Mixed use of piled and strip foundations 3

[Teamdesign]

Hi to all,

I am a junior structural engineer from UK. Will appreciate some help from more knowledgeable engineers.
I am designing the structure for a three extensions - to the side,rear and front of one house.
The side one is two-storey the rear and front just one. Location is in predominantly clay soil area.
Because of the extension builders will have to remove a huge 15m Cypress tree and logically the water content will be higher in the future therefore building regulations guidance are for 2.5+ meters deep foundations designed by eng.

I have specified RC beam on 3m bored piles with min 90kN load capacity at 2m c/c for the side and rear extension nearer the tree to be removed. For the front extension (12+ meters away) a trench fill foundation to 1.5m depth. Also specified on the plans that foundation depth is subject to local site conditions and building control officer approval.

Original house has strip foundation.

The comment on the letter from building control :
"Mixed use of piled and strip foundations are not accepted and should be avoided. Please amend."

I tried to call the officer few times, but he was always on site etc.

Any thoughts appreciated. What foundation you think will be more suitable?

Thanks for the help

 

[msquared48]

He is worried about differntial settlement due to the different types of foundations as you are mixing apples and oranges with the strip footings and piling. The pipings are a hard point and will not settle (generally) whereas the spread foootings will with any extra load. The presence of the clay complicates matters too.

As you have no comment with regard to the involvement of a geotech here, it would be a good time to engage one if you have not already. I am in the US, but the same basic engineering principles apply.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Teamdesign]

Hi MSQUARED48,
Thanks for the reply.
But isn't that why we provide movement joints when connecting new to existing structure?

 

[msquared48]

Mainly for lateral movement. Verical move3ment is frowned on as it gets into discontinuities of floor levels, crtacked sheetrock, unintentioned shears and stresses, and tripping hazards. Ideally, we want the entire structure to settle uniformly, as some settlement will almost always occur, and should work our design toward that goal.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Michel60]

Good morning Teamdesign,

msquared is correct, mixed foundation systems (deep vs. shallow) will settle differently and are generally avoided.

But as an aside, one exception to that rule is when both systems are founded in a common "firm" layer. For example, a sharply dipping layer of bedrock might be exposed on the surface at one end of a building pad and buried at another. Under those circumstances a spread footing on the exposed bedrock and drilled piers into the buried bedrock can be considered, IF recommended as appropriate by a geotechnical engineer.

You circumstance doesn't seem to fit this exception but you do still have some limitations on what is suitable for the site conditions, I would expect you need recommendations from a local geotechnical firm to sort this one.

regards,
Michel

 

[BAretired]

Removal of the cypress tree could be the biggest problem. If the clay is expansive, it will swell with an increase in moisture content. Any new or existing grade slabs or strip footings in the vicinity of the tree will be subject to heaving.

It seems very unlikely that a 3m deep bored pile would be adequate to sustain a load of 90 kN. How did you arrive at this?

Whether the foundation under the additions is founded on footings or piles, any grade slabs near the old cypress tree will be subject to heaving due to swelling soil.

A geotechnical investigation is strongly recommended.

BA

 

[Teamdesign]

Thanks Michael,

Good afternoon from London. I will try to speak with the building control officer tomorrow and ask for his advice as well,
but from what you say guys I assume we will have to go for a deep footing. Will update the thread once I have the advice from the BCO.

Kind Regards

 

[Teamdesign]

BAretired,

I just say that the piles specified have this capacity, the actual unfactorized load is around 20-25kN/m and in 2m c/c piles it comes to 20-25kN shear per pile. It's a small project and typically the soil and suitable depth is checked by the BCO
just before concrete is poured. I also checked a map today that states there is a gravel deposit in this area, but because of the water table I don't believe this will help much.
Thanks for all your comments, will update after talking to BCO.

 

[JAE]

I think if you use a complete expansion joint (floors,walls,roof) at the interface, then the piling could be considered. But you would want to notify the building owner of the risk involved in the new piling-addition not moving and the existing building on the strip footings possibly moving relative to the new.

In some cases the existing building has already "settled in" over time and future movements might not be likely.

 

[msquared48]

That's where you need the Geotech's involvement.

Personally, I would expect some additional settlement with any additional long term dead load, even considering time induced settling (creep), on a strip footing foundation, but would revert to the opinion of the Geotech.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[1freewun]

Hello Team Design,
If the two extensions aren't connected and not adjacent I don't see a problem.
Piles are sometimes used for circumstances where a deep strip is not practicable and deep strips are used where trees dictate the depth and are the cheapest option. The house presumably has been there for a while and is unlikely to be subject to further significant settlement and therefore piles are eminently suitable as they settle less than strip footings if correctly designed and constructed.
As you say extensions are normally built with movement joints against exg. construction and these will cater for the range of movement likely from new work on correctly designed strip foundations.