Repair of "Pumping" Areas 2

[Rjeffery]

In a parking lot, a contractor, after a failing proof roll, removed about a foot (`300 mm)of overly moist material and placed Mirafi (500 I think) in the bottom and placed a single lift of (crushed) recycled concrete AASHTO 21 gradation into the excavation.

Yes, I know that the lift is to thick .

My concern is that eventually water is going to make it into the depression that was filled with this granular material. This area is the lowest spot (now) of the parking lots subgrade...

Is this something that the owner should be concerned with?

 

[Focht3]

How large is the area? How far did the Mirafi extend beyond the pumping area?

This may be a case where the cost of doing things "right" may outweigh the risk of leaving things "as is."

The lift thickness, while thicker than desirable, isn't the end of the earth (for a parking lot.) It's no worse than the pumping subgrade it replaced.

 

[SirAl]

The 12 inch thick lift of subbase granular used to backfill a subcut of soft soil is not exessive in my opinion. If the material below is as soft as the material removed, you may need all of the 12 inches if not more to bridge the soft subsoil conditions.

With respect to the 'sump' effect, if the location is the low point of the parking lot, then I presume the subgrade would, as well, be the low point. As such, even without the subcut you required, this would have been the collection point of water infiltrating your surfacing structure. The crushed concrete is likely non-frost susceptible. My conclusion is that you are no worse off having provided the measures that you did. In fact, the crushed concrete will offer improved subgrade stregth.

 

[Rjeffery]

Focht3:

The area in question was about 600 square yards (~550 M^2)

If the fabric extended a foot or so beyond the soft area, I would be suprised.

Thank you for your reply. Your posts are, as always excellent.

SirAl:

Thank you as well for your thoughtful reply. I feel a little better now. Here is some add'l information...

When the parking lot is finished, the area in question will be in the interior corner of an 'L' shaped building. The area's finished grade, once the full paving section is in place, will be above the curb inlet while the lowest point of the excavation (subcut) is lower than any other subgrade elevation.

The area was not proof rolled after the subcut was made because there is 48" (1200 mm) HDPE retention piping only 2 feet (600 mm)or so below.

My concern is that rain or landscaping water could migrate down to this area (the HDPE) was pressure tested and no leaks were found) and begin to fail.

 

[Ron]

Rjeffery...I concur with the other comments. Had they placed sand, crushed limerock (indigenous to your area), or other materials I would be more concerned. The crushed, recycled concrete was a good choice. It will actually re-hydrate to some degree and offer a high modulus bridge as Sir Al noted.

I would not be overly concerned about the low spot. The crushed, recycled concrete retains high stability when wet, assuming it was reasonably compacted.

 

[Focht3]

Rjeffery:

Thank you for your kind words.


Since the low spot is "near" the re-entrant corner of an L-shaped building, I would also suggest that you review this condition with your geotechnical engineer. The combination of the Mirafi and crushed concrete will create a french drain - that might have its preferred discharge point toward the building. It is worth reviewing, anyway.