Implementation of the OM
Extending the Process of Progressive Modification
The implementation of the method for the Limehouse Link broadly followed the process summarised by Peck (1969). Design uncertainties, combined with a lack of case history data, needed careful assessment. This particularly concerned the perceived level of risk - both to the safety of the tunnel construction and to adjacent structures. It was therefore considered inappropriate to start on site with a design directly based on an assessment of the most probable conditions - that is, a design which would have marginal factors of safety at best in the event of significantly adverse variations from the assumed most probable conditions. Instead, as described later, starting from the original design, changes were introduced sequentially based on observations of actual performance. The OM was thus implemented through a process of progressive modification. The background to the development of applications of the OM through this process is discussed in Chapter 1.
The potential modes of soil-structure interaction, relating to each method and sequence of construction and the varying ground conditions, were carefully assessed. Conceptual evaluation was supplemented by simple plane- strain analysis to assess likely factors of safety under actual conditions.
Parametric studies were undertaken to provide an indicative range of wall deflections. However, no undue reliance was placed on this pre-construction analysis. The validity and safety of the assumptions were tested during construction in a controlled step-by-step sequence. This was essential for the ongoing assessment of not only the structural implications for the tunnel and its support works but also any associated risks deriving from induced ground movements on adjacent structures. Monitoring and back-analysis were coordinated with construction progress.
Sequence for Each Implementation of the OM
(a) Examination and logging of the actual ground conditions and properties during top-down excavation, including that for the diaphragm walls and central piles, were undertaken.
- (b) Based on this information, and using the original site investigation data, the most probable conditions and the most unfavourable deviations from them were then assessed. The focus was on the likely short-term performance from the soil/structurc interaction. This centred on the probable generation of much lower earth pressures on the active side combined with fully mobilised passive support resulting from the unloading of the cohesive soil (Figure 4.5). The concept was analogous to that developed for the Channel Tunnel cut and cover tunnels (Chapter 2). It was supported by careful field observations and a judgement on what level of conservatism was required to justify the implementation of the OM. However, as noted, the guiding approach was to achieve this within a comfortable margin of safety. In other words, to avoid dependence on potentially optimistic criteria based on predictions of the most probable conditions. Such predictions tend to vary quite widely and are likely to need ongoing revision from the observations of actual performance.
- (c) Proceeding on this basis, using site-specific data at given chainages, mixed effective and total stress analyses were undertaken. A reduction factor of 0.7 on the passive pressures was applied but no allowance was made for softening in the upper layers or for over-excavation. The resulting factors of safety on bending moments in the diaphragm walls were in the range of 1.3-1.5. These are rather lower than would normally be adopted. It was noted that Padfield and Mair (1984) recommended a factor of safety of 2 for total stress analysis and expressed caution on
Figure 4.5 Top-down construction: basis for earth pressures assumed for OM design.
the implications of mixed analysis when used outside direct experience with specific clays. Thus, the designs developed for the OM were significantly less conservative than would result using the criteria of Padfield and Mair for moderately conservative parameters. However, it was considered that the assumptions for the OM design would still be significantly more conservative than those based on the then current estimate for the most probable conditions. As subsequently demonstrated by measurements of actual wall movements, this consistently proved to be the case despite the wide range of soil conditions, spatial arrangements and excavation depths. It is pertinent to note here that restricting the term ‘most probable’ to apply only to soil properties risks being over- prescriptive. The actual conditions and nature of the soil/structure interaction will inevitably depend on more than just the soil properties. A further benefit of implementing the OM through progressive modification is that it is not essential to rely on developing designs for commencing the OM closely based on estimates of the most probable conditions. It also helps to avoid controversy since there is always likely to be a range of predictions across the parties involved as to what constitutes the most probable conditions. These issues are discussed further in Chapters 1 and 14. For Limehouse it was necessary in the back-analysis to increase c' from 0 to 20 kN/m 2 and to reduce K0 to unity to achieve a reasonable correlation with the measured wall movements. This was based on a simple plane strain analysis which, conservatively, took no account of three-dimensional effects. While the precise values of propping forces developed within the roof slab were not known, this basic analysis indicated actual factors of safety in excess of 3.
- (d) However, the OM designs were not implemented immediately. That would have effectively sanctioned proceeding directly with no midheight props. At this stage, given the contractual environment, that would have been pre-emptively too far to satisfy all stakeholders. Instead, the OM was initiated at each working front with the props of the original design in place but with the gap creating a ‘soft prop’ as noted in Section 4.3.2. In every case, the trial was successfully concluded with the gap between the props and the diaphragm walls all remaining open.
- (e) The next step was the selection of the quantities to be measured, at which locations and at what frequencies. These observations included ground movements and changes in porewater pressure, but the key focus for risk control was the rate and magnitude of wall movement. Predicted values for wall movements were based on the estimates for conditions that were significantly less onerous than those adopted for the original design.
- (f) The implications for the most unfavourable conditions were also assessed. This included the worst-case scenario which would have been a critical inadequacy of passive support to the diaphragm walls without the props in place, thus leading to unacceptable wall and ground movements. To assess and contain such a risk, each new working front was commenced by limiting the exposure to the walls within a 5 m bay length. Adverse deflection trends could be tracked and actions taken to arrest them before any unacceptable conditions were reached.