10' trencher?

Is there such a thing- or should I try to build one.
There is an old 10x10 cistern in my basement. It is under a slab, but two walls are exposed. The soil is clay. I've cut a door in it and drained it, but there is still seepage from somewhere. I want to trench 4-6" channels along the buried walls and fill with stone.
I haven't seen anything that long, but I'm thinking I could fabricate a ditchwitch style trencher that ran off a 1HP electric motor geared down. I don't care if it takes a month to dig one side.
Has anyone seen a long trencher- or plans to build one? Or have a better idea on how to drain the clay before it gets to the cistern walls?
thanks,, Jim
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