aside from preloading soil, what other methods to minimize possilble foundation settlement?

[delagina]

If I'm trying to minimize possible future settlement for my foundation. I will put 6" compacted subgrade below the foundation. Other than that what else can be done. I was told to preload the soil with 5 to 10 feet of soil then remove it before putting the foundation. How much time are we looking at this if we want to do this approach? What other method can we use? Any Geotech here want to take a shot?

 

[delagina]

I'm talking cheap methods not that will require a drill rig or heavy equipment because we talking of only 1 foundation here.

 

[KootK]

You could oversize your foundations considerably.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[oldestguy]

What is "one foundation" there? A whole building, one footing, one flag pole??? What kind of building? Number of stories?

Then, what is the stuff below? Soft highly plastic clay? loose sand? Old landfill? How thick?

Length of time for pre-loading can vary. In most cases I am familiar with, such as peat, I shoot for one month. I have had surcharges siting for a year, but then it didn't take as high a pile.

Another technique, such as uncompacted fill, removing only part and replacing with compacted fill works.

6" of compacted gravel below a foundation would only serve to stabilize so you can work there. Not effective for reducing footing size.

 

[GeoPaveTraffic]

You really need to talk to the project geotechnical engineer. Pre-loads can be very effective if done properly.

As for other options, you could always use a deep foundation to rock. That pretty much minimizes the settlement potential.

As for the suggestion to oversize the footing, might work; but it depends on the soils. I've seen cases where making the footing larger actually increases the settlement.

Mike Lambert

 

[PEinc]

You didn't give any information about the foundation(s), the loads, the soils, or the ground water. Maybe you can underpin the footing before you actually construct it or install driven or drilled piles, helical piers, or micropiles. Maybe you can do dynamic compaction by dropping a very heavy weight on the footing area. Maybe you can do compaction grouting or permeation grouting to improve the soil. You could possibly install one or more drilled shafts.

http://www.PeirceEngineering.com

 

[JStephen]

I assume this is in reference to this thread:

http://www.eng-tips.com/viewthread.cfm?qid=404935

That would be a question for the geotech. In addition to preloading, I have seen soil reports that recommended vibratory compaction, but that'd be questions for the geotech as well.

With the circumstances described in that previous thread, my initial thought is to design for that very low bearing, make the slab high enough above grade to allow for large settlement, design it so it can be supported in the center only or two edges only without failure so it won't much matter how it moves around. It wouldn't be cheap, but still likely cheaper, quicker, and simpler than the alternatives. I haven't tried running any numbers to see how practical, though.

 

[oldestguy]

Assuming your other post is correct for this question (20 kip 3' diam. tank), soft soil, consider undercutting say 5 feet,and about 8' x 8' area, or 300 psf down there, and then place a slab footing maybe 4 x 4 on compacted fill. Check the relative costs of these alternatives and go that way.

 

[manstrom]

dynamic compaction?

wood piles are fairly cheap

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Ron]

Where is the water table?

 

[kc9508]

Maybe rammed aggregate piers if there is a competent layer of soil below?

 

[PEinc]

Menard seems to be doing a lot of similar projects with their CMC's - controlled modulus columns (rigid inclusions). Check out Menard's web site.

http://www.menardgroupusa.com/

http://www.PeirceEngineering.com

 

[BigH]

As Oldestguy says, you can undercut and then place your footing but need to ensure that the stresses induced into the softer clay are minimal to inducing additional settlement. You indicated that the bearing would be 700 psf (35 kPa) - this implies an undrained shear strength of 350 psf (18 kPa) or so. This is soft clay. The problem with undercutting is that the replacement fill, typically sand or sand and gravel, is heavier than the clay it is replacing. You will need to take into account the additional weight of the replacement fill and its effect on the overall settlement issue. Ron asks about where the water table is located - since you had indicated that the clay is soft - without other information, it must be presumed that the soil has no effective desiccated crust - and hence the water table is at or very near the ground surface. All these issues - location of water table, net increase of stress due to replacement fill, etc. must be taken into account. The use of preload has been used extensively and is a true and tried method.