Building over rail infrastructure and terminus stations is helping to relieve the pressure of increasing city density in London.
The idea of overbuilding has gained momentum in recent years. In London, where land values are high and there is pressure to increase city density, a number of significant projects are in progress or in the pipeline of planning and development.
WSP | Parsons Brinckerhoff has particularly strong capabilities, a track record, and most importantly – the imagination to create value from this somewhat specialist area of development.
London has a history of developing over rail infrastructure and typically at rail terminus stations. Good examples from the 1980s include Liverpool Street and Charing Cross. In both instances, major commercial offices were constructed over the challenging rail environment to create significant value concentrated on a transport hub. These projects were technically complex but also very successful. Other examples were constructed and many opportunities examined, but the business case often dominated the scene with many schemes failing. The usual failure points comprise:
- Decking costs being too high (span width, special foundations, etc.);
- Rail restrictions creating risk and uncertainty; and
- Extended programme periods associated with approvals and possessions.
More recently, and especially since 2012 in London, the economic climate has improved significantly. At the top of the residential market, London is regarded as a leading investment location, attracting large quantities of overseas as well as London-based funding. The overall profile and prosperity of London is also causing population growth which in turn is creating housing demand in excess of 40,000 homes per year for the next ten years.
These factors have created the conditions where the task of rafting over rail assets is increasingly seen as viable and attractive. Viable in the sense that the deck costs are in better proportion to the overall development value, and attractive because the commercial benefits to the rail asset owners can be reinvested in the infrastructure the city needs.
WSP | Parsons Brinckerhoff is now seen as a leading thinker and promoter of rail overbuild and is involved with almost 20 different sites. The examples below highlight the issues and technical challenges associated with this expanding workload.
Network Rail “Red Box”
In 2012, Network Rail appointed WSP | Parsons Brinckerhoff to undertake a generic feasibility study on decking over track. The brief was to consider twin and four track scenarios at grade and also in a cutting situation (see Figure 1). We were asked to challenge standards and innovate throughout. The client encouraged the team to examine a very wide range of deck options including arches, prefabricated trusses, semi-mechanical swinging arrangements, and erection methodologies. The study also considered different overbuild scenarios in terms of building type and height. Other factors including fire, acoustics, vibration, safety, possessions, and systems impacts were also considered (see Figure 2). The output of this study led to some key conclusions:
- The enclosure box should be as small as possible with walls constructed inside the 'impact zone'.
- Side walls of solid concrete are preferred to individual columns.
- Deck (horizontal structure) should be concrete to help address fire, robustness, and maintenance objectives.
- The deck and support structure strength should be designed only to respond to the overbuild masterplan and not to a 'design standard'.
- Vibration control should be addressed outside the rail enclosure and not as part of the base rail works.
Royal Mint Gardens, London
The Royal Mint Gardens development (see Figure 3) consists of a 14-storey high mixed-use development. WSP | Parsons Brinckerhoff carried out concept and schematic design services as well as Category 3 checking of enabling and preparation works. The development is constructed over:
- The main lines into Fenchurch Street Station;
- High level Docklands Light Railway line to Tower Gateway Station; and
- Low level Docklands Light Railway line to Bank Station.
The rail infrastructure prior to redevelopment is shown in Figure 4 with Docklands Light Railway (DLR) in the foreground centre.
Complex transfer structures, rail encapsulation, and isolated foundation solutions are proposed to overcome acoustic and vibration issues. Construction logistics over and adjacent to the railway are being carefully considered in order to adhere to Docklands Light Railway and Network Rail requirements and standards.
The project is on site and the construction value exceeds £250 million
Principal Place, London
Principal Place is situated on Shoreditch High Street, in close proximity to Liverpool Street Station in Central London (see Figure 5).
Our client, a joint venture comprising Brookfield Property Partners LP, Concord Pacific and W1 Developments, implemented a scheme to develop land adjacent to an open section of the railway, approximately 200 metres north of Liverpool Street Station. WSP | Parsons Brinckerhoff’s role is civil and structural engineering designer plus the Network Rail interface management. The scope of the first phase of this project was to cover the open ‘cutting’ with a steel and concrete deck. This enables pedestrian access to future phases of the development. The steelwork comprising the infill deck is shown in Figure 6.
The majority of these challenging works required night and weekend possessions and closures coordinated with changes to Network Rail’s infrastructure. Consequently, precise programming and planning were crucial to the efficient and timely execution of the works.
The project scope included:
- Demolition of disused buildings;
- Geotechnical surveys of existing ground conditions;
- Pile cap construction;
- Steel beam installation (using a 1000-foot mobile crane, one of only two in the UK);
- Steel deck installation; and
- Concrete deck construction, waterproofing, and associated drainage.
We applied our knowledge to the constraints of construction in an operational railway environment. This resulted in efficient production of Network Rail compliant and validated designs which led to prompt receipt of approvals from Network Rail.
We also provided high quality reports in a standardised format to assist our client in understanding the difficulties of the project and recommending further actions.
Chelsea Football Club New Stadium, London
For the proposed new Chelsea Football Club stadium in London (see Figure 7), the intention is that the club will increase the spectator capacity and remain in the same location. To achieve this, two significant areas of rail decking are required. To the east the deck covers the West London Line which is a twin track arrangement in a shallow cutting. To the North West is the London Underground (Tube) District Line. Again, this is twin track in a slightly deeper cutting with a station at one end and a tunnel portal at the other. In all, around 500 linear metres of decking are needed. WSP | Parsons Brinckerhoff is providing a combination of civil, structural, geotechnical, and rail systems engineering to achieve this.
Our design strategy for the rail enclosure envisages the following ingredients:
- Piled foundations on both sides of the rail corridor to carry load from the deck and walls.
- Solid concrete walls on both sides designed for lateral impact loads as well as high level ventilation and emergency escape routes. The walls are 500 millimetres thick.
- Deck structure in precast or reinforced in situ concrete. In some locations there are also deep transfer beams to support columns from the east stand of the stadium. This type of scenario extends over around 100 metres. The deck will provide a clearance over the rails of around 5.5 metres. This is just under the normal minimum of 5.8 metres on the basis of recently approved decking over a site to the northwest of Earl’s Court Station and a low bridge to the south. The cross section in Figure 8 shows how the rail corridor is protected and passes under critical load bearing elements of the stadium.
The strategy is to enclose the rail corridor in the smallest space that can be provided on the basis of minimising wall heights and span widths. This enclosure or 'box' has the effect of minimising space compromised by the rail infrastructure whilst maximising the zone outside the box in a more normal construction environment.
The projects outlined in this paper demonstrate a wealth of experience and knowledge in the dual challenges of railways and overbuilding.
Projects range from small areas of infill to very significant enabling works to create decking for redevelopment and regeneration.
Our proven knowledge and understanding of London Underground and Network Rail requirements and procedures gives confidence to the rail authorities and allows mutually beneficial design strategies to be developed.
This type of work is complex and challenging, usually resulting in extended programmes and higher overall development costs. WSP | Parsons Brinckerhoff can advise on engineering solutions to mitigate these based on current activity in the UK and especially in London.