Shallow Foundation/Underpinning Options?

[psychedomination]

Hi there,

I'm working on a project where we will be adding a 2nd storey onto a residential house. The existing house is very old so I wasn't expecting it to have any proper foundations.

As a result, I was intending on underpinning the existing 8" hollow CMU house wall with a 2' wide x 1' deep shallow unreinforced mass concrete foundation.

The line loads were quite low at 65kN/m, so an unreinforced mass concrete foundations seemed to work fine. The 2'x1' footing size resulted in a bearing pressure of around 100kPa. Typically for soils in the area, I would assume a max of 150kPa.

The contractor started to excavate one of the underpin bays today and we found that the house was actually extended down ~3' below grade to a soft rock bearing material and it did appear to have a concrete foundation. The rock appeared to be soft sandstone or limestone. I tried to dig it up with a shovel but the shovel was unable to properly penetrate the rock. If hitting with the pointy edge of the shovel, it may go in like 1/4" The existing concrete foundation extended about 4" from the outside of the house wall but when I came back to the site later that day I noticed that the contractor removed this when he excavated deeper... which I wasn't expecting him to do. See the image below :

I would like some advice on what I should do next in this situation.

I was assuming that the soft rock would have a bearing capacity of around 400kPa, which may be conservative based on this literature : Link . There is no geotech in my area, so I need to make a reasonable and safe assumption.

With the 4" concrete nib removed and assuming there is no nib on the other side, an 8" block wall bearing directly on the soft rock would have a bearing pressure of 320kPa, which based on the above literature, the soft rock should be able to handle quite easily.

My question is :

1. Since the bearing capacity seems adequate, should I just leave this foundation as is (note, the 4" offset was only removed on this one 3' bay, the rest of the bays would likely have that 4" concrete foundation nib/extension) and just build the 2nd storey on top?

Or

2. Should I try to re extend the footing width to get the 4" back and more. Something like this?

or

3. Should I forget about the existing concrete foundation and build up the underpin with hardcore, break open a few masonry courses and pour a new foundation like what I was initially planning to do? This would require full underpinning for all the bays.

or

4. An alternative someone else can offer

Obviously underpinning is slow and expensive, so I'm more inclined to not do it if I don't need to but checking to see if there is any obvious reason that I should be doing it that I am missing. My main initial concern was settlement, but as it's on soft rock, that shouldn't be a problem.

Any guidance would be greatly appreciated.

Regardless of the option chosen, I'm still going to get the contractor to excavate another bay further from this one that was marked for underpinning to confirm that the soft rock is consistent at the same level.

 

[BAretired]

I was assuming that the soft rock would have a bearing capacity of around 400kPa, which may be conservative based on this literature : Link . There is no geotech in my area, so I need to make a reasonable and safe assumption.

Your assumption may be reasonable, but how do you expect us to know what is reasonable and safe? There must be a geotech somewhere in Bermuda, but if not, you will have to do your own testing. To rely on an assumption is a dangerous plan. You could find yourself in a heap of trouble if there are excessive settlements. At the very least, try to find out what others have used for bearing in the soft sandstone in the area.

 

[psychedomination]

@BAretired thanks for the response and you make some fair points.

Unfortunately, there isn't even one geotech here. However, in 1994 a geotech company did a report on the entire country but this report seems somewhat general and it doesn't give the bearing capacity allowance/settlement information for this specific area. The report relies on getting SPT data (which I don't have).

It's quite frustrating as it forces the engineers here to assume for smaller projects. For very large projects a geotech company would typically be flown in.

However, I did some digging and I managed to find a document that another local engineer did for the government, where a few trial pits were dug and they found a similar sand stone that was very soft/weak. They recommended a bearing capacity of 200kPa.

With this in mind, I'll go ahead with the underpinning scheme *Option 3 - keeping original 2'x1' size, which would end up having a bearing pressure less than the above recommended.

You mentioned "do your own testing", would you be able to expand on how you would go about doing that?

 

[BAretired]

Part of testing is to drill more than one hole to determine if conditions are typical over the site. There must be drillers available on the island which could do that part for you. As far as carrying out specific soil tests, I don't have any experience with that. The pointy shovel test, penetrating to a depth of only 1/4" would seem to me to suggest a material firmer than soft sandstone, but it's not a good measure of bearing value.

Perhaps you will get some useful advice from geotechnical members of Eng-Tips. You may be coming up against similar problems in the future, so you really need a method of testing the soil.

 

[Greenalleycat]

Can you just buy yourself a Scala penetrometer and bang it in?
They're pretty cheap and if you had one on hand you'd be able to do your own investigations and do away with needing a geotech
Heck, you could upskill and start charging people for it and become the geotechy guy in the area....

 

[milkshakelake]

This is so risky to do without a geotech. But I'm speaking from a standpoint of privilege, where we have those things available. I know things are done differently in other countries, and this seems too small to fly in a geotech.

The first thing is to use a penetrometer, like Greenalleycat suggested. The second thing is to get more information about the soil. Meaning you need to do a boring, which needs equipment and a lab. But if you can't do a boring, at least dig a few test pits at a few different elevations and test the soil there. Then you'll know how far down to go. You might even come up with a different solution, like expanding the footing. The test pits will also help you determine if it's the same condition everywhere.

The applied bearing pressure is quite low. So I wouldn't be too worried. But that being said, I really have no idea about the soils in that area, so I don't know how much to be worried or not. In my area, we have a long record of what causes settlement and what doesn't based on the experience of many, many engineers and buildings. Maybe you can see what others have done in a similar situation, and visit those houses to see how they're holding up. You might be able to get old drawings from some contractors or public records. Maybe people just add stories on top and nothing happens.