Geotechnical Engineers and Geophysicists – the perfect match
Geotechnics and geophysics IS a perfect match. Or at least it SHOULD be. What do you say? We say: “it depends”. Sometimes the answer is “yes” and sometimes “no”, depending on both the geotechnician, the geophysicist and the country you work in.
In Sweden and United Kingdom the use of geophysical investigation methods as a complement to geotechnical investigations, especially in larger infrastructure projects, has slowly increased during the last 10 years. Aside from the gradual increasing familiarity with geophysics amongst planning engineers, one of the main reasons is probably the increasing use of planning tools in 3D, which need better data coverage to be truly effective. However, geophysics is still not counted as a standard method in the toolbox for ground investigations and the number of geophysical investigations carried out in Swedish and British infrastructure projects differ a lot, due to several reasons.
Make the phone call
Many would say that the combination of our title characters, the geotechnical engineer, keen on drilling and digging and the geophysicist, with the divining rod in the pocket, is the only way to achieve the best knowledge of our subsurface conditions without needing to excavate the whole area. So don’t make an assumption about whether something will work or not, just call each other, as geophysics will most likely provide the following pros to a geotechnical project:
- Efficiency (both in time and economically). For instance, if you have several route corridors, commonplace for a new highway construction, geophysics can provide rapid information for an overview of all corridors whilst still in the planning stage.
- Data coverage. Geophysical methods provide extensive data coverage compared to traditional drilling. If familiar with both areas of work – try to imagine making a 3D model for a 1×1 km area with only 10 reference drilling points, compared to using the same 10 reference points PLUS 10-15 km of interpreted GPR (which can be achieved in only 2 additional days), or other geophysical data.
- Overall information. Geophysical investigations will not give you centimeter accuracy on the depth to bedrock but, in a short amount of time, you get a feeling of the overall geological situation. Or, as a geotechnical engineer said: “you and our GPR are my extra pairs of eyes”. The same person would always let the GPR go first, and then plan and decide the traditional geotechnical investigations programme after.
- Availability. Geophysical methods are non-destructive, which is a must for areas where no impact is allowed, for example sites under preservation orders due to the nature or heritage assets within them. Further on, geophysical methods can be used in rough terrain with dense forest and large topographical variations, where drilling equipment is hard or impossible get in and move around.
But why is this call not always taking place?
As you all know there are no perfect relationships. For geotechnical engineers the following points are the ones itching the most, certainly when looking at the conditions in Sweden and United Kingdom:
- Bad examples. Since the start of using geophysics for ground investigation in Sweden and United Kingdom, a large number of bad examples has occurred. It may be a problem with the interpretations, or an over-optimistic geophysicist has promised 100% coverage and/or accuracy of data but fails to deliver, which has led to mistrust and disbelief in the methodology.
- Uncertainty. The results are dependent on several different factors (field conditions, experience in interpretation, physical contrast, disturbance in the surroundings, measurements are made for a soil volume etc.). So, yes, as a geophysicist you know this list can be long, but does the client? Which key considerations need to be clarified prior to commissioning of the work? Most often the coverage of reliable data is not 100% but 40-50%.
- Data format. How are the results from geophysical investigations fitted to geotechnical information and planning software such as CAD, GIS or some other 3D software? Today there is often a break in the chain at this point, giving the geotechnical engineers data they do not know how to further process or present. It can be as simple as a question of X and Y-scales, or going from straight lines and flat planes in a geotechnical drawing to very undulating geophysical interpretation (i.e. bedrock from GPR data).
- Competition. It can be a tough pill for a drilling company to hand x% of tender value to a geophysical contractor whilst simultaneously reducing the drilling scope in line with the added information anticipated from the geophysics. Think bedrock mapping for example. You aren’t just taking less potential profit but also putting faith in the delivery with an outside group. For this to make sense you need to clearly know the added benefit. So lets have this conversation.
So, is this match reparable?
What are the most important action points we, as geophysicists, need to promote that will encourage the geotechnical engineer to make the call and continue to call? I would say we need to focus on:
- Purpose and Expectations. The purpose of and expectations on the geophysical investigation need to be specified early in the process and is really important when comparing geophysical investigations to conventional methods.
- Awareness. An awareness of the method itself is needed for both parts in this match (in terms of how it works, how do you do it, what will be the final output), to gain an understanding on which factors may impact the results gained.
- Uncertainty. The discussion of uncertainty must be more open and presented in simple terms, avoiding too much technical ‘jargon’. This can be done by, for instance, showing a span of depths instead of a single line for a bedrock level or give a weighted classification (e.g. based on certainty) to the interpretations that are made. Geotechnical Engineers are aware of and handle uncertainties already and are used to knowing the soil/rock characteristics in one point and then generalizing over the larger area.
- Standards/Method descriptions. Geotechnical methods and results are standardized, and geophysical data are not. Geotechnical methods are based on old long-time knowledge, strong empiricism, high security factors and by that large marginals.
- Tools. Tools for interpretation and visualization of geophysical or geotechnical data are found in uncountable numbers but seldom compatible. By using the same tool for both geophysical and geotechnical data, or any data from the site, the visualization becomes complete and correct assumptions on ground conditions can be made more easily.
- Understanding the Cost Efficiency. Geotechnical engineers and geophysicists should work closely together in the design of the ground investigation. Knowing how these disciplines work together to fulfil the end clients’ requirements will allow for more competitive tenders and help those parties win the work.
Stay tuned for Part II with perfectly matched results for geotechnical purposes.
Harry Higgs, Application Engineer, BSc FGS
Jaana Gustafsson, Applications Specialist, Phd
“This work has also been presented at the NSG2019 conference titled “The use of geophysics in large Swedish infrastructure projects”.