It became clear that sonic had the technological capa- bilities of collecting virtually everything, as it existed in situ, for laying out on the ground to be read like a book. With sonic providing higher- resolution samples and sub- surface information, engi- neers around the world have therefore saved millions on project budgets by avoiding the need to overdesign and overconstruct. Today, sonic has advanced from providing high-quality core samples to serving as a platform capable of installing instrumentation, foundation relief wells and tiebacks in challenging formations, as well as providing foundation grout borings for seepage control in dams, micropiles in contamination- impacted soils, angled wells and drains, freeze piles, and much, much more. In summary, sonic drilling is a very flexible and very versatile drilling platform in the hands of a skilled and experienced driller. In this article, Cascade Environmental presents three case studies to show a few of the many novel ways sonic drilling is currently being implemented. However, let’s first touch briefly on what sonic drilling is and its inherent drilling benefits. Sonic drilling works on the principle of advancing a boring by resonating the entire tool string (core barrel, casing or rods) at a frequency ranging from 50 Hz to 150 Hz (cycles per second) to overcome borehole friction. The frequency is generated by an oscillator that is contained in the drill head. With the help of hydraulically powered motors, two small counterweights spin extremely fast, thereby transmitting a resonating frequency directly down the tool string. At the pulsating bit face, subsurface materials are either sheared and collected inside a core barrel during the drilling process, or become assimilated into the adjacent borehole wall. Depending on the type of material being drilled, sonic drilling can be done without the use of drilling fluids. This unique approach to tool string advancement allows sonic to drill very quickly and efficiently into some of the most challenging subsurface materials, such as urban fill containing thick steel and other mixed materials. In addition, other formations that pose a challenge to conventional rotary and auger drilling techniques, such as boulders and cobbles, are quickly and easily overcome by sonic drilling. The simplest sequence in which the drill tooling is advanced is as follows: first, a core barrel (of sizes ranging from 4.75 in [120 mm] to 10.5 in [266 mm]) is advanced into the subsurface. Then, a slightly larger casing is advanced directly over the core barrel in the over-drilling process to stabilize the borehole so the core barrel can be retrieved without collapse. These casings range in size from 6 in (152 mm) to 12 in (305 mm), with multiple increments possible in between. Also, for special applications, larger tooling can be fabricated. As a result of this telescopic drilling process, borehole deviation and verticality can be greatly constrained to less than 0.5% in many cases. In addition, this tooling can be advanced on angles ranging from vertical to as low as 20 degrees above horizontal. Here are some project applications where sonic drilling was adapted to deliver successful results. These applications demonstrate sonic use with dam foundations, sea wall bulkhead tieback installations, and retrofitting deep angled leachate drains that extend through a landfill’s mixed waste. Dam Foundations When a dam needs to be drilled despite the risks, sonic drilling provides a method that complies with best safety practices. Such projects require taking into consideration that no damage be imposed during the drilling process, whether it’s into an embankment dam or a lock chamber for river operations. Damage could result from elevated pore pressures while using drilling fluids or from drilling that involves an unsupported or uncased borehole. This is why the use is discouraged of water, mud — and particularly air rotary — methods on these types of projects as those technologies can exacerbate voids and embankment fracturing, which could cause dam structure failure. Floating platforms for river lock chamber drilling In December 2017, Cascade Environmental served as a subcontractor that developed a safe and effective method to install a series of foundation relief wells around the perimeter of a river lock DEEP FOUNDATIONS • JULY/AUG 2020 • 17
