Foundations on varying strata 1

[Vinny7]

I have a project where the client excavated foundations to firm bearing for a house. In one corner rock (schist) was encountered over approx. 5-10% of the building footprint. Is it possible to found on the two stratums and provide a movement joint at the junction. If so what issues need considered. Are there any other good practical solutions.

I am also concidering breaking the stone back and providing hardcore compacted in layers (1.0m Dp.) with a raft over the whole house but this might be over kill??

 

[csd72]

What is the depth to rock on the rest of the site?

 

[Vinny7]

I have asked for the contractor to dig a trial pit at the opposite side of the house, If its less than 3m to rock I will get them to found on the rock with trenchfill, I suppose I am wondering about the options if this isn't the case.

 

[FinnB]

Vinny7

Well graded compacted hardcore could be used directly on top of the rock, aprox 250mm (10 inches) thick. Standard founds would be built off the hardcore and no raft would be needed. The hardcore acts as a cushion over the rock when the building on the good bearing material begins to settle.

I received this advise on a number of occasions from geotechnical engineers.

Would be worth getting a geotechs opinion as there are construction costs to consider.

 

[Vinny7]

FinnB,

I was also concidering this option but was loking for some experiance of similar situations. I would have though it would be better to have a deeper cushion of hardcore though?

 

[FinnB]

Vinny7

Try and estimate what the settle will be for the building on the firm bearing material. The design is for a house so the building may not be that heavy. Once you know the settlement it would be easily to estimate the differential settlement and then allowance could be made for the compaction of the hardcore.

On some projects I have been on 250mm of hardcore has been used to cushion rock under 6 storey concrete buildings. Settlements were small on these projects (4-5mm in bolder clay/glacial tills). When you have a feel for the settlement the right decision should be easy to make.