Aerial view of the siteWinner of three industry awards, embankment stabilisation works for Thrandeston Bog designed by Mott MacDonald represent the first use of an innovative ground treatment technique - dry soil mixing - to stabilise a key part of the UK rail network. The works mark a UK first in terms of the application of dry soil mixing immediately adjacent to a live railway for the treatment of up to 10m of peat and very soft clay whilst maintaining the full use of the railway. The project was delivered to a cost of some 50% less than the client’s original scheme, saving more than £6 million, and achieved a more environmentally sustainable solution with a lower carbon footprint for the site, located in a rural and environmentally sensitive part of Suffolk.

Site conditions at Thrandeston, located on the strategically important London to Norwich mainline, have historically proved problematic, dating back to the construction of the original Great Eastern Railway in 1847. Constrained by the ground conditions at the site, comprising over 10m of peat and very soft clays, the original construction was subject to repeated slope failure and excessive embankment settlement.

More than 150 years on, the challenges at Thrandeston Bog continued for Network Rail. Instability of the embankment, some 650m in length and up to 5m high was ongoing with movement of the both the tracks and overhead power line stanchions. The asset required a high degree of maintenance and monitoring while the possibility of embankment failure would threaten the continued operation of this mainline railway.

In 2006 Network Rail employed Mott MacDonald to provide a detailed design for the stabilisation of the railway embankment, increasing the design life of the structure for a further 120 years. There was also a desire to provide a solution aesthetically sympathetic to the rural setting of the site that could be implemented without adversely affecting a wet meadow Site of Special Scientific Interest located along the boundary of the site.

Dry soil mixing rig in operation adjacent to live railway tracksMott MacDonald proposed the use of dry soil mixing to realise the project aims, strengthening the peat and soft clay upon which the railway embankment was founded and effectively creating stabilised ground either side of the railway tracks. The technology was applied immediately adjacent to a live railway, up to 2.75m from the existing overhead electrified line, establishing a UK first. This design solution was proposed and accepted by Network Rail in preference to a conventional piling scheme consisting of a line of tubular steel piles installed along both sides of the embankment.

From inception of the dry soil mixing scheme Mott MacDonald provided a suite of services to Network Rail, including preliminary and detailed design, production of specifications, advice on contacts and selection of a preferred contactor and technical support during construction encompassing both the ground engineering works and environmental support. Additionally, Mott MacDonald designed new gantry structures as a replacement for the existing overhead line stanchions and designed a new culvert that was installed beneath the embankment adopting a guided auger-bored installation technique.

Technique and scheme design
Dry soil mixing is used for increasing the strength of very soft soils. The technique injects and then mixes cement powder into the soil to create columns of stronger material, known as soilcrete.

The scheme design involved installation of approximately 11,000 soil mix columns (total meterage 75,000) in panels along both sides of the embankment. The ground treatment effectively buttressed the soft ground beneath the railway lines to prevent it from spreading laterally and the track level setting. Application of the scheme required the following research and studies:

• numerical modelling to validate the long term performance of the treated ground using as a basis the European Soil Stabilisation research findings; and
• laboratory and field studies to demonstrate the effectiveness of the treated ground at the site prior to implementation and for value engineering the design in the field.

Research, laboratory and field studies
The application of the dry soil mixing technique, using the EuroSoilStab project as a basis established the principles for work at Thrandeston Bog. This research was extended by Mott MacDonald to encompass the particular site specific constrains of implementing the works immediately adjacent to a live railway, with particular focus on the long term embankment settlement and the application of recent research on the lateral spreading of soft clays.

Finite element numerical modelling was undertaken, which utilised a non-linear stress-strain-time dependant model to represent the behaviour of the peat and very soft clays which demonstrated that the long term track movements over the lifetime of the earth structure would not exceed tolerable limits.

The second element of the design development entailed extensive laboratory and field studies to demonstrate that the soil mixing would significantly improve the strength of the soft ground and to optimise the cement dosage required to achieve the design strengths. To accomplish these goals a programme of field and laboratory testing in excess of £100,000 was implemented.

Innovation and value
Technical innovation achieved reductions to both cost and environmental impactsInitially the results of the laboratory tests were disappointing as the strength of the soil mix materials were relatively low. However, subsequent field studies demonstrated the suitability of dry soil mixing technique and identified further value engineering opportunities through the use differing cement contents for the treatment of the clay and peat zones along the length of the site. This led to significant savings of around £100,000.

The application of the dry soil mixing in favour of a conventional piled solution also created a more environmentally sustainable solution with manufacture of the raw material generating 80% less embodied energy at a cost of less than half of the pile solution, saving the scheme more than £6 million – demonstrating the clear advantages of the dry soil mixing solution.

The project’s accomplishments demonstrate Mott MacDonald’s commitment to delivering innovative solutions, illustrating the value of investing in developing technologies and undertaking applied research. Industry recognition of the project has followed since completion, with the project succeeding in winning three awards.

The completed embankmentThe project triumphed at the 2009 ACE Engineering Excellence Awards, claiming the prize for Research, Studies and Consulting, while the Ground Engineering Awards saw the scheme take the top award for Project with a Geotechnical Value over £1 million where it was also highly commended in the Innovation Award category. At the Institution of Civil Engineers, East of England Merit Awards the project succeeded in winning in the Innovation and Technical Excellence Award category and the project was also Highly Commended in the National Rail Awards, Civil Engineering Achievement of the Year award category.