LONDON UNDERGROUND BOND STREET Station is located in the heart of London, on the west end of Oxford Street. The station was first opened in the 1900s for the Central Line followed by the Jubilee Line, which wasn’t constructed until the 1970s, 13m deeper than the existing Central Line.

Bond Street Station suffers from severe congestion and currently serves 155,000 passengers per day. This is predicted to rise by 30 per cent by 2018 once Bond Street Crossrail Station opens. The main aim of the project is to provide congestion relieve and provide step free access. The major work for the project started in 2011 and is due to complete in early 2017.

Atkins was instrumental in the development of the concept design having developed the RIBA C design for London Underground in 2007. This design underpins the inclusion of the Bond Street works in the Crossrail Act and among other things settled on the election for Sprayed Concrete Lining for the majority of works carried out in BSSU.

Atkins was supported by Dr Sauer & Partners (DSP) as SCL specialist sub- consultant and subsequently delivered the RIBA D pre-final-design to London Underground in 2008/09. Among other things, these works demonstrated the feasibility of some of the more challenging tunnelling works, which include tunnelling under the Central Line’s platform and sensitive escalators. When Costain Laing O’Rourke (Color) won the NEC Option C Contract Design & Build, they selected Halcrow and Atkins Joint Venture (HAT JV) as designer with DSP as SCL specialist sub-consultant and this joint venture drew on Atkins’ knowledge of the Bond Street works together with Halcrow and DSP’s delivery of the Tottenham Court Road Station upgrade design. The HAT JV delivered a fully integrated multi discipline design that included tunnelling ground improvement, ground movement assessment, structure engineering, architecture, mechanical and electrical and fire engineering. In addition to the SCL works, the temporary works and square works designers are Alan Auld Group and Donaldson Associates.

DESCRIPTION OF PROJECT

Prior to the construction of the two shafts, two new basement levels were constructed including opening of the shafts. A six-storey frame for the cver site development building was built to provide working space for the project. The top three floors provided an engineers’ office, workshops and welfare facilities, which are above two 17.5 tonne gantry cranes in the first three storeys.

In Figure 1, the dark green shows existing structures, which are the Central Line, Jubilee Line and an existing Post Office Tunnel. The orange shows the existing ticket hall. BSSU will provide congestion relieve by providing a new entrance to the station, a new satellite ticket hall, additional escalators, lifts and passageways. The project will also improve fire safety in the station and a connection to the Bond Street Crossrail Station.

The tunnelling work was split into three areas, Northern Tunnels, Southern Tunnels and Crossrail Link Passage. Northern Tunnels refers to the northern end of the Jubilee Line, which includes a new inclined escalator barrel, crossing under the post office tunnel, the Jubilee Line Overbridge and the new binocular tunnels.

Southern tunnels refers to the Central Line and the southern end of the Jubilee Line, which consist of low level passage, a new lift shaft and a new stairway to the Jubilee Line (Figure 2). The new Crossrail Link Passage refers to a series of connecting tunnels linking the Southern Tunnels to the existing station passageway and Crossrail station, which is currently in construction in the south of Oxford Street.

GEOLOGY

The geology at Bond Street station is a typical London geology. A majority of the tunnels is in London Clay with the deepest part a SCL tunnel 30m below street level in Upper Mottled Clay of Lambeth Group.

KEY CHALLENGES

There are many challenges on BSSU. The project includes approximately 400m of very complex tunnel geometry and alignment due to the limit of deviation and existing structures. There are approximately 25 different cross sections with transitions between each of the cross sections. The longest straight part of the tunnels was 4/209 and the pilot tunnel for Connection Chamber 1, which was 45m long. During the construction of these tunnels, both underground lines have remained operational.

There were two tight corners that were a challenge to construct. There was also potential to find sand channels in the lower part of the tunnels.

Due to the existing building foundations, there was only 4m distance between the crown of the tunnel and the bottom of the foundation pile.

Compensation grouting was implemented for ground settlement mitigation. Compensation grouting pipes were installed to be able to react if more settlement than expected was to occur.

All tunnel works were carried out on a limited site in the footprint of a demolished bank building on Oxford Street. The site includes three silos, two access shafts and other equipment. DESIGN

The monitoring system for the project was installed in 2010. The design of the SCL also started in 2010 with the review from the outline design of the tender document. The design team carried out value engineering and came up with safety improvements and then focused on critical design issues.

After this stage, the design team carried out the conceptual design, which included the initial sequencing assumptions, lining thickness calculation, 2D Finite Element Analysis and internal space-proofing validation.

This was then followed by detailed design that included 3D Finite Element Analysis and 3D CAD modelling, staged constructability co-ordination, coordination with the compensation grouting designer, primary lining and secondary lining steel fibre reinforced sprayed concrete and sprayed waterproofing membrane.

SCL DESIGN CONCEPT

The concept is that the primary lining has to support the short-term load and the secondary lining to support the long-term load, assuming that the primary lining will deteriorates in time. The secondary lining is fully tanked with a sprayed waterproofing membrane design. The design loads include full water pressure and ground load and additional surcharge load.

The Finite Element specialist divided the project into three models to accommodate the large size of the station and optimise the performance of the analysis because it was such a big model (Figure 3).

Construction of tunnelling started in mid-2013 with shaft one excavation and SCL tunnelling started in January 2014. The most important process during SCL construction is the Required Excavation and Support (RES) process, which includes RES meeting; review of tunnel ground conditions; tunnel, surface and surface structure monitoring; SCL material performance and agree plan for tunnel advance, for which a RES Sheet is issued with details of advance and support requirement. Shift managers carried out the RES briefing to shift engineers and workforce including miners, pitboss and occasionally to London Underground inspectors and SCL engineers.

The Jubilee Line Overbridge is a hybrid SCL and square work construction. The SCL top heading resting on the square work bench. The trains for the Jubilee Line were in operation during construction of the overbridge. During construction, it was decided to complete the secondary lining for the top heading with a cast in-situ lining.

The post office underpass was also a challenge because of the complex geometry and transition. It wasn’t perpendicular so it was a challenge for the surveying team to input the setting out information correctly.

At the end of the Jubilee Line are the binocular tunnels. The construction sequence was chosen to minimise the deformation of the Jubilee Line tunnels. The right-hand side was excavated first followed by primary lining and secondary lining, then excavation of left-hand side followed by breakout of the side wall.

The interesting part was to the connection of the waterproofing and secondary lining. The complex sequen The construction of the Lift 3 shaft was redesigned to improve construction sequence. The original design was to build Lift 3 Shaft from Connection Chamber 2 to connect to 6/210. This didn’t work for the programme and the construction sequence was redesign by one of CoLOR’s engineer to start construction of part of Lift 3 shaft from the Southern tunnels and then finished the shaft after completion of the construction of Connection Chamber 2.

Another redesign is on top of Lift 2 shaft. The original square work design was changed to a SCL design between two live Central Line tunnels. There was very limited space with access issue. Monitoring for all existing cast iron segment of the platform tunnel adjacent showed results well within the calculation and expected range.

Challenges relating to the ground conditions included one water bearing lens that was breached and created problems with water ingress in Lift 3 shaft. In the end, the team used a pocket excavation method, with immediate SCL application using sheet pile support, straw and sand bags to stop the water inflow.

ONE TEAM APPROACH

TFL purchased one building for the BSSU site, which was a bank built in the 1960s. The building was bought with the power of the Crossrail Act. Demolition had to be carried out extremely carefully, as the building’s neighbours included the Tanzanian High Commission. The Crossrail Act did not apply to the Tanzanian High Commissions.

The project worked collaboratively from the beginning. Responsibility for delivery and cost remains with contractor and the LU team to observe and accept.

As times went on, the commercial differences developed and the relationship between the contractor and client became strained. At a later stage in 2013, it was decided by senior management of the client and contractor that a new approach was required and the ‘one team’ was born. The one team approach allowed the construction team to focus on delivery and facilitated workshops for agreeing on completion dates. It promoted a more collaborative way of working and a combined team to jointly responsible for delivery.

This approach worked well especially for productivity and the information flow improved dramatically together with a joint approach for problems solving.

CONSTRUCTION CONSTRAINTS

Constraints included:

– Congestive site

– Tunnelling under the Central Line

– Tunnelling over the Jubilee Line

– Tunnelling with six Victorian sewers in the area:

¦ Kings Scholars’ Pond sewer

¦ Westminster sewer

¦ Mid-level sewer

¦ Stratford Place Sewer

¦ Tyburn sewer

¦ Marylebone Lane sewer

As well as the sewers, there were also 24-inch and 36-inch water mains that were realigned. There were also gas main, fibre optics, and other LU assets.

A pedestrian is 36 times more likely to get knocked down by a car on Oxford Street so the project team, including miners, must only cross with the green man.

Other stakeholders included the Botswana High Commission, above the binocular tunnels as well as a private members club and a Kabbalah centre.

The project also introduced behavioural based safety technique.

Collaboration is a journey. To achieve the best safety culture, the best quality culture, it takes time and requires regular engagement, positive reinforcement, inclusion of all parties and most importantly regular checks that it is on the right path. ‘What changed after the one team plan was launched?’ was asked to the team members and the answers by the team were as follows:

¦ Joint ownership

¦ Improved efficiency

¦ Increased familiarity and interaction

¦ More trust

¦ Clearer communications

¦ Sharing of knowledge and experience

¦ Improved working environment