In geotechnical engineering, different drilling methods are used to better understand the soil beneath the surface. There are three common methods to consider: solid stem auger, hollow stem auger and mud rotary. Here is what you should know about each method.
Solid Stem Auger
Solid stem auger (SSA) uses continuous flight augers which mechanically excavate and continuously transport cuttings to the surface. Augers are available in diameters of 3 to 14 inches. Solid stem augering has a number of advantages. It produces a moderate amount of easily contained cuttings and little or no fluid is required in the drilling process. Thin wall and split barrel sampling operations are supported. Because of lower torque requirements for solid stem augering, smaller drill-rigs can be used, simplifying site maneuvering and often incurring lower costs.
However, this drilling method also has a number of disadvantages. In order to obtain soil samples, the solid stem auger must be removed from the hole and therefore, is limited to stable soils which will not collapse when the augers is removed. Soil sampling during solid stem auger drilling is labor intensive, especially in deeper holes because the augers must be removed from the hole during each sampling procedure.
Hollow Stem Auger
Hollow Stem Auger (HAS) are commonly used to set ground water monitoring wells for environmental and geotechnical applications, but can also be used to obtain soil samples. Hollow stem auger drilling uses large diameter (up to 14 inch outside diameter) continuous flight augers which mechanically excavate and continuously transport cuttings to the surface. A center bit, which is attached to the drill rod and bolted to the auger drive cap, is inserted through the cutter head to excavate the center of the boring. As the boring is advanced by adding sections of auger, sections of drill rod are added, maintaining the center bit at the face of the cutter bit.
Hollow stem augering has a number of advantages. The augers act as a temporary casing during and at the completion of drilling to facilitate the soil and water sampling and the installation of monitoring wells. It is relatively rapid and little or no fluid is required in the drilling process. Additionally, this drilling method readily supports thin wall and split barrel sampling.
Hollow stem augering has a handful of disadvantages. For example, it is limited to drilling in poorly lithified to unlithified sediments and to a maximum depth of 150 feet. Shallow bedrock or other hard to drill materials may reduce this depth significantly. High hydrostatic pressures in the bore hole can cause problems with sand heaving up into the auger during soil sampling and well installation.
The mud rotary method of drilling uses a drill bit that is mounted on the end of a drill rod and is advanced by the rapid rotation of the bit. The cuttings are removed by pumping drilling fluid (water or water mixed with bentonite or other fluid enhancers) down through the drill rods and bit and up the annulus, between the bore hole and the drill rods. The drilling fluid also serves to cool the drill bit and stabilize the bore hole walls.
Mud rotary drilling has a number of benefits. It is a very fast and efficient means of drilling—efficient drill-rigs can produce several hundred feet of hole per day. Mud rotary is adaptable to a wide range of geologic conditions. However, exceptionally large, poorly stabilized boulder conditions are unsuitable for mud rotary drilling. Sediment sampling is broadly supported and thin wall and split barrel sampling is available.
Mud rotary has a number of disadvantages. The use of drilling fluids may require support vehicles to properly manage and contain both the cuttings and the drilling fluids. The use of drilling fluids may also invade permeable zones compromising the validity of the monitoring well sampling.
RMG’s highly skilled team of geotechnical engineers have the knowledge and experience to assess which drilling method is best for your project. Contact us today to learn more about our services!