Monitoring of ground movements is playing a key role in delivery of Crossrail, but at Plumstead it is also enabling work to be carried out close to a busy railway line. GE reports
Constructing a deep foundation box, especially one equivalent at its deepest point to a fivelevel basement, is never a task to be undertaken lightly in an urban setting. But what made construction of the foundation box for Crossrail’s Thames tunnel portal at Plumstead in south-east London super critical is the fact that it is situated immediately alongside Network Rail’s North Kent Line. It was clear from the outset that monitoring was going to be a key part of ensuring the work went smoothly.
Crossrail’s Thames tunnel - the only one being bored through Chalk - between North Woolwich and Plumstead is being constructed as contract C310 by a joint venture of Hochtief and Murphy (HMJV). But before the main tunnelling work could get underway, work on the southern portal at Plumstead, from which the tunnel boring machines (TBMs) will be launched, needed to be completed by contractor Cementation Skanska.
Piling at Plumstead was just 3m from an active railway at its closest point. With up to 16 train movements an hour, the project team were required to apply for a derogation from Network Rail - essentially an exemption from its standard rules with respect to working close to a live railway. That of course meant proving to an exceptional level that every technical and logistical risk was fully considered, assessed and mitigated.
The Plumstead portal - a 320m long box structure - is being used as the launch chamber for the Thames tunnel TBMs and will take the rail line from ground level to 15m below ground where it will enter the tunnel. The box has been constructed using a variety of techniques but essentially diaphragm walls have been used where the excavation depth exceeds 7.5m, and cased continuous flight auger (CFA) secant piles have been adopted at
the eastern end, where the retained height shallows as the track level rises to the surface.
Additionally, rotary bored tension piles resist uplift forces on the base slab.
The work at Plumstead has been acknowledged as a new benchmark for the installation of large-scale ground engineering in proximity to live railway infrastructure
Ground conditions at the Plumstead end generally comprise 4m of made ground overlying approximately 2.5m of alluvium, then 8m to 9m of sand and gravel river terrace deposits, overlying 3m of dense Thanet Sand with the Chalk - through which the twin tunnels are being bored - coming in at around 14m depth.
Solutions for the portal construction method were derived through detailed early discussions between the stakeholders, where Cementation and HMJV had to demonstrate competence and agree the additional measures, explains Cementation project manager Patrick Savage.
As a result, lift plans, method statements and inspection and test plans were all reviewed and approved by Network Rail. HMJV installed an asset protection barrier and, together with Cementation, developed detailed crane lift plans.
According to Cementation, the work at Plumstead has been acknowledged as a new benchmark for the installation of large scale ground engineering in close proximity to live railway infrastructure.
An essential activity was the extensive use of instrumentation and monitoring equipment installed to measure and give early warning of any ground movements occurring during the foundation installation stage. The same systems will be used to constantly check that movement during the subsequent excavation stage. A further complication was the potential effects of extensive dewatering needed throughout the dig stage.
Largely, instrumentation was geared around inclinometer tubes installed in both the diaphragm wall and secant pile walls, along with levelling surveys.
The approach to monitoring allowed for a number of independent systems. So for instance, inclinometer tubes were fitted with vertical shape arrays linked via radios to the site office, providing 24-hour, real-time monitoring. In addition, manual inclinometer data was taken periodically to cross check the automated data. In the excavation stage, movements were derived by monitoring the props. Load cells were installed in the hydraulic props and strain gauges in the steelfabricated props. Should deflections or prop loadings have exceeded the trigger levels, the contingency would have been to install more props and failing that a rapid backfilling of the critical areas.
Monitoring was also applied to a Grade II listed industrial building, which originally housed one of the UK’s first waste-to-energy plants, close to the north retaining wall. A buried incinerator base, some 7m below the site working platform, had to be removed and the backfilled area improved by compaction grouting ahead of diaphragm wall panels in this area.
Some 20 tilt meters, fitted with wireless tilt sensors, were installed on the building. The wireless connectivity enabled the monitoring system to be set up and configured without the need to run power and signal cables around the site. Data was sent remotely via a 3G link to a website allowing HMJV to monitor the building as the construction proceeded.
With the Plumstead portal complete and the TBM successfully launched in January, the project team is hoping for an equally smooth and incident-free experience across the river at North Woolwich, where Cementation is currently installing the foundation elements for the TBM reception chamber and portal structure, some 2.6km west of Plumstead portal.
In terms of its design and construction, the North Woolwich portal at the northern end of the tunnel is essentially a repeat of Plumstead and Cementation is using the same project team for the work, ensuring lessons learnt are carried through directly.
The box is slightly smaller and of course the work doesn’t have to contend with the North Kent railway line. Nevertheless, it is located in a busy mixed industrial and residential area, and the two main long-axis retaining walls are right up against public roads; one of which is the Woolwich Ferry approach road. The wall is also located very close to the Royal Docks Sewer, which required a special section of hard-hard fully cased secant pile wall to remove the risk of a pile bore collapse undermining the sewer.
The North Woolwich portal is not quite as deep. There are fewer diaphragm wall panels and the secant piles are generally 750mm rather than 900mm in diameter. However the near-surface ground conditions are worse. It is much closer to the river and the alluvium is thicker. This is partly compensated by the fact that the Thanet Sand is not present and the Chalk comes in at around 12m depth.
Because the ground level is lower than at Plumstead, the groundwater is higher relative to the working platform, which means the performance of the support fluid is critical - use of polymer fluid has been dropped, and bentonite is being used throughout for the diaphragm walls.
And the challenges are not purely technical - archaeologists believe recently discovered buried tree trunks found within the alluvium might be part of a Viking engineered riverside walkway.
Plumstead portal required 2,300t of reinforcement to be fixed on site and 22,000m3 of structural concrete, which at the peak of activity led to five rigs were used at once and a combined Cementation Skanska /HMJV workforce of up to 80.
The portal structure is formed from 121 diaphragm wall panels with six headwall panels reinforced with carbon-fibre and a further eight “soft” panels adjacent to the head wall to create a seal around the TBM to facilitate its launch.
The 730 secant piles for the shallower part of the structure were installed using Bauer’s DKS cased continuous flight auger system. With pile lengths of up to 18.5m and 900mm diameter, the system was operating to its maximum capabilities. The project is, reports Cementation project manager Patrick Savage, the first time Cementation has used the system to produce this number of fully cased piles of this diameter with these cage lengths.
The other major foundation components were rotary bored tension piles, constructed using a polymer support fluid, marking another first at Plumstead, in that it is the first time it has used polymer at this scale.