Temporary Works support for a Bridge Drive-out
Temporary works design support — granular access ramp, trackway transition, machine control, and setting-out models
Commercial Client
Project Overview
The Severus Footbridge forms part of the York Central infrastructure programme delivered by John Sisk & Son, providing a new pedestrian and cycle link across the East Coast Main Line alongside the existing Severus Road bridge.
The completed span was assembled on site and moved into its lift position on Mammoet self-propelled modular transporters (SPMTs) before being lifted over the railway by tandem crawler cranes. The whole operation sat inside a single fixed window combining a rail possession with a full closure of Water End.
That drive-out phase depended entirely on the temporary running surface beneath it. The bridge had to be carried from the assembly area, up a temporary granular ramp, and out onto the public highway across a matted transition. Keystone Black was engaged to work alongside Sisk’s internal Temporary Works Department, providing additional design capacity for the access ramp and its transition onto Water End.
A Collaborative Approach
Sisk retained overall temporary works coordination, approval, and control throughout. Keystone Black was brought in as focused design support to develop the access ramp and trackway transition, working directly to Mammoet’s drive route requirements and to the fixed programme date.
The value of that arrangement was in how quickly the two teams could work together. With survey, design and setting-out held by a single party, the ramp could be developed, checked against the move requirements, issued to machine control, and handed to the matting installer without the loss of information that usually sits between those steps.
Severus Footbridge — three-span weathering steel pedestrian bridge, approximately 75m
The Challenge
The move presented a set of constraints that had to be resolved together rather than in sequence:
- An immovable date, set by the rail possession and the agreed road closure.
- Tight geometric tolerance — SPMT transport is sensitive to longitudinal gradient, crossfall and to sudden changes between the two.
- Existing ground varying by approximately five metres across the ramp footprint.
- A transition from a construction platform onto live public highway, crossing kerb, channel, and road crown.
- A design that had to be buildable with the plant, materials, and time available on site
- Setting out that had to be verifiable in the field, at night, under closure conditions.
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Keystone Black
Our Solution
4.1 Survey-Led Baseline
The starting point was a topographic survey of the existing extents. Rather than a simple point cloud, the survey was string-modelled to pick up the features that mattered to the move — kerb face, channel line, road crown, edge of the existing haul road, footpath and fence line — so that every constraint was carried into the design model as real geometry.
Figure 3 Existing ground survey and the developed design surface for the temporary granular access ramp.
4.2 Integrating the Drive Route
Mammoet’s drive route requirements were then layered over the survey. Working from those requirements, the available linear and transverse tolerances were assessed along the length of the route, identifying where the existing ground could be accepted, where it needed to be regulated, and where a designed ramp was the only viable answer.
4.3 Ramp Design Development
The granular access ramp was developed as a single design surface across approximately 1,950 m² of formation, requiring in the order of 1,500 m³ of engineered fill. The climb was held at a consistent 7.0 per cent (1 in 14.3) across successive intervals, deliberately avoiding step changes in gradient, and terminated in a level receiving platform at 16.410 m AOD for the transporter to sit on. Gabion units were modelled along the western flank where fill depths were greatest, and a build-out was designed at the north-east corner where the route swung onto the road.
Figure 4 Ramp long section — a sustained, consistent gradient onto a level receiving platform.
4.4 Transition onto Water End
The existing road geometry was assessed for its suitability to carry the move, and the transition between the ramp head and Water End was modelled and analysed as part of the same exercise. A 4.90 m running width was set out over a 54 m run, at a consistent 2.7 per cent longitudinal fall with a held 2.0 per cent crossfall, so that the change from temporary surface to public highway was made without an abrupt change in geometry.
4.5 The Trackway Model
A dedicated trackway model was then developed for the matting itself. The layout was set to the optimal orientation for the SPMT driveline, so that the mats ran with the transport rather than across it through the change in direction at the corner.
Because the layout was modelled rather than estimated, it also gave the number of mats required, which allowed the correct quantity to be procured ahead of installation, and it gave every mat a defined position for controlled placement on site.
Figure 5 Trackway model — mat orientation and layout from the ramp head onto Water End, with corner build-out and gabion positions held in the same model.
4.6 Delivery Through Machine Control
The completed design surface was fed directly into GPS machine control. Plant on site worked to the model to complete the final trim, producing a prepared formation to the required tolerance and giving Trackway a compliant surface to install the matting onto. Removing the intermediate step between design and plant took a significant amount of interpretation, and therefore risk, out of the build.
4.7 A Single Coordinated Setting-Out Model
The matting layout, transition design, corner build-out, gabion positions and site constraints were all held in one setting-out model. That gave the site team a single reference for the works, rather than a set of separate drawings that would each have needed reconciling on the night.
Figure 6 Left, the bridge under way on SPMTs over the temporary surface towards Water End; right, the span supported on temporary trestles at Water End.
5. Key Design Outcomes
- Approximately 1,950 m² of designed formation, in the order of 1,500 m³ of engineered fill
- Sustained ramp gradient held at 7.0 per cent (1 in 14.3) with no step changes.
- Level receiving platform at 16.410 m AOD.
- 4.90 m matted running corridor over 54 m at 2.7 per cent longitudinal fall and 2.0 per cent crossfall
- Mat orientation set to the SPMT driveline, with quantities and positions taken from the model.
- Design issued directly to GPS machine control for final trim.
- Survey, design, setting out and matting layout coordinated in a single model.
6. Outcome and Client Value
6.1 Programme Certainty
- Design delivered inside a compressed window against a fixed, externally driven date.
- Ramp and transition completed and handed over in time for the drive-out.
6.2 Buildability
- Designed around the plant, materials, and working conditions actually available on site.
- Machine control delivery removed interpretation between the model and the trim.
6.3 Tolerance Assurance
- Existing road geometry assessed for suitability rather than assumed.
- Clear, defensible geometry provided to the move contractor before the operation began.
6.4 Collaboration
- Additional design capacity provided alongside the client’s own temporary works team.
- One coordinated model rather than several separate information sources
The bridge was driven out and installed as planned.
Figure 7 The completed span on transporters at York Central, ahead of the lift over the East Coast Main Line.
Why Keystone Black?
This project demonstrates our ability to:
- Step into a live temporary works programme and add design capacity without disrupting it.
- Move from survey through design to machine-ready model quickly and without information loss.
- Design to the real tolerances of specialist plant, not to generalised assumptions.
- Produce clear, defensible outputs that site teams can build to and check against
- Work collaboratively alongside main contractor temporary works teams and specialist subcontractors.
Services Delivered
- Topographic and string-modelled survey of existing extents
- Temporary works design — granular access ramp for SPMT transport
- Trackway matting transition design and setting out.
- Drive route integration and tolerance assessment.
- Earthworks modelling and quantity assessment.
- GPS machine control model preparation
- Coordinated setting-out models for site delivery.
Looking Ahead
The same approach — survey-led modelling, tolerance-driven design, and direct delivery into machine control — applies wherever heavy or abnormal loads have to be moved across constrained ground to a fixed date. It is a method that shortens the distance between design intent and what is actually built.
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