Geology, risk, hypothesis, model, investigation
Introduction
First of all, what is a hypothesis? You might remember writing a hypothesis in high school chemistry class or a science fair, like ‘will chilling an onion before cutting it keep you from crying’? As defined by Google Dictionary (Oxford Languages), a hypothesis is ‘a supposition or proposed explanation made on the basis of limited evidence as a starting point for further investigation’. It should explain what you anticipate, be easily understandable, and be testable.
So, what does a hypothesis have to do with our geotechnical and geological work? Well, quite a lot it turns out. When undertaking a site investigation or geotechnical assessment, it is fundamental to have a hypothesis. Otherwise, how do we know what we are, and are not, meant to be looking for. We are much more likely to misinterpret the ground conditions if we don’t have a hypothesis. Misinterpretation of ground conditions is our biggest risk and is the most common reason for liability claims made against geotechnical engineers (Lucia et. al. 2017).
State of play
I have written this piece, because in my view most geotechnical projects are lacking a hypothesis and have room for improvement in this space. I believe that often ground models are not reviewed critically during design. It is all too easy to focus on the geotechnical engineering facets of a design, this calculation or that, when the ground model is clearly illustrated on cross sections and plans subdividing a block coloured stratigraphy using discrete lines. It is also very difficult to see beyond a cross section, to understand the potential for different ground conditions and incorporate that risk prudently into the design.
A robust hypothesis developed at the start of the project, to refine and challenge over the project lifecycle will help form a better ground model and manage project risk. Note that a hypothesis can be similar or analogous to a conceptual engineering geological model (Parry et. al. 2014).
Figure 1 depicts three trend lines for the development of site geological knowledge over the basic stages of a project.
Figure 1: Development of site geological knowledge over the basic stages of a project. Reproduced from Fookes et. al. (2015).
The blue line I think shows the typical process in New Zealand. There is no hypothesis developed at project commencement, limited desktop study and only a brief site walkover where little site observations are recorded. The (observational) ground model developed for design therefore heavily relies on, and is essentially fitted to suit the results of physical ground investigations, such as boreholes, Cone Penetration Tests etc. This is a key pitfall, because somewhat blindly fitting the model to the investigation results might not necessarily make geological sense. This could result in a ‘mechanical’ interpretation such as that shown in Figure 2. These mechanical interpretations seem to be common across our profession, noting that curved lines can be mechanical too, not just straight ones.
The green line and to a slightly lesser extent the red line, has a hypothesis developed at the outset and a rigorous desktop study has been completed. The hypothesis is challenged during the investigation process and the physical investigation results are integrated with the hypothesis (conceptual model), to build the observational model used for design. This is practically the reverse of the process followed by the blue line and is significantly more robust i.e. ‘fitting’ the investigation results within the conceptual model framework as opposed to fitting the model to the investigations.
Figure 2: Example of a mechanical interpretation (upper section) by somewhat blindly fitting the cross section to the investigation data, and a more realistic interpretation (lower section) by integrating the investigation data with geological concepts. Reproduced from Baynes et. al. (2020).
Need more convincing?
- An insurance company in the US undertook a study of liability claims against geotechnical engineers, over the 25 year period from 1988 to 2013 (Lucia et. al. 2017). The main findings were:
– Out of 1500 total claims against engineering companies, 897 were for geotechnical engineering.
– The majority of claims against geotechnical engineers related to site investigation and inadequate site characterisation, accounting for 56% of the total loss severity ($69.4M of the total $124.1M from the 897 claims). The study states that these claims originate primarily from ‘incorrect characterisation of the subsurface stratigraphy (including missing a geologic feature), groundwater conditions, and the associated engineering properties of each layer’.
– The critical recommendation made by the study to mitigate geotechnical risk and demonstrate that ‘standard of care’ is met, is to develop a hypothesis of the expected site conditions prior to physical site investigations, based on desktop study information. - Over 80% of 71 major tunnel failures worldwide which occurred between 1964 and 2015 were attributed to unexpected geological or hydrogeological conditions (Hong Kong geotechnical engineering office 2015).
How to develop a good hypothesis
I’m no expert on geology or conceptual models, but here are some ideas:
- Undertake a thorough desktop study. What is the regional geological setting? Consider geomorphology and not just geology maps, look at soil maps too. Review historical photos, it is easy to make stereo-photos using freeware. Retrolens.nz is a great resource for historical vertical aerials, so are sources such as the National Library of New Zealand for historical oblique aerials.
- Learn about geological concepts and processes, to be able to define more realistic subsurface profiles. I keep a personal library of photos of soil and rock exposures for reference.
- Consider the conditions we can anticipate (Skipper, 2017):
– Environments of deposition
– Post depositional changes (chemical, biological, gravity)
– Tectonic (faulting, uplift)
– Quaternary changes e.g. sea level rise and fall downcutting and infilling paleochannels, temperature changes, erosion, slope failures
– Anthropocene effects such as construction, earthworks, agriculture - Illustrate or write down your hypothesis and concepts. Don’t keep it locked up in your mind, you’d be surprised of the gaps and improvements only noticeable once it is down on paper. Drawing a conceptual cross section or block diagram without any ground investigation data will force in-depth brainstorming.
- Justify the physical ground investigations and laboratory testing, by type and location. For example, create a table with proposed scope in the left side column, and purpose of scope in the right side column.
- Communicate geological and geotechnical risks associated with the hypothesis to the project team using the help of pictures.
- Involve both engineering geologist and geotechnical engineer on projects to bridge the knowledge gap, and ensure collaboration between the two. What does the engineering geologist want to know? How does that relate to the goals of the geotechnical engineer and vice versa? Apply perspective, what part of the ground do we want the hypothesis to focus on, this will likely depend on the type of project, whether a subdivision, bridge, building, tunnel, or something else.
Conclusions
To finish, the story of The Blind Men and an Elephant is shared below, from the John Godfrey Saxe (1816-1887) poem of ancient Indian story. It is about a group of blind men who have never seen an elephant, and must conceptualise the shape of the elephant by touch. This is a pertinent metaphor for the obscured lens through which engineering geologists and geotechnical engineers see the earth. To me it also highlights the importance of experience and teamwork, to manage risk and help practitioners avoid mistaken deductions.
It was six men from Indostan
To learning much inclined,
Who went to see the elephant
(Though all of them were blind),
That each by observation
Might satisfy his mind.
The first approached the elephant,
And happening to fall
Against his broad and sturdy side,
At once began to bawl:
“God bless me! – but the elephant
Is very like a wall!”
The second, feeling of the tusk,
Cried, “Ho, what have we here
So very round and smooth and sharp!
To me ‘tis mighty clear
This wonder of an elephant
Is very like a spear!”
The third approached the animal,
And happening to take
The squirming trunk within his hands,
Thus boldly up and spoke:
“I see,” quote he, “the elephant
Is very like a snake!”
The fourth reached out an eager hand,
And felt about the knee.
“What most this wondrous beast is like
Is mighty plain”, quote he;
“Tis clear enough the elephant
Is very like a tree!”
The fifth who chanced to touch an ear,
Said “E’en the blinded man
Can tell what this resembles most;
Deny the fact who can
This marvel of an elephant
Is very like a fan!”
The sixth no sooner had begun
About the beast to grope,
Than, seizing on the swinging tail
That fell within his scope,
“I see”, quote he, “the elephant
Is very like a rope!”
And so these men of Indostan
Disputed loud and long,
Each in his own opinion
Exceeding still and strong,
Though each was partly in the right,
And all were in the wrong!
References
- F.J. Baynes, S. Parry and J. Novotný. Engineering geological models, projects and geotechnical risk. Quarterly Journal of Engineering Geology and Hydrogeology, 23 September 2020.
- P. Fookes, G. Pettifer, T. Waltham. Geomodels in Engineering Geology – An Introduction. Whittles Publishing 2015.
- Hong Kong geotechnical engineering office, 2015. Catalogue of notable tunnel failures. Case histories up to April 2015. Mainland east division, Civil engineering and development department, Government of the Hong Kong Special Administrative Region.
- P.C. Lucia, L. Yabusaki, J.T. DeJong, D.L.J. Coduto. Claims against Geotechnical Engineers. Geostrata magazine July/August 2017.
- S. Parry, F.J. Baynes, M.G. Culshaw, M. Eggers, J.F. Keaton, K. Lentfer, J. Novotný & D. Paul Engineering geological models: an introduction: IAEG Commission C25. Bulletin of Engineering Geology and the Environment, February 2014.
- J. Skipper, 2017. The 18th Glossop Medal lecture – Variability and ground hazards: how does the ground get to be ‘unexpected’?