Good morning to everyone
A technique I've seen used to go under a road or surface obstruction is to
dig work-pit holes on opposite sides and use a hydraulic-jack laying on its
side with its base pressing against a steel-plate or block of wood
positioned against the back-side of the access pit. The jack's
extending-post then presses against the end of a 'penetration pipe' whose
length is increased by adding additional sections.
If the soil is basically rock-free you could use a garden hose with one of
those axially-thin adjustable spray nozzles set to generate a thin jet of
water and feed it under the object. This technique might require a nozzle
fitted on the end of a two-foot long guide-pipe to keep the path pretty
straight. The eroded dirt flows back out of the hole along the hose/pipe
into your work pit.
As a kid I lost about 20-feet of hose digging a 'hole' straight into the
ground after turning off the water and the slurry settled in around the
UR RST IS ... ... ..9 QSB QSB - hw? BK