This is a form of continuous-flight auger where the helices are wound around and welded to a tubular center stem or axle. Drilling proceeds essentially as in solid-stem drilling. When the sections are connected however, the hollow-stem auger will present a smooth, uniform bore throughout its length thus providing an open, cased hole in which samplers can be used or well installation can be performed. Other drilling methods can also proceed within the hollow stem, which can be used as temporary casing to prevent caving.
The use of hollow-stem auger, although common in geotechnical drilling, is limited in groundwater applications. Augers with diameters ranging from 6"-13" can be found (often drillers discuss HSA diameters as the inside diameter of the auger since the outside diameter varies with use; the outside is abraded with use.), with depths reaching up to 120 feet depending on the size used. The method is limited to unconsolidated to semi-consolidated formations. The method applies particularly well to clayey formation and is reasonably "good" to collect representative soil samples.
Sonic Drilling
This relatively new drilling method allows the collection of core samples with speed and precision, and without the need of drilling fluids or air. One of the main advantages of the sonic drill is its ability to continuously core unconsolidated and some consolidated formations with a minimal amount of disturbance and compaction. The samples can be analyzed to provide a detailed stratigraphic profile of unconsolidated formations, including: dry or wet sands and gravels, cobbles and boulders, clays, silts and hard tills. Applications include environmental borings, monitoring well installation, aggregate exploration, rock exploration, methane probes, conductor casing installation, extraction wells, or other applications requiring a borehole diameter of less than 12-inches and depth of less than 500 feet.
High frequency mechanical oscillations, developed in a special drill head, are transmitted as resonant vibrations, along with a rotary action, through the tooling to the bit. The vibratory action fluidizes the soil particles, destroying the shear strength and pushing the particles away from the drill bit and along the sides of the drill string. Similar to a casing hammer or Odex system, the sonic rig drives an outer drill casing and an inner string consisting of drill rods and core barrel.
While coring, the core barrel is advanced before the outer drill casing, without fluid or air. The outer drill casing is then advanced to the same depth; this is best accomplished with water, which aids in pushing soil particles away from the drill bit, and also helps to cool the core barrel. Although dry casing advancement is possible, "drilling dry" can generate heat that affects the sample integrity. The use of water also provides an effective means of combating heaving sands without drilling mud or bentonite. With the outer casing left in place to hold the borehole open, which decreases the possibility of cross-contamination by cave-in of up-hole material, the core barrel is extracted and the samples are vibrated out of the barrel into plastic sleeves, stainless steel sample trays, wooden core boxes, or other containers.
The outer drill casing also holds the borehole open while installing monitoring wells, piezometers, vents, observation wells, instrumentation, or other down-hole equipment. Outer drill casing sizes include nominal diameters of 6 and 8 inches, allowing sufficient space to install the common monitoring well sizes of 2 and 4-inches. While constructing wells, the vibratory effect reduces "bridging" of the filter pack and seal, and also reduces the potential problem of "sand locking" and inadvertently removing the well as the outer drill casing is extracted. This positive placement of well construction materials allows for controlled well installations.
Methods that use drilling fluids:
Rotary (direct) Drilling
This method makes use of a constantly rotating bit to penetrate any type of formation to depths that can exceed 1,000 feet. As drilling proceeds, cuttings are removed by a continuous circulation of fluid (either air or water based) that flows down inside the pipe string and up-hole along the annular space between the borehole walls and the pipe string. The penetration rate is often faster and the bit life longer when using air as compared with water based drilling fluids. A drag bit is normally used to penetrate unconsolidated to semi-consolidated sediments; while a cone-type or roller bit is used to drill consolidated rock. The bit can be rotated either by a top-drive or a table-drive system. The rotation speed is adjusted according to the hardness of the formation material.
The drilling fluids serve several functions, which are principally to: lift and transport drill cuttings to the surface; stabilize borehole walls and prevent caving by the action of pressure and; cool and clean the bit. In some instances however, air tends to cause loosen unconsolidated formations. The method is limited to borehole diameter of less than 24 inches, due to the fluid's viscosity and up-flow velocity that make it difficult to clean out the cuttings.
Reverse Circulation Rotary Drilling (RC)
Reverse circulation rotary drilling uses the same principles as direct rotary drilling, except that the flow pattern of the drilling fluid is reversed. In this method, the drilling fluid (air or water) is pushed down in the annular space between the borehole walls and the pipe string, and is expelled upward within the pipe string. The cuttings are pumped to a collection facility at the surface by the use of an inverse coupling that carries cuttings through a discharge pipe. The method has seen few applications in groundwater monitoring work.
Reverse circulation drilling is mostly used in consolidated formation but can also be used in soft consolidated rock. I can also be used in hard rock if using both air and water as drilling fluids. The drilling mud is best described as muddy water and additives may or may not be added to the water. Engineering the correct mud chemistry and viscosity can be critical for some projects. Reverse circulation is a method often used to drill water wells since borehole diameter in excess 24 inches can be drilled to depths greater than for the direct rotary method. Reverse circulation rigs can be either table or top head driven.
Dual-wall Reverse Circulation Drilling
Dual-wall reverse circulation is a sub-type of rotary drilling similar to RC, in which two concentric drill pipes are assembled as a unit to create a controlled annulus. This method is widely used in groundwater monitoring work. The drilling fluid (air- or water-based) is pumped through an outer swivel down through the annulus of the bit where it is deflected upward into the center pipe. The cuttings are carried upward through the inner pipe and surface swivel, and can be collected as a sample or pile up on the ground. The method allows collection of geologic samples of known depth within the formation, which is delivered through a cyclone at the surface.
The method is applicable to both rotary drilling and percussion (down-the-hole) drilling methods. Because the fluids at all time circulate within a controlled space, the problem of lost-circulation is minimized and cuttings can be recovered at all times. The method is applicable in virtually any type of geologic formation and does not require the use of a surface casing. In general, borehole diameters do not exceed 10 inches and depths can reach in excess of 1,400 feet.
Cable-tool percusion
Cable-tool drilling is a method in which a bit, hammer or other heavy tool is alternately raised and dropped to strike through the formation by breaking the soil or rock. Cuttings are recovered by adding water to the borehole (slurry), which is intermittently bailed out of the borehole. This method is widely used in water wells but of limited application in monitoring work mainly because the method is slow.
In unconsolidated formations, the pipe (or casing) is advanced behind the bit. The casing diameter is slightly larger than the diameter of the bit and is equipped with a drive shoe at the end of the casing. No casing is necessary in consolidated formations and drilling proceeds by open-hole methods. The minimum borehole diameter is 6 inches because of the need to use large, heavy bits. The maximum diameter is ~ 24 inches, because of the lifting limitation of the hoist structure. Depths can reach several 1,000's feet, but the method is increasingly slow as the blowing impact looses force due to the increased friction between the casing and walls of the borehole.
