Achieving Agreement to Use the OM
As noted in Section 6.1, the OM was being extensively implemented in the СТА. This applied to the construction both before and after the collapse of the SCL station tunnels. Many hard lessons were quickly learnt in the detailed investigation that followed and prominent amongst these were the vital issues of effective communication and ownership. Correcting these failures was paramount. It is a real testament to the vision of the owner, BAA, that, following the collapse, a retreat into onerous conservatism was overtly eschewed and instead the enlightened development of the ‘Single Team’ approach was fostered (Powderham and Rust D’Eye, 2003). As described in the HEX cofferdam case history, this engendered a very conducive environment to introduce and implement the OM. Apart from aspects intrinsic to the tunnels, the OM was applied to the structures within their zones of influence and the associated construction including the HEX cofferdam and access shafts. Given the continued and even enhanced positive disposition by BAA towards the use of the OM, it was relatively straightforward to achieve agreement to apply it to the MSCPs.
Implementation of the OM
Traffic Lights, Trigger Levels and Contingencies
As with other applications for structures in the СТА, including the other MSCPs and the cofferdam, the OM was implemented using a traffic light system with the trigger levels defining the green, amber and red zones set on a reasonably conservative basis in relation the predicted performance. The actual performance within these parameters was measured and assessed progressively in line with the tunnelling construction. As was used for the OM at Mansion House, the primary system of instrumentation was based on strings of beam-mounted electro-levels and precise levelling. Similarly, the focus was on assessment of the angular distortion developed and the maintenance of this within an acceptable level of risk. These measurements were supplemented with condition surveys in the overall assessment. In the event of an adverse trend developing, this would be addressed in advance of reaching critical conditions by implementing contingency measures such as reducing the volume loss from tunnelling and/or installing an appropriate form of structural strengthening. At least this was the plan. Such plans are fully feasible when the generated ground movements and the response of the structure are mutually progressive. If there is a sudden or brittle event in either, then it is not generally possible to avoid the associated damage. That, quite unexpected, response proved to be the case for MSCP 1A. As described below, such sensitivity was not indicated in advance by the detailed risk assessments. In fact, of the three MSCPs in the СТА, MSCP 1A appeared to be exposed to the lowest risk. The predicted and measured values for the induced ground settlement for this building were quite close with the angular distortion being recorded at 1/1,000.
The risk assessments for Stage 1, based on Rankin (1988), were undertaken in early 1994 to identify which buildings might be adversely affected by the HEX construction at Heathrow. These involved several MSCPs, including MSCP 1, 1A and 3 (Figure 6.1).
Stage 2 assessments were then developed to provide more specific evaluations for individual structures. These assessments produced maximum predicted settlements, ground slopes and maximum tensile strains from calculations of ground movements based on expected tunnelling volume losses. They were undertaken in the same year but before the collapse of the СТА Station tunnels in October 1994 (HSE, 2000).
The Stage 2 assessments were based on the ‘greenfield’ settlement troughs. This conservatively assumes no beneficial effects from the modification of the trough from the soil/structure interaction with the affected buildings. Accordingly, the maximum ground slopes from the ‘greenfield’ settlement trough beneath the buildings were used to estimate the angular distortion. This evaluation was undertaken on a bay-by-bay basis. The maximum tensile strain was determined from the sum of the maximum bending strain in the building and the average tensile strain in the ground across the estimated width of the settlement trough. For MSCP 1A, this
Figure 6.3 MSCP IA looking north, showing five storey structure and lift towers.
Figure 6.4 СТА Station tunnels in relation to car park MSCP I A.
width was determined as 30 m. A check was also made for the combination of diagonal shear and horizontal strain. These are not directly additive but combined using a Mohr’s circle. It is important to note, in this context, that the process of summing strains is based on the idealisation of the building (or specific parts of it such as a flank wall) as a simple elastic beam (Burland and Wroth, 1974). The depth of this notional beam would generally be taken as the full height of the main structure. Consequently, such an idealisation does not directly model the individual components of a framed structure but represents a generalised risk. Thus, in such a global assessment, diagonal tensile strains, for example, would typically relate to shear walls or infill panels. Such features were absent in the zone of the car park under consideration and so did not feature in the risk assessment. However, in practice, shear and bending failures were indeed a critical issue.
The combination of angular distortion and horizontal strain from the ‘greenfield’ computations was related to levels of risk of damage as set out by Boscardin and Cording (1989). Thus, although no direct measurements of horizontal strain were made, it was taken into account in the risk assessment using this approach. This essentially followed that developed for the Mansion House as described in Chapter 3 (see Figure 3.8 in that case history).
These assessments, which, as noted above, were derived on a conservative basis, indicated that MSCP 1A had the lowest predicted risk of damage for the three car parks while MSCP 1 had the highest. For MSCP 1, the predictions produced maxima of 33 mm settlement, 1:310 ground slope and tensile strain of 0.16%. Correlation with Boscardin and Cording (1989), placed it well into the risk category of ‘moderate’ damage. The comparative values for MSCP 1A were 15 mm, 1:790 and 0.06%, respectively, placing this building in the risk category of ‘very slight’ damage. However, with the proximity of the Piccadilly Line tunnels and the potential for some differential movement that might lead to some separation of the towers, it was decided to upgrade the risk of damage for MSCP 1A to ‘slight’.