Site Investigation on Existing 2 Storey House

[Contraflexure74]

Hi All,

I have a scenario where I'm not sure what to do.

I have a 2 storey detached domestic house which is situated in an area well known for having poor ground. There are some 1mm cracks in the property but nothing that worries me. The house was built the circa 1970's.

We did some site investigation works to date and to be honest the results are a bit confusing. We have established that the strip footings are 2m down and are 1.1m wide.

The foundations were too far down to expose the bottom of foundation to see what type of soil the strips sits on.

We also carried out some dynamic probing and all 4 probes are consistently bad. At 2m depth we have a n value of 2 (per 100mm) which is extremely pour. This by my calcs gives about 40kN/m2 of bearing capacity at best. The soil is a soft clay. We got refusal from the probes at 4.5m down.

The client wants to convert the attic into a bedroom and add a 2 storey extension to the rear of the property. I'm worried about modifying the existing house with these probe results.

I'm thinking of going down the route of installing a piled raft within the existing house and piling the extension but this will be expensive. Is this overkill?

Should I dig a larger trial hole to visually see what the ground is like at strip footing level??? Or is this a waste of time bearing in mind the poor probe results.

Your thoughts are welcome.

 

[XR250]

Converting an attic into bedroom would not significantly change the load on the foundation so I would not lose sleep over that. I would be more concerned about the addition and how to support it.

 

[WARose]

We did some site investigation works to date and to be honest the results are a bit confusing.

Does "we" consist of a geotechnical professional? If it doesn't, I would highly advise getting one involved because some of the judgment calls involved in such a project should be left to them. (Especially if piles are going to come into play.)

I'm thinking of going down the route of installing a piled raft within the existing house and piling the extension but this will be expensive. Is this overkill?

It might be a good idea on the extension......but it may be unnecessary on the existing house. (As it may be (for the most part) done settling.) You might be able to get away with just having a pile foundation for the extension. (Micropiles would probably be most feasible if you've got good bedrock to anchor to.)

If the original home continues to settle (creating a differential settlement), a Ram Jack solution might be able to fix it (on down the line).

It's just something to work out based on what the geotech tells you.

 

[Contraflexure74]

Hi Guys,

Good points and advice there. No I'm not a geotechnical Engineer. I'm civil/structural. The probes and trial pits were carried out by a local piling contractor.

The thing I can't understand is that if the house has done all it's settling why are the probe results right beside the footings so poor?

 

[WARose]

The thing I can't understand is that if the house has done all it's settling why are the probe results right beside the footings so poor?

That's a question for the geotech......and it may not be done settling.

 

[KootK]

The thing I can't understand is that if the house has done all it's settling why are the probe results right beside the footings so poor?

Could be any number of reasons:

- if you tested at 2m beside a 2m footing there's little reason to assume that the soil there would have been improved by the continuously loading from the footing.

- 1.1 m is a pretty wide footing for residential. As crappy as the soil is, it still may not be working too hard.

In my area 75 kPa is what we use for material lacking a geotechnical blessing. 40 kPa's not that bad.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dik]

Your bearing is approx 1Ksf... which is slightly stiffer than 'loon sh*t'... I would be reluctant to add anything to the structure... you could be looking at long term 'ugly cracks'. Often changes in load regimes cause unanticipated cracking/settlement. I'd caution the owner strongly against adding to the structure.

Dik

 

[oldestguy]

As others have advised, get an experienced geotech involved. This quote explains why >. "The thing I can't understand is that if the house has done all it's settling why are the probe results right beside the footings so poor?"

 

[Contraflexure74]

Thanks to all.

Your views are quite varied on this particular item; from the ground should be okay to the ground is poor. One thing that seems to be a common thread though is to get a geotech professional involved which is what I'm going to do. Thanks to all for your time. Much obliged.

 

[GeoPaveTraffic]

Are you sure that the probes indicating poor ground are located in natural soil? Could they be in uncompacted backfill from the original construction?

Mike Lambert

 

[Contraflexure74]

Hi Mike/All,

We have taken 4 dynamic probes; 3 right beside the existing foundations and 1 out in the back garden away from the existing house. All 4 show a blow count of 1 or 2 every 100mm until about 3m depth then we start to get a value of 5 and rising which is what I would normally look for (for good ground). We have established that the bottom of existing foundation level is at 2.1m depth stepping down to 2.4m depth. So there appears to be a layer of circa 600mm - 900mm of poor ground according to the probes.

Are you suggesting that the site was over dug and back filled?

What I think happened is they kept digging and found that the ground was not getting any better and decided to put in a wider strip footing but I'm only guessing.

Your thoughts/comments are welcome. Thanks.

John.

 

[LittleInch]

For the footings you have the total bearing load for a 1m length is circa 4 tonnes.

I did a quick calc for bricks and foundation for a 5m high double skinned wall and get about 2 to 2.5 tonnes. How much you add on for floors and roof I don't know, but spread around the perimeter of a house I can't think it's going to exceed your 4 tonnes per metre.

However additional roof load and floor load in the roof may just tip it over the edge.

Thought about grout injection under the existing foundations?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Contraflexure74]

Hi,

I did the bearing calc and I calculate that there is 50kN/m2 under the existing strip (unfactored dead + live). The depth of foundation is too deep for grouting + clay is not really suitable for grout injection in my view.

 

[XR250]

Once you finish out the bedroom, what would your bearing be?

 

[Contraflexure74]

Its works out at 52kN/m2. Minor difference I know which has already been said. My point is the blow count at bottom of foundation level indicates a bearing capacity of 40kN/m2 at best and I have zero safety factor.

 

[SlideRuleEra]

...I calculate that there is 50kN/m2 under the existing strip (unfactored dead + live).
...blow count at bottom of foundation level indicates a bearing capacity of 40kN/m2 at best...

Make a realistic estimate of the existing actual live load (based on the contents of the house, NOT code requirements). You may find the unfactored dead + actual live load is < 40kN/m².
Of course, this could be just a reason the house has not settled much, not a solution.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[XR250]

I think ASCE7 has a table of actual recorded live loads in a residence. IIRC, it was like 6 PSF in a bedroom.

 

[dik]

I don't think it's a bearing issue, but, one of settlement...

Dik

 

[dik]

XR259,,Baker's Steel Skeleton had a summary of 'real' loads...

Dik

 

[XR250]

I don't think it's a bearing issue, but, one of settlement...

Converting an attic to a bedroom won't change that. As long as the owner is made aware that settlement is occurring and he may see some cracks in the finishes (in the new bedroom and other areas), then I do not see a problem with it.