With small drills, our depth capability is much more dependent on the site specific geology than with larger, conventional drills. There is no easy answer for this question, because of the inherent variations in subsurface conditions and circumstances, particularly in southern Ontario where glaciation has provided a very wide diversity of soil conditions. With our equipment, it can range from 6-7 feet with the Pionjar in a very hard, dry, clay till material to over 60-80 feet with the larger machines.
With the Pionjar drill, the PEP, and the Rota-Hammer, you are generally restricted to depths of less than 15-20 feet. In very hard materials you may reach refusal much sooner.
The Geoprobe 420 can access similar areas, but is much more powerful and capable of penetrating these harder formations to greater depths than the hand held units. Although both of these methods are suitable for environmental sampling, they are not capable of obtaining SPTs, commonly required when doing geotechnical drilling.
The MiniProbe, equipped with a very powerful 175 ft lb probe hammer on a small rubber track carrier is a more powerful direct push probe method, capable of better penetration than the Geoprobe 420. This machine is commonly used to depths of approximately 20-30 feet for environmental sampling.
The Big Beaver is generally more suited to depths greater than 10 feet, because there is more equipment required and longer set up time to use it, but you greatly increase the penetration ability with this drill over the hand held percussion units and the Geoprobe 420. The Big Beaver uses a 140lb hammer to pound a 2″ split spoon connected with AW rods – this is the same process as a conventional geotechnical/environmental truck mounted drilling rig. Therefore the ability to penetrate soils with the split spon is identical to a regular rig. The Big Beaver though, does not have the weight (~700lbs) or engine power of a truck mounted rig for the augering process. The depth capability of the Big Beaver is highly dependent on the soil conditions, but it is most commonly used in areas where there is no choice but to use this drill due to the access limitations. It is most often used for 10-30ft holes, but has been used to advance over 60ft in ideal conditions.
While the MiniMole has approximately 5 times the weight and power of the Big Beaver, it still does not match the substantial power of the CME55 and CME75 truck mounted drills commonly used in Southern Ontario for areas without access restrictions. The penetration ability will also depend on the size of auger used and whether it is a solid stem or a hollow stem. Although 20-40ft holes are the most common, drilling to 60-80ft has been accomplished in specialized situations.
The Geoprobe 6620 is our largest and most powerful drill, and therefore has the greatest penetration capabilities. If depth and formation are challenges at the site, and you can fit this drill to the borehole location, it has your best chances to get to the greatest depth. Although not as heavy and powerful as the larger truck mounted drills, it is in the same league as many standard truck mounts in terms of depth capabilities.
Therefore the short answer to the question of depth is: it depends!