The Skanska Costain STRABAG (SCS) JV is building 13 miles of twin-bore tunnels on the HS2 route, connecting London to the Midlands, North and Scotland. Valued at four billion pounds, this new high-speed railway will have its southern terminus at Euston. Given that 95% of the journey is underground, the project involves six Tunnel Boring Machines (TBMs), with five operating simultaneously.

The preliminary studies carried out as part of the HS2 Euston Tunnels planning phase concluded that various bridges along the HS2 tunnels route could be potentially affected by tunnelling-induced ground movements. BG&E was appointed by OTB Engineering (working for SCS JV) to provide bridge engineering services, including site inspection, surveys and detailed structural analysis to review the results of the initial studies completed by the permanent works designers.

The approach followed by BG&E to maximise value for the Client was to

• Refine the structural analysis and incorporate soil-structure interaction considerations to maximise the opportunities to demonstrate that the existing bridges could accommodate the predicted ground movements without mitigation measures.
• Propose and develop the most efficient mitigation schemes only when strictly necessary.

Working in partnership with OTB Engineering (tunnelling specialist), BG&E analysed a large number of bridge structures, providing recommendations on the predicted level of damage. A summary of BG&E’s scope of work is highlighted below.

Phase 3 ground movement assessments (GMAs)

• Site surveys and inspections.
• 27 nos. refined GMA Reports (bridge assessments).
• For 8 nos. Transport for London (TfL) assets, BG&E also provided the additional deliverables to TfL and HS2 standards:

• • • Bridge Approval in Principle (AiP) for Assessment.
    • Structural Assessment Reports.
•  For 4 nos. Network Rail (NR) assets, BG&E also provided additional deliverables to NR and HS2 standards:
• • • Forms A, B and C.
    • Forms AA and BA.
    • CRE Civils.

Mitigation design
• 3 nos. bridge mitigation design schemes to TfL, NR and HS2 standards.

Phase 3 GMA independent Cat 3 checks
• 7 nos. bridge GMA Reports Cat 3 Checks.
• 2 nos. bridge mitigation scheme design Cat 3 Checks to TfL and NR standards.

As part of the 27 nos. bridge Phase 3 refined GMA reports, full structural assessments to the asset owner’s standards were carried out to determine the load capacity of all key structural components with and without the ground movement effects.

The typologies of the rail and road bridges and footbridges assessed included

• Reinforced and restressed concrete decks.
• Composite decks.
• Steel trusses.
• Half-trough steel decks.
• Masonry arches.
• Highly skewed decks.
• Reinforced concrete portal frames.
• Multi-span continuous decks.

As a result of the refined analysis, the mitigation works required ahead of the TBM crossing were significantly reduced from the initial 27 bridges to only three.

The significant positive contribution made to the overall project can be summarised as follows

• 90% reduction in the number of structures requiring mitigation (from 27 to only three).
• Reduction of the mitigation scheme scope.
• Significant construction programme risks removal or reduction.
• Reduction in the carbon footprint by eliminating almost entirely the initial mitigation schemes identified.
• Minimal disruption to third parties, asset owners and asset users for some of the key traffic and rail networks in the London area.

BG&E’s team had demonstrated, throughout the duration of the Asset Protection Framework, the ability to undertake complex analysis, apply sound engineering and risk management principles, understand the key and most essential project requirements and expectations, and provide a whole range of services to maximise the additional value delivered to the Client, stakeholders and the community.