Settlement calculations for organic soils 1

[4lbSledge]

A bit about the project: An existing facility in New England located adjacent to a river is looking to build an addition with full basement onto an existing building. The finish floor elevations will be the same, and the new building basement will be accessed via the existing building's basement. In preliminary discussions with the client, we became aware of settlement issues at the existing building. After drilling, we discovered a loose organic silt/sand layer on the order of 5 - 8 feet thick at a depth below the proposed subgrade that makes it impractical to remove prior to construction. Cost is king, and piles are expensive. Add to that the existing building is NOT on piles, and we're left trying to figure out how to determine settlement of the organic layer under both the existing and proposed building sections. Any method / computation suggestions for how to evaluate this potential settlement and differential movement between the two buildings? Thanks in advance for any help!

- 4lbSledge

 

[lovethecold]

I am not too sure you will find any help with what you are asking for. Have you looked into underpinning the existing home with micropiles or helical piers and support the addition on the same? These are cheaper and quite practical if competent soils are not too deep, although it seems they are capable of going quite deep with relatively heavy loads. You can go to

http://www.abchance.com,

they should be able to help locate a local design/build firm who could help on this.

Good luck. Others should be able to provide better guidance.

 

[4lbSledge]

Thanks for the suggestion. Underpinning of the existing structure really isn't an option here - it's an existing municipal/industrial building on a structural mat slab that has other issues with the structural mat slab basically falling apart that are outside the scope of this work. We had been considering the Helical Peir option for the new structure; it's nice to hear suggestions that support our initial thoughts on a solution.

Our main concern was how to evaluate settlement of organic soils should the client decide against helical piers for some reason. I've seen a lot of information on calculating settlement of high organic content layers like peat, but nothing that suggests how to approach a layer with only 4-5% organic content. The organic content may be negligable in this case, but it never hurts to have calculations to back up that assumption

 

[BigH]

Piling may be expensive but so is remedial works if the new addition settles or causes settlement of the existing building. Why not consider stone columns - I don't think that you have to use "rammed" stone columns or vibro-stone columns. You want to reinforce the loose layer without creating unwanted effects on the existing building.

 

[TXStructural]

Do you think that the new building will have a weight greater than that of the soil removed for the basement? If not, you might not expect further consolidation of the deeper layers, but you could have the opposite problem.

If you are building with a similar structural system as the existing (mat/raft slab), you might try to determine how soils under the existing have performed. If there are survey benchmarks for comparison.

Also, you mention being next to a river, so consider that you may lose soil volume due to expression of water under the new weight of the building (or float the new building if too light.) Or, if the building is on soil higher than the river, is there a soil stability issue to deal with?

 

[5234185]

Can you surchrage this area? Also, you can monitor the settlement using 2’ x 2’ x ½” steel plates.
Thanks

 

[BigH]

5234185 - what would the surcharge effect be on the existing building? Has to be considered.