The hydraulic digger (or backhoe) is useful as a means of revealing the nature and extent of shallow overburden material on site. It can excavate a trench up to 3–4 m deep in soft or moderately hard material, at a fast rate and cheaply.
The substantial cross-section of material then revealed for inspection can be more informative than samples from a few borings. However, trenches of this depth must be securely supported before access for inspection is allowable.
A sampling tube can sometimes be pushed into the base of the trench using the digger bucket, and then dug out by the same machine.
The auger may be used for boring holes in soft materials. A lining may be required to keep a hole open during and after boring. Large augers, machine driven, are used for sinking shafts for the formation of in situ concreted piles. For site investigations, the hand-auger is a simple little tool, usually of 75–150 mm diameter, for penetrating shallow depths of soft material. About 300 mm of material is penetrated at a time before the tool is withdrawn and the material taken out of it and examined. Two men are usually required to twist the auger, the hole being watered from time to time if necessary in order to reduce friction. Penetration is usually of the order of 1.5–2.5 m; to get a hole deeper than 3 m the ground has to be very soft. Gravel or cobbles cannot be penetrated. The tool is useful for locating the extent and depth of shallow, very soft overburden material.
As an alternative to rotary core drilling the same rig can be used for open hole drilling where the drill bit cuts all material within the hole. Casing may be needed in unstable ground. This can be a rapid means of reaching a required depth to carry out a test or install instruments.