shell-and-auger drilling

[img_assist|nid=12006|title=|desc=|link=none|align=right|width=157|height=184]A type of percussive drilling used for site investigation which is particularly useful in obtaining samples of sand and gravel from below the water table but, by using a chisel in conjunction with the shell, the hardest rocks may be penetrated. The weight of the tools and rods is carried by shear legs or three-pole derrick but the drilling rig may also be truck-mounted.