The Observational Method in Civil Engineering

Minimising Risk, Maximising Economy

Alan Powderham, Anthony O'Brien

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000164855
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

The Observational Method (OM) is a natural and powerful technique that maximises economy while assuring safety. Its key features are highlighted in The Observational Method in Civil Engineering through twelve case histories from major infrastructure projects. They cover protection of adjacent structures including buildings and railway systems, bored and jacked tunnels, shafts and cofferdams, retaining walls, embankments, deep foundations, ground improvement and groundwater control. They illustrate how the OM can achieve more effective collaboration between the client and the design and construction teams, as well as how it can enhance the industry's ability to learn from experience, thus improving future practice and stimulating innovation.

Despite these advantages, the OM is significantly underused. The book demonstrates how the full potential of the OM can overcome a wide range of concerns and constraints. Other chapters address the advantages and limitations of the OM, the key role of progressive modification, the art of achieving agreement and the commercial and contractual environment.

The book will appeal to a range of construction professionals, including civil, structural and geotechnical engineers, contractors and owners. It will also be of interest to students and researchers.

Chapter 1

The Art of Achieving Agreement

1.1 Key Factors

One of the most frequently asked questions in Q&A sessions following presentations on the observational method (OM) that the authors have given around the world is: ‘How was agreement to use the method achieved?’ In practice, the process is quite variable, requiring a flexible approach to address the circumstances particular to a given project and the specific concerns of each stakeholder. How agreement is reached will also be influenced by whether it is an ‘ab initio’ or ‘best way out’ scenario. As illustrated in the featured case histories, the starting position on site for the OM design can have quite widely varying levels of conservatism between projects in order to satisfy specific stakeholder concerns.

A reference set of abstracts is included in Section 1.3, which summarise the key aspects of each case history. These are described in much greater detail in the respective chapter and include a separate section devoted to this issue of achieving agreement to implement the OM.

Achieving agreement can present a substantial challenge. The OM demands commitment by all stakeholders and is fundamental to ensuring success in any application. Consequently, when there is no overriding driver for change, for example, in an ‘ab initio’ application, there is typically little incentive to depart from convention or ‘business as usual’. The parties are inclined to remain within their respective comfort zones. By contrast, with projects that have reached a crisis, there will be a marked absence of comfort generally. This will be evident in polarised and often strongly conflicting perspectives between the parties. Here the OM would be proposed as the ‘best way out’ solution. Peck (1969) noted that such applications were much more familiar, indicating, in turn, that resistance to change in the absence of a crisis is quite common. Even with a crisis as a catalyst, it often takes sustained and convincing advocacy by the proposer to secure the ‘buy-in’ by the other parties. They will probably be in varying degrees of disagreement and will therefore not readily reach a consensus that the OM does indeed offer the best way forward. It will test the commitment, confidence and ability of the proposer. This crucial aspect is discussed further in Chapter 13 on the advantages and limitations of the OM. Resilience is another important quality as it can take detailed development, strong advocacy and, as noted in many of the case histories featured in this book, substantial time and effort to reach agreement. The key factors involved are as follows:

• Convincing business case
  The benefits of adopting the OM must be clearly established and communicated to the stakeholders – particularly to the main stakeholder on whose approval the agreement to implement depends. The advantages of the OM need to be sufficiently convincing when evaluated against the established base design case or other potential alternatives.
• Sound technical basis
  Applications of the OM inherently involve some form of soil/structure interaction. Both the geotechnical and the structural aspects must be appropriately assessed and understood. This understanding must be manifest. In short, OM practitioners need to have commitment, competence and clarity. This must be evident to the stakeholders.
• Risk management
  Maintaining and demonstrating an acceptable level of safety is essential. Moreover, OM practitioners require an active and broad appreciation of the stakeholders’ perspectives on risks and constraints including those relating to commercial, programme and contractual issues.
• Trust
  Central to any application of the OM are interpersonal relationships in which trust between the parties plays a key role. This trust is not a given but has to be earned.

1.2 The Power of Progressive Modification

One theme common to all of the case histories in this volume was the implementation of the OM through progressive modification. With its additional focus on enhancing and demonstrating safety, whether it will be for ‘ab initio’ or ‘best way out’ situations, progressive modification provides greater comfort to the stakeholders in their concerns about risk. As illustrated in Figure 1.1, progressive modification can address all types of category of the OM. In practice, there is basically no difference technically between the categories in implementing the OM. But there is a big difference contractually and politically since ‘best way out’ situations, as opposed to ‘ab initio’ ones, involve crisis and consequently demand radical solutions. Once agreement has been achieved to use the OM, then each application can effectively proceed on a common conceptual basis. However, it is essential to appreciate that each application of the OM is bespoke and, beyond the broad concepts, each case

Figure 1.1 OM categories – all can be addressed by progressive modification.

needs to be addressed in detail for the particular conditions. In this, progressive modification brings a very effective flexibility. Consequently, there is no need to commence construction with an unduly optimistic design or one based, with little margins, on estimations of the most probable conditions. Instead, the conservatism of the starting condition can be chosen to accommodate the comfort levels of the stakeholders for each specific case. It is fundamental that every application of the OM should commence from a safe position. Progressive modification, as shown in the featured case histories, does not need to be constrained by any prescriptive requirement to implement and complete an application of the OM with a fixed level of conservatism in the design. It is essential, however, to have a realistic appreciation of the potential range of conditions from the most probable to the most unfavourable. Such understanding is central to establishing the viability of any application of the OM. This assessment requires, as well as site investigation and analysis, experience and engineering judgement. Accordingly, applications of the OM are not routine and it can be very risky to treat them as such – even for apparently very similar situations. OM designs must be carefully developed to suit each case and are consequently not well served by a prescriptive approach. For example, CIRIA Report 185 (1999) recommends that the OM uses:

• ‘most probable’ and ‘moderately conservative’ conditions for deformation and load predictions – serviceability limit state (SLS) designs;
• ‘most unfavourable’ conditions for ultimate limit state (ULS) designs and for robustness check during risk assessment.

Such recommendations, while made with the best of intentions to provide consistency for future applications, risk being too prescriptive and are also a recipe for over-complicating the OM (see also the discussion in Chapter 13). Peck was keenly aware of this and, while he included caveats in his Rankine Lecture, expressed his dissatisfaction with his attempt to formalise the method which he felt was too prescriptive (Dunnicliff and Deere, 1984). There is always the initial issue of how to establish in advance what the likely conditions for most probable and most unfavourable are likely to be. There are inevitably unknowns and our predictions have often been inaccurate. A recent comprehensive review of databases of retaining wall case studies concluded that ‘ground movements cannot be predicted exactly’ (Gaba et al., 2017). Considerable scatter was observed in maximum retaining wall deflections for similar wall types in similar ground conditions. The scatter was attributed to a range of factors, including the impact of variations in construction sequence, with the timing of establishing wall supports and local construction details. These construction issues are not amenable to prediction by a designer. The uncertainties associated with underground construction mean that, even with unlimited analytical power at their disposal, engineers should not expect to be able make accurate predictions of soil/structure interaction behaviour. While such surveys question our abilities to predict the most probable conditions, such a disparity also implies a warning about those for the most unfavourable. Moreover, while it is generally understood how the terms ‘most probable’, ‘moderately conservative’ and ‘most unfavourable’ may be addressed in terms of design parameters for soils, it is less clear how they relate to soil/structure interaction and the range of structural responses that may be involved. For example, as described in Chapter 3, the key issue for the Mansion House was the degree and nature of the response of the building to imposed settlements. While it was expected to be within SLS conditions, it was quite unacceptable to the building owners to commence the OM with a design based on estimates of the most probable or even those for moderately conservative conditions. In this case, only a level of slight risk of damage was acceptable. Agreement to implement the OM was therefore achieved by commencing on site at the much lower level of very slight risk. This recognised that the level of risk would progressively increase as tunnelling construction progressed and ground movements increased within the zone of influence. The success of this case history presents a compelling endorsement for implementation through progressive modification where, in practice, the starting position for the OM was demonstrated to be one of virtually negligible risk.

It is important to note that design modifications do not always have to be contingent measures of a corrective kind, although this is often assumed. It is implied, for example, in the relevant clauses relating to the OM in Eurocode 7. This assumption is more explicitly expressed by Muir Wood (2000) who also finds the procedures set out by Peck in his Rankine Lecture (1969) to be unnecessarily cumbersome. It is likely that Peck would have had some sympathy with this concern as indicated in his commentary on his Rankine Lecture (Dunnicliff and Deere, 1984). The concern is understandable given the tendency to treat Peck’s list of procedures too prescriptively. However, the authors have found throughout their own experience of applying the OM that, if considered flexibly, Peck’s procedures provide an excellent guide. A nuanced reading of Peck’s paper can reveal his pragmatic approach underlined by the need to exercise engineering judgement to each case individually.

Rather than design modifications being restricted to corrective contingencies, progressive modification inherently offers the potential for a fully flexible process. Through its step-by-step approach, it creates the opportunity to maximise value by facilitating the introduction of beneficial design changes in a safe and controlled manner. And, if the OM is commenced from a demonstrably safe conservative base, most of the design changes are likely to be beneficial. Moreover, this managed incremental approach also facilitates a very effective way to deal with unexpectedly unfavourable conditions. Thus, with this ability to satisfy safety concerns while also maximising value, progressive modification creates a fertile basis on which to achieve agreement to implement the OM. The advantages of adopting progressive modification are highlighted in the case histories and are further discussed in Chapter 13.

1.3 Summaries of Featured Case Histories

1.3.1 The Channel Tunnel (1988–1991)

A hybrid ‘ab initio’ and first overt application of what has become known as progressive modification: It was applied to cut and cover works in Gault Clay. Comprising bottom-up and top-down construction with contiguous piled walls, it involved tunnel portals and crossover works within an active and very complex landslip. It was also the first example of the use of blinding struts in the OM.

The OM is an inherently natural way to address uncertainty and one that engineers would instinctively adopt (Peck, 1969). In that context, this first case history could be considered as an exemplar. It developed naturally to address the basic question: ‘How could we do this better?’ Only in retrospect did the realisation dawn that it was essentially an example of the OM. It did not fall neatly into either an ‘ab initio’ or ‘best way out’ application, although in spirit it was closer to the former. There was no crisis to act as a catalyst for corrective change. On the contrary, the application of the OM simply evolved from the early rapport established between the contractor and designer. It fostered creative teamwork facilitated by the harmonious interaction of aligned objectives and was introduced and managed by the original designer.

1.3.2 The Mansion House (1989–1991)

‘Best way out’ application: Protection of a Grade I listed masonry building of national importance from potential damage caused by tunnelling. This involved complex soil/structure interaction from a range of both shallow and deep bored tunnels constructed in London Clay within the zone of influence of the building. Its success marked a significant development of the implementation of the OM through progressive modification. The prime objective was to safely protect the building from any unacceptable damage while, at the same time, avoiding the contingencies of expensive and time consuming preventative works. This was achieved by commencing construction with negligible risk and demonstrating, at each stage in a sequential process, that the accumulative risk would be maintained within acceptable levels. It was the first application of real-time electronic data capture of the critical observations combined with the traffic light system for the OM. It was introduced and managed by an independent specialist team.

1.3.3 Limehouse Link (1991–1993)

‘Best way out’ application to cut and cover highway tunnels: This case history, like the Mansion House, highlights key obstacles to achieving agreement to implement the OM. Here the immediate problem was the contractual constraints to introducing design changes during construction. Such limitations make the OM a complete non-starter. It was unlocked by adding a value engineering clause to the contract and was the first example in the United Kingdom of the OM being introduced through such a clause. This was added as a variation to the design and build contract well after construction had started. The OM was applied on multiple fronts involving top-down construction wit...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Foreword
• Authors
• Glossary
• Abbreviations
• Notation
• Introduction
• 1. The Art of Achieving Agreement
• 2. Channel Tunnel Cut and Cover Works (1988–1990)
• 3. Mansion House (1989–1991)
• 4. Limehouse Link (1991–1993)
• 5. Heathrow Express Cofferdam (1994–1995)
• 6. Heathrow Airport Multi-Storey Car Park 1A (1995–1996)
• 7. Boston Central Artery Tunnel Jacking (1991–2001)
• 8. Irlam Railway Embankment (1996–1998)
• 9. Heathrow Airport Airside Road Tunnel
• 10. Raising the 133 m High Triumphal Arch at the New Wembley Stadium (2002–2004)
• 11. Crossrail Blomfield Box (2012–2015)
• 12. Crossrail Moorgate Shaft (2012–2014)
• 13. Reflections on the Advantages and Limitations of the Observational Method
• 14. Some Observations on the Way Forward
• Index