Foundations for building additions

[Okiryu]

Hi, I would like to ask for your opinions for the below situation:

I have a new project for a building addition. The existing building is a 1-story office building and it was constructed 55 years ago and it has 1.7m x 1.7m shallow square footings where Df is at GL-1.5m. Those footings were designed for an allowable bearing pressure of 4,500 psf (200 KPa) based on a bearing layer of silty sandy gravel soils.
Based on our soil borings we confirmed the silty sandy gravel soils, however, we found that at GL-1.5 the gravel soils were loose (SPT N-value 4) or were replaced by cohesive fill soils. It may appear that this could happen due to past construction activities. Now, we found the competent gravel layer to be at an approximately depth of GL-2.0m~3.0m.
We are in the process to provide our recommendations to the structural and at those depths (GL-2.0m~3.0m) we might be able to validate/recommend the previous 4,500 psf for allowable bearing pressure.
Of course, for the new foundation for the additions adjacent to the existing building, the concern is if we excavate deeper than GL-1.5m we will be removing support to the existing footings. The preliminary structural plans for the new additions call for a bearing wall on a continuous footing adjacent to the existing wall, so in order to avoid excavation we are planning to recommend soil improvements using soil cement columns (I am in Japan and in my area soil cement improvements are common)or helical steel piles to a depth of GL-3.0m to support the new continuous footing. The structural is also concerned if we excavate for the new foundations we will undermine the existing slab-on-grade.

Do you think that there are other options? I made a sketch so you can visualize better the situation.

Thanks for your help.

 

[BigH]

I'll bite . . . there are a couple of possibilities as to why the old building was able to use 200 kPa spread footings at 1.5 m and you find suitable bearing to be at 2 to 3 m. Perhaps, they dug down to a depth of 1.5 m everywhere and then used heavy compactors to densify the sands and gravels. Seems a reasonable approach and they might have done it too for ensuring that floor settlements would be minimal - backfilling with the same sand and gravel from the site in layers. Secondly, the more competent sand and gravel layer might just be dipping - or the upper zones were loosened during original construction.

As for a thought on your foundation . . . - the existing building is on spread footings - but you do not say at what bay spacing - 6 m? Why not put your footings in between theirs - i.e., stagger yours. Excavation for your footing could be cut similar to the picture attached (from Kaniraj's Design Aids in Soil Mechanics and Foundation Engineering). If the existing footings are wide enough apart, you might be able to squeeze yours in at the building line. - anyway - just a thought.

 

[Ron]

Agree with BigH's approach.

Be sure to do a very careful condition assessment of the existing building before you start any construction. If there are existing cracks, map those out and mark their ends so you'll be able to tell if the new construction causes any difference. Use video and still photography to document, as well as copious notes. During construction, consider monitoring construction vibrations if you are planning on using reasonably large compaction equipment. The purpose is twofold...to prevent damage and to protect your liability. Protect the existing building and its occupants from danger and damage during construction.

Adjacent construction is always a challenge. Do as much as you can to minimize the effects of the adjacent construction on the existing building.

 

[Okiryu]

BigH and Ron, thanks for your comments.

BigH, The bay spacing is 6m so I think that squeezing the new footings there might be an option, however the structural wants a continuous footing to support a bearing wall adjacent to the existing bldg., so that makes the foundation more challenging. Also, my concern is that if we staggered the new footings it will be a strange configuration at the corner of the existing and new bldgs. (if we do not install a footing at the corner, the walls will be cantilevered)

Also the structural is concerning about remove support of the slab on grade if we excavate for the new footings. That's also why we are trying to minimize excavation operations adjacent to the existing bldg.

Ron, I agree with your advice. We always add similar recommendations in our report when dealing with adjacent construction.

 

[dik]

Whenever I do an addition where there are new foundations in proximity, I usually send the client a letter indicating that the loading regime in the vicinity of the existing building has changed and there may be minor cracking due to loading and differential settlement. In addition, in these environs, we usually check for snow loading on the existing as well as snow accumulation if roofs (rooves?) are at different elevations.

Dik

 

[BUGGAR]

Well, I tried to think up a really good solution to your foundation problem and I think you have already found it - soil cement columns or helical piles. I've used both and if they are reasonable in cost for you and the soils are suitable, I say go for either.

As you know, soil cement columns are started out with test columns to "tune" their installation methods to minimize disruption to adjacent construction and verify their strength. I used them below and alongside crane rail grade beams with no movement induced in the grade beams. Worst case was when grouted soil bubbled up a few feet from the installed column but no damage was done. Generally, our soils were impermeable enough to contain the slurry mixture.

It would be great if you could let us know how this project goes, construction problems and solutions, and everything you can share. Thanks.

Bob

 

[Okiryu]

Thanks for your replies. I will present different options to the structural based on the above suggestions and will let you know how the project goes.

 

[Mercycity]

Dear 0kiryu,
The underlying soils and the existing foundation plus the proposed building are making things complication. Myself i have a second thought. In consideration that the exiting building is a single story and old, why not advice the developer to demolish it and pave way for the new building founded in good soils as recommended in a geotechnical report.

regards,