Small Dean Viaduct
Small Dean Viaduct
Client: HS2 Ltd
Main contractor: EKFB JV (Eiffage, Kier, Ferrovial Construction, BAM Nuttall)
Design partner: ASC (Arcadis, SETEC and COWI joint venture)
Architect: Moxon
Record-breaking deck slide across operational infrastructure in North Chilterns .
The 345 metre Small Dean Viaduct carries HS2’s high‑speed rail line across the A413 and Chiltern railway south of Wendover, Hampshire.
We undertook a Category 3 check of the temporary works for the 4,500 tonne deck slide, the heaviest and longest single‑stage viaduct deck slide on the HS2 project. Our role also included structural assessments, reinforcement stability analysis, and geotechnical design throughout construction. The operation required sliding the entire assembled deck in one continuous four-day stage over a live road and railway.
The 345 metre Small Dean Viaduct carries HS2’s high‑speed rail line across the A413 and Chiltern railway south of Wendover, Hampshire.
We undertook a Category 3 check of the temporary works for the 4,500 tonne deck slide, the heaviest and longest single‑stage viaduct deck slide on the HS2 project. Our role also included structural assessments, reinforcement stability analysis, and geotechnical design throughout construction. The operation required sliding the entire assembled deck in one continuous four-day stage over a live road and railway.
345m
length of the viaduct deck slid in a single operation
4,500
tonnes total weight of assembled deck moved during record-breaking, slide operation
49m
deep piled foundations carrying high-speed trains up to 250mph on curved alignment
Our Role
Our RoleWentworth provided independent technical support to EKFB, peer reviews of designs and technical advice.
Our work included:
This ensured a fully integrated engineering output throughout the project.
Our work included:
- structural stability assessments of bridge girders and precast U-beams during temporary conditions
- design of temporary support systems to maintain overall stability
- design of temporary works to ensure the reinforcement stability of abutment walls and foundations
- geotechnical engineering input covering working platforms, earth retaining structures, slope stability
- utility and ground movement assessments.
This ensured a fully integrated engineering output throughout the project.
Capabilities
Capabilities- Bridges & Highways
- Infrastructure
- Structural Engineering
- Geotechnics
- Construction Method Engineering
- Temporary Works
- 3D Modelling and Detailing
- Heavy Lift and Logistics
Verifying 4,500 tonne deck slide
Verifying 4,500 tonne deck slideThe Small Dean deck slide moved a fully assembled 340 metre, 4,500 tonne viaduct deck across the A413 and Chiltern railway line.
The weathering steel, double‑composite structure was built in full before being slid into position during a four‑day road and rail closure, with temporary works ensuring safe load transfer at speeds of up to 10 mph.
We completed a Category 3 check of the temporary works design, verifying the steel bearing system, PTFE pad calculations, cable control system, and structural performance, enabling EKFB to complete the slide nine hours ahead of schedule.
The weathering steel, double‑composite structure was built in full before being slid into position during a four‑day road and rail closure, with temporary works ensuring safe load transfer at speeds of up to 10 mph.
We completed a Category 3 check of the temporary works design, verifying the steel bearing system, PTFE pad calculations, cable control system, and structural performance, enabling EKFB to complete the slide nine hours ahead of schedule.
Assessing stability during temporary construction
Assessing stability during temporary constructionThe precast Y‑shaped piers and deck components experienced different loading conditions during their temporary condition compared to their in-use state. Each temporary phase posed distinct stability challenges, requiring detailed structural analysis and temporary support design.
Our team carried out stability assessments of bridge girders and precast U‑beams throughout all temporary conditions, analysing structural behaviour at each stage. Temporary support systems were designed to ensure stability during lifting, positioning, and assembly.
Our assessments enabled EKFB to safely construct the innovative double‑composite deck, with each temporary phase fully validated before progressing to the next stage.
Our team carried out stability assessments of bridge girders and precast U‑beams throughout all temporary conditions, analysing structural behaviour at each stage. Temporary support systems were designed to ensure stability during lifting, positioning, and assembly.
Our assessments enabled EKFB to safely construct the innovative double‑composite deck, with each temporary phase fully validated before progressing to the next stage.
Designing geotechnical solutions
Designing geotechnical solutionsThe viaduct piers sit on 49-metre-deep piled foundations, requiring substantial working platforms and excavation support. Construction adjacent to a railway line and the A413 demanded careful ground movement assessment and utility protection, while varying ground conditions along the 345 metre alignment required tailored earth-retaining and slope stability solutions.
We designed working platforms for heavy piling rigs, developed earth-retaining structures, and conducted slope stability, utility, and ground movement assessments to protect nearby infrastructure.
We also analysed reinforcement stability for the abutment walls and foundations, ensuring temporary conditions safely transitioned to the permanent state, giving EKFB confidence throughout construction.
We designed working platforms for heavy piling rigs, developed earth-retaining structures, and conducted slope stability, utility, and ground movement assessments to protect nearby infrastructure.
We also analysed reinforcement stability for the abutment walls and foundations, ensuring temporary conditions safely transitioned to the permanent state, giving EKFB confidence throughout construction.
Outcomes
OutcomesThis project demonstrates the value of comprehensive construction engineering involvement from early stages through to construction completion. Our multi-disciplinary engagement ensured construction knowledge informed every phase before problems could emerge on site.
Key results:
Key results:
- Record-breaking achievement: Successfully verified the temporary works for HS2’s heaviest and longest single-stage viaduct deck slide.
- Programme acceleration: Enabled the slide to be completed nine hours ahead of schedule.
- Contractor confidence: Integrated design services, including geotechnical through structural and temporary works, gave EKFB certainty throughout construction.
- Supported innovation: Structural assessments and temporary works verification supported the UK's first double-composite railway viaduct construction, cutting embedded carbon by more than half.
- Infrastructure protected: Ground movement assessments and utility evaluations kept the Chiltern railway and A413 operational throughout 49-metre deep foundation construction.
The Small Dean Viaduct deck slide was completed in February 2025. Wentworth's independent verification contributed to one of the most significant engineering achievements on HS2's central section.