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Four recent case histories

Powderham presented an overview of four recent applications of the observational method through 'progressive modification' He agreed strongly with Peck's views, emphasising that simplicity was at the heart of the method. In this context, there had not been major changes in its application as set out in Peck's 1969 Rankine lecture. However, there had been some important developments, including the enhanced role of teamwork and partnering and the strong linking of the observational method with value engineering.

The progressive modification approach was particularly useful in addressing the risk concerns of all stakeholders, and flourished in a team environment. The step-by-step procedure provided a simple framework to manage risk and demonstrate that safety was being contained at an acceptable level throughout construction.

Although the case histories presented were complex major projects, the common critical issue focused on the control of ground movement. This involved the exploitation of the short-term stiffness of the ground, typically over-consolidated stiff clays, and an appropriate level of monitoring to achieve communication of the right information to the right people at the right time.

Such a system should be as simple as practicable, to ensure that critical observations are reliably obtained. For the Channel Tunnel and the Limehouse Link, the primary system was essentially tape extensometers. The Mansion House and the Heathrow Express cofferdam projects used a system of electrolevels - partly because of the critical importance of subsurface measurements but also to enable trends of movements to be identified at an early stage. However, in these days of electronic data capture it is too easy to have a proliferation of instrumentation. Too much information can be misleading. The four projects were presented chronologically to highlight the development of the method over the past ten years:

The Channel Tunnel at Castle Hill was the first use of the observational method on a progressive modification basis for cut and cover construction. The implementation of the method occurred naturally within a team environment on this design and build project. There was no crisis; the opportunity simply arose in a climate of change and innovation. Thus a common-sense, step by step approach led to the elimination of more than 2000t of temporary support strutting for the cut and cover construction within an active landslip.

The Mansion House in the City of London is a sensitive structure, both structurally and politically. In 1989, the integrity of the building was threatened by tunnelling works from the extension of the London Docklands Light Railway. The aim was to contain the risk of damage at an acceptably low level and so avoid unnecessary major preventive measures such as a £13M underpinning scheme.

To simplify the approach in a complex environment, it was decided to adopt angular distortion of the building as the key critical observation. Tunnelling levels were set at values acceptable to all parties. This was the first example of the traffic light system being applied with alert and trigger levels. The boundaries between green and amber were set for an angular distortion relating to a negligible risk of damage and between amber and red a very slight risk of damage. This simple arrangement was then placed within a framework for overall risk management relating the present condition of the building to condition surveys and the full reporting system to the city engineer. The observational method was applied comprehensively through the sequential phases of tunnelling. The approval to proceed on each successive phase was dependent on demonstrating acceptable safety in the previous phase. In this way, the progressive modification approach was able to successfully demonstrate that no major expensive preventive measures were necessary to complete the tunnelling.

In 1991, the Limehouse Link road tunnel was a major project in crisis. It was well over budget and behind programme.

Introducing a value engineering clause to the contract conditions created substantial opportunities for cost and time savings. It also encouraged the confrontational environment to change, improving communications and teamwork. A range of value engineering changes were introduced but the greatest cost and time savings derived from the observational method. It was applied on a progressive modification basis to eliminate the temporary propping. In essence it was similar to the Channel Tunnel cut and cover but included some innovations such as 'soft'struts. Success was substantial, leading to the elimination of 5,000t of temporary steel strutting and cutting six months off the programme.

Construction of the Heathrow Express cofferdam began in mid-1995.At 60m diameter and 30m deep, it was a dramatically simple solution to re-establish the connections for the collapsed tunnels. Use of the observational method was planned from the start - so the extra benefits of symmetry and a simple repetitive sequence in construction were identified early in the design concept. This would make it easy to identify trends in ground behaviour and structural response and thus aid prediction and risk management. The high degree of success resulted from an integrated approach to design and construction through a single team. During the construction phase the design team was co-located to site. Major cost and time savings were achieved and the seven Egan targets were either met or exceeded. The key benefits from the observational method were avoidance of any contingencies, early tunnel breakthroughs and programme acceleration by 20%.

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