Bridge analysis, design + assessment

Case Study

BEBO Arch System Modelling

  • First BEBO arch structure to be built in the UK

  • Soil-structure interaction modelling

  • Staged construction analysis of all backfilling and overfilling stages

Dobwalls Bypass arch tunnel under construction

BEBO Arch International AG specialises in the development and application of the BEBO System and the design of overfilled reinforced concrete arch structures in general. BEBO used LUSAS Bridge analysis software to carry out a staged construction analysis of its detailed design for an 85m long, overfilled arch tunnel for the Dobwalls Bypass scheme. This was the first BEBO Arch structure to be constructed in the UK, and is just one of many structures worldwide designed by BEBO with the aid of LUSAS software.

Dobwalls Bypass

As part of the Dobwalls Bypass scheme a re-alignment of a 1km length of the A390 to the west of Dobwalls required a suitable structure to be built to carry the road over the main London to Penzance railway line. The chosen solution was to construct a BEBO Arch structure, the first of its type to be built in the UK. This solution was considered to have the lowest life cycle cost and cause minimal disruption to the railway during construction and maintenance. It also offered a fast and safe installation procedure. BEBO's proposed design not only met the many project requirements, but had to undergo one of the most rigorous testing and approval processes conducted on a public access development before being authorised for construction.

Foundations were placed such that construction of the piles and pile cap could be done from a position of safety of at least 3 metres back from the nearest rail. Following piling and pilecap construction, the precast arch units of 15.5m span and 5.6m rise were lifted into place and installed over the live railway during a series of 7 hour overnight full track possessions. The 38 standard arch pairs used (76 arch units in total) were installed in just four track possessions, giving an average output of 19 units per possession. Once constructed, backfilling took place symmetrically using prescribed types and thicknesses of fill.

Arch installation during track closure Bevelled ends to tunnel

Modelling with LUSAS

The existing railway embankment was modelled sitting on layers of clay, weathered mudstone and mudstone. Then, staged construction modelling in LUSAS using nonlinear analysis replicated the entire construction process including the installation of the piles and pilecap; the addition of a fill layer to the level of the existing railway embankment; the installation of the concrete arch segments and initial backfilling to the top of pedestal level. This was then followed by the addition of a series of roller compacted fill layers of 0.5m thickness up to arch crown level with, finally, two, 1.5m overfill layers completing the embankment construction. 

During backfilling, measurements of arch deflections were taken and the LUSAS input parameters, mainly for the subsoil, were updated to understand the unexpected behaviour of the piled foundation. Excellent agreement between LUSAS and the monitoring results was found reassuring all parties that the construction method and the structure itself would be safe for the designed loading. 

LUSAS was used for the entire analysis and design of the 85m long Dobwalls Bypass tunnel, including the innovative and client-requested bevelled ends. For these, the options available in LUSAS for defining local axes made the designing of the bevels using Clarke-Nielsen results very straightforward.

Dobwalls Bypass tunnel : modelling of backfill procedure

Modelling of the tunnel backfilling and overfilling stages in LUSAS

Dobwalls Bypass tunnel : modelling of backfill procedure

Staged construction modelling of the arch system installation and backfilling process

Dobwalls Bypass tunnel : crown heave verses fill height

LUSAS calculated and on-site measured values for crown heave versus fill height

Completed and named Sperritt Tunnel in 2010. 

Seremban Senawang Expressway, Malaysia

This example of another installed BEBO tunnel is located approximately 1 hour South of Kuala Lumpur,  serves the widening of the Seremban Senawang Expressway and the crossing of an access road. As it runs parallel to a railway, the tunnel arches could not be backfilled or overfilled in the usual manner so a Mechanically Stabilized Earth (MSE) wall was used to hold the fill in place. 

LUSAS Bridge analysis software was used to assist with the design of the arches and to confirm the design of the MSE-wall and piled foundations. LUSAS was used on this project because of the number of sections that had to be investigated and because modifications to the geometry, such as angle of battered piles, could be so easily modified and re-analyzed. Results for one section along the length were cross-checked against another software package proving the Mohr-Coulomb model that was used to be adequate for this type of soil stress history.


Seremban Senawang Expressway tunnel modelling

Model geometry

Seremban Senawang Expressway tunnel modelling

Material assignments to the LUSAS model

"BEBO engineers have a lot of experience in the use of finite element analysis software, not only for the design of projects involving soil-structure interaction, but also for a variety of materially nonlinear analysis, particularly of reinforced concrete structures. To assist us with the design of our range of BEBO Arch Bridge Solutions, and for our general structural analysis requirements, LUSAS meets our needs perfectly."

Gian Nick, Managing Director, BEBO Arch International AG


The BEBO SystemThe BEBO System

The BEBO System is a pre-engineered system of precast reinforced concrete arch elements and spandrel wall and wing wall elements, all assembled and secured to a prepared concrete foundation. Straight arch ends are shown on the accompanying image but bevelled ends, as first requested on the Dobwells Bypass project, can also be used.


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Any modelling, design and analysis capabilities described are dependent upon the LUSAS software product, version and option in use.