reinforcement cage, through which each column was to be placed. During the construction of Panel 8.20, the verticality of the panel excavation, final position of the rebar cage and position of the internal shear links within the cage were all at the tolerance limits and the combined effect resulted in the cage clashing with the plunge column positioning. Consequently, the plunge column was installed with a verticality tolerance 1:104. Unfortunately, this column was designed Schematic of the diaphragm wall plunge column frame placing and positioning the columns. In the final working condition, each plunge column was to be encased in concrete; however, in the short term, each column was designed to carry the temporary construction loads, as the ground within the station box was excavated and the roof support structure was constructed. Therefore, installation tolerances were critical: 1:300 verticality, +/-10 mm (0.4 in) in plan and +/-5 mm (0.2 in) vertically. The installation frame needed to be sufficiently robust to (1) carry the self-weight of each column (2) be sufficiently stable and rigid to provide adequate reaction to move and position each column into final position and (3) provide a mechanism suitable to make fine adjustments to the column’s position. The successful installation of each plunge column relied on a number of elements of the panel construction being within acceptable limits of deviation, most importantly was the final position of the as the main support for a temporary bridge over the station box. Back analysis of the design demonstrated that the column now failed in the temporary condition. Due to the accuracy of the construction records, as- built details were input into the project’s Building Information Modeling (BIM) model and remedial works to strengthen the column were performed in advance of bulk excavation. This was a good example of proactive use of BIM to assist in the timely resolution of a construction issue, which minimized cost and program impacts. Conclusion This 60 m (197 ft) deep diaphragm wall is the deepest ever constructed by Cemen- tation Skanska and is one of the deepest constructed in the U.K. using hydraulic grab technology. The di f f icul t ies encountered during excavation of the panels through what is considered to be well known, well understood and typical of the London Basin, demonstrated that the relationship between ground conditions and excavation technique can never be taken to be mutually exclusive. Many of the successes resulted from the accuracy of as-built records. Although the benefit of this level of data collection is often questioned, the data collected and the, often, real-time analysis undertaken on this project highlighted that, with the proper focused approach, data can be a powerful tool. The positive impact of BIM and 3D modelling on a project’s costs and program by informing and changing construction strategies as the project works continue was clearly demonstrated on this project. The innovative and bespoke concrete discharge chutes not only mitigated hun- dreds of concrete delivery truck journeys into a cramped work area, reducing congestion and improving productivity, but set the tone that safety was important. Ultimately, the contractor delivered a high- quality diaphragm wall with zero lost time accidents, on time and on budget. Acknowledgements The authors would like to thank the Ferrovial Laing O’Rourke JV Northern Line Extension Project Team, Mott MacDonald, London Underground and many other colleagues within Cementation Skanska Ltd for their support and professionalism throughout the construction of diaphragm walls for the Battersea Station box. Paul Wiltcher is the operations director at Cementation Skanska. He has more than 20 years of experience in the European piling and foundations industry, having worked on many notable projects, including CTRL Phase 1, Crossrail, Northern Line Extension, USA Embassy (London) and the Amsterdam Noord Zuidlijn Metro. An exaggerated schematic of as-built position of the plunge column in Panel 8.20 Schematic from BIM model showing remedial solution to out of verticality plunge column in Panel 8.20 Peter Handley is the director - plant & resource at Cementation Skanska. He has more than 25 years of experience in foundation engineering, including esti- mating, design, operations and plant management. DEEP FOUNDATIONS • NOV/DEC 2018 • 95
