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Total geological history: a model approach to understanding site conditions

By Peter Fookes, consultant engineering geologist, UK, Frederick Baynes, consultant engineering geologist, Australia, and John Hutchinson, Emeritus Professor, Imperial College, London.

One of the principal uncertainties in geotechnical engineering is the risk of encountering unexpected geological conditions.This is because geological materials are often irregularly arranged and highly variable in their properties. Failure to anticipate ground conditions generally results from an inadequate geological understanding. This note forms a summary of a paper that presents an approach to site evaluation designed to assist anticipation of all potential geological conditions, from desk study to project construction, that is based on developing an understanding of the total geological history of the site.

This invited paper was presented at the GeoEng2000 conference held in Melbourne, Australia,19-24 November 2000.The full paper is published in Volume 1 of the Conference Proceedings, pp 370 to 460.

Ground conditions at any site are a product of its total geological and geomorphological history (generally abbreviated to 'total geological history') which includes the stratigraphy, the structure and the past and present geomorphological processes and climatic conditions.

The total geological history is responsible for the mass and material characteristics of the ground. To understand this history, the development of a site specific geological model is required, based on consideration of the regional and local geological and geomorphological history and the current ground surface conditions (Fookes, 1997). The engineering performance of the site during and after construction results from the influence of the engineering works on the total geological history.

The object is to describe and evaluate how:

Initial desk study knowledge of the engineering geology environment can help in the anticipation of geological and geomorphological conditions at a project site.Such anticipation is used to help construct the preliminary geological model for the site, plan the investigation and assist geologists and engineers in the site investigation and design of the project.This model develops progressively to be site specific as the understanding of the local geology improves during the project.

Improved understanding of the site as observations are made during investigation and construction also helps the anticipation and definition of ground conditions.

This paper is targeted at site investigators and financial decision makers who will probably never be aware of its existence.

The approach The essence of the approach involves understanding the geology and geomorphology to evaluate anticipated conditions.To do this and to understand the models, there must be some appreciation of the geological history of the world.

The approach starts with a series of simple, related geological and geomorphological models to generate questions about the site and to provide the basis for a checklist.The models represent the end members of a continuous range of possibilities.

One or more of these initial models will be identified to represent the particular site, enabling the earliest planning to take account of the broadly anticipated geology and geomorphology.The geological concepts embodied in the selected initial models and the specific checklists for the site are then investigated thoroughly as the site specific engineering geology model develops during the project studies.

To help anticipate the regional scale geology, 10 two-dimensional initial global scale tectonic models based on the concepts of plate tectonics are considered.To help anticipate the local scale geology, 17 three-dimensional initial site scale geological models are considered, each with an anticipation list, encompassing the rock-forming environments and tectonic and diagenetic modifications to these environments. To help anticipate the local geomorphology, eight 3D initial geomorphological landform models are considered, each again with an anticipation list, related to climate and geomorphological processes.

The approach allows the broad anticipation of the geology and geomorphology from a desk study of the site location, where the knowledge of site conditions may be minimal.Using this approach, it is necessary to:

form an understanding of what is revealed by the desk study anticipate what geology conditions might be encountered at the site develop the understanding through subsequent stages of the investigation, design and construction.

Conventional approaches to desk studies for the feasibility and early phases of site evaluation typically start using local maps and literature which commonly exist for many developed areas around the world.The Transport Research Laboratory Report (TRL 192),1996, by Perry and West, Sources of Information for Site Investigations in Britain (Revision of TRL Report LR 403) is a good example from the UK of these approaches. Such works also generally describe various forms of maps and remote sensing for engineering purposes.

In some locations little may be known, or much may be known already.At sites where geological maps do exist or there are good air photographs or other imagery, the initial local geology can be quite well anticipated by site specific models prior to any preliminary inspection (see Fookes 1997, page 348 and Figures 10,41a to 41e, and Table 5).

The site inspection, preliminary and full ground studies can be progressed quite quickly at locations where much is known, to give as detailed a picture of the geology as is considered sufficient or necessary.Nevertheless, this may still result in some shortfall in the final anticipation of the total geological picture if a fundamental understanding of the regional and local geology has not been formed.

At sites where little is known, the procedures followed are the same, but more work may be required in the early stages.

Anticipating the total geological picture means that all the geological and geomorphological characteristics of the site have been considered together with the range of possible variation (sizes, locations, properties) of the characteristics identified. Ideally there should be no condition that comes as a surprise during construction.

To illustrate the usefulness of the approach, Figure 1 is a crude approximation of how well-designed conventional site investigation studies develop increasing geological and geotechnical knowledge.Note that it is suggested that by using this approach geological knowledge rises more quickly than geotechnical in the earlier stages of a project: the gathering of geotechnical data develops faster when insitu and laboratory testing and rigorous description is introduced during the latter stages of investigations (Fookes, 1997). Relatively few publications describe investigation activity during construction but the principles and techniques are essentially those used in pre-construction investigations. For guidance, see Eddleston et al (1995).

Anticipating site conditions from the models The global tectonic models provide the setting for the other two groups of models.Only the site scale geomorphological and geological models have annotations and key descriptions to form the basis for the checklists, since it is these models which provide the initial basic conceptual picture of the local potential conditions. Full understanding of the actual geology and geomorphology of the site must come from the subsequent ground investigation.

It is essential that an experienced engineering geologist or geomorphologist is involved in important projects. Their training and experience is needed to interpret and develop the engineering geology history and to contribute to the planning and the investigation design.

Desk study stage The relationship of the initial models for identifying and building the site checklist is shown in Figure 2.When the preliminary site engineering geology environment model has been developed, it is used for overall planning and for design of the preliminary ground investigation, or the full ground investigation should there be no preliminary stage.

Ground investigation stage(s) A well designed ground investigation should now progressively identify site conditions, answer checklist questions and give information to build the real site model(s). It is not the intention here to elaborate on the development of the specific site geological/geotechnical model(s) and the conduct of the ground investigations - guidance is given in numerous publications - but it is worth emphasising that the site checklist should continue to be systematically evaluated and developed, using geological and geomorphological judgement and investigation findings.

It is also necessary to emphasise that the investigation should not consist solely of boreholes. Adequate engineering geology/ geomorphology mapping should be carried out or be in place early in the investigations. Trial pits, trenches and other high return ground observation techniques should figure largely. Engineering geological and geomorphological interpretation must be continuous and early observation and deductions continuously reviewed, objectives defined and questions asked (see Stapledon,1983 and 1996).This process must continue into construction and the service life of the project.

The initial models and anticipations are offered as aides-memoire for competent geologists, geomorphologists and engineering geologists.

Engineering geological environments around the world, which began to be shaped long ago, can present a bewildering array of conditions that can impact on projects: a lifetime is not long enough to see them all.The models offer assistance to those breaking potentially new ground and therefore will need constant review and addition. Some of the annotations and text on the models can, with little time or effort, be dismissed as inappropriate for a particular site: others may take extensive and expensive investigation to prove or disprove their presence and relevance.

Very little geology or geomorphology on a site will be unforeseen if the evaluation is done properly - all geological conditions affecting the engineering performance of the ground should be reasonably foreseen by a considered investigation.What can be unforeseeable is the detailed variation in their location, form and size or specific engineering characteristics (the potential range of which must be established) which might not be capable of evaluation within time and money constraints.

A simple example of this would be the presence of a karstic cave system anticipated by the model and proved by drilling.The precise configuration of the cave system would require either underground mapping, which might not be feasible, or an enormously large number of boreholes, which would be impracticable.The cave system has been foreseen and allowed for in the contract arrangement, but its detail is unforeseeable. In such circumstances, adoption of the observational method would overcome potential engineering problems.

Figure 3 summarises, as a flow path, the activities required during a large site investigation to complete a total geology-based site investigation.

Figure 4 compares the total geology method (TM) with the conventional or 'predetermined'method (PM) and the observational method (OM). The figure shows where the emphasis comes in the three principal stages: the preliminary investigation, the main investigation and during construction. As can be seen, the TM is more prominent in the relatively inexpensive preliminary investigation stage, the PM has the greater prominence in the main investigation stage and the OM has the potential to be the most prominent in construction stage.Arbitrary judgements on the quality of the performance rated excellent to poor have been added to the figure at the end of the investigation stages and at the end of construction stage.

The full paper includes 31 selected case histories that are used to illustrate how the geological model helped, or could have helped, to anticipate ground conditions that proved to be critical to the project engineering.Thirty-eight initial models are also presented (Table 1).

Concluding remarks From the numerous case histories examined, the overwhelming conclusion was that for well over a century there has been a repetition of common reasons which, individually or in association with the others, have led to failure to anticipate geological conditions.This in turn has led to failure in the project engineering.Little new was learnt in this respect and many authors have reached similar conclusions before.

There was no statistical evaluation of causes, nor of features leading to causes of the failures, since the case histories differ widely in degree of detail available, and in the numbers of various types of case.However, some broad principal conclusions on the conduct of a project investigation were reached:

There must be a specific and determined endeavour to understand the engineering geology and geomorphology environment of the site and to incorporate that understanding into the project design.

Around the world it is often the problems related to the Quaternary geology and geomorphology events that dominate.

The engineering geologist must be competent, well trained, experienced and a good geologist if they are to be either the lead engineering geologist in the team, or the only engineering geologist in the team.

The engineering geologist must have a good knowledge of geomorphology and on occasion will need to work with a geomorphologist. Appropriate experience is most important in the professional development of qualified geologists and geomorphologists.

Each site evaluation, however small, should ideally have at least one engineering geologist involved in the work.This does not always occur in practice.

The preliminary stage is when the engineering geologist probably can have the most significant influence on the project by indicating potential hazards and their consequence on the economy of design, matters of construction and expected performance of the works.

References

Fookes PG (1997). The First Glossop Lecture. Geology for engineers: the geological model, prediction and performance. Quarterly Journal of Engineering Geology,30,293-431.

Eddleston M, Murfin RE and Walthall S (1995).The role of the engineering geologist in construction. In Eddleston M, Walthall S, Cripps JC and Culshaw MG (Eds).Engineering geology of construction, Geological Society Engineering Geology Special Publication No.10,389-401.

Emery D and Myers KJ (eds) (1997).Sequence stratigraphy, Blackwell Science, Oxford.

Moores EM and Twiss RJ (1995).Tectonics, WH Freeman and Company, USA.

Perry J and West G (1996). Sources of information for site investigations in Britain.TRL Report 192 (Revision of TRL Report LR403).The Transport Research Laboratory, Crowthorne.

Ritter DF (1986).Process geomorphology (2nd Edition), Wm C Brown Publishers, USA.

Stapledon DH (1983). Keynote address: Towards successful waterworks. Proceedings of the symposium for engineering for dams and canals. Institution of Professional Engineers, New Zealand.1.3-1.15.

Worked example Use of the approach can be illustrated by a coastal exposure in north Cornwall of strongly folded and faulted Carboniferous age thick sandstones, thin turbidite sandstones and shales (Figure 5).

The starting point is a map and section of the world, subdivided into the major geological components, forming a global setting of the site (see full paper).The regional tectonic setting of the site during the tropical shallow/deep marine deposition and subsequent deformation of the original sandy beds is that of a convergent plate boundary with fold and thrust belts (Figure 6).

These rocks were subsequently affected by subtropical weathering during the Tertiary when the plate on which Britain was moving passed north of the Equator, on the way to its present position.This was followed by periglacial weathering which strongly affected the ground in the Quaternary ice ages.The different processes can be depicted separately in four site scale geological and geomorphological models (Figures 7-10 overleaf ), but in geological time have successively combined and result in the complex and variable ground conditions shown in Figure 5.

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