Appraisal and civil engineering design
Dennis Gedge MCInstCES, Consulting Engineer
IN architecture an artistic visualisation of the completed project has always been seen as part of the construction job itself, but in civil engineering, working drawings have served more as a practical means to an end. Current ways of producing images have blurred this distinction. Civil engineering is a profession scattered within an industry, and surveys are themselves scattered within that profession.
The verb ‘to survey’ implies more than just recording or looking at something, the implication is that it is an appraisal. That is to say it is an assessment of how to proceed. The UK’s national mapping organisation Ordnance Survey (OS), was founded for this reason in the 18th century, it was a military project. Maps were a missing part of the country’s armoury at the time, so they had to be provided. OS today still sees maps as underpinning decisions made by policymakers1, but now on an international basis. In same way, surveying is part of civil engineering; it is not a different job, it is essential to the assessment and to the forming of concepts for projects.
Learn as-you-go
An example of just how much civil engineering design and survey are interwoven is the observational method (OM) for soil mechanics (geotechnics). An example of just how much civil engineering design and survey are interwoven is the observational method (OM) for soil mechanics (geotechnics). Soil mechanics is not a precise science, site investigations are only ever a snapshot of the points sampled.
Ground conditions can vary widely across the site, civil engineers need no reminding of that. The actual ground conditions are only fully known when construction starts. The OM is described as:
“An approach that enables a design to be changed during construction as a result of observations that are made about the way a material or structure is behaving.”
OM is often assumed to be a reactive process, to be used when unforeseen events occur, but it was actually devised to reduce overdesign and at the same time to guard against failure. It is a logical approach to design. In practice, this means taking site observations i.e. surveying shapes in three dimensions, and detecting changes in them as a project is constructed. In other words, it is necessary to produce a survey in four dimensions, namely one which includes the dimension of time.
The father of soil mechanics Karl Terzaghi described this technique2 in 1945 as learn as you go. All professions are notoriously resistant to accepting changes to established practices. Those in charge are always politically adept at fending off criticism of the working ways of the day. This may have been one of the reasons that it took some time for OM to become a more commonplace procedure on projects, when previously it had mainly been seen on the ones personally supervised by Terzaghi himself, but in the 1940s there were limitations in the ways structural distortions were able to be measured on-site.
Structural distortions are, however, only able to be understood by the same theorems of trigonometry as those used in topographical surveys which are on a much larger scale.
Structural distortions are, however, only able to be understood by the same theorems of trigonometry as those used in topographical surveys which are on a much larger scale.Instrumentation enabling distortions in construction to be measured practically became much more readily available from the 1970s onwards, and these instruments have rapidly developed since then.
The OM method was included in the European civil engineering design codes Eurocode 7 in 2004. Its application is now often reported. The major temporary works on the construction of the central part London’s Crossrail project used this approach.
Although the philosophy of the method was proposed some years ago, it is only now that sophisticated measurement techniques such as ‘shape accel arrays’, strain gauges, inclinometers, and extensometers can all be combined and the data from them easily collected and interpreted. The value of this approach to civil engineering design is now seen more widely.
Different kinds of surveys
A similar technique on site using very precise measurements is often used when the stability of an existing structure is in question, for example the monitoring and assessment of an old bridge, when remedial work is needed to be designed. A branch of civil engineering surveying specialises in this type of work.
The Client Guide to Instrumentation and Monitoring3 sponsored by a consortium of professional institutions and instrument makers was published in 2017. One of the stated aims of this guide is to confirm design model predictions. This is a subtle but distinct difference to actually informing design, prior to its being done.
The resulting reporting and professional advice following the site investigations are inextricably linked.
Specialist surveys of this type and typical site surveys all depend upon a series of three-dimensional measurements requiring interpretation, but these special monitoring surveys are not just based on the application of trigonometry, in addition to that they also require a large element of appraisal.
Conventional site surveys are sometimes seen as an entity in themselves, but they too are also intertwined with a need for appraisal in the same way as these more specialist ones.
When it comes to the concept of any civil engineering design, the starting point is a model devised in the mind of the designer, that is the creative or human part.
Despite the feeling today that within the civil engineering profession it is almost obligatory to reach for automated things to solve problems, hand-drawn sketches still come first; technology has not overtaken us yet on that point. It is the conceptual model which is the foundation, and it is the absolutely essential part of any civil engineering design.
Surveying the concept of a design
Having arrived at an embryonic model it must then be rendered correct in terms of its shape, size and location with respect to other things; these must all be in the right proportions before considerations of any of the other physical dimensional parameters such as fluid viscosity, or strength of materials can be made.
Surveying is fundamental to civil engineering just as anatomy is to medicine. The suitably accurate model must then be subjected in theory to the adverse conditions it is expected to withstand in practical use. If the stresses, deflections, its capacity to convey fluids, its qualities to resist decay or failure in other ways are beyond acceptable limits, or if the model has an excessively large safety factor, or perhaps it would be uneconomic for other reasons, it must be altered and then re-analysed until it is optimised.
Design is an iterative process; the conceptual model is refined and re-refined in stages. It is therefore a process of appraising or ‘surveying’ a model. Thomas Telford the first president of the Institution of Civil Engineers was known as a surveyor.
Unlike some other branches of engineering, in civil engineering the theoretical design model itself cannot exist without a physical survey in the first place, because each job is a prototype or unique, it is related to the individual site. Surveying is fundamental to civil engineering just as anatomy is to medicine
The design models of today rely very much upon maths and analytical geometry, but the earlier projects still had empirical rules to follow. Alignments of tunnel headings from each side of a mountain, or the shapes and positioning of complicated foundations for the structure of a massive dam just have to be mathematically consistent if the job is to work. Analysis by way of manual calculations aided by books of mathematical tables has served many a civil engineering project in the past. The accuracy with which they were built, and their quality can still be clearly seen today.
The electronic aids to our design methods today still rely on the same theorems of Newtonian mechanics and kinetics from the 17th century, and they also rely on the mathematical theory of probability which also came to us from the 18th century. The current conundrums in the 21st st century of reconciling relativity and quantum theory which apply to the two extremes of spatial measurement certainly exist, but they have no bearing on today’s practical civil engineering. Our current methods of design still use the earlier classical principles.
Analytical and conceptual
Civil engineering design has been largely analytical since the latter part of the 19th th century, it is the tools which have changed. But in some cases like the complex hydraulics such as a river estuary, or structures that defy reliable analysis, physical scale models still have to be tested. Mathematical analysis has limitations. The reason that the conceptual models used for civil engineering design are founded on site surveys is because they must combine an understanding of the existing surroundings and the merging of the proposed construction with it.
These two aspects come in varying proportions on different types of job. Some, like flood alleviation schemes will require a far greater understanding of the existing surroundings than others. At the other extreme, a standard pre-fabricated building requires far less. All design models need the fundamental location and spatial definition of the site.
The current conundrums in the 21st st century of reconciling relativity and quantum theory which apply to the two extremes of spatial measurement certainly exist, but they have no bearing on today’s practical civil engineering.These things may generally be measured in just three dimensions, but other parameters such as wind speed, intensity and probability of rainfall, ranges of operating temperatures, probability of increase in traffic loading, are also parts of an engineering site survey.
As methods of mathematical analysis have become much more readily available this raises some interesting questions about the concept of actually surveying civil engineering sites, and how this operation might develop in the future. To be a civil engineering surveyor in the accepted sense today one does not need to understand the complete theory of how electromagnetic distance measurement (EDM) works, nor is it necessary to understand the inner workings of satellite positioning (GNSS). A grasp of the principles is enough. These questions of understanding have been with us for some time and academics and the professions generally accept a practical view about specialist tools like these.
It is possible that these tools will become more automated and simpler to operate and the 3D site survey as we know it might become a component of specialist surveys where design is involved. We are already seeing developments like this where drainage networks are surveyed internally by remotely controlled means. The prime object of surveys of this kind is to identify structural faults and to prepare designs to put them right. In other words the appraisal of the results is seen as the main part of the survey, the need for accurate 3D location is just taken as read.
This is to the credit of civil engineering surveyors who have made good progress in the past fifty years bringing a more general realisation of this point, to those with no understanding of engineering principles. The best structural assessment of a pipeline in the world is no good unless we know exactly where it is under the ground.
Back to the future
Civil engineering surveying may now perhaps expand in unexpected ways, it may diffuse becoming more scattered within the civil engineering profession. Structural and hydraulic analyses, are examples of how quite complex assessments might also be aided by computer methods.
These are similar specialist tools to EDM or GNSS. This might lead to multiple special branches of site surveying developing in the future, and other types of surveys perhaps might include much more of an element of appraisal, but these too rely fundamentally upon accurate 3D location. Perhaps these might be seen as wider applications of the observational method.
This is different to the development of surveying becoming responsible for more and more sophisticated data handling about what exists, or is proposed to be built. BIM has a different focus, it is to unify and to avoid confusion and to improve design and construction processes. Fundamental engineering design springs from ingenuity which is different to innovation of this type.
A civil engineering survey for a proposed flood alleviation project will call for cross sections of rivers which are then normally passed to civil engineers to assess the flow capacity of a river. It is essential to know this in order to appraise (or survey) the risk the surrounding land has to flood. The same applies to the network of an existing drainage system, once its physical dimensions and condition have been measured (or surveyed in civil engineering parlance), however, should the site survey include an appraisal of the probability of its being inundated by rainstorms?
These two examples do not actually infringe upon the process of a conceptual design, which may follow; they would merely take the survey to a different level, so the designer would be better informed.
Computers having taken the burden out of topographic detailing, may restore the links between civil engineering survey and design in unexpected ways.
Dennis Gedge MCInstCES,
Consulting Engineer
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1 Ezechie J, See your city smile. Civil Engineering Surveyor November 2022
2 The term learn as you go was used in the Introduction to Soil Mechanics in Engineering Practice which was written in 1945. It was mentioned by Ralph Peck in his Rankine Lecture No9 Geotechnique 1969 pp171-187
3 A Client Guide to Instrumentation and Monitoring. Published by The Survey Liaison Group, comprising the Chartered Institution of Civil Engineering Surveyors, the Institution of Civil Engineers, the Royal Institution of Chartered Surveyors, and The Survey Association, November 2017