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An olympic ambition

Last year’s London 2012 Olympics was pitched as being the greenest Games ever. Zeena Farook looks at the role played by geotechnical software in Arup’s work to deliver this aim.

the London 2012 Olympic and Paralympic Games was planned to regenerate part of the city and included a commitment to achieving “the most sustainable games ever”. While most people saw the impact of this commitment during the event, it also set a challenge for the design and construction of the Olympic Park.

With tight budgets and an unmoveable deadline, the Olympic Delivery Authority (ODA) set targets for the design work to meet this green ambition from the outset. Starting from the ground up meant that the geotechnical design element was the first to meet the challenge and has resulted in case studies that demonstrate sustainable geotechnics.

All infrastructure at the park also had to achieve a CEEQUAL accreditation, which is an objective measure of sustainability

To keep its promise, the ODA set engineers working on the Olympics project with specific design targets, which included using 20% (by value) recycled construction materials, 25% (by weight) of all aggregates used to be recycled and 90% of demolition waste to be reused and recycled within the development.

olympics

The green im for the Olympics site started with geotechnical design

All infrastructure at the park also had to achieve a CEEQUAL accreditation, which is an objective measure of sustainability as it has to be evidence based, so designers were required to provide drawings, calculations and meeting minutes, and to record their actions.

Design detail

The Olympic Park involved extensive landscaping and a large amount of the geotechnical design for this incorporated reinforced earth techniques. Splayed reinforced soil design was selected for the bridge abutments, not only for the aesthetic aspects, but for its contribution to ecology. The splayed reinforced soil reduces the “urban heat island” effect - where urban areas have higher ambient temperatures than rural areas - and provides opportunities for planting and habitat creation.

The strengthened earthworks design also provided opportunities to incorporate sustainability but, for such an extensive site with interweaving rivers, this only added to the geotechnical constraints.

The geotechnical design for the earthworks around bridges and rivers incorporated basal reinforcement and, where there was a significant retained height, the reinforced soil sloped were founded on vibro concrete columns (VCC).

Initially, 2,700 VCCs were proposed to meet the design demands and uncertainty regarding the ground conditions. On-site testing was proposed by Arup, which meant that the number of VCCs required could be more accurately calculated.

On-site testing resulted in the number of VCCs needed being reduced to 2,000 and, with each column costing around £500 to construct, the costs were reduced and also meant there were significant time savings for the construction programme.

Slope solution

Engineers also used Oasys Slope geotechnical software to assess slope stability, which helped incorporate the basal reinforcement into the slope design and allowed the risk of flooding to the slopes to be assessed. The design guidance also allowed for the departure from standard materials such as Class 6I fill to cater for the target to re-use material. Consequently, where polymer geogrids were used for shallow slopes, materials from on-site fill sources were considered and it was critical to assess the impact of these materials on the slope stability of the proposed strengthened earthworks.

As well as the design tasks, the software also played a critical role in understanding the site geomorphology. A number of cross sections were considered for each slope to assess the extent of VCCs, basal reinforcement and the suitability of on-site fill instead of standard reinforced earth fill.

“For the geotechnical analysis the soil, embankments, abutments, piers and bridge deck must all be considered as one compliant system and this presents a challenge for load distribution calculations”

Building bridges

Using bridges with integral bridge abutments was a key part of meeting the demand for sustainable solutions at the park, but geotechnical analysis of such structures is not straightforward.

Bridges with integral abutments have two major advantages: they don’t have expansion joints or bearings, which reduces maintenance and whole life cost of the bridge; and with the superstructure restrained at the end supports the end moments counteract the field moments resulting in a slimmer bridge deck, further saving on construction materials.

However, for the geotechnical analysis the soil, embankments, abutments, piers and bridge deck must all be considered as one compliant system and this presents a challenge for load distribution calculations.

An important aspect of integral bridge design is the modelling of the soil-structure interaction. For abutments with vertical piles, and particularly those with only a single row, shortening of the superstructure due to creep and shrinkage and movements arising from traction forces, induces bending moments which are directly related to the combined stiffness of the pile and the surrounding soil. To calculate bending moments in the piles, the stiffness of the soil must be included.

Arup needed a simple method of analysing the soil-structure interaction to identify the effect and sensitivity of critical design parameters and to quickly develop a variety of options. The design needed be to such that the most suitable approach could be identified and taken through the approvals and detailed design stage with minimum changes.

The analysis of soil structure interaction needed to address several different loading conditions. Each of these conditions can have a critical effect on the integral bridge and involve the interaction of deck movements and the soil resistance on the back of the abutment.

The issues analysed included deck rotation due to vehicle loads resisted by soil stiffness behind abutment; increased longterm soil pressure due to deck expansion/thermal ratcheting; thermal contraction and reduced soil pressures; and breaking loads resisted by soil pressure.

Software package Oasys Frew was used with guidance from BA 42/96 and this enabled the relatively quick and conservative analysis of the complex problem to be carried out. The design experience gleaned from the Olympic Park project led to further development of the Frew software package to incorporate integral bridge design analysis.

According to Arup, the Olympic Park design was an opportunity to showcase innovation but also set a new sustainability bar for the designers. Geotechnical engineers are not only affected by these goals but can overcome them to achieve something beyond good design. The company, and its software developer Oasys, believe that research, cuttingedge analysis, and exemplary design can go hand in hand with sustainability.

The impact made by the design and running of the games has been recognised by organisations outside of the ODA. World Wildlife Fund UK chief executive David Nussbaum said: “We are pleased to report that London has set the bar higher and has moved faster than previous comparable events. But as the batons are passed to those responsible for delivering a legacy from London 2012, and for putting on future Games, we expect that a stronger commitment to sustainability will continue.”

Arup hopes that by setting the bar high for London means that this level of design will be sustained by geotechnical engineers in the future.

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