It's actually known as a trenching tool.
(
barryheadwound Mul-ti-pass? Multipass!,
Thu 1 Dec 2011, 14:48,
archived)
Or "tool, trenching" to be precise.
(
barryheadwound Mul-ti-pass? Multipass!,
Thu 1 Dec 2011, 14:48,
archived)