Mel

Generally raspberries are shallow rooted and the approach you are
taking should be fine. Raspberries are forest fruits - naturally
growing in woodland clearings. This tells us a little about their
soils needs - free draining, cool roots and moisture. The muck will
help improve the soil. Come spring mulch the row with organic
material, which should be removed in the autum so it does not harbour
raspberry beetle or other raspberry bests (it can be dug in elswhere in
the garden)

Having said that I got some bare rooted autumn raspberries 'Polka'
which must have about 3ft of roots. If that is not a sign that they
are going to be vigourous!

Good luck and Happy growing

Clifford
Bawtry, Doncsater, South Yorkshire