Sonic drilling is a subsurface exploration
technique that strongly reduces friction on the drill string and drill bit due
to liquefaction, inertial effects, and a temporary reduction of the porosity of
the soil generated by a high-frequency vibration applied directly to the drill
stem at the drill head. In addition to vibration, sonic drilling uses both the
rotation and downforce of the drill rig and drill casing to advance the
borehole. Sonic drill stems incorporate both an inner core barrel and an outer
sonic drill casing to penetrate the substrate, stabilize the borehole, and
collect continuous, relatively undisturbed soil samples.
Sonic drilling
techniques can recover a fairly large sample specimen (depending upon the
casing diameter employed), successfully sample large-diameter granular soils
(gravel and cobbles), and are somewhat effective at recovering in-tact samples
of frozen soils. Sonic drilling is also
fairly efficient, results in limited ground disturbance, and is devoid of drill
cuttings. Sonic drill rigs can also be
equipped with standard geotechnical sampling equipment (SPT/MPT, Shelby tubes,
etc.). However, sonic drilling is a fairly new, equipment-intensive technology,
and therefore, is generally fairly cost-prohibitive for moderate to small-scale
geotechnical exploration projects.