Probably i have misled you guys - it's actually black cotton soil plot and not clay soil..... and VERY far away from Njiru!!It's off Thika Rd actually.

Let's keep to the topic though: Those who have done the workings or have some experience with this- which is the cheaper alternative given this kind of bed rock depth and is any of the two technically superior?

Refer to post no 7.

The engineer may do a design for both and cost them accordingly in line with the loading of the structure. Only then can which is better(economical) be determined. No quick answer to your query in my view.

I hope by pillars you mean piles, the kind being done by Chinese to support the overpasses on Thika road. Piling is expensive and the few contractors available charge highly. Your Engineer will make a decision based on the soil bearing capacity . I don't have much data but it seems you'll go the raft way.

The panacea to your issue is to follow your engineer's advice. Looks you'll have to invest heavily on your black cotton plot. If it were possible it would be better if there was a floor down there... whats the point of excavating and then filling it up. But then you'll have to contend with flooding.

For me I would go the pillar way with suspended structure,,,.. but don't give up