You will have to brace the cut.

thanks chipB for your valuable interest. but this is exactly my problem. my current idea is 2 drill into the clay a series of 1.0m diameter piles at circa 2 to 3m intervals depending on the anlalysis. this done i would run a cross member to tie the whole lot together against the lateral load being imposed by the upper load. assuming enough pile anchorage i would then be able to excavate the site to prepare for my new raft foundation. what do u reckon??

you said an 8m cut.  is this 8m below the bottom of the existing fdn or from the top?  is your new raft fdn. going to be 8m deep?  if you're already taking about piles, why not install them where your new fdn is going and cantilever your raft over these piles towards your villa.  would it still need to be 8m deep?

Indeed the depth of excavation is 8m below the bottom of d existing raft foundation of the upper villa. after excavating the 8m depth of foundation i will be in a position to cast my raft foundation which presumably is going to be circa 1.0m deep. the pillars will by this time be subjected to a considerable side thrust which i need to derive to be able to size and detail the pillars.. compriende?? :)

Here, we have push piers, that could be pushed into the ground until they acheived the required capacity.  They cost about $1000 each.  This would take the verical forces away from the immediate soil directly under the existing footing, thereby signifacantly reducing the thrust due to fdn load.  Push piers I like to use can be seen here: http://www.griptite.com/pushpier.html

You can't do a cut this deep without bracing it somehow. Too much of a risk for soil failure, even with clay. I just did a 24 foot excavation and used sheet piles which were braced at the top edge via steel beams.

yr right but how do i achieve bracing if the site is 40m wide? and by the way we ve no push piers over here. i guess i ve to rely on reinforced concrete piles driven into the terrain prior to excavation deep enough for good anchorage. about twice as much as the length for the cantilever arm.

Braced sheet piles.  Use steel beams approximately 1m from the top to handle the forces.  Every so often, you'll have to put a strut in to cancel out the buildup of the shear.  You might be able to design it as a cantilevered sheet pile, but 8m is pretty significant.