Geotechnical engineers often specify the use of deep drilling to assess soil conditions before commencing complex construction projects. Choosing the right drilling method is one of the most important decisions and can directly impact project outcomes.
When deeper boreholes or high quality soil samples are required, particularly in more difficult geological conditions, sonic drilling can be an ideal solution.
Sonic drilling uses high frequency resonant technology that produces a very straight drill point which penetrates most ground formations including cobbles, rock layers and boulders. It can be undertaken in almost all types of soil and ground conditions, from gravel and landfill, to softer formations such as sandstone, limestone or weathered basalt.
The high vibration frequency causes a very thin layer of soil particles directly surrounding the drill string and bit to lose structure and temporarily behave like a fluid powder or paste. When retrieving the drill, the suction and vibration enables the soil to reform.
While the technology was originally developed to speed up the process of digging oil wells, it was also found to have an outstanding ability to take an accurate representation of a continuous core sample of almost any material, even boulders and bedrock.
With extremely fast penetration speed, it is able to extract quality, undisturbed samples with minimal impact to surrounding soil and less environmental disturbance or contamination.
Deep core soil samples inform remediation solutions
In some cases, deep core soil samples can help to better inform remediation solutions. This was the case when an elevated railway bund at a Queensland coal export facility displayed signs of differential settlement just four years after construction.
Sonic drilling was used to extract samples that could be analysed in order to identify weak soil layers in the ground beneath the railway.
Using samples collected to a depth of 10m underneath the rail beam, an appropriate solution was identified to help improve the soil density and bearing capacity in weak layers beneath the rail while avoiding contamination of adjacent coal stock.
Innovative placement technique solves the unsolvable
Sonic drilling also played a key role in an Australian first mine shaft rehabilitation project.
An abandoned mine shaft located beneath a motel carpark in regional New South Wales had collapsed and voids began to form, resulting in serious subsidence affecting both the carpark and the motel building.
The technically and physically challenging remediation project had already undergone prior failed attempts before Mainmark was engaged to help resolve the issue, using a solution to plug the shaft at 30m underwater, then cap, and seal the mineshaft.
The sonic drill rig was instrumental, enabling the Mainmark crew to successfully remediate the site by combining bespoke cementitious grout and an innovative placement technique.
During the carefully staged project, the sonic drill rig was used to install three 76mm ID steel casings to a depth of 60m to provide access for a temporary or ‘sacrificial’ plug, allowing technicians to implement the required grouting solution in order to seal the mineshaft.
This unique approach resulted in Mainmark solving an extremely complex problem that had previously been considered unsolvable.
By Dr Phil Irwin
Dr Phil Irwin is Mainmark’s Area Manager, servicing Victoria and Tasmania. Phil has a PhD in earth sciences and has extensive experience in geotechnical investigations, soil testing, drill rig operations, and site surveying.