Foundations in changing ground types.

[engstructural]

I have been asked to look as a domestic foundation solution for a site. the ground conditions are generally good (firm under foot) sandy gravelly ground. In one area of the building the contractor has gone through this layer into a different gravelly layer but of similar firmness.

To avoid the potential of bifferential settlement I was going to propose the strip foundations between both ground types are reinforced with extra rebar to effectively bridge the junction if there was differential settlement and hopefully avoid any cracks.

Would this be the best solution or have any of you done anything different in this situation?

 

[ash060]

Get a geotech on board, and see what he/she recommends. A couple of soil borings can only help the picture.

Also, how large are your loads? If they are small you probably won't have to do anything special, you will just have isolated footings.

 

[csd72]

If what you say is true and they are both gravelly layers of similar firmness then I doubt there will be a problem but I do agree that it would be prudent to consult a geotech if you are unsure.

It would be worthwhile to see if the top layer thins gradually to nothing or if it is an abrupt change in material. The later could be an issue but the former is most likely not. If the top layer is only a maximum of 1 or 2 feet thick then you have to think about how much settlement you are really going to get in such a small thickness.

Bars across the junction would simply ensure that any cracks would appear past the ends of the rebar and would not necessarily solve anything.

 

[engstructural]

I was proposing to use bar throughout but at the junction and for approx. 2-3m each side I felt that adding additional bar and designing this section to bridge between would act like a transfer slab and make the potential movement more gradual hopefully preventing serious cracks.