This paper will outline one key aspect of the Bank

Station Capacity Upgrade (BSCU) project. The pile load transfer structures beneath 6-8 Princes Street are some of the first documented instances where tunnels have been constructed through live end-bearing piles. My role on the project was to develop the empirical design for the transfer structures and also to oversee the construction phase.

1.1. Bank Station Capacity Upgrade

Frequently described as a rabbit warren beneath the city, the Bank–Monument complex has presented many challenges for tunnellers for over a century. Bank Station Capacity Upgrade in the heart of the City of London, is a £550m (US$692m) project that aims to match projected ridership, improve journey experience within the station and provide step-free access to the Northern Line and Docklands Light Railway (DLR). The current upgrade seeks to thread even more tunnels into the complex layers of existing assets, which will be constructed above, below, and even through many of the existing operational assets.

The BSCU client is London Underground (LU), with Dragados acting as both principal contractor and principal designer. I joined the project in January 2015, at the start of the detailed design, working on the sprayed concrete lined (SCL) tunnels delivered by Dr Sauer & Partners (DSP).

BSCU includes the construction of a new Northern Line southbound running tunnel (NLRT) allowing the existing platform to be used as a concourse tunnel and alleviating congestion at Northern Line level. Due to the concentration of structures in the area, the new running tunnel will intercept the piles of a number of buildings along the route. At the northern end of the BSCU scheme, the NL RT passes under 6-8 Princes Street, which is founded on large diameter under-reamed piles (see diagram Figure 1).

During the development of the Transport and Works Act Order (TWAO), agreements were made with building owners to mitigate the impact of the tunnelling works on their structures. For many, detailed instrumentation and monitoring was installed based on building damage assessments (BDAs).

BDA reviews are a staged process, which eliminate structures at different levels, ensuring the appropriate level of detail is carried out – with more ‘at risk’ structures undergoing more rigorous assessment. No 6-8 Princes Street is owned by the Worshipful Company of Grocers, and removing a part of the foundation would be heavily impacted by the project. As a result, the most detailed level of assessment was undertaken: specifically, detailed modelling of the tunnelling impact and the building response.

Under the TWAO, London Underground (LU) and the building owner of 6-8 Princes Street (see Figure 2) had an agreement that the building foundation capacity would not just be retained, but future development capacity would also be met. To do this, load transfers structure have been provided, replacing sections of the end-bearing piles for four intercepted piles: numbers 4, 14, 24 and 25.


The construction of the transfer structures was particularly complex, as the design relied heavily on adhering to a strict construction sequence. It was essential that throughout the project development, there was a strong marriage between the principles of design and the construction methodology. London Underground, Dragados and Dr Sauer & Partners worked closely together to ensure that all requirements were met. The Worshipful Company of Grocers was also closely involved, even taking the time to visit site to witness the construction.

The team anticipated the complete removal of under-ream and shaft of four piles and the possible clash and partial removal of three additional piles, based on the locations shown in Figure 3.

During design it was concluded that three transfer structures should be provided. These were made up of:

  • An SCL primary lining, to provide the access for the transfer structure construction;
  • A concrete arch load-transfer structure;
  • And a final tunnel lining, which was structurally separate. The lengths of these were varied to account for the differing pile loads and to avoid clashing with additional piles. This solution balanced consistency and sustainability in the design and streamlined the variations during construction.

2.1. Concept design proposal

The first pile interception proposal was included in the concept design. This solution involved handworks tunnelling above the running tunnel to create an access gallery to provide a jacking system at the base of the piles to transfer the loads before the piles were cut. At the start of detailed design, alternative solutions were explored, including some on the basis that the piles could be cut without detriment to the structure in its current condition. The main advantages of the proposed solution were as follows:

  • No handworks – resulting in a safer construction methodology;
  • Structural separation between the tunnel and the building foundation – a requirement under LU Standards;
  • No maintenance requirements of a jacking system – resulting in a more sustainable design.

3.1. Pile locations

The nature of pile construction resulted in a degree of uncertainty as to where the piles would be intercepted by the running tunnel. To carry out the design, assumptions were made regarding the pile locations and related tolerances. A 525mm deviation in any direction from the as-built location was allowed for (see Figure 3). When these tolerances were applied, it could be seen clearly which of the piles would be intercepted by the NLRT alignment.

3.2. Design loads

The design load input was taken from the TWAO agreement with the building owner. This included the current loads in the building and proposed future loads for development of the site.

The short-term loads considered are calculation of current pile loads. The as-built loads were based on 1970s building standards, not in line with current guidance, so assessment was done to convert these loads to current standards. I used this value, rather than the future loads, to avoid being too conservative and allow a more sustainable design.

The long-term loads are those included in the legal agreement, which account for future building development at 6-8 Princes Street.


The construction was carried out in stages. This reflected the sequence modelled as part of the Stage 3 BDA, carried out by Dr Sauer & Partners and Robert Bird Group. The assessment found that the pile load transfer structures were not required under the current building loads; they were only required to provide additional transfers for any future buildings on the site. As a result, the transfer structures were constructed as part of the normal sprayed concrete excavation sequence.

The primary lining of the structure was installed, and the piles were cut as the tunnel advanced. However, to ensure that there would be no excess damage to the building, the four fully intercepted piles were grouped into two groups of two. This resulted in a four-stage construction sequence of which Stages 1 and 2 are shown in figure 4.

In Stage 1, the excavation progressed until both Pile 23 and Pile 15 were exposed. These were two of three piles with the potential to require partial removal. This was chosen to allow the project team to employ a solution, should it be required. When they were exposed, it could be seen that the piles would require removal of the under-ream at the end of the load-bearing foundation (see figure 5). Detailed survey was carried out on the exposed piles, which resulted in the conclusion that the shaft of the piles would not be affected. As a result, a separation material (see figure 6) was installed to allow structural separation between the building and the piles but no transfer structure was required. Stage 2 was the installation of reinforcement and cast in-situ concrete for the first two transfer structures. These were built in stages, as detailed in Section 9.

Once the concrete for transfer structures 1 and 2 reached 28-day strength, the construction proceeded with Stage 3; this required the excavation of the running tunnel to within 500mm of the existing operational Northern Line Southbound running tunnel, as can be seen in figure 7.

Stage 4 was the construction of the final pile load transfer structure, which was provided for both piles 24 and 25. This was due to the proximity of their locations and the required bearing capacity to support the loads.

5.1. Initial geometry

The preferred initial detailed design solution proposed a ring-shape structure installed at the base of the piles, transferring the pile load around the tunnel and into the ground below. This is shown in figure 8.

During design, confirmation of the original toe level of the piles was received. Using this, an assessment was carried out to establish the zone of influence around the end bearing piles. Using a 45° angle around the base of the piles, it was concluded the ring shape would undermine the stability of other piles within the foundation. As a result, a geometry was developed based on a horseshoe shape that resolved this problem by raising the invert.

During construction, the toe level of each of the piles was recorded to validate the design assumptions.

5.2. Detailed design basis

It is well known that a tunnel arch is insufficient support in soft ground and full ring closure is required. This is why a closed form SCL tunnel shape was provided around the outside of the concrete transfer structure arch. This shape transferred the load to the ground (refer to figure 10).

The SCL excavation geometry considered the restrictions of sprayed concrete lining design.

To ensure stability, I checked the long- and short-term and bearing capacity of the London Clay. This dictated the length of the transfer structures and the initial assumption of width of the concrete footing. The short-term load case employs a formula (see equation 1) developed by Brinch Hansen2 for foundations, considering ?=0.

Below, equation 1:

Short-term bearing capacity equation from Brinch Hansen:

As the footing is assumed to sit on the surface of the clay, the depth factor d?c and the ground factor g?c are assumed to be 0.

The base of the transfer structure is assumed to be horizontal with the provision of ‘elephant’s feet’. This gives a base factor, b?c of 0.

As the load is assumed to act at the neutral axis, there is no eccentricity or inclined loading and hence, the inclination factor, i?c is 0.

For the long-term load 1 case I used an adjusted version of the short-term formula (see equation 2), considering the behaviour of a pile.

Where Ap is the area of the base of the pile. For conservatism, I assumed the area of the base of the pile to be equal to 1m2.

Below, equation 2:

Long-term bearing capacity equation from Brinch Hansen

Additionally, structural analysis calculations were carried out to ensure that the depth of the concrete arch would sufficiently resist the concentrated loads from the piles. Finally, finite element analysis was carried out to ensure that the solution was effective in 3D and that all stresses and deformation had been allowed for.

Two mirrored cross-sectional geometries were used; this reflected the fact that for piles 4 and 14 the load would be concentrated on the right-hand side and for piles 24 and 25 on the left-hand side.

6.1. Sprayed concrete lining

The excavation was carried out using a stepped SCL sequence with a top heading and invert excavation split (see figure 11). Once complete, the excavation was surveyed to ensure that the profile was correct.

The exposed ground was sealed using an SCL primary lining. Two mirrored cross-sectional geometries were used, reflecting the fact that for piles 4 and 14, the load would be concentrated on the RHS and for piles 24 and 25 on the LHS. The main excavation challenge was the complex angles of the transitions, which had been dictated by the location of the other piles in foundation. Due to the restricted space for excavation around the pile, a variety of construction plant was used.

6.2. Monitoring

An extensive monitoring system was established to observe the movements during construction. This was to ensure that the piles, the surrounding ground, and the building above were behaving in line with predictions from the BDA. To generate the monitoring predictions, finite element model outputs were calibrated against empirical values and movements from the earlier stages of the BSCU project.

In particular, monitoring targets were installed onto the pile above and below the cut line so that the movements could be compared before and after the piles were cut (see figure 12).

7.1. Design considerations

The cut level for the pile separation was dictated by the detailed design to ensure load transfer from the pile to the concrete structure occurred in the centre of the concrete arch. As a result, precision was required when selecting a separation method. Coring was selected as the pile separation method. This gave the required control for pile cut location. Additionally, coring minimised the impact to the building above through vibration transfer and damage to the integrity of the pile.

Once the pile and the building were structurally separate, piles were broken out using a mechanical breaker with no vibration transfer to the building above. Any exposed ground was sealed with SCL.

7.2. Coring the piles

The primary focus when planning for the coring of the piles was safety. During the design when assessing the CDM risks, I highlighted that safe access to the cut location was paramount. As a result, a scaffold tower was installed for each pile as shown in Figure 15.

The tower was selected as it could give access to the whole pile shaft at once and would not require adjustment during the separation stage. It was not only for coring but also for the installation of monitoring onto the pile and to allow close inspection of the pile cut.

Separation of the pile was carried out once the majority of the pile structure was exposed, eliminating the access issues that could result from excavating behind the pile.

7.3. Pile 25

When exposed, it could be seen that pile 25 was located very predominantly to the left-hand side of the cross section (see figure 13). As a result, the decision was made to cut this pile in a stepped arrangement. This ensured the load transfer mechanism would match that assumed in the design.

8.1. Reinforcement design

The pile interception load transfer structure was designed as a concrete arch with a point-load acting at the neutral axis as shown in figure 14. The formulas used were taken from Roarke1.

The moment was calculated within the structure at x, a point on the arch (see figure 15). The formula for this varies before and after the point at which the point load, P, is applied. In all pile cases, I found that the maximum moment occurs at the location of the point load, LP . Additionally, shear calculations were carried out to check that the pile would not punch through the concrete beam.

Once calculated, I used the maximum moment to provide initial detail for the reinforcement in accordance with Eurocode 2. This was later supported by detailed finite element modelling carried out by other members of the Dr Sauer & Partners Team.

The modelling showed some additional stress concentrations within the structure as a result of ground loading, for which additional reinforcement was provided. The final reinforcement design configuration can be seen in figure 16.

8.2. Installation of reinforcement

The reinforcement directly under the piles was designed to create a steel cradle beneath the pile (this can be seen in figure 17). This transferred the pile load into the surrounding concrete structure, reducing the concentration of load and ultimately into the ground below (see figure 10).

As this was a fundamental mechanism for the load transfer, the design and construction teams worked closely to ensure that every bar was positioned correctly.

During the construction stage, minor adjustments were made to the reinforcement to facilitate the construction of this key element. Additional bars were added allowing easier continuation between stages.

9.1. Design principles

The cast in-situ concrete structures within the SCL lining were layered to serve differing purposes (see figure 18). The pile load transfer structure (shown in a cream colour) was positioned outside of the main running tunnel structure (shown in cream). This allowed independent behaviour of the building foundation and the LU asset. The transfer structure arch was designed for the pile loads, and the tunnel lining designed to take all ground and hydrostatic loads. The structural separation material (purple) and the waterproofing substrate (blue) were installed between the two layers. A common shared invert was installed transferring loads into the ground.

9.2. Construction

The concrete transfer structure arches were poured in stages. This simplified the installation of the reinforcement and shuttering. Bespoke shuttering was installed for each pour. To simplify the shutter design and the installation of the separation material, the internal faces of the transfer structure arch were flat.

The inverts were cast first, giving a solid working base. This was followed by the walls and finally the crown. Each stage was closely monitored by the design and construction teams. The 28-day strength of one stage was required ahead of striking the shutter for the next stage. To simplify the logistics on site and to meet the design requirements, the same mix used for the SCL was used in the transfer structure concrete arches. Pouring the crown last, as well as making logical sense, maximised the amount of monitoring data that could be collected from the base of the exposed piles.


The completion of the construction of the pile load transfer structures at 6-8 Princes Street was carried out with no visible damage to the building structure. The tenants at 6-8 Princes Street were able to remain in the building throughout the construction. Through collaboration between London Underground, Dragados, Dr Sauer & Partners, Robert Bird Group, and through frequent and open communication with the Worshipful Company of Grocers, a solution was found that will ensure the future-proofing for both Bank Station and the site at 6-8 Princes Street.


I would like to thank the entire design team at Dr Sauer & Partners, in particular Ali Nasekhian, for continued guidance and support. Also, all parties involved in the detailed design at BSCU, especially London Underground, Dragados, and the Worshipful Company of Grocers.