Achieving the ‘Greenest Games Ever’, the Geotechnical Engineers Challenge

The bid to host the 2012 Olympic and Paralympic Games included a commitment to achieving “the most sustainable games ever”. This set a challenge for the design and construction of the Olympic Park that would have a significant impact on every aspect of design.

But, with cost and time limits, how would the Olympic Delivery Authority be able to achieve such goals and how could geotechnical engineers adapt their design to incorporate these challenges. This article introduces the general process used to make London 2012 the ‘greenest games ever’ and geotechnical case studies showing how even simple designs incorporated sustainable goals.

Achieving Sustainability

To keep to their promise, the ODA developed a set of 12 Sustainability Objectives that they require all their delivery partners to support the implementation of.

The engineers working on the Olympics were tasked with incorporating these into their design and the following targets were set to give more specific requirements for design:

  • 20% by value of reused or recycled construction materials to be used.
  • 25% by weight of recycled aggregates to be used (as a proportion of total aggregate)
  • 90% of demolitions materials to be reused and recycled
  • All infrastructure to achieve a CEEQUAL accreditation

The requirement for CEEQUAL accreditation was particularly permanent as any claims has to be evidence based so designers were required to provide drawings, calculations and meeting minutes and record their actions accordingly.

Reinforced Soil Design

The Olympic Park involved extensive landscaping. A large amount of geotechnical design incorporated reinforced earth design. 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 and provided 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 strengthened earthworks around bridges and rivers incorporated basal reinforcement and for significant retained heights, the reinforced soil sloped were founded on Vibro Concrete Columns (VCC). Initially 2,700 VCC’s were proposed to satisfy design standards and account for design standards and uncertainty in the ground conditions. On site testing was proposed, which meant that the VCCs needed could be measured precisely. On-site testing resulted in a significant reduction in the number of VCCs required across the project from 2,700 to 2,000. This resulted in cost saving at an average cost of £500 per unit and significant time saving for the construction programme.

Engineers also used Oasys Slope to assess slope stability. Oasys Slope enabled easy incorporation of basal reinforcement into the design of slopes and the ability to assess the risk of flooding to the slopes. 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 incorporating the aspects of design mentioned, understanding the geometry of the site was critical and 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.

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Integral Bridge Design

Bridges with integral abutments are characterized by two major advantages. Firstly that expansion joints and bearings are omitted, reducing the maintenance and whole life cost of the bridge. Second that the superstructure is restraint at the end supports such that end moments counteracting to the field moments leads to bridges with a small construction height of the superstructure. This saves on material, meeting a crucial requirement for sustainability.

However, the geotechnical analysis of integral bridge abutments is not straightforward. 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, induce bending moments which are directly related to the combined stiffness of the pile and the surrounding soil. In order to calculate bending moments in the piles, the stiffness of the soil must be included.

The Engineers needed to use a simple method of analysing the soil-structure interaction to enable identification of the effect and sensitivity of critical design parameters and to quickly scheme up a variety of options. The design should be to such a level that the most suitable scheme could be identified and the chosen scheme could be taken through approvals and detailed design with the minimum of design changes.

The analysis of soil structure interaction needed to address several different loading conditions, each of which can have a critical effect on the integral bridge and involve the interaction of deck movements (displacements and/or rotations) and the soil resistance on the back of the abutment.

These can be summarised as:

  • Deck rotation due to vehicle loads resisted by soil stiffness behind abutment.
  • Increased long-term soil pressure due to deck expansion/thermal ratcheting.
  • Thermal contraction and reduced soil pressures.
  • Breaking loads resisted by soil pressure.

Oasys Frew was used with guidance from BA 42/96 and this enabled the relatively quick and conservative analysis of a complex problem. Further to this, the design experience gleaned from the Olympic Park Project was used to develop Oasys Frew, which incorporates integral bridge design.

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 Summary

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 examples discussed show how research, cutting edge analysis and exemplary design can go hand in hand with sustainability. The following quote shows the enduring legacy of a sustainable games and this legacy translates to sustainability being the keyword in future Olympic bids.

Setting the bar high means that this level of design will need to be sustained by Geotechnical Engineers in the future.