Project outline

Murphy Ground Engineering was employed by North Midland Construction to deliver the Ffrydd Wood section of the Elan Valley Aqueduct upgrades following two previous successful phases carried out during 2017.

The works involved:

  • Designing and constructing one temporary and one permanent structure that included a Transition Shaft and a Tunnel Boring Machine (TBM) Drop Shaft

  • Teams from the UK and Ireland working alternative shifts around the clock to meet anticipated installation rates due to difficult ground conditions

  • Utilising a recently procured Bauer BG30 LDP piling rig to complete works without the need for an external oscillator or casing extractor

  • Installing 100 1180/1080mm fully cased secant piles up to 31.7M, four gravel piles, 26 GU21 sheet piles and various sheet pile pre bores
Project delivery and innovations:

Due to its expertise and experience in segmental piling and secant shafts Murphy Ground Engineering was able to offer advice early on in the project which led to a scheme that could be built in accordance with the specification.

For the piles to meet the specified 100-year design life whilst left un-lined in its permanent condition, a Hard/Firm secant option was adopted. This ensured the female piles could meet the required design life of the structure without any need for a secondary lining.

To maximise space within the drop shaft hoop, compression was taken into consideration between the piles which streamlined the design of the shaft. Ring beams were also utilised at various depths through the shaft to spread the loads between all of the piles in the wall. This eradicated the need of any props spanning across the shaft which would have caused issue with lowering and launching the TBM.

The DSI ‘Superlatch’ cage splicing system was adopted for this project due to the complexity and weight of the cages being installed. This splicing system removes the need for any of the site operatives putting their hands within the suspended cages as was previously required with more traditional methods. As one cage is lowered over the preceding cage the receiving band is pushed past the jaws of the latch, the integral spring closes the latch automatically thereby forming a connection between the two cages.

Key challenges

Rock levels varied across the project by up to 13.5m with rock strengths increasing up to 200Mpa which is at the limit of what a piling rig with conventional tooling can drill.

Due to the design and proposed use of the drop shaft, the piles could not be based on a rock socket length and had to be installed to a pre-determined toe level. This was due to the shaft being fully exposed to, and beyond the toe of the 31.7m piles and to maintain hoop compression to toe depth.

Further challenges were encountered whilst designing the transition shaft high level break out portal.

The need for a 3.5m wide break out meant that two of the male piles would need a section breaking out to allow follow on construction works. This led to Murphy Ground Engineering designing a 880mm x 370mm rectangular reinforcement cage with bespoke spacers that could be positioned towards the outer edge of the pile to facilitate the required break out.

This ensured the performance and stability of the piles in the area were not compromised as no reinforcement was cut or damaged.

Key facts
  • 100 1180/1080mm Rotary Bored Segmental piles installed
  • 26 GU21 sheet piles driven
  • 48m of secant guide wall installed
  • 3400m3 of concrete poured
  • Cages up to 8 tonne in weight installed
  • 6 month construction period
  • Bauer BG30 at Elan Valley.
    Bauer BG30 at Elan Valley.
  • Cages in use at Elan Valley.
    Cages in use at Elan Valley.
  • Shaft excavation at Elan Valley.
    Shaft excavation at Elan Valley.

Related projects

Bauer BG30 machine.

Building from the ground up

The Ground Engineering team at Murphy offers in-house pile design, value engineering and bespoke technical solutions throughout the UK and Ireland. We have a proven track record of successfully delivering both minor and major works across a range of sectors.

Ground Engineering