Settlement Issue with Proposed Building Site on Mine Spoil

[chrisrosebud2001]

I have a proposed building site that is a reclaimed surface mining area and the building is to be placed on approximately 200 feet of mine spoil fill. The fill has been in place since approximately 1992, so it is expected that most of the soil consolidation has already taken place. However, it is still recommended by the geotechnical report to improve the site bearing conditions using a deep dynamic compaction procedure. Upon completion of this procedure the report recommends that shallow foundations may be used with a net allowable bearing capacity of 2ksf. It is also noted that this procedure will only be able to effectively compact approximately the upper 20’ of the fill material and therefore, significant differential settlements are expected in the fill below. The report suggests that we design for 1½” of differential settlement over 20’. Myself and the architect for this project do not feel that it is likely feasible to design for this amount of settlement. Has anyone dealt with a similar situation in which it was required for the structure to withstand that much settlement or would it be better to improve the soil conditions further?

Thanks in advance for any advice.

 

[born2drill]

Anything I've seen built over mine spoils has been put on micropiles. I think it would be wise to do the same here.

 

[chrisrosebud2001]

Structurally speaking, I prefer the idea of deep foundations, but it is likely that any deep foundations would need to bear below the mine spoil to reach more suitable soils which is 200 feet below existing grade. That is why I believe the geotech did not recommend this as an economical solution.

 

[cvg]

"mine spoils" is a very broad and somewhat ambigous term. However, it is very possible (and dependant on the type of spoils or tailings or other material and method of placement) that not all of the soil consolodation has taken place and differential settlement is highly likely. I would go back to your geotech and ask for another solution

 

[Dirtguy4587]

I'd suggest that the geotech's recommendations are reasonable, both from a technical and cost perspective. Any methods to get you deeper compaction (and less settlement) would be very very expensive. I also think the 1.5" over 20' is reasonable as well, assuming we are talking about conventional mine spoil (and not tailings/sludge etc...).

 

[msquared48]

This scenario has "Mat Footing Foundation" written all over it.

Mike McCann
MMC Engineering

 

[FixedEarth]

The following suggestion assumes max. total settlement of about 3 inches with dynamic compaction improvement & about 1.5 inch differential settlement.

It is not a cheap solution. Once you complete dynamic compaction, you can preload the site by placing a stockpile (Say 3 ksf pressure) and install settlement plates so you know when to remove the surcharge. Then remove upper 2B soil depth beneath the slab elevation and place one sack slurry mix. Then design the foundation as isolated footings and connect them to each other with grade beams in x direction, y direction and diagonal directon. Use only 2 ksf bearing pressure and flexible pipe connections. Also make the slab span between grade beams. This will reduce differential settlement but make sure material from 20 to 60 ft is not WOH (weight of hammer) material, i.e. very loose, soft or highly organic.

http://www.FoundEng.com

 

[emmgjld]

It is hard to make recommendations when several, big unknowns are in the shadows.
-- Just how big is the footprint of new structure?
-- What type of structure is planned, Wood, Masonry, Steel or Concrete Framed?
-- What is to be the ultimate Use? How important is building/slab straightness?
What is the proposed life-span of the building?

 

[chrisrosebud2001]

The building is approximately 50,000 square feet and a single story masonry strucuture. The building will fall under occupancy category IV per ASCE. Life span of the building is 100 yrs with an estimated service life of 40 yrs. The building will be a national guard facility and building/slab straightness will be important especially for some architectural components.

 

[fattdad]

jam a dilatometer into the ground, measure the modulus and do your own calculations.

f-d

¡papá gordo ain’t no madre flaca!

 

[cvg]

since this was posted in the geotech forum, the recommendations will be to either improve the foundation or do more geotechnical analysis. I would not recommend designing a large, heavily occupied, essential structure out of masonry on a waste material pile without doing one or both. If you want another opinion, either provide some data or ask your geotechnical engineer.

 

[Mccoy]

I would be in favour of a detailed geotechnical investigation.
detailed but optimized as to the cost. The building is not tall but it is pretty large, so a number of relatively shallow CPT's would do, possibly completed by a few soil samples to calibrate the correlations and maybe one or two mor expensive tests as dilatometer as fattdad suggests.

In this way you would have a final picture of the soil strenght fluctuations below the building, carry out an estimate of the differential settlements with several foundations options and finally decide which foundation is best with an informed knowledge.

In such a case every additional expense in the foundation structure would be justified as necessary

 

[sirsoil]

Where is the fill and what type of building. you may not have to do anything different than a conventional site from a bearing capacity construction standpoint. however, the big issue with mine spoil sites in settlement due to water infiltration about a year or two after construction. The fill type, sandstone v. shale will have a big impact on the performance of the building.