In our Drilling 201 webinar held in August, we discussed sonic drilling methodologies and best practices. Our goal was to provide a working knowledge of this highly viable drilling methodology to all of those unfamiliar with sonic drilling and it’s typical applications. If you missed the webinar, don’t worry! What follows is a detailed breakdown of all the information we covered. In the meantime, don’t hesitate to sign up for upcoming webinars in the series.
When we choose the method of drilling, we’re really weighing the method of penetration, the method of removing cuttings, and the method of stabilizing the boring against the geological constraints of the site and the project’s overall goals. With this is mind, it can be said that sonic drilling is most often used in scenarios when the drilling (whether through particular ground materials, or to a particular depth) is rather difficult and the integrity of the core sample is extremely important.
It’s important to distinguish the drilling method from the rig platform. They are not one in the same. Just because a rig is considered auger or direct push, doesn’t mean it’s always tied to that method. Geoprobe rigs, for example, are capable of accommodating a number of different drilling methods. One of the major benefits of sonic is the fact that we can use it on a variety of rig platforms that it can be done from multiple different rigs or platforms and it can support many different tool types.
Sonic gets its name from the mechanism that drives the drill bit. There are two oscillators in the sonic drill head, that are essentially out of tune, and act as counterweights working against one another. Once you have the two oscillators tuned the way you want, you’re generating resonance down your tool string to your bit, and that’s what drives the drill downward.
Rotation and resonance
Recovered as core sample or displaced into outside of borehole. We typically slip these into a polyethylene sample, or lexan liner, for storage. These lines are are also good for samples that might have a longer shelf life.
Core and case - we then run the core barrel into the ground, then we run or override the casing over the core barrel before retraction. Sometimes we need to use water to do that if you are working with higher depths. However, it is prefered to do it dry, as it allows us to determine where the groundwater is.
The continuous core is inherent to the sonic drilling method. A continuous core provides a detailed look at the soil at the depths drilled. This results in a better understanding of the subsurface conditions.
Once the core sample is retrieved from the subsurface, a polyethylene sleeve (bag) is placed over the core barrel and the sample is extruded out by resonating the drill pipe. Core samples are usually kept in 3 foot bags so that they’re easily manageable.
Casing Sizes - 5, 6, 7, 8, 9, 10, 12 inch
Sample Sizes - 3,4,5,6,7,8 inch
Depths are dependent up on casing size and formations. The typical size is 4x6 - a 4 inch core overridden by a 6 inch casing. Most state regulations require a minimum 6 inch hole for a 2 inch monitoring well. Larger casings are often used in telescope or step down mode to reach greater depths. It’s common to start with a 10 inch casing, move down to 8, and end with 6. Additionally if the smaller casing gets stuck, a larger casing can be used to get it loose.
Sonic drilling carries with it a very specific set of advantages:
It is a quick way of obtaining samples, especially at shallow depths.
It provides a lot of really good sample information relative to other drilling methods.
Very little waste is generated with sonic, about a 60-70% reduction compared to hollow stem auger.
It’s generally safer and more stable than comparable drilling methods.
It’s a very clean method, especially with dry drilling. On contaminated sites, there’s very little that comes to the surface. Most of the return that does come up is easily contained.
For the most typical applications, like in unconsolidated formations up 700 feet, sonic has very low to no refusal. Sonic is capable of drilling weathered rock pretty well, but it’s fairly slow through hard rock and may compromise the samples, which is why it’s not always best for drilling bedrock. However, we do have the ability to add on a speed increaser and use rock core tooling, which is pretty common to use within a 6 inch hole for setting instruments or obtaining a bedrock sample.
One of the main concerns with sonic drilling is the price. Yes, when dealing with upfront costs, sonic is typically a more expensive drilling method. But, when you take into account the overall value produced by the dynamic capabilities of this method, it’s easy to see sonic’s overall value.
As we’ve stated before, one of the foremost advantages of the sonic method is the wide variety of applications to which it is well suited. What follows is a brief overview of the typical applications for sonic drilling, and details as to how sonic outweighs conventional drilling methods in that scenario.
Sonic drilling provides information to enhance conventional methods, or bridge the gap where conventional methods fail. Poor or no recovery, inability to penetrate, inefficiency, tolerances, and alignment are all issues of conventional drilling that are overcome by sonic.
In any geotechnical or environmental investigation, the integrity and quality of core samples is paramount. With sonic, you can stay within less than 2% borehole deviation, oftentimes we’re closer to 1%. Sampling options include:
Earthen embankments present a number of unique challenges that are successfully mitigated with sonic. For instance, in any Army Corp of Engineering project, where fluid or air-based methods of drilling aren’t allowed, sonic presents a viable method of accomplishing the project within the necessary constraints.
As you might expect, sonic drilling presents a number of advantages for environmental remediation projects as well. Both low mobility/compaction grouting and high-mobility for grout curtains are achievable with sonic.
Sonic is ideal for obtaining accurate subsurface information with minimal impact. It is often used for the installation of instruments in a fully cased plumb borehole. Instruments include:
Sonic is perhaps the best drilling method for obtaining a groundwater profile. A subsurface water pump is lowered into the borehole. This allows the technician to draw water to the surface without compromising the integrity of the boring.
When dealing with a particularly difficult drill site, heat will likely become an issue. Friction heats the drill bit as it is driven into the subsurface. Reducing this heat is important to maintain the integrity of the sample. Here are some of the ways sonic helps mitigate heat:
Additional applications for sonic include::
Because of its inherent flexibility and the various advantages that this platform carries with it, there are a number of markets where sonic works well:
In conclusion, sonic drilling is, in its most basic sense, a resonating drill string used to overcome friction and drive a bit to a specific depth. To recap, here are the specific benefits of sonic drilling:
There you have it. We’ve provide a brief overview of our Drilling 201 Webinar, in which we covered the fundamentals and typical applications sonic drilling. We covered a lot in this post, but the truth is that there’s always more to learn. If you’re interested in learning more about sonic drilling at Cascade, or have a project you need help with, please contact us here. An expert will be right with you.