December 6, 2018
Writing something new about sonic drilling is a difficult task; many practitioners have already authored excellent articles documenting both the benefits and some of the drawbacks. Whilst the technology itself is nothing new, having its origins all the way back in the 1970’s, it wasn’t until the 2000’s that sonic drill rigs really began arriving on UK shores.
Initially the technology was reserved for ground investigations with unique and challenging circumstances or locations; clients who valued the techniques’ increased production rates and ability to offer risk mitigation against difficult ground conditions. However, in the present day, acceptance of sonic drilling has grown steadily to the point where it has now reached mainstream use across the UK in the geotechnical, environmental and mineral exploration markets. So why is this the case and what learnings have been made since its introduction that has facilitated this growth?
The primary reason is a much better understanding of the technique by both practitioners and specifiers, much of which revolves around the samples recovered from sonic drilling and the versatility of a modern sonic rig to complete a range of sampling options as well as sonic sampling and their applicability to the geotechnical ground investigation market in terms of their relation to the quality classes for soil samples as set out in EN ISO 22475-1.
There is often an assumption made when discussing sonic drilling that the only option for sample recovery is ‘soil sampling by drilling’, as referred to in EN ISO 22475-1. In this instance, the achievable sample categories are quoted as 4 for cohesive deposits and 5 for granular deposits. However the versatility of a sonic drilling rig to accommodate most of the major dedicated soil samplers during formation of a borehole and recover what are often Class 1 undisturbed samples now has a wider understanding. It is very important to note that these samples are obtained without the use of any sonic vibration, and depending on the rig type, samplers are advanced statically or dynamically in accordance with the standards.
For low strength cohesive deposits, sonic drilling and sampling on its own is unlikely to produce a high quality sample for subsequent laboratory testing, principally because of the effects of resonance, but also because the majority of sonic tooling widely available is relatively crude and does not meet the requirements of EN ISO 22475-1. However, the use of dedicated samplers for recovery of undisturbed samples in conjunction with sonic drilling for rapid profiling of the strata is often a viable and cost effective alternative to the traditional techniques.
Sonic drilling in higher strength over consolidated cohesive deposits with a suitably experienced operator often challenges the perceived wisdom by recovering very high quality samples with the use of resonance. Specifications in these deposits are often challenging as thin walled samplers simply cannot be pushed or will be destroyed trying, and triple tube rotary coring whilst excellent in more homogenous deposits can result in poor recovery due to the influence of water flush where granular lenses or higher proportions of granular materials are present in the deposits. Lined sonic samples in these deposits can offer an alternative to the other methods of sample collection and many engineers and geologists are, upon examination, happy enough to send these sonic samples for strength and compressibility testing on the basis of their assessed quality.
It is generally accepted that recovery of undisturbed samples within granular deposits requires very specialised methods and is extremely rare within UK practice. Standard sonic samples of granular deposits have the advantage over other techniques of providing continuous core samples with a very high percentage recovery allowing subsampling to take place with a focus on representative subsamples. Depending on the exact specification of the rig and experience of the sonic driller, it is possible to recover delicate sand seams and other easily altered geology which has helped to grow the technique in the minerals and aggregates markets.
Again, the effectiveness of sonic drilling in a wide range of superficial deposits coupled with the ability to recover undisturbed samples and integrate other techniques such as triple tube rotary coring using the same rig mean that it is often an attractive package for a broad spectrum of ground investigations.
Many mistakenly believe that because of the ability to progress through difficult ground conditions and overcome obstructions within superficial materials that sonic drilling will be just as good through solid strata. Sonic drilling should be principally considered as an overburden technique and as such excels in most overburden formations, What a sonic rig can bring to the table however, is excellent recovery through weathered horizons and transition zones into competent rock where all too often it can either be difficult to recover core or difficult to make progress depending on which drilling method is adopted. Many sonic rigs also have the option to rotate the drill string only without the use of resonance and simply be used as a rotary coring rig.
Often, resistance to specifying sonic drilling or reticence of clients to use the equipment comes in the form of a perception of increased cost. In a metre for metre for comparison with other techniques then a sonic rig is more expensive, but with that slight increase in costs to clients an overall program reduction with less days on site can be achieved leading to reduced overhead costs. There are significantly reduced risks from geology or buried obstructions, high installation quality is achieved, versatility is introduced with the ability to sample as any other rig as well as sonic sample and improved site cleanliness can be expected with less arisings to deal with.
Additionally clients using sonic rigs can benefit from complete accurate geological information enabling precise site investigations, visibility of micro features within easily altered geology, an absence of any flush during drilling when required undertaken by an experienced sonic contractor to help avoid sample bias and cross contamination, as well telescoping capability to selectively isolate contamination.
With all that being said when choosing to specify sonic or selecting a sonic contractor you should be aware of some of the learnings from sonic contractors operating sonic machinery in the last thirteen years in the UK market. Sonic drilling is tough on machinery and poorly maintained sonic rigs can be susceptible to increased downtime. The current breed of sonic machines are large due to associated hydraulic power required for the sonic systems and are not suited to all sites. Sonic rigs have high parts and associated maintenance costs. Market capacity as growth in the market continues – there are a lot less sonic rigs in the UK than conventional techniques. Finally operator training is key; poor training can lead to poor results so when selecting a contractor pay attention to sonic specific training programs employed with the selected contactor.
Author: Callum Whitelaw, BDA Technical and Standards Committee