus of supporters for a geotextile. We have had several projects where we could have either "dug to China" to find a suitable subgrade or just remove a foot or two of the soft soil, install a geotextile, then start placing suitable fill material. Saves time and money. Geotextile is relatively inexpensive, in the neighborhood of $0.10 to $0.25 per square foot installed.

 

[fattdad]

Any possibility to scarify and recompact? If not, a 12 to 18 in undercut with separation geotextile and compacted structural fill should work.

Is this a case of saturated clay by virtue of the water table or is this a perched water condition?

f-d

¡papá gordo ain’t no madre flaca!

 

[trey25624]

I don't think this is a perched condition. The groundwater is just that high on this site.

 

[fattdad]

Wow! Must have quite a wetland permit!

f-d

¡papá gordo ain’t no madre flaca!

 

[Mickney]

The first question that comes to mind is where is groundwater in relation to finished grade? If within a couple of feet, then I would consider including permanent trench drains with the fix, particularly if the groundwater levels rise and fall with seasonal changes.

If groundwater is over 4 to 5 feet below the finished grade, it is likely capillary rise causing the high moisture condition and you would need to break the capillary rise. Lime and/or overexcavation and backfilling with a compacted graded stone layer will work for this.

 

[JML4TCM]

If you are using a geotextile you will probably want a geotextile with good tensile strength for reinforcement and good permittivity (>0.25 sec-1) to help drain and dissapate the pore water pressure present in the subgrade soil. The Mirafi HP-series geotextiles would be a good option with anywhere from 12-24 inches of aggregate placed on the fabirc would probably do the job.

 

[DirtEstimator]

Problems like this are why Lime Stabilization is used. I would go 8% to a depth of 8" and be done with it.