Stacked drifts as presupport Apr 2021

Large diameter caverns for the Euston terminus station in London for the HS2 highspeed rail project in the UK is the latest project planned to adopt the stacked drift method of presupport. Seven interlocking smaller diameter headings around the profile of the caverns will provide the presupport and security for excavation of the core to create the station under the existing surface rail Euston Station, which must remain operational during the full construction period of the new HS2 infrastructure (Fig 1).

Fig 1. Stacked drift plan for the Euston Station approaches and cavern for the HS2 project in London

Publication of the plan in an interview article with HS2 engineers by TunnelTalk reporter Jonathan Rowland opened the discussion of where and how the stacked drift method has been used in the past and how their applications differ from the proposed application at Euston.⁽¹⁾

One of the first well-recorded applications of the method was for the Mount Baker Ridge highway tunnel in Seattle, Washington, USA. The story goes that planning application for the short 1,500ft long (460m) underground route at shallow depth beneath the homes and urban infrastructure of the area above was being problematic. There was objection to the project in general and of the proposed multi-tube design for the four traffic lanes plus provision for a future light rail connection through the tunnel and across the Lake Washington highway bridge onto Mercer Island, a project that is now being finalised as the East Link of the Sound Transit LRT system.⁽²⁾ A proviso that all be accommodated in one single tube, and that a pedestrian and cycle way be included as well, would kill off the project, resulted instead in a stacked drift design to create that single tube solution (Fig 2).

In an outstanding design and construction operation, 24 x 9.5ft diameter x 1,332ft long shield driven drifts were excavated to permit excavation of the impressive 63.5ft diameter interior space within which the four lane highway, the now the two tracks of the East Link LRT line, and the pedestrian and cycle ways in the upper levels, have been built and have been operating since the underground infrastructure opened in 1988.^(3,4)

The next application of the method was for the Río Piedras Station on the Tren Urbano project in Puerto Rico. Funded 30% by the USA Federal Transit Administration (FTA), Tren Urbano was one of five demonstration projects selected by the FTA in the 1990s to research application of the turnkey design-build procurement method to control costs and time overruns on FTA funded public transit projects. It was also an early application of NATM in the USA.

Fig 3. A rectangular stacked drift design as applied to the Río Piedras Station for the Tren Urano project in Puerto Rico

For Río Piedras, the Kiewit/Kenny/Zachry (KKZ) JV and its underground design engineer Jacobs Associates (now McMillen Jacobs) adopted the stacked drift method, after use of the method also by Kiewit for the Eisenhower highway tunnel through squeezing rock conditions in Colorado, USA. In Puerto Rico the design was needed to create the 240m² Río Piedras Station cavern, at 17m wide x 17m high x 150m long, and under a shallow cover below the surface.⁽⁵⁾

The design comprised 15 interconnecting rectangular drifts of about 3m x 3m, excavated to the full 150m length of the station and back-filled with concrete to create a massive pre-installed lining for the core excavation (Fig 3). Compensation grouting by subcontractor Soletanche-Bachy was selected to limit settlement caused by excavation of the drifts.

Also in the 1990s, the method was applied to excavation of the undersea cavern for the crossover facility on the French side of the Channel Tunnel railway connection between the UK and France. A presupport of 11 contiguous headings of 170m long x 1.7m in dimension and 4.2m wide at the base was excavated ahead of core excavation to create a cavern of 20m wide at the base and 350m² in cross section (Fig 4).⁽⁶⁾

Fig 4. Complex presupport required for excavation of the French crossover cavern on the Channel Tunnel undersea rail connection

An initial three drifts were excavated from access adits from the advancing TBM service tunnel drive and were back filled before the two 8.7m o.d. running tunnel TBMs reached the crossover at 12.5km under the Channel from the French coast. After passage of the running tunnel TBMs, work continued to excavate the remaining eight drifts to permit core excavation and breakout of the 7.6m i.d. segmentally lined TBM drives (Fig 4).

Another example of a presupport sequence is the method adopted for the Alonso Martinez and Tribunal Stations for the Madrid Metro in Spain (Fig 5).

Presupport using smaller diameter microtunnel excavations and the method of presupport pipe umbrellas and spiling are also well-established design concepts. Both have been used to successful effect to presupport and control poor ground conditions in the crown of open face excavations and to control the risks of surface settlement under shallow cover in critical situations.

A recent application of small diameter microtunnel presupport is for the twin tube railway underpass of a 21-lane highway in Toronto, Canada. The 180m long x 80m² rail tunnels were presupported by a canopy of 13 x 813mm diameter steel pipes installed using an auger boring system to limit surface settlement on the operating highway above to 25mm maximum (Fig 6).⁽⁷⁾

The sequence of excavation for a stacked drift concept is a particular topic of debate and experience. Do you start with the lower drifts and work upwards, or start at the top and work down?

In Seattle, the sequence started in the bottom and advanced in a sequence using two custom built shields, one either side, to complete the joining drift number 24 in the crown.

In Puerto Rico, the design submitted for approval noted that the sequence would start with the two invert drifts and advance up either side to the crown. During excavation of the access shafts, it was considered that top down excavation of the drifts would be more convenient, but the change was not accepted by Tren Urbano. As a result, access ramps together with temporary backfill in the working shaft and decking was required to provide access for excavation the higher level drifts.

Fig 5. Presupport excavation for enlarging the caverns for stations on the Madrid Metro in Spain

Fig 6. Recent application in Toronto of smaller diameter microtunnelling presupport of a twin rail pass under a highway in Canada

While the detailed design of the Euston Station stacked drift method is being developed at present, the concept illustration has the drifts numbered with the first in the crown, perhaps suggesting a top down sequence (Fig 1). TunnelTalk will publish further news as it becomes available.

Another concern in the stacked drift concept debate focuses on the control of settlement during the excavation of the drifts. Face loss will be experienced with excavation of the drifts and this settlement risk will require equal control and management ahead of core excavation.

A final consideration is the added cost of the presupport concept and the time needed to excavate and complete the system. The Euston Station drifts look quite large in size by comparison to the cavern excavations that they will presupport.

Once excavated, the drifts of the concept are backfilled with concrete ahead of core excavation. This adds time and cost to the application and as one commentator posted on the TunnelTalk LinkedIn post about the Euston Station stacked drift concept: “Buy one, get seven free”. Other comments on the LinkedIn post mentioned the application of the concept on other projects around the world and raised other points about the application in London including a note that other similar sized caverns have been excavated in London without the need for a presupport design. These include the crossover and Y junction caverns excavated at Stepney Green for the Crossrrail project in south London.⁽⁸⁾

The 17m high x 16.5m x 120m long cavern was designed and excavated as an SCL topheading, bench and invert sequence, dividing further into single and double side-wall drifts as the span of the Y junction cavern increased to its widest point. The Stepney Green caverns are largest excavated by SCL in the UK (other than the UK crossover chamber on the Channel Tunnel, which is slightly larger and also excavated as an SCL or NATM multi drift sequence). Both the Crossrail and the Channel Tunnel caverns were fully excavated before arrival and pull through of the running tunnel TBMs.

The stacked drift design being developed for the HS2 Euston Station caverns might well be considered part of the “gold-plating” of the project design that Doug Oakervee, as Chair of a panel of experts set up by the Government to undertake an independent review of the HS2 project, highlighted in its report.⁽⁹⁾ The report noted that: “there has been considerable over-specification and gold-plating in HS2 contracts with much of the design seemingly done on a worst-case, risk-averse scenario”. To counter the comment, the excavation objective of the Euston Station caverns in particular is to assure the safety and security of the surface and adjacent underground infrastructure in the congested Euston area of the London.

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