Clay rarely moves if there's nothing porous above or below it. And it also depends on which clay it is. A photo and a location would help! And as DM and Dove say, it's ground water not rain which can be an huge problem
water seepage will be the only major problem, in clay when it gets wet (from rain) you get a thing called soil creep - this is a very slow motion landslide - it might only move 6 inches over 5 years but if your garage is in the way it might be a problem. see - http://en.wikipedia.org/wiki/Downhill_creep
However if there is a source of water in or above the slope, in the form of a spring or the end of an old land drain, then an earthflow might be a problem (and if it goes it will take the garage with it!), - http://en.wikipedia.org/wiki/Earthflow
I would grow something spreading with deep roots if you can to bind as much of the clay together as possible.
I saw a very spectacular bank (can't remember where) it was steepish and was covered in hypericum calycinum (Rose of Sharon). It creates quite a firm tangle of greenery, flowers all summer and just needs a haircut in early spring.
Always thought if I ever had a tricky bank I would replicate it..
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Very good point Dave - digging into the bank may very likely have cut into the water table and the effect may be that of having created a spring!
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Clay rarely moves if there's nothing porous above or below it. And it also depends on which clay it is. A photo and a location would help! And as DM and Dove say, it's ground water not rain which can be an huge problem
water seepage will be the only major problem, in clay when it gets wet (from rain) you get a thing called soil creep - this is a very slow motion landslide - it might only move 6 inches over 5 years but if your garage is in the way it might be a problem. see - http://en.wikipedia.org/wiki/Downhill_creep
However if there is a source of water in or above the slope, in the form of a spring or the end of an old land drain, then an earthflow might be a problem (and if it goes it will take the garage with it!), - http://en.wikipedia.org/wiki/Earthflow
I would grow something spreading with deep roots if you can to bind as much of the clay together as possible.
I saw a very spectacular bank (can't remember where) it was steepish and was covered in hypericum calycinum (Rose of Sharon). It creates quite a firm tangle of greenery, flowers all summer and just needs a haircut in early spring.
Always thought if I ever had a tricky bank I would replicate it..