Overexcavation of a footing 1

[jgailla]

I just became involved with a renovation project. The existing structure is two-story masonry on shallow footings. An elevator is to be added to the side of the existing building. Originally, the elevator pit and walls were to rest on a mat bearing three feet below adjacent grade. The contractors dug to this three feet and hit a layer of sandy clay. The top layer was fine sand. They came back the next day to find the hole a foot deep in water. They expected to dig below the sandy clay and encounter sand which could be dewatered. They went down to 10 feet below adjacent grade (of course without adequate supervision), didn't hit sand, and called us. I did one auger boring, and hit sand at about 13 feet below adjacent grade. Further excavation is impractical, and I am suspicious the existing building will undergo detrimental settlement already due to the overexcavation. They want to remove standing water, strip the top 6" of soil, and place gravel (57) into the hole. I am concerned about fines migration and unbalanced pore water pressure, which can be solved with a geofabric, and dynamic loading on what is essentially a gravel column held laterally by existing soils. Dewatering is a problem due to the high groundwater level (about 4 to 5 feet below ground surface now) and the high fines content of the sandy clay. I need to write a proposal for fixing the problem reasonably, and I'm not even sure what tests I want to run. We'll do an SPT to 30 feet to get some general information.

 

[Ron]

Fill the hole back with select sand backfill, compacted to 95 % of Modified Proctor. Fines migration will be minimal.

Dewatering the hole will probably cause more settlement than filling with porous material. Unbalanced poor pressures are not a big deal.

 

[jgailla]

I don't know how they can get compaction on the lower lifts. The bottom of the hole, even with pumping, will stay mucky. If they simply dump sand into the hole about two feet thick uncompacted, they could probably get compaction on the layers above. This would produce about five feet of compacted fill above two feet of loose material. I'm comfortable with the sand backfill, but I have to convince my senior to buy it. Thanks for your quick post.

 

[Ron]

jgailla....fill the bottom of the hole with combination of gravel and sand until you can use sand only. The gravel will help reduce the pumping and you can get at least some reasonable compaction, though probably not to a 95% level. Don't worry about testing it until you get to sand lifts only, then test for compaction. If you get a couple of feet of the sand-gravel mix in place and well compacted sands above it, you should be comparable to the surrounding material, at least.

 

[DRC1]

If you are backfilling below water use crushed stone so that compaction is not necessary. If you eliminate the clay, you should eliminate the perched water table. Thus the normal water table gradient will generally be so flat that migration of fines will not be a problem. However, putting down some filter fabric before you fill with stone is a good idea and cheap insurance. If your footing is founded on the sand, long term settlments should not be too bad. I would back fill right away to stabilize the footing.How close was the excvation to the footing?

 

[Zambo]

Lean mix concrete would be just the job.

Zambo

 

[Ron]

jgailla...I understand your excavation to be about 50 or 60 cubic yards by now with the necessary side slopes. While lean concrete will work, it is expensive. You might consideer CLSM if speed is a consideration.

 

[SlideRuleEra]

A lower cost alternate to lean concrete is flowable fill. Here are a couple of links for information

http://www.wrmca.com/Flowable Fill.htm

http://www.gcpa.org/FlowableFillSpec.pdf