Solid Stem Auger (SSA)
This type of auger consists of a solid stem or shaft with a continuous spiraled steel flight welded on the outside.
An auger bit connected to the bottom disturbs soil material when rotated and the helical flights transport cuttings to the surface. At the desired depth the entire auger string is removed to gain access to the bottom of the borehole.
This auger method is used in cohesive and semi-cohesive soils that do not have a tendency to collapse when disturbed. Boreholes can be augured to depths of 30 metres or more (depending on the auger size), but generally boreholes are augered to depths less than 30 metres.
Solid augers drilling is generally straight forward and consistent. The bit cuts efficiently and the large flights are effective at clearing cuttings in nearly all soil types.
There are many bit styles available for this auger type and each has its individual strengths and weaknesses.
Solid stem augers are pinned together and driven via a hexagonal insert that does not transfer pressure to the pin. The hex system removes the need for hydraulic or manual breakout equipment
Numac carries solid 125mm solid augers with a 150mm cutting bit as stanrd on environmental drilling rigs. Larger 250mm augers are available upon request.
Hollow Stem Auger (HSA)
Environmental drilling standards often require that wells are drilled without the introduction of fluids. To achieve this goal "hollow" augers are utilised in unstable formations where solid stem augers are not effective.
This type of auger consists of a hollow, steel stem or shaft with a continuous, spiraled steel flight, welded onto the exterior.
Hollow augers are generally used on our Geoprobe 7822 machines due to the high torque requirements. A hollow auger bit, generally with carbide teeth, disturbs soil material when rotated, whereupon the spiral flights transport the cuttings to the surface. This method is best suited in soils that have a tendency to collapse when disturbed.
Bottom plugs can be used where no soil sampling is conducted during the drilling process. The bottom plug is wedged into the bottom of the auger bit and is knocked out at depth with drill pipe or the weight of the casing and screen assembly. The plug material should be compatible with the project screening requirements.
A bottom plug or pilot bit assembly can be used at the bottom of the augers to keep out most of the soils and/or water that have a tendency to enter the bottom of the augers during drilling.
A monitoring well can be installed inside of hollow-stem augers with little or no concern for the caving potential of the soils. If caving sands exist during monitoring well installations, a drilling rig must be used that has enough power to extract the augers from the borehole without having to rotate them.
Potable water may be poured into the augers during drilling to equalize pressure so that the inflow of formation materials will be held to a minimum. Water-tight center bits are not acceptable because they create suction when extracted from the augers. This suction forces or pulls cuttings and formation materials into the augers, defeating the purpose of the center plug.
Using a Geoprobe machine hollow augers are generally used to depth under 30mbgl butt generally boreholes are augured to depths less than 20 metres. For bores in unconsolidated formations deeper then 20m we generally recommend using a sonic rig which is far more efficient at these depths.
Numac runs two standard hollow auger sizes; 108mm ID (194mm OD) for installation of 50mm Monitoring wells and 159mmID (244mm OD) for installation of 100mm Monitoring wells.