Sonic Drilling and Resonant Pile Driving
Infamously, in the 1940s, the Tacoma Narrows Bridge collapsed due to self-exciting resonance caused by winds of only 35 miles per hour. The bridge collapse shows the immense power that can be generated through vibrations that match the natural frequency of structures called resonance. Mechanical engineering typically chooses to dampen these forces so that the harmful vibrations are reduced. However, in some cases resonance can actually be productive - take for example sonic drilling or resonant pile driving.
What is Resonance?
The easiest way to understand resonance is the swingset example. As anyone who has pushed a swing set before, they intuitively understand that there is a certain window at which you must push the person on the swing to maximize how high the person will swing. When you push the person in this window of time, the person will swing higher each time. If you push the person too early, you end up taking energy from the system and the person does not swing as high. If you push too late, the push does not put as much energy into the system and the energy input is not efficient. The swing set example shows that there is a certain frequency that forces can be inputted in a system to maximize the amplitude of vibration and therefore the energy of the system (resonance).
This video explains the example in much greater detail and is a great introduction to resonance. https://www.youtube.com/watch?v=vpA4leFhQzc
There are a few main ideas to take away from resonance:
Energy input can be optimized so that the input cycles match the natural frequency and thus create larger amplitude vibrations than normal in the material.
Resonance in drill steel or piles will result in repeated cycles of lengthening and contracting. These high frequency cycles of lengthening and shortening is what will allow the drill steel or pile to advance through the earth.
The primary damping force that opposes the input force/wave force is the soil/rock pushing upwards on the drill steel or the pile.
How Sonic Drills use Resonance
Sonic drills are often used in water well drilling, core sample recovery and are almost exclusively used in dam remediation projects. Sonic drills are favored over rotary drills in dam applications because they are able to drill effectively without water or air flushing which prevents fracturing the formation and creating pathways for water to flow. Sonic drills are also known for making quick work of steel or concrete that would give conventional rotary drills issues.
As the name suggests, sonic drills rely on resonance. Resonance allows the sonic drill to drill without water as the high frequency vibrations will fluidize most soils within .25 inches of the drill string. This allows for the drill to push the hollow drill string through the soil with ease. Another benefit of resonance is that the energy transfer into harder materials such as rock is more effective than conventional rotary drills. Rock will absorb the vibrational energy, undergo brittle failure and will not reflect the energy back up the drill string which would interfere with resonance. This efficient energy transfer is how sonic drills will make easy work of rock and other brittle materials. Lastly, since resonance is a vertical movement of the drill string, the holes are incredibly plumb as compared to rotary drills which often have a tendency to “walk” and skew plumbness.
The disadvantages of using resonance is that it can be difficult to manually stay in resonance. Drill operators often do not have a great understanding of resonance. One of the biggest mistakes that is made when drilling with a sonic rig is thinking that applying crowd force will increase penetration rates. Too much down force can cause a decrease in energy transfer because the drill string is not able to move through its full amplitude - analogous to pushing the swing too early. At certain drill lengths, it may even be necessary to slightly pull up on the drill to prevent too much crowd from the weight of the drill string. There is a great research article that is linked below that provides an automated drill control solution that would keep the sonic drill on resonance without relying on operator control.
Resonance in Pile Driving
Most vibratory hammers are not resonant pile hammers. Standard hammers operate at frequencies ranging from 1 to 50Hz which is often too low to resonate a pile. One of the most common side effects of vibratory pile driving, especially in unconsolidated sands, is localized settlement around the pile being driven. The soil is able to couple to the pile while it is being driven and thus the vibration propagates through the soil causing settlement and may also affect other nearby structures that are sensitive to vibration.
Resonant Pile Drivers operate at much higher frequencies than standard vibratory hammers which are too high to allow for the soil to couple to the pile. The higher frequency thus keeps the vibrations localized in the pile and allows for pile driving near sensitive structures. This lack of vibration is demonstrated in multiple videos online where a glass of water is placed next to a pile being driven. Although the pile is advancing in the ground, the water is still.
Resonance is not necessarily an emerging technology in the foundation world, however it is a technology that is not often covered outside of niche circles. Resonance can be difficult to understand at first however once understood, it is intuitive as to the benefits the phenomenon provides for the foundation industry.