Earth Embankments - Part 2: Sonic Drilling
August 1, 2007
Sonic drilling uses high frequency mechanical oscillations, developed in the special drill head, to transmit resonant vibrations and rotary power through the specially designed drill tooling to the drill bit, allowing it to achieve drilling penetration without the need for drilling fluids or air. Frequencies in the range of 50 Hz to more than 180 Hz are generated. The driller adjusts the frequency to match the natural frequency of the drill tooling, causing no dampening of the vibratory wavelength to the bit. Exceptional drill pipe acceleration rates are transmitted to the drill bit face to create an effective cutting action. The sonic vibratory action fluidizes the soil particles, destroying the shear strength and pushing the particles away from the tip of the drill bit and along the sides of the drill string. This localized liquefaction process allows for penetration of overburden formations. The drill bit can be designed to either push all the soils into the borehole wall or modified to allow a continuous core to enter the steel pipe of the drill. Core samples can be continuously retrieved of both unconsolidated and consolidated formations with significant detail and accuracy. The core samples can be analyzed to provide a precise and detailed stratigraphic profile of any overburden condition, including dry or wet saturated sands and gravels, cobbles and boulders, clays, silts and hard tills. Recovery of a sample is as consistently close to 100 percent as any other boring methodology.
The sonic rig utilizes a dual line of drill pipe. The inner string of drill rods has the core barrel(s) attached. All overburden core sampling is done ahead of the outer string of drill casing with no fluid or air added to ensure accurate, representative, undiluted samples. After the core barrel has been advanced, the outer drill casing is advanced to the same depth. This can best be accomplished with water; however, dry casing advancement methods also can be employed. With the outer casing left in place to hold the hole open, the core barrel then is removed from the borehole. The core sample can then be extracted into plastic sleeves, stainless steel sample trays, wooden core boxes or virtually any container. The outer drill casing ensures there is no sample contamination from uphole material by sealing it off prior to each sample run. Various sample diameters can be acquired with this method; however, for drilling in embankments, a minimum sample diameter of 4 inches is recommended.
The outer casing also serves to hold the borehole open for installation of monitoring wells, piezometers, vents, observation wells, instrumentation or other downhole equipment. The outer drill casing has nominal diameters of 6 inches and 8 inches, allowing ample space to install 2- and 4-inch wells with a 1- or 11⁄4-inch tremie pipe to place sand packs, seals, slurries and grouts into the annular space between the well screen/riser and the outer casing and borehole annulus. The drill bits used on the outer drill casing are open and are 57⁄8 inches through 81⁄2 inches in diameter, depending on borehole size requirements. Most practitioners also are capable of performing conventional sampling through the sonic drill string.
An experienced operator is required for this method. Equipment maintenance, downtime and mobilization costs typically are higher with sonic drilling than with other drilling techniques; however, for deep holes in well-compacted soil, this method usually will outperform the alternatives from the standpoint of production rate, sample quality and overall cost per foot.
Circulation of drilling fluid is not required with sonic drilling. Water may be used for two main reasons – to facilitate penetration in very tough soil materials when the in-situ moisture content is not high enough to facilitate the shearing of the material, and to control heave in the bottom of the hole. Using water for sonic drilling in embankments should be avoided, and should only be used only as a last resort to facilitate the penetration in very tough, dry material. The amount of water used for any purpose should be gravity-fed to the collar of the hole. The volume of water introduced should be closely monitored, and held to no more than would raise the water level in the hole to 15 feet above the phreatic (water table) water level.
When a drill bit is used, sonic drilling forces most of the soil cuttings into the borehole wall, which may create problems for subsequent logging, in-situ permeability test-ing and monitoring well performance. Based upon moderate field experience, some bottom heave and sample growth problems have been reported. The typical set-up requires two large trucks positioned end-to-end, so site access and space issues must be considered, although smaller, modular (trailer- or skid-mounted) sonic drills are available with limited capabilities.
This article is provided through the courtesy of the U.S. Army Corps of Engineers.