For the past four years, GeoSonic Drilling has been supplying specialist sonic drilling services to all of the main ground investigation contractors on the current multi-supplier framework agreement for the dualling of the carriageway of the A9 road in Scotland between Perth and Inverness. Late 2018 and early 2019 has seen a peak in demand for GeoSonic's services with multiple concurrent phases of ground investigation with a strong emphasis on sonic drilling within the scope of intrusive works.
With around 500 sonic boreholes completed by GeoSonic across this timeframe, a total drilling meterage approaching 10km and the completion of some boreholes with depths exceeding 100m, the firm has collectively amassed a wealth of experience of the linear site with all its challenges and complexities.
The geological setting along the route of the A9 consists of various sporadic quaternary river terrace and regolithic deposits overlying materials that are principally glacial in origin. These vary across the route but include glaciofluvial ice contact deposits and sheet deposits, lodgement tills and many forms of glacial diamicton that are lithologically diverse, often with significant proportions of cobbles and boulders.
Demand for sonic drilling has steadily increased on the project to the point where on several of the sections, sonic drilling overwhelmingly dominates the specified drilling methods employed. This is largely attributed to the ability of sonic rigs to penetrate the deposits and provide a very high degree of confidence that samples will be recovered over the full depth profile of the borehole, ensuring few, if any, data gaps and that the scheduled depths will be achieved.
Sonic benefits
Analysis of GeoSonic's drilling metrics indicates sample recovery across the whole project is generally upwards of 94.5 per cent. Sample quality is maintained by ensuring that minimal if any, volumes of water are used during advancement of the core barrel. This helps to ensure that a representative sample is recovered and that fines are not washed out of the recovered sample.
The multi-disciplinary nature of GeoSonic's rig fleet has meant that much of the rotary core follow-on drilling at the sonic borehole locations undertaken by GeoSonic has been undertaken with the same rig. Given the difficulties with swapping different rigs on site and depending on the scheduled coring depth, this can be a beneficial exercise in saving time and minimising required resources.
In general, across the route, the depth of overburden does not usually exceed 30-40m below ground level. The most obvious exceptions to this coincide with the major river crossings where depth to rock head can sometimes exceed 50m depth. Sonic boreholes located adjacent to these areas can often be critical as there are usually bridges or structures required to transport the carriageway across the rivers. Often with little or no previous ground investigation knowledge of these areas, anticipated depths to rock head can be an informed estimate at best. On numerous occasions, GeoSonic has drilled and fully sampled the overburden and successfully located rock head in areas of deep overburden ranging from 50m all the way down to 97m at the deepest point.
As with any linear infrastructure project, the nature of the access to the exploratory hole positions becomes of the utmost importance in order for the main contractors to plan, implement and complete the works in a timely manner. Throughout the length of the A9, surface conditions at the exploratory hole locations vary from hardstanding on carriageways to dense woodland, steep slopes, peat bogs and water crossings with a marked increase in difficulty in the northern half of the route. On some sections, aside from boreholes located on or near the existing carriageway itself, it is prudent to make an assumption that the use of wheeled vehicles or plant of any sort is just not a realistic proposition.
GeoSonic has modified rigs to allow access to remote locations
GeoSonic proposed the use of modified drilling plant and specialist support equipment in order to help meet the challenges posed by the difficult ground with the aim of offsetting or reducing some of the necessary preparatory works that may be implemented by the main contractors running the project.
Those rigs within GeoSonic's fleet tasked with the most difficult positions were fitted with specialist wide tracks. By increasing the footprint of the rig significantly, it greatly reduces the pressure exerted on sensitive landscapes and transforms the rig into an item of true low ground pressure plant with the ability to cross terrain that can be difficult to walk across. Support plant for the transport of tooling largely consists of the ever popular Hagglund vehicles and where conditions are more favourable, tracked skid/steer loaders or tracked dumpers. GeoSonic's drill crews liaise closely with clients at all times, especially so where access is hazardous as often the main contractors have gone to considerable effort and expense to facilitate access for both their own plant and other subcontractors. The crew's expertise comes to the fore as they carefully assess the nature of access routes prior to mobilising in order to minimise the chances of either damage to the rig or project delays resulting from rigs stuck in inaccessible locations.
Secondary challenges
The access challenges posed by the site also present secondary challenges in supplying appropriate resource to complete the scope of sonic drilling works in the specified timescales. With so much non-drilling time associated with moving plant and equipment (especially so with preliminary or less detailed investigations with wider borehole spacings), the average sonic drilling production rates are significantly down on what GeoSonic would consider normal. To this end, more rigs are generally required than is usual for any given scope of works and timeframe. GeoSonic endeavour to anticipate this demand from the outset and works with its clients throughout each project to ensure the required level of resource is available.
Perhaps more than anything, the prevalence of sonic drilling in ground investigation work on the A9 dualling programme signifies a growing industry acceptance of the technique as an integral method of collecting quality subsurface geotechnical and geological information, particularly in challenging ground conditions. The benefits of GeoSonic's small role, in what is a massive ground investigation project, is hopefully resulting in quality information which main contractors and consultants can use to inform less conservative designs and perhaps save on construction costs. There is no getting away from the fact that sonic drilling tends to be much more expensive but where timescales are short and the provision of accurate, repeatable data is key, maybe, just maybe, the tide is turning in favour of alternative solutions that offer a cost saving over the lifespan of the whole construction project, not just the narrow window of the intrusive works element of a ground investigation.
Callum Whitelaw is contracts manager at GeoSonic Drilling Limited which offers a range of sonic drilling solutions
